Impact theory

Asteroids that can intersect the orbit of the earth are discussed, which include Aten asteroids (semimajor axis (a) less than 1 AU, aphelion greater than 0.983 AU), Apollo asteroids (a greater than 1 AU, perihelion less than 1.017 AU), and Amor asteroids (perihelion distance between 1.017 and 1.3 AU). The principal sources of earth-crossing asteroids appear to be extinct comet nuclei and collision fragments from regions in the main asteroid belt. The total population of earth-crossers is estimated at 13,000, of which approximately 8% are Atens, 50% are Apollos, and 40% are Amors,and the present collision rate of such asteroids with the earth is estimated at about 3.5 objects, to absolute magnitude 18, per million years.

@article{openalexw1615946943,
    author = "Shoemaker, E. M. and Williams, J. G. and Helin, E. F. and Wolfe, R. F.",
    title = "Earth-crossing asteroids - Orbital classes, collision rates with earth, and origin",
    year = "1979",
    journal = "NASA Technical Reports Server (NASA)",
    abstract = "Asteroids that can intersect the orbit of the earth are discussed, which include Aten asteroids (semimajor axis (a) less than 1 AU, aphelion greater than 0.983 AU), Apollo asteroids (a greater than 1 AU, perihelion less than 1.017 AU), and Amor asteroids (perihelion distance between 1.017 and 1.3 AU). The principal sources of earth-crossing asteroids appear to be extinct comet nuclei and collision fragments from regions in the main asteroid belt. The total population of earth-crossers is estimated at 13,000, of which approximately 8\% are Atens, 50\% are Apollos, and 40\% are Amors,and the present collision rate of such asteroids with the earth is estimated at about 3.5 objects, to absolute magnitude 18, per million years.",
    openalex = "W1615946943"
}

Platinum metals are depleted in the earth's crust relative to their cosmic abundance; concentrations of these elements in deep-sea sediments may thus indicate influxes of extraterrestrial material. Deep-sea limestones exposed in Italy, Denmark, and New Zealand show iridium increases of about 30, 160, and 20 times, respectively, above the background level at precisely the time of the Cretaceous-Tertiary extinctions, 65 million years ago. Reasons are given to indicate that this iridium is of extraterrestrial origin, but did not come from a nearby supernova. A hypothesis is suggested which accounts for the extinctions and the iridium observations. Impact of a large earth-crossing asteroid would inject about 60 times the object's mass into the atmosphere as pulverized rock; a fraction of this dust would stay in the stratosphere for several years and be distributed worldwide. The resulting darkness would suppress photosynthesis, and the expected biological consequences match quite closely the extinctions observed in the paleontological record. One prediction of this hypothesis has been verified: the chemical composition of the boundary clay, which is thought to come from the stratospheric dust, is markedly different from that of clay mixed with the Cretaceous and Tertiary limestones, which are chemically similar to each other. Four different independent estimates of the diameter of the asteroid give values that lie in the range 10 ± 4 kilometers.

@article{alvarez1980extraterrestrial,
    author = "Alvarez, Luis W. and Alvarez, Walter and Asaro, Frank and Michel, Helen V.",
    title = "Extraterrestrial Cause for the Cretaceous-Tertiary Extinction",
    year = "1980",
    journal = "Science",
    abstract = "Platinum metals are depleted in the earth's crust relative to their cosmic abundance; concentrations of these elements in deep-sea sediments may thus indicate influxes of extraterrestrial material. Deep-sea limestones exposed in Italy, Denmark, and New Zealand show iridium increases of about 30, 160, and 20 times, respectively, above the background level at precisely the time of the Cretaceous-Tertiary extinctions, 65 million years ago. Reasons are given to indicate that this iridium is of extraterrestrial origin, but did not come from a nearby supernova. A hypothesis is suggested which accounts for the extinctions and the iridium observations. Impact of a large earth-crossing asteroid would inject about 60 times the object's mass into the atmosphere as pulverized rock; a fraction of this dust would stay in the stratosphere for several years and be distributed worldwide. The resulting darkness would suppress photosynthesis, and the expected biological consequences match quite closely the extinctions observed in the paleontological record. One prediction of this hypothesis has been verified: the chemical composition of the boundary clay, which is thought to come from the stratospheric dust, is markedly different from that of clay mixed with the Cretaceous and Tertiary limestones, which are chemically similar to each other. Four different independent estimates of the diameter of the asteroid give values that lie in the range 10 ± 4 kilometers.",
    url = "https://doi.org/10.1126/science.208.4448.1095",
    doi = "10.1126/science.208.4448.1095",
    number = "4448",
    openalex = "W2110619496",
    pages = "1095-1108",
    volume = "208",
    references = "doi101007bf00212446, doi1010160016703773900665, doi1010160031018268900473, doi101038242032a0, doi101038267403a0, doi1010970001069419540800000019, doi101126science18441411079, doi10113000167606197788367ucmsag20co2, doi10113000167606197788374ucmsag20co2, doi10113000167606197788383ucmsag20co2, doi101146annurevea07050179001115, hays1971faunal"
}

@misc{alvarez1980extraterrestrial1,
    author = "Alvarez, L. W. and Alveraz, W. and Asaro, F. and Michel, H",
    title = "Extraterrestrial cause for the Creataceous-Tertiary extinction",
    year = "1980",
    howpublished = "Science, v. 208, p. 1095- 1108",
    note = "talkorigins\_source = {true}; raw\_reference = {Alvarez, L. W., Alveraz, W., Asaro, F., and Michel, H., 1980, Extraterrestrial cause for the Creataceous-Tertiary extinction: Science, v. 208, p. 1095- 1108.}"
}

@article{alveraz1980extraterrestrial4,
    author = "Alveraz, L. W. and Alveraz, W. and Asaro, F. and Michel, H. V",
    title = "Extraterrestrial cause for the Cretaceous-Tertiary extinction",
    year = "1980",
    journal = "Science, v. 208, p. 1095-1108; See also Letters and authors' reply, Science , vol. 211, pp. 648-656",
    note = "talkorigins\_source = {true}; raw\_reference = {Alveraz, L. W., Alveraz, W., Asaro, F., and Michel, H. V., 1980, Extraterrestrial cause for the Cretaceous-Tertiary extinction: Science, v. 208, p. 1095-1108; See also Letters and authors' reply, Science , vol. 211, pp. 648-656.}"
}

@misc{crossref1982geological,
    title = "Geological Implications of Impacts of Large Asteroids and Comets on the Earth",
    year = "1982",
    url = "https://doi.org/10.1130/spe190",
    doi = "10.1130/spe190",
    openalex = "W1568935506"
}

@misc{sepkoski1982mass7,
    author = "Sepkoski, J. J. and Jr",
    title = "Mass Extinctions in the Phanerozoic Oceans, in Silver, L. T., and Schultz, P. H., eds., Geological Implications of Impacts of Large Asteroids and Comets on the Earth, 190 of Geological Society of America Special Paper",
    year = "1982",
    howpublished = "Boulder, Colorado, Geological Society of America, p. 283-289",
    note = "talkorigins\_source = {true}; raw\_reference = {Sepkoski, J. J., Jr., 1982, Mass Extinctions in the Phanerozoic Oceans, in Silver, L. T., and Schultz, P. H., eds., Geological Implications of Impacts of Large Asteroids and Comets on the Earth, 190 of Geological Society of America Special Paper: Boulder, Colorado, Geological Society of America, p. 283-289.}"
}

@misc{silver1982geological8,
    author = "Silver, L. T. and Schultz, P. H",
    title = "Geological implications of impacts of large asteroids and comets on the earth",
    year = "1982",
    howpublished = "Boulder, Colorado, Geological Society of America, 528 p.; Geological Society of America Special Paper, No. 190",
    note = "talkorigins\_source = {true}; raw\_reference = {Silver, L. T., and Schultz, P. H., 1982, Geological implications of impacts of large asteroids and comets on the earth: Boulder, Colorado, Geological Society of America, 528 p.; Geological Society of America Special Paper, No. 190.}"
}

@misc{thierstein1982terminal10,
    author = "Thierstein, H. R",
    title = "Terminal Cretaceous Plankton Extinctions, in Silver, L. T., and Schultz, P. H., eds., Geological Implications of Impacts of Large Asteroids and Comets on the Earth",
    year = "1982",
    howpublished = "Boulder, Colorado, Geological Society of America, p. 385-399; Geological Society of America Special Paper No. 190",
    note = "talkorigins\_source = {true}; raw\_reference = {Thierstein, H. R., 1982, Terminal Cretaceous Plankton Extinctions, in Silver, L. T., and Schultz, P. H., eds., Geological Implications of Impacts of Large Asteroids and Comets on the Earth: Boulder, Colorado, Geological Society of America, p. 385-399; Geological Society of America Special Paper No. 190.}"
}

Two classes of solid bodies large enough to be detected by telescopes occur in orbits that overlap that of the Earth. These bodies are the Earth-crossing asteroids and comet nuclei. Although their orbits only rarely intersect the Earth's, the probabilities of their collision with the Earth are nevertheless finite and calculable. Systematic telescopic surveys carried out over the past two decades show that the flux of asteroids and comet nuclei in the Earth's neighborhood is sufficiently high that the effects of occasional collisions should be recognizable in the geological record. During these same two decades, an intensive international search for ancient impact structures has gone forward. The actual rate of bombardment of the Earth during the last half-billion years has been found to be roughly consistent with the present rate predicted from astronomical observations. Within a factor of about two, the average rate of bombardment of the Earth during the last half­ billion years also appears to be consistent with the average rate of bombardment of the Moon over the last 3.3 billion years. Spectacular new lines of study have developed in recent years leading to the recognition of rare large impact events that produce geochemical anomalies on a global scale. The possible effects of these large impacts on the Earth's biota have become the subject of vigorous debate. In this paper, I first review the astronomical and geologic evidence concerning the history of bombardment and then discuss the physical effects of large impacts, as they may apply to both the inorganic and organic worlds.

@article{doi101146annurevea11050183002333,
    author = "Shoemaker, Eugene M.",
    title = "ASTEROID AND COMET BOMBARDMENT OF THE EARTH",
    year = "1983",
    journal = "Annual Review of Earth and Planetary Sciences",
    abstract = "Two classes of solid bodies large enough to be detected by telescopes occur in orbits that overlap that of the Earth. These bodies are the Earth-crossing asteroids and comet nuclei. Although their orbits only rarely intersect the Earth's, the probabilities of their collision with the Earth are nevertheless finite and calculable. Systematic telescopic surveys carried out over the past two decades show that the flux of asteroids and comet nuclei in the Earth's neighborhood is sufficiently high that the effects of occasional collisions should be recognizable in the geological record. During these same two decades, an intensive international search for ancient impact structures has gone forward. The actual rate of bombardment of the Earth during the last half-billion years has been found to be roughly consistent with the present rate predicted from astronomical observations. Within a factor of about two, the average rate of bombardment of the Earth during the last half­ billion years also appears to be consistent with the average rate of bombardment of the Moon over the last 3.3 billion years. Spectacular new lines of study have developed in recent years leading to the recognition of rare large impact events that produce geochemical anomalies on a global scale. The possible effects of these large impacts on the Earth's biota have become the subject of vigorous debate. In this paper, I first review the astronomical and geologic evidence concerning the history of bombardment and then discuss the physical effects of large impacts, as they may apply to both the inorganic and organic worlds.",
    url = "https://doi.org/10.1146/annurev.ea.11.050183.002333",
    doi = "10.1146/annurev.ea.11.050183.002333",
    openalex = "W2137685264",
    references = "openalexw1615946943"
}

@misc{davis1984extinction5,
    author = "Davis, M. and Hut, P. and Muller, R. A",
    title = "Extinction by periodic comet showers",
    year = "1984",
    howpublished = "Nature, v. 308, p. 715-717",
    note = "talkorigins\_source = {true}; raw\_reference = {Davis, M., Hut, P., and Muller, R. A., 1984, Extinction by periodic comet showers: Nature, v. 308, p. 715-717.}"
}

A computer search for earth-approaching comets among those listed in Marsden's (1983) updated orbit catalog has identified 36 cases at which minimum separation distance was less than 2500 earth radii. A strong representation of short period comets in the sample is noted, and the constant rate of the close approaching comets in the last 300 years is interpreted to suggest the lack of long-period comets intrinsically fainter than an absolute magnitude of about 11. A comet-earth collision rate derived from the statistics of these close encounters implies an average period of 33-64 million years between any two events. This rate is comparable with the frequency of geologically recent global catastrophes which appear to be associated with extraterrestrial object impacts, such as the Cretaceous-Tertiary extinction 65 million years ago and the late Eocene event 34 million years ago.

@article{doi101086113494,
    author = "Sekanina, Z. and Yeomans, D. K.",
    title = "Close encounters and collisions of comets with the earth",
    year = "1984",
    journal = "The Astronomical Journal",
    abstract = "A computer search for earth-approaching comets among those listed in Marsden's (1983) updated orbit catalog has identified 36 cases at which minimum separation distance was less than 2500 earth radii. A strong representation of short period comets in the sample is noted, and the constant rate of the close approaching comets in the last 300 years is interpreted to suggest the lack of long-period comets intrinsically fainter than an absolute magnitude of about 11. A comet-earth collision rate derived from the statistics of these close encounters implies an average period of 33-64 million years between any two events. This rate is comparable with the frequency of geologically recent global catastrophes which appear to be associated with extraterrestrial object impacts, such as the Cretaceous-Tertiary extinction 65 million years ago and the late Eocene event 34 million years ago.",
    url = "https://doi.org/10.1086/113494",
    doi = "10.1086/113494",
    openalex = "W2035996596"
}

Clay samples from three Cretaceous-Tertiary boundary sites contain 0.36 to 0.58 percent graphitic carbon, mainly as fluffy aggregates of 0.1 to 0.5 micrometers-apparently a worldwide layer of soot. It may have been produced by wildfires triggered by a giant meteorite. This carbon, corresponding to a global abundance of 0.021 +/- 0.006 gram per square centimeter, could have greatly enhanced the darkening and cooling of the earth by rock dust, which has been suggested as a cause of the extinctions. The surprisingly large amount of soot (10 percent of the present biomass of the earth) implies either that much of the earth's vegetation burned down or that substantial amounts of fossil fuels were ignited also. The particle-size distribution of the soot is similar to that assumed for the smoke cloud of "nuclear winter," but the global distribution is more uniform and the amounts are much greater, suggesting that soot production by large wildfires is about 10 times more efficient that has been assumed for a nuclear winter. Thus cooling would be more pervasive and lasting. No trace of meteoritic noble gases and no meteoritic spinel were found in these carbon fractions. Accordingly, limits can be set on the mass fraction of the meteorite that escaped degassing (</=3 x 10(-5)) or vaporization (</=0.04). Thus it seems unlikely that comets contributed significant amounts of prebiotic organic matter to the primitive earth.

@article{doi101126science2304722167,
    author = "Wolbach, Wendy S. and Lewis, R. S. and Anders, Edward",
    title = "Cretaceous Extinctions: Evidence for Wildfires and Search for Meteoritic Material",
    year = "1985",
    journal = "Science",
    abstract = {Clay samples from three Cretaceous-Tertiary boundary sites contain 0.36 to 0.58 percent graphitic carbon, mainly as fluffy aggregates of 0.1 to 0.5 micrometers-apparently a worldwide layer of soot. It may have been produced by wildfires triggered by a giant meteorite. This carbon, corresponding to a global abundance of 0.021 +/- 0.006 gram per square centimeter, could have greatly enhanced the darkening and cooling of the earth by rock dust, which has been suggested as a cause of the extinctions. The surprisingly large amount of soot (10 percent of the present biomass of the earth) implies either that much of the earth's vegetation burned down or that substantial amounts of fossil fuels were ignited also. The particle-size distribution of the soot is similar to that assumed for the smoke cloud of "nuclear winter," but the global distribution is more uniform and the amounts are much greater, suggesting that soot production by large wildfires is about 10 times more efficient that has been assumed for a nuclear winter. Thus cooling would be more pervasive and lasting. No trace of meteoritic noble gases and no meteoritic spinel were found in these carbon fractions. Accordingly, limits can be set on the mass fraction of the meteorite that escaped degassing (</=3 x 10(-5)) or vaporization (</=0.04). Thus it seems unlikely that comets contributed significant amounts of prebiotic organic matter to the primitive earth.},
    url = "https://doi.org/10.1126/science.230.4722.167",
    doi = "10.1126/science.230.4722.167",
    openalex = "W2067579175",
    references = "doi101073pnas802627, doi101126science22346411183, doi101126science22546661030"
}

@misc{taylor1985lunar9,
    author = "Taylor, G. J",
    title = "Lunar origin meeting favors impact theory",
    year = "1985",
    howpublished = "Geotimes, v. 30, no. 4, p. 16-17",
    note = "talkorigins\_source = {true}; raw\_reference = {Taylor, G. J., 1985, Lunar origin meeting favors impact theory: Geotimes, v. 30, no. 4, p. 16-17.}"
}

@misc{alvarez1986toward2,
    author = "Alvarez, W",
    title = "Toward a theory of impact crisis",
    year = "1986",
    howpublished = "Eos, v. 131, p. 248-250",
    note = "talkorigins\_source = {true}; raw\_reference = {Alvarez, W., 1986, Toward a theory of impact crisis: Eos, v. 131, p. 248-250.}"
}

@misc{alvarez1990iridium3,
    author = "Alvarez, W. and Asaro, F. and Montanari, A",
    title = "Iridium profile for 10 million years across the Cretaceous-Tertiary boundary at Gubbio (Italy)",
    year = "1990",
    howpublished = "Science, v. 250, no. 4988, p. 1700-1702",
    note = "talkorigins\_source = {true}; raw\_reference = {Alvarez, W., Asaro, F., and Montanari, A., 1990, Iridium profile for 10 million years across the Cretaceous-Tertiary boundary at Gubbio (Italy): Science, v. 250, no. 4988, p. 1700-1702.}"
}

It has long been speculated that Earth accreted prebiotic organic molecules important for the origins of life from impacts of carbonaceous asteroids and comets during the period of heavy bombardment 4.5 x 10(9) to 3.8 x 10(9) years ago. A comprehensive treatment of comet-asteroid interaction with the atmosphere, surface impact, and resulting organic pyrolysis demonstrates that organics will not survive impacts at velocities greater than about 10 kilometers per second and that even comets and asteroids as small as 100 meters in radius cannot be aerobraked to below this velocity in 1-bar atmospheres. However, for plausible dense (10-bar carbon dioxide) early atmospheres, we find that 4.5 x 10(9) years ago Earth was accreting intact cometary organics at a rate of at least approximately 10(6) to 10(7) kilograms per year, a flux that thereafter declined with a half-life of approximately 10(8) years. These results may be put in context by comparison with terrestrial oceanic and total biomasses, approximately 3 x 10(12) kilograms and approximately 6 x 10(14) kilograms, respectively.

@article{doi101126science11538074,
    author = "Chyba, Christopher F. and Thomas, Paul J. and Brookshaw, Leigh and Sagan, Carl",
    title = "Cometary Delivery of Organic Molecules to the Early Earth",
    year = "1990",
    journal = "Science",
    abstract = "It has long been speculated that Earth accreted prebiotic organic molecules important for the origins of life from impacts of carbonaceous asteroids and comets during the period of heavy bombardment 4.5 x 10(9) to 3.8 x 10(9) years ago. A comprehensive treatment of comet-asteroid interaction with the atmosphere, surface impact, and resulting organic pyrolysis demonstrates that organics will not survive impacts at velocities greater than about 10 kilometers per second and that even comets and asteroids as small as 100 meters in radius cannot be aerobraked to below this velocity in 1-bar atmospheres. However, for plausible dense (10-bar carbon dioxide) early atmospheres, we find that 4.5 x 10(9) years ago Earth was accreting intact cometary organics at a rate of at least approximately 10(6) to 10(7) kilograms per year, a flux that thereafter declined with a half-life of approximately 10(8) years. These results may be put in context by comparison with terrestrial oceanic and total biomasses, approximately 3 x 10(12) kilograms and approximately 6 x 10(14) kilograms, respectively.",
    url = "https://doi.org/10.1126/science.11538074",
    doi = "10.1126/science.11538074",
    openalex = "W2076605024",
    references = "doi1010079789400972223, doi101016001670378990286x, doi1010160019103589901292, doi101016002199918790074x, doi101029jb091ib02p01921, doi101038190389a0, doi101038332691a0, doi101038333313a0, doi101038342139a0, doi101038342255a0, doi101038343129a0"
}

Topics addressed include: Cretaceous-Tertiary mass extinctions; geologial indicators for meteorite collisions; carbon dioxide catastrophes; volcanism; climatic changes; geochemistry; mineralogy; fossil records; biospheric traumas; stratigraphy; mathematical models; and ocean dynamics.

@book{doi101130spe247,
    title = "Global Catastrophes in Earth History; An Interdisciplinary Conference on Impacts, Volcanism, and Mass Mortality",
    year = "1990",
    booktitle = "Geological Society of America eBooks",
    abstract = "Topics addressed include: Cretaceous-Tertiary mass extinctions; geologial indicators for meteorite collisions; carbon dioxide catastrophes; volcanism; climatic changes; geochemistry; mineralogy; fossil records; biospheric traumas; stratigraphy; mathematical models; and ocean dynamics.",
    url = "https://doi.org/10.1130/spe247",
    doi = "10.1130/spe247",
    openalex = "W370642989",
    references = "crossref1982geological, doi1010079783642708312, doi101029jb088ib03p02485, doi101086628623, doi101111j136530911979tb00935x, doi101130spe239p1, doi1023073514751, openalexw606525048"
}

Approximately 90 Earth-crossing asteroids had been discovered through September 1989. Discovery is thought to be complete at absolute V magnitude (H) = 13.2 (the magnitude of the brightest known object, diameter ∼8.1 km), and about 6 percent complete at H = 17.7 (typical diameter about 1 km). The calculated mean probability of collision of Earth-crossing asteroids with Earth is (4.2 ± 1.7) × 10 −9yr −1. When multiplied by the estimated population of 1030 ± 470 at H = 17.7, this probability yields a collision rate of (4.3 ± 2.6) × 10 −6yr −1for asteroids larger than about 1 km in diameter. At H = 15.8, roughly equivalent to asteroid diameters more than 2 km, the estimated collision rate is ≈7 × 10 −7yr −1, and at 8-km diameter, the rate is ≈3 × 10 −9yr −1. Comet nuclei with diameters more than 2.5 km are estimated to strike the Earth at the rate of ≈ 10 −7yr −1; comets larger than 10 km in diameter probably strike at a rate ≈10 −8yr −1. Impact of asteroids probably dominates the production of craters smaller than 30 km in diameter, whereas comet impact probably forms most craters larger than 50 km. The production rate for craters larger than 20 km in diameter, estimated from the astronomical evidence, is (4.9 ± 2.9) × 10 −15km −2yr −1; this rate is consistent with the cratering rate estimated by Grieve from the geologic record for the last 120 m.y.

@incollection{doi101130spe247p155,
    author = "Shoemaker, Eugene M. and Wolfe, R. F. and Shoemaker, C. S.",
    title = "Asteroid and comet flux in the neighborhood of Earth",
    year = "1990",
    booktitle = "Geological Society of America eBooks",
    abstract = "Approximately 90 Earth-crossing asteroids had been discovered through September 1989. Discovery is thought to be complete at absolute V magnitude (H) = 13.2 (the magnitude of the brightest known object, diameter ∼8.1 km), and about 6 percent complete at H = 17.7 (typical diameter about 1 km). The calculated mean probability of collision of Earth-crossing asteroids with Earth is (4.2 ± 1.7) × 10 −9yr −1. When multiplied by the estimated population of 1030 ± 470 at H = 17.7, this probability yields a collision rate of (4.3 ± 2.6) × 10 −6yr −1for asteroids larger than about 1 km in diameter. At H = 15.8, roughly equivalent to asteroid diameters more than 2 km, the estimated collision rate is ≈7 × 10 −7yr −1, and at 8-km diameter, the rate is ≈3 × 10 −9yr −1. Comet nuclei with diameters more than 2.5 km are estimated to strike the Earth at the rate of ≈ 10 −7yr −1; comets larger than 10 km in diameter probably strike at a rate ≈10 −8yr −1. Impact of asteroids probably dominates the production of craters smaller than 30 km in diameter, whereas comet impact probably forms most craters larger than 50 km. The production rate for craters larger than 20 km in diameter, estimated from the astronomical evidence, is (4.9 ± 2.9) × 10 −15km −2yr −1; this rate is consistent with the cratering rate estimated by Grieve from the geologic record for the last 120 m.y.",
    url = "https://doi.org/10.1130/spe247-p155",
    doi = "10.1130/spe247-p155",
    openalex = "W2149644358"
}

The effect of the atmosphere on impacting asteroids and comets is investigated. It is found that the atmosphere is inefficient in preventing impact damage to the ground when the radius of a stony asteroid exceeds 100 m and that of a comet exceeds 500 m. For iron meteorites that impact at greater than 20 km/s, the critical radius is about 20-30 m. For low-velocity iron meteorites that hit at 11.2-15 km/s, the critical radius is only 2 m. While the dissipation of energy in the atmosphere protects the ground from impact damage, it can enhance the damage done by the airburst. The area of destruction produced by the airburst during the impact of small asteroids can be up to twice as large as would have been the case had the same energy been released at sea level. The plumes of stony meteorites with radii over about 60 km break out of the atmosphere and float around the earth, carrying entrapped dust whose deposition may provide a counter for large meteorite impacts.

@article{doi101086116499,
    author = "Hills, J. G. and Goda, M. P.",
    title = "The fragmentation of small asteroids in the atmosphere",
    year = "1993",
    journal = "The Astronomical Journal",
    abstract = "The effect of the atmosphere on impacting asteroids and comets is investigated. It is found that the atmosphere is inefficient in preventing impact damage to the ground when the radius of a stony asteroid exceeds 100 m and that of a comet exceeds 500 m. For iron meteorites that impact at greater than 20 km/s, the critical radius is about 20-30 m. For low-velocity iron meteorites that hit at 11.2-15 km/s, the critical radius is only 2 m. While the dissipation of energy in the atmosphere protects the ground from impact damage, it can enhance the damage done by the airburst. The area of destruction produced by the airburst during the impact of small asteroids can be up to twice as large as would have been the case had the same energy been released at sea level. The plumes of stony meteorites with radii over about 60 km break out of the atmosphere and float around the earth, carrying entrapped dust whose deposition may provide a counter for large meteorite impacts.",
    url = "https://doi.org/10.1086/116499",
    doi = "10.1086/116499",
    openalex = "W2047757483"
}

@article{doi101038367033a0,
    author = "Chapman, C. R. and Morrison, David",
    title = "Impacts on the Earth by asteroids and comets: assessing the hazard",
    year = "1994",
    journal = "Nature",
    url = "https://doi.org/10.1038/367033a0",
    doi = "10.1038/367033a0",
    openalex = "W2057273337",
    references = "doi101038331612a0, doi101038361040a0, doi101126science2575072954, doi105860choice293880"
}

In 1993, the U.S. Department of Defense declassified information dealing with frequent explosions in the upper atmosphere caused by meteoric impact. It is estimated that impacts have occurred of a magnitude equivalent to the atomic bomb detonated at Hiroshima. Not all such space voyagers meet their end in the atmosphere, however; huge craters attest to the bombardment of earth over millions of years, and a major impact may have resulted in the extinction of dinosaurs. An impact in Siberia near the beginning of this century proves that such events are not confined to geologic time. Hazards Due to Comets and Asteroids marks a significant step in the attempt to come to grips with the threats posed by such phenomena. It brings together more than one hundred scientists from around the world, who draw on observational and theoretical research to focus on the technical problems related to all aspects of dealing with these hazards: searching for and identifying hazardous comets and asteroids; describing their statistics and characteristics; intercepting and altering the orbits of dangerous objects; and applying existent technologies rocket boosters, rendezvous and soft-landing techniques, instrumentation to such missions. The book considers defensive options for diverting or disrupting an approaching body, including solar sails, kinetic-energy impacts, nuclear explosives, robotic mass drivers, and various propulsion systems. A cataclysmic impact posing a threat to life on Earth is a possibility that tomorrow's technology is capable of averting. This book examines in depth the reality of the threat and proposes practical measures that can be initiated now should we ever need to deal with it.

@article{doi105860choice330281,
    title = "Hazards due to comets and asteroids",
    year = "1995",
    journal = "Choice Reviews Online",
    abstract = "In 1993, the U.S. Department of Defense declassified information dealing with frequent explosions in the upper atmosphere caused by meteoric impact. It is estimated that impacts have occurred of a magnitude equivalent to the atomic bomb detonated at Hiroshima. Not all such space voyagers meet their end in the atmosphere, however; huge craters attest to the bombardment of earth over millions of years, and a major impact may have resulted in the extinction of dinosaurs. An impact in Siberia near the beginning of this century proves that such events are not confined to geologic time. Hazards Due to Comets and Asteroids marks a significant step in the attempt to come to grips with the threats posed by such phenomena. It brings together more than one hundred scientists from around the world, who draw on observational and theoretical research to focus on the technical problems related to all aspects of dealing with these hazards: searching for and identifying hazardous comets and asteroids; describing their statistics and characteristics; intercepting and altering the orbits of dangerous objects; and applying existent technologies rocket boosters, rendezvous and soft-landing techniques, instrumentation to such missions. The book considers defensive options for diverting or disrupting an approaching body, including solar sails, kinetic-energy impacts, nuclear explosives, robotic mass drivers, and various propulsion systems. A cataclysmic impact posing a threat to life on Earth is a possibility that tomorrow's technology is capable of averting. This book examines in depth the reality of the threat and proposes practical measures that can be initiated now should we ever need to deal with it.",
    url = "https://doi.org/10.5860/choice.33-0281",
    doi = "10.5860/choice.33-0281",
    openalex = "W1612135897"
}

@misc{hills1996the,
    author = "Hills, J.",
    title = "The detectability of asteroids and comets before earth impact",
    year = "1996",
    url = "https://doi.org/10.2172/378682",
    doi = "10.2172/378682",
    openalex = "W126792934"
}

We review the major impact‐associated mechanisms proposed to cause extinctions at the Cretaceous‐Tertiary geological boundary. We then discuss how the proposed extinction mechanisms may relate to the environmental consequences of asteroid and comet impacts in general. Our chief goal is to provide relatively simple prescriptions for evaluating the importance of impacting objects over a range of energies and compositions, but we also stress that there are many uncertainties. We conclude that impacts with energies less than about 10 Mt are a negligible hazard. For impacts with energies above 10 Mt and below about 10 4 Mt (i.e., impact frequencies less than one in 6 × 10 4 years, corresponding to comets and asteroids with diameters smaller than about 400 m and 650 m, respectively), blast damage, earthquakes, and fires should be important on a scale of 10 4 or 10 5 km², which corresponds to the area damaged in many natural disasters of recent history. However, tsunami excited by marine impacts could be more damaging, flooding a kilometer of coastal plain over entire ocean basins. In the energy range of 10 4 –10 5 Mt (intervals up to 3 × 10 5 years, corresponding to comets and asteroids with diameters up to 850 m and 1.4 km, respectively) water vapor injections and ozone loss become significant on the global scale. In our nominal model, such an impact does not inject enough submicrometer dust into the stratosphere to produce major adverse effects, but if a higher fraction of pulverized rock than we think likely reaches the stratosphere, stratospheric dust (causing global cooling) would also be important in this energy range. Thus 10 5 Mt is a lower limit where damage might occur beyond the experience of human history. The energy range from 10 5 to 10 6 Mt (intervals up to 2 × 10 6 years, corresponding to comets and asteroids up to 1.8 and 3 km diameter) is transitional between regional and global effects. Stratospheric dust, sulfates released from within impacting asteroids, and soot from extensive wild‐fires sparked by thermal radiation from the impact can produce climatologically significant global optical depths of the order of 10. Moreover, the ejecta plumes of these impacts may produce enough NO from shock‐heated air to destroy the ozone shield. Between 10 6 and 10 7 Mt (intervals up to 1.5 × 10 7 years, corresponding to comets and asteroids up to 4 and 6.5 km diameter), dust and sulfate levels would be high enough to reduce light levels below those necessary for photosynthesis. Ballistic ejecta reentering the atmosphere as shooting stars would set fires over regions exceeding 10 7 km², and the resulting smoke would reduce light levels even further. At energies above 10 7 Mt, blast and earthquake damage reach the regional scale (10 6 km²). Tsunami cresting to 100 m and flooding 20 km inland could sweep the coastal zones of one of the world's ocean basins. Fires would be set globally. Light levels may drop so low from the smoke, dust, and sulfate as to make vision impossible. At energies approaching 10 9 Mt (>10 8 years) the ocean surface waters may be acidified globally by sulfur from the interiors of comets and asteroids. The Cretaceous‐Tertiary impact in particular struck evaporate substrates that very likely generated a dense, widespread sulfate aerosol layer with consequent climatic effects. The combination of all of these physical effects would surely represent a devastating stress on the global biosphere.

@article{doi10102996rg03038,
    author = "Toon, O. B. and Zahnle, Kevin and Morrison, David and Turco, R. P. and Covey, Curt",
    title = "Environmental perturbations caused by the impacts of asteroids and comets",
    year = "1997",
    journal = "Reviews of Geophysics",
    abstract = "We review the major impact‐associated mechanisms proposed to cause extinctions at the Cretaceous‐Tertiary geological boundary. We then discuss how the proposed extinction mechanisms may relate to the environmental consequences of asteroid and comet impacts in general. Our chief goal is to provide relatively simple prescriptions for evaluating the importance of impacting objects over a range of energies and compositions, but we also stress that there are many uncertainties. We conclude that impacts with energies less than about 10 Mt are a negligible hazard. For impacts with energies above 10 Mt and below about 10 4 Mt (i.e., impact frequencies less than one in 6 × 10 4 years, corresponding to comets and asteroids with diameters smaller than about 400 m and 650 m, respectively), blast damage, earthquakes, and fires should be important on a scale of 10 4 or 10 5 km², which corresponds to the area damaged in many natural disasters of recent history. However, tsunami excited by marine impacts could be more damaging, flooding a kilometer of coastal plain over entire ocean basins. In the energy range of 10 4 –10 5 Mt (intervals up to 3 × 10 5 years, corresponding to comets and asteroids with diameters up to 850 m and 1.4 km, respectively) water vapor injections and ozone loss become significant on the global scale. In our nominal model, such an impact does not inject enough submicrometer dust into the stratosphere to produce major adverse effects, but if a higher fraction of pulverized rock than we think likely reaches the stratosphere, stratospheric dust (causing global cooling) would also be important in this energy range. Thus 10 5 Mt is a lower limit where damage might occur beyond the experience of human history. The energy range from 10 5 to 10 6 Mt (intervals up to 2 × 10 6 years, corresponding to comets and asteroids up to 1.8 and 3 km diameter) is transitional between regional and global effects. Stratospheric dust, sulfates released from within impacting asteroids, and soot from extensive wild‐fires sparked by thermal radiation from the impact can produce climatologically significant global optical depths of the order of 10. Moreover, the ejecta plumes of these impacts may produce enough NO from shock‐heated air to destroy the ozone shield. Between 10 6 and 10 7 Mt (intervals up to 1.5 × 10 7 years, corresponding to comets and asteroids up to 4 and 6.5 km diameter), dust and sulfate levels would be high enough to reduce light levels below those necessary for photosynthesis. Ballistic ejecta reentering the atmosphere as shooting stars would set fires over regions exceeding 10 7 km², and the resulting smoke would reduce light levels even further. At energies above 10 7 Mt, blast and earthquake damage reach the regional scale (10 6 km²). Tsunami cresting to 100 m and flooding 20 km inland could sweep the coastal zones of one of the world's ocean basins. Fires would be set globally. Light levels may drop so low from the smoke, dust, and sulfate as to make vision impossible. At energies approaching 10 9 Mt (>10 8 years) the ocean surface waters may be acidified globally by sulfur from the interiors of comets and asteroids. The Cretaceous‐Tertiary impact in particular struck evaporate substrates that very likely generated a dense, widespread sulfate aerosol layer with consequent climatic effects. The combination of all of these physical effects would surely represent a devastating stress on the global biosphere.",
    url = "https://doi.org/10.1029/96rg03038",
    doi = "10.1029/96rg03038",
    openalex = "W1997782776",
    references = "alvarez1980extraterrestrial, doi1010079781489921246, doi1010160016703789901506, doi101016001670378990286x, doi101038361040a0, doi101126science22246301283, doi101126science25049881669, doi1011300091761319910190867ccapct23co2, doi101130spe247p155, doi1011751520046919640210361teotaw20co2, doi1011751520046919670240241teotaw20co2, doi105860choice330281"
}

A comprehensive analysis of volatiles in the Chicxulub impact strongly supports the hypothesis that impact-generated sulfate aerosols caused over a decade of global cooling, acid rain, and disruption of ocean circulation, which contributed to the mass extinction at the Cretaceous/Tertiary (K/T) boundary. The crater size, meteoritic content of the K/T boundary clay, and impact models indicate that the Chicxulub crater was formed by a short period comet or an asteroid impact that released 0.7-3.4 x 10(31) ergs of energy. Impact models and experiments combined with estimates of volatiles in the projectile and target rocks predict that over 200 gigatons (Gt) each of SO2 and water vapor, and over 500 Gt of CO2, were globally distributed in the stratosphere by the impact. Additional volatiles may have been produced on a global or regional scale that formed sulfate aerosols rapidly in cooler parts of the vapor plume, causing an early, intense pulse of sulfuric acid rain. Estimates of the conversion rate of stratospheric SO2 and water vapor to sulfate aerosol, based on volcanic production of sulfate aerosols, coupled with calculations of diffusion, coagulation, and sedimentation, demonstrate that the 200 Gt stratospheric SO2 and water vapor reservoir would produce sulfate aerosols for 12 years. These sulfate aerosols caused a second pulse of acid rain that was global. Radiative transfer modeling of the aerosol clouds demonstrates (1) that if the initial rapid pulse of sulfate aerosols was global, photosynthesis may have been shut down for 6 months and (2) that for the second prolonged aerosol cloud, solar transmission dropped 80% by the end of first year and remained 50% below normal for 9 years. As a result, global average surface temperatures probably dropped between 5 degrees and 31 degrees K, suggesting that global near-freezing conditions may have been reached. Impact-generated CO2 caused less than 1 degree K greenhouse warming and therefore was insignificant compare to the sulfate cooling. The magnitude of sulfate cooling depends largely upon the rate of ocean mixing as surface waters cool, sink, and are replaced by upwelling of deep ocean water. This upwelling apparently drastically altered ocean stratification and circulation, which may explain the global collapse of the delta 13C gradient between surface and deep ocean waters at the K/T boundary.

@article{doi10102997je01743,
    author = "Pope, Kevin and Baines, Kevin H. and Ocampo, Adriana and Ivanov, B. A.",
    title = "Energy, volatile production, and climatic effects of the Chicxulub Cretaceous/Tertiary impact",
    year = "1997",
    journal = "Journal of Geophysical Research Atmospheres",
    abstract = "A comprehensive analysis of volatiles in the Chicxulub impact strongly supports the hypothesis that impact-generated sulfate aerosols caused over a decade of global cooling, acid rain, and disruption of ocean circulation, which contributed to the mass extinction at the Cretaceous/Tertiary (K/T) boundary. The crater size, meteoritic content of the K/T boundary clay, and impact models indicate that the Chicxulub crater was formed by a short period comet or an asteroid impact that released 0.7-3.4 x 10(31) ergs of energy. Impact models and experiments combined with estimates of volatiles in the projectile and target rocks predict that over 200 gigatons (Gt) each of SO2 and water vapor, and over 500 Gt of CO2, were globally distributed in the stratosphere by the impact. Additional volatiles may have been produced on a global or regional scale that formed sulfate aerosols rapidly in cooler parts of the vapor plume, causing an early, intense pulse of sulfuric acid rain. Estimates of the conversion rate of stratospheric SO2 and water vapor to sulfate aerosol, based on volcanic production of sulfate aerosols, coupled with calculations of diffusion, coagulation, and sedimentation, demonstrate that the 200 Gt stratospheric SO2 and water vapor reservoir would produce sulfate aerosols for 12 years. These sulfate aerosols caused a second pulse of acid rain that was global. Radiative transfer modeling of the aerosol clouds demonstrates (1) that if the initial rapid pulse of sulfate aerosols was global, photosynthesis may have been shut down for 6 months and (2) that for the second prolonged aerosol cloud, solar transmission dropped 80\% by the end of first year and remained 50\% below normal for 9 years. As a result, global average surface temperatures probably dropped between 5 degrees and 31 degrees K, suggesting that global near-freezing conditions may have been reached. Impact-generated CO2 caused less than 1 degree K greenhouse warming and therefore was insignificant compare to the sulfate cooling. The magnitude of sulfate cooling depends largely upon the rate of ocean mixing as surface waters cool, sink, and are replaced by upwelling of deep ocean water. This upwelling apparently drastically altered ocean stratification and circulation, which may explain the global collapse of the delta 13C gradient between surface and deep ocean waters at the K/T boundary.",
    url = "https://doi.org/10.1029/97je01743",
    doi = "10.1029/97je01743",
    openalex = "W2066886748",
    references = "alvarez1980extraterrestrial, doi1010160016703789901506, doi101038359819a0, doi101111j194551001995tb01113x, doi101126science21545391501, doi101126science22246301283, doi101126science23547931156, doi101126science2555043423, doi1011300091761319910190867ccapct23co2, doi1011300091761319920200099tbdwcu23co2, doi1011300091761319950230873ynssia23co2, doi101130spe247p155, doi1017159caj1991847309, doi105860choice330281, openalexw2139291338"
}

Abstract Why do mass extinctions occur? The demise of the dinosaurs has been discussed exhaustively, but has never been out into the context of other extinction events. This is the first systematic review of the mass extinctions of all organisms, plant and animal, terrestrial and marine, that have occurred in the history of life. This includes the major crisis 250 million years ago which nearly wiped out all life on Earth. By examining current paleontological, geological, and sedimentological evidence of environmental changes, the cases for explanations based on climate change, marine regressions, asteroid or comet impact, anoxia, and volcanic eruptions are all critically evaluated.

@book{doi101093oso97801985491780010001,
    author = "Hallam, A. and Wignall, Paul B.",
    title = "Mass Extinctions and Their Aftermath",
    year = "1997",
    abstract = "Abstract Why do mass extinctions occur? The demise of the dinosaurs has been discussed exhaustively, but has never been out into the context of other extinction events. This is the first systematic review of the mass extinctions of all organisms, plant and animal, terrestrial and marine, that have occurred in the history of life. This includes the major crisis 250 million years ago which nearly wiped out all life on Earth. By examining current paleontological, geological, and sedimentological evidence of environmental changes, the cases for explanations based on climate change, marine regressions, asteroid or comet impact, anoxia, and volcanic eruptions are all critically evaluated.",
    url = "https://doi.org/10.1093/oso/9780198549178.001.0001",
    doi = "10.1093/oso/9780198549178.001.0001",
    openalex = "W4388328712"
}

We present results of numerical simulations that show that Earth's tidal forces can both distort and disrupt Earth-crossing asteroids that have weak “rubble-pile” structures. Building on previous studies, we consider more realistic asteroid shapes and trajectories, test a variety of spin rates and axis orientations, and employ a dissipation algorithm to treat more accurately collisions between the particles that make up the model asteroid. We explore a large parameter space, including the asteroid's periapseq, encounter velocity with the Earthv∞, spin periodP, initial spin axis orientation, and body orientation at periapse. We parameterize the simulation outcomes by the amount of mass stripped from the asteroid during a flyby. Our most severe disruptions result in fragment trains similar in character to the “string of pearls” created when Comet D/Shoemaker–Levy 9 was disrupted near Jupiter in 1992. Less catastrophic disruptions cause material to be stripped off in more isotropic fashion, leaving a central remnant with a characteristic distorted shape. Some ejecta can enter into stable orbits around the remnant, creating a binary or multiple system. Even when no mass is lost tidal forces and torques can modify the asteroid's shape and spin. Our results show that mass loss is enhanced for small values ofq,v∞, andP, and depends to a certain extent on the body's initial spin orientation (for example, retrograde rotation reduces mass loss). An elongated asteroid was found to be far easier to disrupt than a spherical one, though the orientation of the ellipsoid at periapse can noticeably change the outcome. The size and orbital distribution of the ejecta are discussed, along with the applications of this technique towards an understanding of doublet craters, crater chains, and asteroids with peculiar shapes and spins.

@article{doi101006icar19985954,
    author = "Richardson, D. C. and Bottke, W. F. and Love, Stanley G.",
    title = "Tidal Distortion and Disruption of Earth-Crossing Asteroids",
    year = "1998",
    journal = "Icarus",
    abstract = "We present results of numerical simulations that show that Earth's tidal forces can both distort and disrupt Earth-crossing asteroids that have weak “rubble-pile” structures. Building on previous studies, we consider more realistic asteroid shapes and trajectories, test a variety of spin rates and axis orientations, and employ a dissipation algorithm to treat more accurately collisions between the particles that make up the model asteroid. We explore a large parameter space, including the asteroid's periapseq, encounter velocity with the Earthv∞, spin periodP, initial spin axis orientation, and body orientation at periapse. We parameterize the simulation outcomes by the amount of mass stripped from the asteroid during a flyby. Our most severe disruptions result in fragment trains similar in character to the “string of pearls” created when Comet D/Shoemaker–Levy 9 was disrupted near Jupiter in 1992. Less catastrophic disruptions cause material to be stripped off in more isotropic fashion, leaving a central remnant with a characteristic distorted shape. Some ejecta can enter into stable orbits around the remnant, creating a binary or multiple system. Even when no mass is lost tidal forces and torques can modify the asteroid's shape and spin. Our results show that mass loss is enhanced for small values ofq,v∞, andP, and depends to a certain extent on the body's initial spin orientation (for example, retrograde rotation reduces mass loss). An elongated asteroid was found to be far easier to disrupt than a spherical one, though the orientation of the ellipsoid at periapse can noticeably change the outcome. The size and orbital distribution of the ejecta are discussed, along with the applications of this technique towards an understanding of doublet craters, crater chains, and asteroids with peculiar shapes and spins.",
    url = "https://doi.org/10.1006/icar.1998.5954",
    doi = "10.1006/icar.1998.5954",
    openalex = "W2139359797",
    references = "doi102307jctv1v3gr3r6, openalexw3113098276"
}

@article{doi101016s0032063397002328,
    author = "Steel, Duncan",
    title = "Distributions and moments of asteroid and comet impact speeds upon the Earth and Mars",
    year = "1998",
    journal = "Planetary and Space Science",
    url = "https://doi.org/10.1016/s0032-0633(97)00232-8",
    doi = "10.1016/s0032-0633(97)00232-8",
    openalex = "W2168822636"
}

Differences between the carbon isotopic values of carbonates secreted by planktic and benthic organisms did not recover to stable preextinction levels for more than 3 million years after the Cretaceous-Tertiary mass extinction. These decreased differences may have resulted from a smaller proportion of marine biological production sinking to deep water in the postextinction ocean. Under this hypothesis, marine production may have recovered shortly after the mass extinction, but the structure of the open-ocean ecosystem did not fully recover for more than 3 million years.

@article{doi101126science2825387276,
    author = "D’Hondt, Steven and Donaghay, Percy L. and Zachos, James C. and Luttenberg, Danielle and Lindinger, Matthias",
    title = "Organic Carbon Fluxes and Ecological Recovery from the Cretaceous-Tertiary Mass Extinction",
    year = "1998",
    journal = "Science",
    abstract = "Differences between the carbon isotopic values of carbonates secreted by planktic and benthic organisms did not recover to stable preextinction levels for more than 3 million years after the Cretaceous-Tertiary mass extinction. These decreased differences may have resulted from a smaller proportion of marine biological production sinking to deep water in the postextinction ocean. Under this hypothesis, marine production may have recovered shortly after the mass extinction, but the structure of the open-ocean ecosystem did not fully recover for more than 3 million years.",
    url = "https://doi.org/10.1126/science.282.5387.276",
    doi = "10.1126/science.282.5387.276",
    openalex = "W2099270219",
    references = "doi1010160016703757900248, doi1010160198014988901264, doi101016037783989390045y, doi1010160377839896000035, doi101016c20120016547, doi10102994jb03098, doi10102997je01743, doi101038337039a0, doi101038337061a0, doi101126science28053661039, doi101146annureves19110188000315"
}

We have deduced the orbital and size distributions of the near-Earth asteroids (NEAs) by (i) numerically integrating NEAs from their source regions to their observed orbits, (ii) estimating the observational biases and size distribution associated with asteroids on those orbits, and (iii) creating a model population that can be fit to the known NEAs. We predict that there are approximately 900 NEAs with absolute magnitude less than 18 (that is, kilometer-sized), of which 29, 65, and 6% reside on Amor, Apollo, and Aten orbits, respectively. These results suggest that roughly 40% of the kilometer-sized NEAs have been found. The remainder, on highly eccentric and inclined orbits, are more difficult to detect.

@article{doi101126science28854742190,
    author = "Bottke, W. F. and Jedicke, Robert and Morbidelli, Alessandro and Petit, Jean-Marc and Gladman, Brett",
    title = "Understanding the Distribution of Near-Earth Asteroids",
    year = "2000",
    journal = "Science",
    abstract = "We have deduced the orbital and size distributions of the near-Earth asteroids (NEAs) by (i) numerically integrating NEAs from their source regions to their observed orbits, (ii) estimating the observational biases and size distribution associated with asteroids on those orbits, and (iii) creating a model population that can be fit to the known NEAs. We predict that there are approximately 900 NEAs with absolute magnitude less than 18 (that is, kilometer-sized), of which 29, 65, and 6\% reside on Amor, Apollo, and Aten orbits, respectively. These results suggest that roughly 40\% of the kilometer-sized NEAs have been found. The remainder, on highly eccentric and inclined orbits, are more difficult to detect.",
    url = "https://doi.org/10.1126/science.288.5474.2190",
    doi = "10.1126/science.288.5474.2190",
    openalex = "W2022978242",
    references = "doi101006icar19941039, doi101006icar19975872, doi101006icar19985954, doi1010160019103583901276, doi10102992je01246, doi101029jb074i010p02531, doi101038367033a0, doi101086115978, doi101086300632, doi101126science2775323197"
}

An Öpik-based geometric algorithm is used to compute impact probabilities and velocity distributions for various near-Earth object (NEO) populations. The resulting crater size distributions for the Earth and Moon are calculated by combining these distributions with assumed NEO size distributions and a selection of crater scaling laws. This crater probability distribution indicates that the largest craters on both the Earth and the Moon are dominated by comets. However, from a calculation of the fractional probabilities of iridium deposition, and the velocity distributions at impact of each NEO population, the only realistic possibilities for the Chicxulub impactor are a short-period comet (possibly inactive) or a near-Earth asteroid. For these classes of object, sufficiently large impacts have mean intervals of 100 and 300 Myr respectively, slightly favouring the cometary hypothesis.

@article{doi101046j13658711200104747x,
    author = "Jeffers, S. V. and Manley, S. P. and Bailey, Mark and Asher, D. J.",
    title = "Near-Earth object velocity distributions and consequences for the Chicxulub impactor",
    year = "2001",
    journal = "Monthly Notices of the Royal Astronomical Society",
    abstract = "An Öpik-based geometric algorithm is used to compute impact probabilities and velocity distributions for various near-Earth object (NEO) populations. The resulting crater size distributions for the Earth and Moon are calculated by combining these distributions with assumed NEO size distributions and a selection of crater scaling laws. This crater probability distribution indicates that the largest craters on both the Earth and the Moon are dominated by comets. However, from a calculation of the fractional probabilities of iridium deposition, and the velocity distributions at impact of each NEO population, the only realistic possibilities for the Chicxulub impactor are a short-period comet (possibly inactive) or a near-Earth asteroid. For these classes of object, sufficiently large impacts have mean intervals of 100 and 300 Myr respectively, slightly favouring the cometary hypothesis.",
    url = "https://doi.org/10.1046/j.1365-8711.2001.04747.x",
    doi = "10.1046/j.1365-8711.2001.04747.x",
    openalex = "W2160418275",
    references = "alvarez1980extraterrestrial, doi101006icar19965637, doi101006icar19975872, doi101007bf00899820, doi1010160031920184900736, doi101016s0032063397002328, doi10102992gl02713, doi10103824322, doi101126science2825390927, doi105860choice330281"
}

The fossil record has been used to support the origin and radiation of modern birds (Neornithes) in Laurasia after the Cretaceous-Tertiary mass extinction event, whereas molecular clocks have suggested a Cretaceous origin for most avian orders. These alternative views of neornithine evolution are examined using an independent set of evidence, namely phylogenetic relationships and historical biogeography. Pylogenetic relationships of basal lineages of neornithines, including ratite birds and their allies (Palaleocognathae), galliforms and anseriforms (Galloanserae), as well as lineages of the more advanced Neoves (Gruiformes, (Capimulgiformes, Passeriformes and others) demonstrate pervasive trans-Antarctic distribution patterns. The temporal history of the neornithines can be inferred from fossil taxa and the ages of vicariance events, and along with their biogeographical patterns, leads to the conclusion that neornithines arose in Gondwana prior to the Cretaceous Tertiary extinction event.

@article{doi101098rspb20001368,
    author = "Cracraft, Joël",
    title = "Avian evolution, Gondwana biogeography and the Cretaceous–Tertiary mass extinction event",
    year = "2001",
    journal = "Proceedings of the Royal Society B Biological Sciences",
    abstract = "The fossil record has been used to support the origin and radiation of modern birds (Neornithes) in Laurasia after the Cretaceous-Tertiary mass extinction event, whereas molecular clocks have suggested a Cretaceous origin for most avian orders. These alternative views of neornithine evolution are examined using an independent set of evidence, namely phylogenetic relationships and historical biogeography. Pylogenetic relationships of basal lineages of neornithines, including ratite birds and their allies (Palaleocognathae), galliforms and anseriforms (Galloanserae), as well as lineages of the more advanced Neoves (Gruiformes, (Capimulgiformes, Passeriformes and others) demonstrate pervasive trans-Antarctic distribution patterns. The temporal history of the neornithines can be inferred from fossil taxa and the ages of vicariance events, and along with their biogeographical patterns, leads to the conclusion that neornithines arose in Gondwana prior to the Cretaceous Tertiary extinction event.",
    url = "https://doi.org/10.1098/rspb.2000.1368",
    doi = "10.1098/rspb.2000.1368",
    openalex = "W2110007690",
    references = "doi1010160040195188902697, doi101016b978012249408650011x, doi101016s0012821x98002829, doi10102996rg03038, doi101038377301a0, doi101038381226a0, doi101111j1469185x1997tb00024x, doi101126science2585084975, doi10113008137233291, doi102307jctt1xp3v3r, doi105281zenodo16171435, doi105860choice343307, openalexw2135985426, openalexw2607033038"
}

Terrestrial climates near the time of the end-Cretaceous mass extinction are poorly known, limiting understanding of environmentally driven changes in biodiversity that occurred before bolide impact. We estimate paleotemperatures for the last approximately 1.1 million years of the Cretaceous (approximately 66.6-65.5 million years ago, Ma) by using fossil plants from North Dakota and employ paleomagnetic stratigraphy to correlate the results to foraminiferal paleoclimatic data from four middle- and high-latitude sites. Both plants and foraminifera indicate warming near 66.0 Ma, a warming peak from approximately 65.8 to 65.6 Ma, and cooling near 65.6 Ma, suggesting that these were global climate shifts. The warming peak coincides with the immigration of a thermophilic flora, maximum plant diversity, and the poleward range expansion of thermophilic foraminifera. Plant data indicate the continuation of relatively cool temperatures across the Cretaceous-Paleogene boundary; there is no indication of a major warming immediately after the boundary as previously reported. Our temperature proxies correspond well with recent pCO(2) data from paleosol carbonate, suggesting a coupling of pCO(2) and temperature. To the extent that biodiversity is correlated with temperature, estimates of the severity of end-Cretaceous extinctions that are based on occurrence data from the warming peak are probably inflated, as we illustrate for North Dakota plants. However, our analysis of climate and facies considerations shows that the effects of bolide impact should be regarded as the most significant contributor to these plant extinctions.

@article{doi101073pnas0234701100,
    author = "Wilf, Peter and Johnson, Kirk R. and Huber, Brian T.",
    title = "Correlated terrestrial and marine evidence for global climate changes before mass extinction at the Cretaceous–Paleogene boundary",
    year = "2003",
    journal = "Proceedings of the National Academy of Sciences",
    abstract = "Terrestrial climates near the time of the end-Cretaceous mass extinction are poorly known, limiting understanding of environmentally driven changes in biodiversity that occurred before bolide impact. We estimate paleotemperatures for the last approximately 1.1 million years of the Cretaceous (approximately 66.6-65.5 million years ago, Ma) by using fossil plants from North Dakota and employ paleomagnetic stratigraphy to correlate the results to foraminiferal paleoclimatic data from four middle- and high-latitude sites. Both plants and foraminifera indicate warming near 66.0 Ma, a warming peak from approximately 65.8 to 65.6 Ma, and cooling near 65.6 Ma, suggesting that these were global climate shifts. The warming peak coincides with the immigration of a thermophilic flora, maximum plant diversity, and the poleward range expansion of thermophilic foraminifera. Plant data indicate the continuation of relatively cool temperatures across the Cretaceous-Paleogene boundary; there is no indication of a major warming immediately after the boundary as previously reported. Our temperature proxies correspond well with recent pCO(2) data from paleosol carbonate, suggesting a coupling of pCO(2) and temperature. To the extent that biodiversity is correlated with temperature, estimates of the severity of end-Cretaceous extinctions that are based on occurrence data from the warming peak are probably inflated, as we illustrate for North Dakota plants. However, our analysis of climate and facies considerations shows that the effects of bolide impact should be regarded as the most significant contributor to these plant extinctions.",
    url = "https://doi.org/10.1073/pnas.0234701100",
    doi = "10.1073/pnas.0234701100",
    openalex = "W1987302968",
    references = "doi101038324148a0, doi101126science1064706, doi1011300091761319980260995adswat23co2"
}

@incollection{doi101007978940071003051,
    author = "Tewari, V. C.",
    title = "Extraterrestrial Impacts on Earth and Extinction of Life in the Himalaya",
    year = "2004",
    booktitle = "Cellular origin and life in extreme habitats",
    url = "https://doi.org/10.1007/978-94-007-1003-0\_51",
    doi = "10.1007/978-94-007-1003-0\_51",
    openalex = "W1535282325",
    references = "alvarez1980extraterrestrial, crossref1998exobiology, doi1010071259409746889, doi101007978940101017744, doi101038230105a0, doi101126science2605108640, doi1017491jgsi2002600515, openalexw280747232, tewari1998earliest"
}

@article{doi101016jpss200312010,
    author = "McInnes, Colin R.",
    title = "Deflection of near-Earth asteroids by kinetic energy impacts from retrograde orbits",
    year = "2004",
    journal = "Planetary and Space Science",
    url = "https://doi.org/10.1016/j.pss.2003.12.010",
    doi = "10.1016/j.pss.2003.12.010",
    openalex = "W2081871229"
}

Although Norman Newell’s pioneering research was published in 1967, general interest in mass extinctions provoked by catastrophic changes in the environment was not aroused until 1980, when a paper appeared in the journal Science proposing that the end-Cretaceous extinction was caused by the impact of a huge asteroid. Before this time several people had suggested an extra-terrestrial cause for particular mass extinctions. Thus, in the middle of the twentieth century, the German palaeontologist Otto Schindewolf, who had long been preoccupied with the marine mass extinction at the end of the Palaeozoic era, concluded on the evidence of fieldwork in the Salt Range of Pakistan that the event must have been a catastrophic one for which he could literally conceive no earthly explanation. He was consequently led to speculate that the causal factor was a nearby supernova explosion. The increased cosmic radiation impinging on the Earth could, he thought, have destroyed the ozone shield and have led to lethal exposure of numerous organisms. A few other such speculations invoking some kind of extraterrestrial factor were put forward at about the same time, and in 1970 Digby McLaren, an expatriate British palaeontologist who had risen to become Director of the Canadian Geological Survey, made a startling proposal. He was an expert on the late Devonian marine mass extinction at the end of the penultimate, Frasnian, stage. Like Schindewolf, he agreed that the event was much too wide spread, dramatic, and ‘geologically instantaneous’ to have been caused by a merely terrestrial process, and he speculated that the world’s ocean of the time had been severely disturbed by the impact of a giant meteorite. Three years later, the American chemist Harold Urey, a Nobel Prize-winner, published a paper in the journal Nature in which he argued that several extinction events within the past 50 million years had been caused by the impact of comets. These various suggestions, together with a few others invoking increases in radiation from outer space, either in the form of cosmic radiation or solar protons, were virtually ignored. This is unsurprising in view of the almost total absence at that time of any supporting evidence, with the possible exception of a few tektite layers in Tertiary deposits.

@incollection{hallam2004impact,
    author = "Hallam, Tony",
    title = "Impact by comets and asteroids",
    year = "2004",
    booktitle = "Catastrophes and Lesser Calamities",
    abstract = "Although Norman Newell’s pioneering research was published in 1967, general interest in mass extinctions provoked by catastrophic changes in the environment was not aroused until 1980, when a paper appeared in the journal Science proposing that the end-Cretaceous extinction was caused by the impact of a huge asteroid. Before this time several people had suggested an extra-terrestrial cause for particular mass extinctions. Thus, in the middle of the twentieth century, the German palaeontologist Otto Schindewolf, who had long been preoccupied with the marine mass extinction at the end of the Palaeozoic era, concluded on the evidence of fieldwork in the Salt Range of Pakistan that the event must have been a catastrophic one for which he could literally conceive no earthly explanation. He was consequently led to speculate that the causal factor was a nearby supernova explosion. The increased cosmic radiation impinging on the Earth could, he thought, have destroyed the ozone shield and have led to lethal exposure of numerous organisms. A few other such speculations invoking some kind of extraterrestrial factor were put forward at about the same time, and in 1970 Digby McLaren, an expatriate British palaeontologist who had risen to become Director of the Canadian Geological Survey, made a startling proposal. He was an expert on the late Devonian marine mass extinction at the end of the penultimate, Frasnian, stage. Like Schindewolf, he agreed that the event was much too wide spread, dramatic, and ‘geologically instantaneous’ to have been caused by a merely terrestrial process, and he speculated that the world’s ocean of the time had been severely disturbed by the impact of a giant meteorite. Three years later, the American chemist Harold Urey, a Nobel Prize-winner, published a paper in the journal Nature in which he argued that several extinction events within the past 50 million years had been caused by the impact of comets. These various suggestions, together with a few others invoking increases in radiation from outer space, either in the form of cosmic radiation or solar protons, were virtually ignored. This is unsurprising in view of the almost total absence at that time of any supporting evidence, with the possible exception of a few tektite layers in Tertiary deposits.",
    url = "https://doi.org/10.1093/oso/9780198524977.003.0007",
    doi = "10.1093/oso/9780198524977.003.0007",
    openalex = "W3105149759"
}

The absolute magnitude and perihelion distributions of long-period comets are derived, using data from the Lincoln Near-Earth Asteroid Research (LINEAR) survey. The results are surprising in three ways. Firstly, the flux of comets through the inner solar system is much lower than some previous estimates. Secondly, the expected rise in comet numbers to larger perihelia is not seen. Thirdly, the number of comets per unit absolute magnitude does not significantly rise to fainter magnitudes. These results imply that the Oort cloud contains many fewer comets than some previous estimates, that small long-period comets collide with the Earth too infrequently to be a plausible source of Tunguska-style impacts, and that some physical process must have prevented small icy planetesmals from reaching the Oort cloud, or have rendered them unobservable. A tight limit is placed on the space density of interstellar comets, but the predicted space density is lower still. The number of long-period comets that will be discovered by telescopes such as SkyMapper, Pan-Starrs and LSST is predicted, and the optimum observing strategy discussed.

@article{doi101086497684,
    author = "Francis, Paul",
    title = "The Demographics of Long‐Period Comets",
    year = "2005",
    journal = "The Astrophysical Journal",
    abstract = "The absolute magnitude and perihelion distributions of long-period comets are derived, using data from the Lincoln Near-Earth Asteroid Research (LINEAR) survey. The results are surprising in three ways. Firstly, the flux of comets through the inner solar system is much lower than some previous estimates. Secondly, the expected rise in comet numbers to larger perihelia is not seen. Thirdly, the number of comets per unit absolute magnitude does not significantly rise to fainter magnitudes. These results imply that the Oort cloud contains many fewer comets than some previous estimates, that small long-period comets collide with the Earth too infrequently to be a plausible source of Tunguska-style impacts, and that some physical process must have prevented small icy planetesmals from reaching the Oort cloud, or have rendered them unobservable. A tight limit is placed on the space density of interstellar comets, but the predicted space density is lower still. The number of long-period comets that will be discovered by telescopes such as SkyMapper, Pan-Starrs and LSST is predicted, and the optimum observing strategy discussed.",
    url = "https://doi.org/10.1086/497684",
    doi = "10.1086/497684",
    openalex = "W2167131971",
    references = "doi101016s001910350200026x, doi101086113494"
}

Abstract— We have developed a Web-based program for quickly estimating the regional environmental consequences of a comet or asteroid impact on Earth (http:www.lpl.arizona.eduimpacteffects). This paper details the observations, assumptions and equations upon which the program is based. It describes our approach to quantifying the principal impact processes that might affect the people, buildings, and landscape in the vicinity of an impact event and discusses the uncertainty in our predictions. The program requires six inputs: impactor diameter, impactor density, impact velocity before atmospheric entry, impact angle, the distance from the impact at which the environmental effects are to be calculated, and the target type (sedimentary rock, crystalline rock, or a water layer above rock). The program includes novel algorithms for estimating the fate of the impactor during atmospheric traverse, the thermal radiation emitted by the impact-generated vapor plume (fireball), and the intensity of seismic shaking. The program also approximates various dimensions of the impact crater and ejecta deposit, as well as estimating the severity of the air blast in both crater-forming and airburst impacts. We illustrate the utility of our program by examining the predicted environmental consequences across the United States of hypothetical impact scenarios occurring in Los Angeles. We find that the most wide-reaching environmental consequence is seismic shaking: both ejecta deposit thickness and air-blast pressure decay much more rapidly with distance than with seismic ground motion. Close to the impact site the most devastating effect is from thermal radiation; however, the curvature of the Earth implies that distant localities are shielded from direct thermal radiation because the fireball is below the horizon.

@article{doi101111j194551002005tb00157x,
    author = "Collins, G. S. and Melosh, H. J. and Marcus, Robert",
    title = "Earth Impact Effects Program: A Web-based computer program for calculating the regional environmental consequences of a meteoroid impact on Earth",
    year = "2005",
    journal = "Meteoritics and Planetary Science",
    abstract = "Abstract— We have developed a Web-based program for quickly estimating the regional environmental consequences of a comet or asteroid impact on Earth (http:www.lpl.arizona.eduimpacteffects). This paper details the observations, assumptions and equations upon which the program is based. It describes our approach to quantifying the principal impact processes that might affect the people, buildings, and landscape in the vicinity of an impact event and discusses the uncertainty in our predictions. The program requires six inputs: impactor diameter, impactor density, impact velocity before atmospheric entry, impact angle, the distance from the impact at which the environmental effects are to be calculated, and the target type (sedimentary rock, crystalline rock, or a water layer above rock). The program includes novel algorithms for estimating the fate of the impactor during atmospheric traverse, the thermal radiation emitted by the impact-generated vapor plume (fireball), and the intensity of seismic shaking. The program also approximates various dimensions of the impact crater and ejecta deposit, as well as estimating the severity of the air blast in both crater-forming and airburst impacts. We illustrate the utility of our program by examining the predicted environmental consequences across the United States of hypothetical impact scenarios occurring in Los Angeles. We find that the most wide-reaching environmental consequence is seismic shaking: both ejecta deposit thickness and air-blast pressure decay much more rapidly with distance than with seismic ground motion. Close to the impact site the most devastating effect is from thermal radiation; however, the curvature of the Earth implies that distant localities are shielded from direct thermal radiation because the fireball is below the horizon.",
    url = "https://doi.org/10.1111/j.1945-5100.2005.tb00157.x",
    doi = "10.1111/j.1945-5100.2005.tb00157.x",
    openalex = "W2143040029",
    references = "alvarez1980extraterrestrial, doi101006icar20026856, doi1010160016703789901506, doi1010160022286070900190, doi1010160031920184900736, doi101016b9780123956729x50012, doi10102996rg03038, doi10102997je01743, doi101038361040a0, doi101126science20844481095, doi101130spe247p155, doi101146annurevea21050193002001, doi105408002213687121, doi105860choice330281, openalexw2139291338"
}

Deep Impact collided with comet Tempel 1, excavating a crater controlled by gravity. The comet's outer layer is composed of 1- to 100-micrometer fine particles with negligible strength (1000 kelvins). A large increase in organic material occurred during and after the event, with smaller changes in carbon dioxide relative to water. On approach, the spacecraft observed frequent natural outbursts, a mean radius of 3.0 ± 0.1 kilometers, smooth and rough terrain, scarps, and impact craters. A thermal map indicates a surface in equilibrium with sunlight.

@article{doi101126science1118923,
    author = "A’Hearn, Michael F. and Belton, M. J. S. and Delamere, W. A. and Kissel, J. and Klaasen, K. P. and McFadden, L. A. and Meech, K. J. and Melosh, H. J. and Schultz, P. H. and Sunshine, J. M. and Thomas, P. C. and Veverka, J. and Yeomans, D. K. and Baca, M. and Busko, I. and Crockett, Christopher and Collins, S.M. and Desnoyer, M. and Eberhardy, C. A. and Ernst, C. M. and Farnham, T. L. and Feaga, Lori M. and Groussin, O. and Hampton, D. L. and Ипатов, С. И. and Li, Jian‐Yang and Lindler, Don J. and Lisse, C. M. and Mastrodemos, Nickolaos and Owen, W. M. and Richardson, J. E. and Wellnitz, D. D. and White, R. L.",
    title = "Deep Impact: Excavating Comet Tempel 1",
    year = "2005",
    journal = "Science",
    abstract = "Deep Impact collided with comet Tempel 1, excavating a crater controlled by gravity. The comet's outer layer is composed of 1- to 100-micrometer fine particles with negligible strength (1000 kelvins). A large increase in organic material occurred during and after the event, with smaller changes in carbon dioxide relative to water. On approach, the spacecraft observed frequent natural outbursts, a mean radius of 3.0 ± 0.1 kilometers, smooth and rough terrain, scarps, and impact craters. A thermal map indicates a surface in equilibrium with sunlight.",
    url = "https://doi.org/10.1126/science.1118923",
    doi = "10.1126/science.1118923",
    openalex = "W2146348165",
    references = "doi1010160019103589901826, doi1010160734743x87900698, doi101029jb088ib03p02485, doi101038248121a0, doi10103845985, doi101111j194551002004tb00342x, doi101126science1097899, doi101126science1098388, doi101126science1118978, openalexw2139291338"
}

On 4 July 2005, many observatories around the world and in space observed the collision of Deep Impact with comet 9P/Tempel 1 or its aftermath. This was an unprecedented coordinated observational campaign. These data show that (i) there was new material after impact that was compositionally different from that seen before impact; (ii) the ratio of dust mass to gas mass in the ejecta was much larger than before impact; (iii) the new activity did not last more than a few days, and by 9 July the comet's behavior was indistinguishable from its pre-impact behavior; and (iv) there were interesting transient phenomena that may be correlated with cratering physics.

@article{doi101126science1118978,
    author = "Meech, K. J. and Ageorges, N. and A’Hearn, Michael F. and Arpigny, C. and Ateş, Ali and Aycock, J. and Bagnulo, S. and Bailey, J. and Barber, R. J. and Barrera, L. and Barrena, R. and Bauer, J. M. and Belton, M. J. S. and Bensch, F. and Bhattacharya, B. and Biver, N. and Blake, Geoffrey A. and Bockelée-Morvan, D. and Boehnhardt, H. and Bonev, B. P. and Bonev, T. and Buie, M. W. and Burton, Michael and Butner, H. M. and Cabanac, R. and Campbell, R. and Campins, H. and Capria, M. T. and Carroll, T. and Chaffee, F. and Charnley, S. B. and Cleis, R. A. and Coates, A. J. and Cochran, A. L. and Colom, P. and Conrad, Al and Coulson, I. M. and Crovisier, J. and DeBuizer, J. M. and Dekany, Richard and de León, J. and Russo, Neil Dello and Delsanti, A. and DiSanti, M. A. and Drummond, J. and Dundon, L. and Etzel, P. B. and Farnham, T. L. and Feldman, P. D. and Fernández, Y. R. and Filipović, M. D. and Fisher, Steven W. and Fitzsimmons, A. and Fong, Diane and Fugate, Robert Q. and Fujiwara, Hideo and Fujiyoshi, T. and Furusho, Reiko and Fuse, T. and Gibb, E. L. and Groussin, O. and Gulkis, S. and Gurwell, Mark and Hadamcik, E. and Hainaut, O. and Harker, D. E. and Harrington, D. M. and Harwit, Martin and Hasegawa, Sunao and Hergenrother, C. W. and Hirst, Paul and Hodapp, K. W. and Honda, Mitsuhiko and Howell, E. S. and Hutsemékers, Damien and Iono, Daisuke and Ip, W.‐H. and Jackson, William M. and Jehin, Emmanuël and Jiang, Zihao and Jones, G. H. and Jones, P. A. and Kadono, Toshihiko and Kamath, U. W. and Käufl, H. U. and Kasuga, Toshihiro and Kawakita, Hideyo and Kelley, Michael S. P. and Kerber, F. and Kidger, M. and Kinoshita, Daisuke and Knight, Matthew M. and Lara, L. M. and Larson, S. M. and Lederer, Susan M. and Lee, Chin‐Fei and Levasseur-Regourd, A. C. and Li, J. Y. and Li, Q.-S. and Licandro, J.",
    title = "Deep Impact: Observations from a Worldwide Earth-Based Campaign",
    year = "2005",
    journal = "Science",
    abstract = "On 4 July 2005, many observatories around the world and in space observed the collision of Deep Impact with comet 9P/Tempel 1 or its aftermath. This was an unprecedented coordinated observational campaign. These data show that (i) there was new material after impact that was compositionally different from that seen before impact; (ii) the ratio of dust mass to gas mass in the ejecta was much larger than before impact; (iii) the new activity did not last more than a few days, and by 9 July the comet's behavior was indistinguishable from its pre-impact behavior; and (iv) there were interesting transient phenomena that may be correlated with cratering physics.",
    url = "https://doi.org/10.1126/science.1118978",
    doi = "10.1126/science.1118978",
    openalex = "W2145682260"
}

Nuclear explosions, and a wide variety of technologies not yet realized, have been proposed to deflect asteroids away from collision with Earth. In contrast, this article presents realistic models for simple kinetic energy impact deflection, using the actual orbital elements of 795 catalogued Potentially Hazardous Asteroids, and impactor masses launched to intercept trajectories by Atlas V HLV rockets or equivalent. The authors take asteroid diameter, density, cratering characteristics, and Earth-collision lead time as parameters whose influence is to be investigated. Assuming asteroids of rocklike density, the article finds deflection off of Earth-collision to be achievable given 5-year lead time with a single kinetic energy intercept for 100% of 250 m diameter PHAs, 20-year lead with a single intercept for 93% of 500 m PHAs, 20-year lead with 5 and 10 intercepts, respectively, for 55% and 94% of 1 km PHAs, or 100-year lead with 1 and 2 intercepts, respectively, for 55% and 94% of 1 km PHAs. Considering likely future lead times for Near-Earth Objects, simple impact deflection using current launch vehicles is therefore a viable strategy for up to kilometer-diameter asteroids. This method has important advantages over other proposals: it requires no new technologies, would not require development or testing of nuclear warheads, and would likely be the least costly, least risky, and fastest to effect.

@article{doi10108008929880701319754,
    author = "Koenig, Jesse D. and Chyba, Christopher F.",
    title = "Impact Deflection of Potentially Hazardous Asteroids Using Current Launch Vehicles",
    year = "2007",
    journal = "Science and Global Security",
    abstract = "Nuclear explosions, and a wide variety of technologies not yet realized, have been proposed to deflect asteroids away from collision with Earth. In contrast, this article presents realistic models for simple kinetic energy impact deflection, using the actual orbital elements of 795 catalogued Potentially Hazardous Asteroids, and impactor masses launched to intercept trajectories by Atlas V HLV rockets or equivalent. The authors take asteroid diameter, density, cratering characteristics, and Earth-collision lead time as parameters whose influence is to be investigated. Assuming asteroids of rocklike density, the article finds deflection off of Earth-collision to be achievable given 5-year lead time with a single kinetic energy intercept for 100\% of 250 m diameter PHAs, 20-year lead with a single intercept for 93\% of 500 m PHAs, 20-year lead with 5 and 10 intercepts, respectively, for 55\% and 94\% of 1 km PHAs, or 100-year lead with 1 and 2 intercepts, respectively, for 55\% and 94\% of 1 km PHAs. Considering likely future lead times for Near-Earth Objects, simple impact deflection using current launch vehicles is therefore a viable strategy for up to kilometer-diameter asteroids. This method has important advantages over other proposals: it requires no new technologies, would not require development or testing of nuclear warheads, and would likely be the least costly, least risky, and fastest to effect.",
    url = "https://doi.org/10.1080/08929880701319754",
    doi = "10.1080/08929880701319754",
    openalex = "W2135375372"
}

Most flowering plants have been shown to be ancient polyploids that have undergone one or more whole genome duplications early in their evolution. Furthermore, many different plant lineages seem to have experienced an additional, more recent genome duplication. Starting from paralogous genes lying in duplicated segments or identified in large expressed sequence tag collections, we dated these youngest duplication events through penalized likelihood phylogenetic tree inference. We show that a majority of these independent genome duplications are clustered in time and seem to coincide with the Cretaceous-Tertiary (KT) boundary. The KT extinction event is the most recent mass extinction caused by one or more catastrophic events such as a massive asteroid impact and/or increased volcanic activity. These events are believed to have generated global wildfires and dust clouds that cut off sunlight during long periods of time resulting in the extinction of approximately 60% of plant species, as well as a majority of animals, including dinosaurs. Recent studies suggest that polyploid species can have a higher adaptability and increased tolerance to different environmental conditions. We propose that polyploidization may have contributed to the survival and propagation of several plant lineages during or following the KT extinction event. Due to advantages such as altered gene expression leading to hybrid vigor and an increased set of genes and alleles available for selection, polyploid plants might have been better able to adapt to the drastically changed environment 65 million years ago.

@article{doi101073pnas0900906106,
    author = "Fawcett, Jeffrey A. and Maere, Steven and de Peer, Yves Van",
    title = "Plants with double genomes might have had a better chance to survive the Cretaceous–Tertiary extinction event",
    year = "2009",
    journal = "Proceedings of the National Academy of Sciences",
    abstract = "Most flowering plants have been shown to be ancient polyploids that have undergone one or more whole genome duplications early in their evolution. Furthermore, many different plant lineages seem to have experienced an additional, more recent genome duplication. Starting from paralogous genes lying in duplicated segments or identified in large expressed sequence tag collections, we dated these youngest duplication events through penalized likelihood phylogenetic tree inference. We show that a majority of these independent genome duplications are clustered in time and seem to coincide with the Cretaceous-Tertiary (KT) boundary. The KT extinction event is the most recent mass extinction caused by one or more catastrophic events such as a massive asteroid impact and/or increased volcanic activity. These events are believed to have generated global wildfires and dust clouds that cut off sunlight during long periods of time resulting in the extinction of approximately 60\% of plant species, as well as a majority of animals, including dinosaurs. Recent studies suggest that polyploid species can have a higher adaptability and increased tolerance to different environmental conditions. We propose that polyploidization may have contributed to the survival and propagation of several plant lineages during or following the KT extinction event. Due to advantages such as altered gene expression leading to hybrid vigor and an increased set of genes and alleles available for selection, polyploid plants might have been better able to adapt to the drastically changed environment 65 million years ago.",
    url = "https://doi.org/10.1073/pnas.0900906106",
    doi = "10.1073/pnas.0900906106",
    openalex = "W2152650625",
    references = "doi101038nature06148, doi101038nrg1711, doi101073pnas0400396101, doi10108010635150390235520, doi101093genetics15141531, doi101093molbevmsm088, doi101093nar22224673, doi101126science1128691, doi101126science29054941151, doi101146annurevgenet341401, doi101371journalpbio0040088"
}

The Cretaceous-Paleogene boundary approximately 65.5 million years ago marks one of the three largest mass extinctions in the past 500 million years. The extinction event coincided with a large asteroid impact at Chicxulub, Mexico, and occurred within the time of Deccan flood basalt volcanism in India. Here, we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction. Notably, a single ejecta-rich deposit compositionally linked to the Chicxulub impact is globally distributed at the Cretaceous-Paleogene boundary. The temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations (for example, darkness and cooling) lead us to conclude that the Chicxulub impact triggered the mass extinction.

@article{doi101126science1177265,
    author = "Schulte, Peter and Alegret, Laia and Arenillas, Ignacio and Arz, José Antonio and Barton, P. J. and Bown, Paul R. and Bralower, Timothy J. and Christeson, Gail and Claeys, Philippe and Cockell, Charles S. and Collins, G. S. and Deutsch, A. and Goldin, Tamara and Goto, Kazuhisa and Grajales-Nishimura, José Manuel and Grieve, R. A. F. and Gulick, S. P. S. and Johnson, Kirk R. and Kiessling, Wolfgang and Koeberl, Christian and Kring, D. A. and MacLeod, Kenneth G. and Matsui, Takafumi and Melosh, J. and Montanari, Alessandro and Morgan, Joanna and Neal, C. R. and Nichols, Douglas J. and Norris, Richard D. and Pierazzo, E. and Ravizza, Greg and Rebolledo‐Vieyra, M. and Reimold, W. U. and Robin, Éric and Salge, T. and Speijer, Robert P. and Sweet, A R and Urrutia‐Fucugauchi, J. and Vajda, Vivi and Whalen, Michael T. and Willumsen, Pi Suhr",
    title = "The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary",
    year = "2010",
    journal = "Science",
    abstract = "The Cretaceous-Paleogene boundary approximately 65.5 million years ago marks one of the three largest mass extinctions in the past 500 million years. The extinction event coincided with a large asteroid impact at Chicxulub, Mexico, and occurred within the time of Deccan flood basalt volcanism in India. Here, we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction. Notably, a single ejecta-rich deposit compositionally linked to the Chicxulub impact is globally distributed at the Cretaceous-Paleogene boundary. The temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations (for example, darkness and cooling) lead us to conclude that the Chicxulub impact triggered the mass extinction.",
    url = "https://doi.org/10.1126/science.1177265",
    doi = "10.1126/science.1177265",
    openalex = "W2160490562",
    references = "alvarez1980extraterrestrial, doi101016jepsl200605041, doi101016jepsl200607020, doi101016jepsl200902019, doi101016jpalaeo200702037, doi101016jpalaeo200709016, doi101017cbo9780511535536, doi1010292008jb005644, doi10102996rg03038, doi10102997je01743, doi101038285198a0, doi101073pnas0802597105, doi101126science1064706, doi101126science20844481095, doi1011300091761319910190867ccapct23co2, doi101130081372356655, doi1011302007242401, doi101146annurevearth27175, doi101146annurevecolsys35021103105715"
}

We analyse the population of near-Earth long-period comets (LPCs; perihelion distances q 10 3 yr). We have considered the sample of LPCs discovered during the period 1900-2009 and their estimated absolute total visual magnitudes H. For the period 1900-1970 we have relied upon historical estimates of absolute total magnitudes, while for the more recent period 1970-2009 we have made our own estimates of H based on Green's photometric data base and IAU Circulars. We have also used historical records for the sample of brightest comets (H < 4.5) covering the period: 1500-1899, based mainly on the Vsekhsvyatskii, Hasegawa and Kronk catalogues. We find that the cumulative distribution of H can be represented by a three-modal law of the form log 10 N <H = C + H, where the C values are constants for the different legs, and 0.28 0.10 for H < 4.0, 0.56 0.10 for 4.0 H < 5.8, and 0.20 0.02 for 5.8 H < 8.6. The large increase of the slope of the second leg of the H-distribution might be at least partially attributed to splitting of comet nuclei, leading to the creation of two or more daughter comets. The cumulative H-distribution tends to flatten for comets fainter than H 8.6. LPCs fainter than H 12 (or diameters D 0.5 km) are extremely rare, despite several sky surveys of near-Earth objects implemented during the last couple of decades, suggesting a minimum size for an LPC to remain active. We also find that about 30 per cent of all LPCs with q < 1.3 au are new (original bound energies 0 < E or < 10 -4 au -1), and that among the new comets about half come from the outer Oort cloud (energies 0 E or 0.3 10 -4 au -1), and the other half from the inner Oort cloud (energies 0.3 10 -4 E or 10 -4 au -1).

@article{doi101111j13652966201220989x,
    author = "Fernández, Julio A. and Sosa, Andrea",
    title = "Magnitude and size distribution of long-period comets in Earth-crossing or approaching orbits",
    year = "2012",
    journal = "Monthly Notices of the Royal Astronomical Society",
    abstract = "We analyse the population of near-Earth long-period comets (LPCs; perihelion distances q 10 3 yr). We have considered the sample of LPCs discovered during the period 1900-2009 and their estimated absolute total visual magnitudes H. For the period 1900-1970 we have relied upon historical estimates of absolute total magnitudes, while for the more recent period 1970-2009 we have made our own estimates of H based on Green's photometric data base and IAU Circulars. We have also used historical records for the sample of brightest comets (H < 4.5) covering the period: 1500-1899, based mainly on the Vsekhsvyatskii, Hasegawa and Kronk catalogues. We find that the cumulative distribution of H can be represented by a three-modal law of the form log 10 N <H = C + H, where the C values are constants for the different legs, and 0.28 0.10 for H < 4.0, 0.56 0.10 for 4.0 H < 5.8, and 0.20 0.02 for 5.8 H < 8.6. The large increase of the slope of the second leg of the H-distribution might be at least partially attributed to splitting of comet nuclei, leading to the creation of two or more daughter comets. The cumulative H-distribution tends to flatten for comets fainter than H 8.6. LPCs fainter than H 12 (or diameters D 0.5 km) are extremely rare, despite several sky surveys of near-Earth objects implemented during the last couple of decades, suggesting a minimum size for an LPC to remain active. We also find that about 30 per cent of all LPCs with q < 1.3 au are new (original bound energies 0 < E or < 10 -4 au -1), and that among the new comets about half come from the outer Oort cloud (energies 0 E or 0.3 10 -4 au -1), and the other half from the inner Oort cloud (energies 0.3 10 -4 E or 10 -4 au -1).",
    url = "https://doi.org/10.1111/j.1365-2966.2012.20989.x",
    doi = "10.1111/j.1365-2966.2012.20989.x",
    openalex = "W1910590492",
    references = "doi101086113494"
}

Ocean acidification may have severe consequences for marine ecosystems; however, assessing its future impact is difficult because laboratory experiments and field observations are limited by their reduced ecologic complexity and sample period, respectively. In contrast, the geological record contains long-term evidence for a variety of global environmental perturbations, including ocean acidification plus their associated biotic responses. We review events exhibiting evidence for elevated atmospheric CO(2), global warming, and ocean acidification over the past ~300 million years of Earth's history, some with contemporaneous extinction or evolutionary turnover among marine calcifiers. Although similarities exist, no past event perfectly parallels future projections in terms of disrupting the balance of ocean carbonate chemistry-a consequence of the unprecedented rapidity of CO(2) release currently taking place.

@article{doi101126science1208277,
    author = "Hönisch, Bärbel and Ridgwell, Andy and Schmidt, Daniela N. and Thomas, Ellen and Gibbs, Samantha J. and Sluijs, Appy and Zeebe, Richard E. and Kump, Lee R. and Martindale, Rowan C. and Greene, Sarah E. and Kiessling, Wolfgang and Ries, Justin B. and Zachos, James C. and Royer, Dana L. and Barker, S. and Marchitto, Thomas M. and Moyer, Ryan P. and Pelejero, Carles and Ziveri, Patrizia and Foster, Gavin L. and Williams, B.",
    title = "The Geological Record of Ocean Acidification",
    year = "2012",
    journal = "Science",
    abstract = "Ocean acidification may have severe consequences for marine ecosystems; however, assessing its future impact is difficult because laboratory experiments and field observations are limited by their reduced ecologic complexity and sample period, respectively. In contrast, the geological record contains long-term evidence for a variety of global environmental perturbations, including ocean acidification plus their associated biotic responses. We review events exhibiting evidence for elevated atmospheric CO(2), global warming, and ocean acidification over the past \textasciitilde 300 million years of Earth's history, some with contemporaneous extinction or evolutionary turnover among marine calcifiers. Although similarities exist, no past event perfectly parallels future projections in terms of disrupting the balance of ocean carbonate chemistry-a consequence of the unprecedented rapidity of CO(2) release currently taking place.",
    url = "https://doi.org/10.1126/science.1208277",
    doi = "10.1126/science.1208277",
    openalex = "W2147331520",
    references = "doi101007978366206278418, doi1010160031018294902518, doi101016jpalaeo200508011, doi101016s0012825200000374, doi101016s0031018298000170, doi1010292001pa000623, doi1010292004gb002247, doi1010292009gc002788, doi101038353225a0, doi101126science1133822, doi101126science1177265, doi101126science1213454, doi101126science29255252310, doi101130g322301, doi101130spe369, doi101146annurevecolsys35021103105715, doi102475ajs2914377"
}

Mass extinctions manifest in Earth's geologic record were turning points in biotic evolution. We present (40)Ar/(39)Ar data that establish synchrony between the Cretaceous-Paleogene boundary and associated mass extinctions with the Chicxulub bolide impact to within 32,000 years. Perturbation of the atmospheric carbon cycle at the boundary likely lasted less than 5000 years, exhibiting a recovery time scale two to three orders of magnitude shorter than that of the major ocean basins. Low-diversity mammalian fauna in the western Williston Basin persisted for as little as 20,000 years after the impact. The Chicxulub impact likely triggered a state shift of ecosystems already under near-critical stress.

@article{doi101126science1230492,
    author = "Renne, Paul R. and Deino, Alan L. and Hilgen, F.J. and Kuiper, Klaudia F. and Mark, Darren F. and Mitchell, William S. and Morgan, Leah E. and Mundil, Roland and Smit, Jan",
    title = "Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary",
    year = "2013",
    journal = "Science",
    abstract = "Mass extinctions manifest in Earth's geologic record were turning points in biotic evolution. We present (40)Ar/(39)Ar data that establish synchrony between the Cretaceous-Paleogene boundary and associated mass extinctions with the Chicxulub bolide impact to within 32,000 years. Perturbation of the atmospheric carbon cycle at the boundary likely lasted less than 5000 years, exhibiting a recovery time scale two to three orders of magnitude shorter than that of the major ocean basins. Low-diversity mammalian fauna in the western Williston Basin persisted for as little as 20,000 years after the impact. The Chicxulub impact likely triggered a state shift of ecosystems already under near-critical stress.",
    url = "https://doi.org/10.1126/science.1230492",
    doi = "10.1126/science.1230492",
    openalex = "W1964523361",
    references = "doi101007s1091400569434, doi101016jchemgeo200503011, doi101016jcretres200805030, doi101016jepsl200902019, doi101016jepsl201107015, doi101016jgca2006061563, doi101016jgca201006017, doi101016jgca201106021, doi101016s0009254197001599, doi101016s0012821x03005570, doi101016s1631071303000063, doi1010292008jb005644, doi101038nature08227, doi101038nature11018, doi1010510004636120041335, doi10105100046361201116836, doi101073pnas802627, doi101126science1116412, doi101126science1154339, doi101126science1177265, doi101126science22346411177, doi101126science25250131690, doi101126science2575072954, doi1011270078042120120020, doi10113000917613198614279ssaedt20co2, doi1011300091761319910190867ccapct23co2, doi101130spe332, doi101146annurevecolsys35021103105715, doi101666070341, openalexw610180004"
}

@article{yeomans2013comparing,
    author = "Yeomans, Donald K. and Chamberlin, Alan B.",
    title = "Comparing the Earth impact flux from comets and near-Earth asteroids",
    year = "2013",
    journal = "Acta Astronautica",
    url = "https://doi.org/10.1016/j.actaastro.2012.03.006",
    doi = "10.1016/j.actaastro.2012.03.006",
    number = "1",
    openalex = "W2158992122",
    pages = "3-5",
    volume = "90",
    references = "doi101046j13658711200104747x, doi101086113494, doi101086185162, doi101111j13652966201118406x, doi101126science1118923, doi101126science28854742190, doi101130spe247p155, openalexw1615946943, openalexw1648269713, openalexw1652940046"
}

Changes in pollen and spore assemblages across the Cretaceous–Paleogene (K–Pg) boundary elucidate the vegetation response to a global environmental crisis triggered by an asteroid impact in Mexico 66 Ma. The Cretaceous–Paleogene boundary clay, associated with the Chicxulub asteroid impact event, constitutes a unique, global marker bed enabling comparison of the world-wide palynological signal spanning the mass extinction event. The data from both hemispheres are consistent, revealing diverse latest Cretaceous assemblages of pollen and spores that were affected by a major diversity loss as a consequence of the K–Pg event. Here we combine new results with past studies to provide an integrated global perspective of the terrestrial vegetation record across the K–Pg boundary. We further apply the K–Pg event as a template to asses the causal mechanism behind other major events in Earths history. The end-Permian, end-Triassic, and the K–Pg mass-extinctions were responses to different causal processes that resulted in essentially similar succession of decline and recovery phases, although expressed at different temporal scales. The events share a characteristic pattern of a bloom of opportunistic “crisis” tax followed by a pulse in pioneer communities, and finally a recovery in diversity including evolution of new taxa. Based on their similar extinction and recovery patterns and the fact that the Last and First Appearance Datums associated with the extinctions are separated in time, we recommend using the K–Pg event as a model and to use relative abundance data for the stratigraphic definition of mass-extinction events and the placement of associated chronostratigraphic boundaries.

@article{doi101016jgloplacha201407014,
    author = "Vajda, Vivi and Bercovici, Antoine",
    title = "The global vegetation pattern across the Cretaceous–Paleogene mass extinction interval: A template for other extinction events",
    year = "2014",
    journal = "Global and Planetary Change",
    abstract = "Changes in pollen and spore assemblages across the Cretaceous–Paleogene (K–Pg) boundary elucidate the vegetation response to a global environmental crisis triggered by an asteroid impact in Mexico 66 Ma. The Cretaceous–Paleogene boundary clay, associated with the Chicxulub asteroid impact event, constitutes a unique, global marker bed enabling comparison of the world-wide palynological signal spanning the mass extinction event. The data from both hemispheres are consistent, revealing diverse latest Cretaceous assemblages of pollen and spores that were affected by a major diversity loss as a consequence of the K–Pg event. Here we combine new results with past studies to provide an integrated global perspective of the terrestrial vegetation record across the K–Pg boundary. We further apply the K–Pg event as a template to asses the causal mechanism behind other major events in Earths history. The end-Permian, end-Triassic, and the K–Pg mass-extinctions were responses to different causal processes that resulted in essentially similar succession of decline and recovery phases, although expressed at different temporal scales. The events share a characteristic pattern of a bloom of opportunistic “crisis” tax followed by a pulse in pioneer communities, and finally a recovery in diversity including evolution of new taxa. Based on their similar extinction and recovery patterns and the fact that the Last and First Appearance Datums associated with the extinctions are separated in time, we recommend using the K–Pg event as a model and to use relative abundance data for the stratigraphic definition of mass-extinction events and the placement of associated chronostratigraphic boundaries.",
    url = "https://doi.org/10.1016/j.gloplacha.2014.07.014",
    doi = "10.1016/j.gloplacha.2014.07.014",
    openalex = "W2079006768",
    references = "doi101007s1091400569434, doi1010160034666780900226, doi101016jcretres200805030, doi101016jepsl200902019, doi101016jpalaeo200702037, doi101016jpalaeo201105050, doi101017cbo9780511535536, doi101038352420a0, doi101038ncomms1815, doi101073pnas1211526110, doi101073pnas1319253111, doi101073pnas802627, doi101080019161222012718609, doi10108003115517708527763, doi10108011035890902924877, doi101130spe247, doi101371journalpone0052455, doi1023073514678, hotton2002palynology, russell2002synopsis"
}

The mass extinction at the Cretaceous-Paleogene boundary, ∼ 66 Ma, is thought to be caused by the impact of an asteroid at Chicxulub, present-day Mexico. Although the precise mechanisms that led to this mass extinction remain enigmatic, most postulated scenarios involve a short-lived global cooling, a so-called "impact winter" phase. Here we document a major decline in sea surface temperature during the first months to decades following the impact event, using TEX86 paleothermometry of sediments from the Brazos River section, Texas. We interpret this cold spell to reflect, to our knowledge, the first direct evidence for the effects of the formation of dust and aerosols by the impact and their injection in the stratosphere, blocking incoming solar radiation. This impact winter was likely a major driver of mass extinction because of the resulting global decimation of marine and continental photosynthesis.

@article{doi101073pnas1319253111,
    author = "Vellekoop, Johan and Sluijs, Appy and Smit, Jan and Schouten, Stefan and Weijers, Johan W.H. and Damsté, Jaap S. Sinninghe and Brinkhuis, Henk",
    title = "Rapid short-term cooling following the Chicxulub impact at the Cretaceous–Paleogene boundary",
    year = "2014",
    journal = "Proceedings of the National Academy of Sciences",
    abstract = {The mass extinction at the Cretaceous-Paleogene boundary, ∼ 66 Ma, is thought to be caused by the impact of an asteroid at Chicxulub, present-day Mexico. Although the precise mechanisms that led to this mass extinction remain enigmatic, most postulated scenarios involve a short-lived global cooling, a so-called "impact winter" phase. Here we document a major decline in sea surface temperature during the first months to decades following the impact event, using TEX86 paleothermometry of sediments from the Brazos River section, Texas. We interpret this cold spell to reflect, to our knowledge, the first direct evidence for the effects of the formation of dust and aerosols by the impact and their injection in the stratosphere, blocking incoming solar radiation. This impact winter was likely a major driver of mass extinction because of the resulting global decimation of marine and continental photosynthesis.},
    url = "https://doi.org/10.1073/pnas.1319253111",
    doi = "10.1073/pnas.1319253111",
    openalex = "W2105771959",
    references = "alvarez1980extraterrestrial, doi101016jgca201005027, doi101016jorggeochem200607018, doi101016jpalaeo200702037, doi101016s0012821x02009792, doi10102997je01743, doi101038285198a0, doi10103835097000, doi101073pnas0802597105, doi101126science1177265, doi101126science2414865567, doi101130081372356655, doi101146annurevearth33092203122654, ganapathy1981iridium"
}

The Chicxulub asteroid impact (Mexico) and the eruption of the massive Deccan volcanic province (India) are two proposed causes of the end-Cretaceous mass extinction, which includes the demise of nonavian dinosaurs. Despite widespread acceptance of the impact hypothesis, the lack of a high-resolution eruption timeline for the Deccan basalts has prevented full assessment of their relationship to the mass extinction. Here we apply uranium-lead (U-Pb) zircon geochronology to Deccan rocks and show that the main phase of eruptions initiated ~250,000 years before the Cretaceous-Paleogene boundary and that >1.1 million cubic kilometers of basalt erupted in ~750,000 years. Our results are consistent with the hypothesis that the Deccan Traps contributed to the latest Cretaceous environmental change and biologic turnover that culminated in the marine and terrestrial mass extinctions.

@article{doi101126scienceaaa0118,
    author = "Schoene, Blair and Samperton, Kyle M. and Eddy, Michael P. and Keller, Gerta and Adatte, Thierry and Bowring, Samuel A. and Khadri, S. and Gertsch, B.",
    title = "U-Pb geochronology of the Deccan Traps and relation to the end-Cretaceous mass extinction",
    year = "2014",
    journal = "Science",
    abstract = "The Chicxulub asteroid impact (Mexico) and the eruption of the massive Deccan volcanic province (India) are two proposed causes of the end-Cretaceous mass extinction, which includes the demise of nonavian dinosaurs. Despite widespread acceptance of the impact hypothesis, the lack of a high-resolution eruption timeline for the Deccan basalts has prevented full assessment of their relationship to the mass extinction. Here we apply uranium-lead (U-Pb) zircon geochronology to Deccan rocks and show that the main phase of eruptions initiated \textasciitilde 250,000 years before the Cretaceous-Paleogene boundary and that >1.1 million cubic kilometers of basalt erupted in \textasciitilde 750,000 years. Our results are consistent with the hypothesis that the Deccan Traps contributed to the latest Cretaceous environmental change and biologic turnover that culminated in the marine and terrestrial mass extinctions.",
    url = "https://doi.org/10.1126/science.aaa0118",
    doi = "10.1126/science.aaa0118",
    openalex = "W2009674195",
    references = "doi101007s0041000203647, doi1010160009254194001404, doi1010160012821x8390211x, doi1010160016703773902135, doi101016b9780080959757003107, doi101016jchemgeo200503011, doi101016jepsl200902019, doi101016jgca201006017, doi101016s0009254197001599, doi101016s0012821x0000159x, doi101016s1631071303000063, doi1010292006gc001492, doi1010292008jb005644, doi101103physrevc41889, doi101126science1097329, doi101126science1154339, doi101126science1177265, doi101126science1215507, doi101126science1230492, doi101126science1234204, doi1011300091761319980260995adswat23co2, doi1011302014250315, doi101130b309291, doi101130g306831, doi101144gsjgs15420265"
}

Abstract Sixty‐six million years ago, the end‐Cretaceous mass extinction ended the reign of the dinosaurs. Flood basalt eruptions and an asteroid impact are widely discussed causes, yet their contributions remain debated. Modeling the environmental changes after the Chicxulub impact can shed light on this question. Existing studies, however, focused on the effect of dust or used one‐dimensional, noncoupled atmosphere models. Here we explore the longer‐lasting cooling due to sulfate aerosols using a coupled climate model. Depending on aerosol stratospheric residence time, global annual mean surface air temperature decreased by at least 26°C, with 3 to 16 years subfreezing temperatures and a recovery time larger than 30 years. The surface cooling triggered vigorous ocean mixing which could have resulted in a plankton bloom due to upwelling of nutrients. These dramatic environmental changes suggest a pivotal role of the impact in the end‐Cretaceous extinction.

@article{doi1010022016gl072241,
    author = "Brugger, Julia and Feulner, Georg and Petri, Stefan",
    title = "Baby, it's cold outside: Climate model simulations of the effects of the asteroid impact at the end of the Cretaceous",
    year = "2016",
    journal = "Geophysical Research Letters",
    abstract = "Abstract Sixty‐six million years ago, the end‐Cretaceous mass extinction ended the reign of the dinosaurs. Flood basalt eruptions and an asteroid impact are widely discussed causes, yet their contributions remain debated. Modeling the environmental changes after the Chicxulub impact can shed light on this question. Existing studies, however, focused on the effect of dust or used one‐dimensional, noncoupled atmosphere models. Here we explore the longer‐lasting cooling due to sulfate aerosols using a coupled climate model. Depending on aerosol stratospheric residence time, global annual mean surface air temperature decreased by at least 26°C, with 3 to 16 years subfreezing temperatures and a recovery time larger than 30 years. The surface cooling triggered vigorous ocean mixing which could have resulted in a plankton bloom due to upwelling of nutrients. These dramatic environmental changes suggest a pivotal role of the impact in the end‐Cretaceous extinction.",
    url = "https://doi.org/10.1002/2016gl072241",
    doi = "10.1002/2016gl072241",
    openalex = "W2566127310",
    references = "doi101073pnas1319253111"
}

Procedures introduced here make it possible, first, to show that background (piecemeal) extinction is recorded throughout geologic stages and substages (not all extinction has occurred suddenly at the ends of such intervals); second, to separate out background extinction from mass extinction for a major crisis in earth history; and third, to correct for clustering of extinctions when using the rarefaction method to estimate the percentage of species lost in a mass extinction. Also presented here is a method for estimating the magnitude of the Signor-Lipps effect, which is the incorrect assignment of extinctions that occurred during a crisis to an interval preceding the crisis because of the incompleteness of the fossil record. Estimates for the magnitudes of mass extinctions presented here are in most cases lower than those previously published. They indicate that only ∼81% of marine species died out in the great terminal Permian crisis, whereas levels of 90-96% have frequently been quoted in the literature. Calculations of the latter numbers were incorrectly based on combined data for the Middle and Late Permian mass extinctions. About 90 orders and more than 220 families of marine animals survived the terminal Permian crisis, and they embodied an enormous amount of morphological, physiological, and ecological diversity. Life did not nearly disappear at the end of the Permian, as has often been claimed.

@article{doi101073pnas1613094113,
    author = "Stanley, Steven M.",
    title = "Estimates of the magnitudes of major marine mass extinctions in earth history",
    year = "2016",
    journal = "Proceedings of the National Academy of Sciences",
    abstract = "Procedures introduced here make it possible, first, to show that background (piecemeal) extinction is recorded throughout geologic stages and substages (not all extinction has occurred suddenly at the ends of such intervals); second, to separate out background extinction from mass extinction for a major crisis in earth history; and third, to correct for clustering of extinctions when using the rarefaction method to estimate the percentage of species lost in a mass extinction. Also presented here is a method for estimating the magnitude of the Signor-Lipps effect, which is the incorrect assignment of extinctions that occurred during a crisis to an interval preceding the crisis because of the incompleteness of the fossil record. Estimates for the magnitudes of mass extinctions presented here are in most cases lower than those previously published. They indicate that only ∼81\% of marine species died out in the great terminal Permian crisis, whereas levels of 90-96\% have frequently been quoted in the literature. Calculations of the latter numbers were incorrectly based on combined data for the Middle and Late Permian mass extinctions. About 90 orders and more than 220 families of marine animals survived the terminal Permian crisis, and they embodied an enormous amount of morphological, physiological, and ecological diversity. Life did not nearly disappear at the end of the Permian, as has often been claimed.",
    url = "https://doi.org/10.1073/pnas.1613094113",
    doi = "10.1073/pnas.1613094113",
    openalex = "W2529501031",
    references = "doi101002gj1090, doi101007978364270831215, doi101016s001282520000026x, doi101016s0012825203000825, doi101017s0094837300013178, doi101130g211551, doi101146annurevearth33092203122654, doi1016660094837320050310006poaeit20co2, doi105860choice435903"
}

Earth-crossing asteroids and comets pose a long-term hazard to life and property on Earth. Schemes to mitigate the impact threat have been studied extensively but tend to focus on asteroid diversion while neglecting the possibility of a comet threat. Such schemes often demand physically intercepting the target by spacecraft, a task feasible only for targets identified decades in advance in a restricted range of orbits. A threatening comet is unlikely to satisfy these criteria and so necessitates a fundamentally different approach for diversion. Comets are naturally perturbed from purely gravitational trajectories through solar heating of their surfaces which activates sublimation-driven jets. Artificial heating of a comet, such as by a high-powered laser array in Earth orbit, may supplement natural heating by the Sun to purposefully manipulate its path to avoid an impact. The effectiveness of any particular laser array for a given comet depends on the comet's heating response which varies dramatically depending on factors including nucleus size, orbit and dynamical history. These factors are incorporated into a numerical orbital model using established models of nongravitational perturbations to evaluate the effectiveness and feasibility of using high-powered laser arrays in Earth orbit or on the ground to deflect a variety of comets. Simulation results suggest that orbital arrays of 500m and 10GW operating for 10 min=d over 1 yr may be adequate for mitigating impacts by comets up to ~500m in diameter. Continuously operating ground-based arrays of 100m and 10GW may be similarly effective when appropriately located.

@article{doi101117122235711,
    author = "Zhang, Qicheng and Lubin, Philip and Hughes, Gary B.",
    title = "Simulations of directed energy comet deflection",
    year = "2016",
    journal = "Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE",
    abstract = "Earth-crossing asteroids and comets pose a long-term hazard to life and property on Earth. Schemes to mitigate the impact threat have been studied extensively but tend to focus on asteroid diversion while neglecting the possibility of a comet threat. Such schemes often demand physically intercepting the target by spacecraft, a task feasible only for targets identified decades in advance in a restricted range of orbits. A threatening comet is unlikely to satisfy these criteria and so necessitates a fundamentally different approach for diversion. Comets are naturally perturbed from purely gravitational trajectories through solar heating of their surfaces which activates sublimation-driven jets. Artificial heating of a comet, such as by a high-powered laser array in Earth orbit, may supplement natural heating by the Sun to purposefully manipulate its path to avoid an impact. The effectiveness of any particular laser array for a given comet depends on the comet's heating response which varies dramatically depending on factors including nucleus size, orbit and dynamical history. These factors are incorporated into a numerical orbital model using established models of nongravitational perturbations to evaluate the effectiveness and feasibility of using high-powered laser arrays in Earth orbit or on the ground to deflect a variety of comets. Simulation results suggest that orbital arrays of 500m and 10GW operating for 10 min=d over 1 yr may be adequate for mitigating impacts by comets up to \textasciitilde 500m in diameter. Continuously operating ground-based arrays of 100m and 10GW may be similarly effective when appropriately located.",
    url = "https://doi.org/10.1117/12.2235711",
    doi = "10.1117/12.2235711",
    openalex = "W2522376979",
    references = "doi101007s1056901092773, doi1010160032063371901802, doi101016jpss200312010, doi10108008929880701319754, doi101086111402, doi1010880004637x7902114, doi101090s0025571897008739, doi101130spe247, openalexw2429971259, openalexw2906926620, yeomans2013comparing"
}

Abstract Potentially hazardous asteroids and comets have hit Earth throughout its history, with catastrophic consequences in the case of the Chicxulub impact. Here we reexamine one of the mechanisms that allow an impact to have a global effect—the release of climate‐active gases from sedimentary rocks. We use the SOVA hydrocode and model ejected materials for a sufficient time after impact to quantify the volume of gases that reach high enough altitudes (> 25 km) to have global consequences. We vary impact angle, sediment thickness and porosity, water depth, and shock pressure for devolatilization and present the results in a dimensionless form so that the released gases can be estimated for any impact into a sedimentary target. Using new constraints on the Chicxulub impact angle and target composition, we estimate that 325 ± 130 Gt of sulfur and 425 ± 160 Gt CO 2 were ejected and produced severe changes to the global climate.

@article{doi1010022017gl074879,
    author = "Artemieva, N. A. and Morgan, Joanna and Party, Expedition 364 Science",
    title = "Quantifying the Release of Climate‐Active Gases by Large Meteorite Impacts With a Case Study of Chicxulub",
    year = "2017",
    journal = "Geophysical Research Letters",
    abstract = "Abstract Potentially hazardous asteroids and comets have hit Earth throughout its history, with catastrophic consequences in the case of the Chicxulub impact. Here we reexamine one of the mechanisms that allow an impact to have a global effect—the release of climate‐active gases from sedimentary rocks. We use the SOVA hydrocode and model ejected materials for a sufficient time after impact to quantify the volume of gases that reach high enough altitudes (> 25 km) to have global consequences. We vary impact angle, sediment thickness and porosity, water depth, and shock pressure for devolatilization and present the results in a dimensionless form so that the released gases can be estimated for any impact into a sedimentary target. Using new constraints on the Chicxulub impact angle and target composition, we estimate that 325 ± 130 Gt of sulfur and 425 ± 160 Gt CO 2 were ejected and produced severe changes to the global climate.",
    url = "https://doi.org/10.1002/2017gl074879",
    doi = "10.1002/2017gl074879",
    openalex = "W2767140213",
    references = "doi101111j194551002005tb00157x"
}

Near-Earth asteroids (NEAs) in the 1–100 meter size range are estimated to be ~1,000 times more numerous than the ~15,000 currently cataloged NEAs, most of which are in the 0.5–10 kilometer size range. Impacts from 10–100 meter size NEAs are not statistically life-threatening, but may cause significant regional damage, while 1–10 meter size NEAs with low velocities relative to Earth are compelling targets for space missions. We describe the implementation and initial results of a real-time NEA-discovery system specialized for the detection of small, high angular rate (visually streaked) NEAs in Palomar Transient Factory (PTF) images. PTF is a 1.2-m aperture, 7.3 deg^2 field of view (FOV) optical survey designed primarily for the discovery of extragalactic transients (e.g., supernovae) in 60-second exposures reaching ~20.5 visual magnitude. Our real-time NEA discovery pipeline uses a machine-learned classifier to filter a large number of false-positive streak detections, permitting a human scanner to efficiently and remotely identify real asteroid streaks during the night. Upon recognition of a streaked NEA detection (typically within an hour of the discovery exposure), the scanner triggers follow-up with the same telescope and posts the observations to the Minor Planet Center for worldwide confirmation. We describe our 11 initial confirmed discoveries, all small NEAs that passed 0.3–15 lunar distances from Earth. Lastly, we derive useful scaling laws for comparing streaked-NEA-detection capabilities of different surveys as a function of their hardware and survey-pattern characteristics. This work most directly informs estimates of the streak-detection capabilities of the Zwicky Transient Facility (ZTF, planned to succeed PTF in 2017), which will apply PTF's current resolution and sensitivity over a 47-deg^2 FOV.

@article{doi10108815383873129973034402,
    author = "Waszczak, A. and Prince, Thomas A. and Laher, Russ R. and Masci, Frank J. and Bue, Brian and Rebbapragada, Umaa and Barlow, Tom A. and Surace, J. and Hélou, G. and Kulkarni, S. R.",
    title = "Small Near-Earth Asteroids in the Palomar Transient Factory Survey: A Real-Time Streak-detection System",
    year = "2017",
    journal = "Publications of the Astronomical Society of the Pacific",
    abstract = "Near-Earth asteroids (NEAs) in the 1–100 meter size range are estimated to be \textasciitilde 1,000 times more numerous than the \textasciitilde 15,000 currently cataloged NEAs, most of which are in the 0.5–10 kilometer size range. Impacts from 10–100 meter size NEAs are not statistically life-threatening, but may cause significant regional damage, while 1–10 meter size NEAs with low velocities relative to Earth are compelling targets for space missions. We describe the implementation and initial results of a real-time NEA-discovery system specialized for the detection of small, high angular rate (visually streaked) NEAs in Palomar Transient Factory (PTF) images. PTF is a 1.2-m aperture, 7.3 deg^2 field of view (FOV) optical survey designed primarily for the discovery of extragalactic transients (e.g., supernovae) in 60-second exposures reaching \textasciitilde 20.5 visual magnitude. Our real-time NEA discovery pipeline uses a machine-learned classifier to filter a large number of false-positive streak detections, permitting a human scanner to efficiently and remotely identify real asteroid streaks during the night. Upon recognition of a streaked NEA detection (typically within an hour of the discovery exposure), the scanner triggers follow-up with the same telescope and posts the observations to the Minor Planet Center for worldwide confirmation. We describe our 11 initial confirmed discoveries, all small NEAs that passed 0.3–15 lunar distances from Earth. Lastly, we derive useful scaling laws for comparing streaked-NEA-detection capabilities of different surveys as a function of their hardware and survey-pattern characteristics. This work most directly informs estimates of the streak-detection capabilities of the Zwicky Transient Facility (ZTF, planned to succeed PTF in 2017), which will apply PTF's current resolution and sensitivity over a 47-deg^2 FOV.",
    url = "https://doi.org/10.1088/1538-3873/129/973/034402",
    doi = "10.1088/1538-3873/129/973/034402",
    openalex = "W2526445631",
    references = "doi101046j13658711200104747x"
}

Long-period comets (LPCs) frequently transit the inner solar system, and like near-Earth asteroids (NEAs), pose a continued risk of impact with Earth. Unlike NEAs, LPCs follow nearly parabolic trajectories and approach from the distant outer solar system where they cannot be observed. An LPC on an Earth-impact trajectory is unlikely to be discovered more than a few years in advance of its arrival, even with significant advancements in sky survey detection capabilities, likely leaving insufficient time to develop and deliver an interception mission to deflect the comet. However, recent proposals have called for the development of one or more large ∼ 1 km laser arrays placed on or near Earth primarily as a means for photon propulsion of low-mass spacecraft at delta-v above what would be feasible by traditional chemical or ion propulsion methods. Such a laser array can also be directed to target and heat a threatening comet, sublimating its ices and activating jets of dust and vapor which alter the comet's trajectory in a manner similar to rocket propulsion. Simulations of directed energy comet deflection were previously developed from astrometric models of nongravitational orbital perturbations from solar heating, an analogous process that has been observed in numerous comets. These simulations are used together with the distribution of known LPC trajectories to evaluate the effect of an operational Earth-based laser array on the LPC impact risk.

@article{doi101117122274726,
    author = "Zhang, Qicheng and Lubin, Philip and Hughes, Gary B.",
    title = "Long-period comet impact risk mitigation with Earth-based laser arrays",
    year = "2017",
    abstract = "Long-period comets (LPCs) frequently transit the inner solar system, and like near-Earth asteroids (NEAs), pose a continued risk of impact with Earth. Unlike NEAs, LPCs follow nearly parabolic trajectories and approach from the distant outer solar system where they cannot be observed. An LPC on an Earth-impact trajectory is unlikely to be discovered more than a few years in advance of its arrival, even with significant advancements in sky survey detection capabilities, likely leaving insufficient time to develop and deliver an interception mission to deflect the comet. However, recent proposals have called for the development of one or more large ∼ 1 km laser arrays placed on or near Earth primarily as a means for photon propulsion of low-mass spacecraft at delta-v above what would be feasible by traditional chemical or ion propulsion methods. Such a laser array can also be directed to target and heat a threatening comet, sublimating its ices and activating jets of dust and vapor which alter the comet's trajectory in a manner similar to rocket propulsion. Simulations of directed energy comet deflection were previously developed from astrometric models of nongravitational orbital perturbations from solar heating, an analogous process that has been observed in numerous comets. These simulations are used together with the distribution of known LPC trajectories to evaluate the effect of an operational Earth-based laser array on the LPC impact risk.",
    url = "https://doi.org/10.1117/12.2274726",
    doi = "10.1117/12.2274726",
    openalex = "W2750731592",
    references = "doi101117122235711, yeomans2013comparing"
}

Abstract We derived 90% confidence limits (CLs) on the interstellar number density () of interstellar objects (ISOs; comets and asteroids) as a function of the slope of their size–frequency distribution (SFD) and limiting absolute magnitude. To account for gravitational focusing, we first generated a quasi-realistic ISO population to from the Sun and propagated it forward in time to generate a steady state population of ISOs with heliocentric distance. We then simulated the detection of the synthetic ISOs using pointing data for each image and average detection efficiencies for each of three contemporary solar system surveys—Pan-STARRS1, the Mt. Lemmon Survey, and the Catalina Sky Survey. These simulations allowed us to determine the surveys’ combined ISO detection efficiency under several different but realistic modes of identifying ISOs in the survey data. Some of the synthetic detected ISOs had eccentricities as small as 1.01, which is in the range of the largest eccentricities of several known comets. Our best CL of implies that the expectation that extra-solar systems form like our solar system, eject planetesimals in the same way, and then distribute them throughout the Galaxy, is too simplistic, or that the SFD or behavior of ISOs as they pass through our solar system is far from expectation.

@article{doi10384715383881aa5c8a,
    author = "Engelhardt, Toni and Jedicke, Robert and Vereš, Peter and Fitzsimmons, Alan and Denneau, Larry and Beshore, Ed and Meinke, Bonnie",
    title = "An Observational Upper Limit on the Interstellar Number Density of Asteroids and Comets",
    year = "2017",
    journal = "The Astronomical Journal",
    abstract = "Abstract We derived 90\% confidence limits (CLs) on the interstellar number density () of interstellar objects (ISOs; comets and asteroids) as a function of the slope of their size–frequency distribution (SFD) and limiting absolute magnitude. To account for gravitational focusing, we first generated a quasi-realistic ISO population to from the Sun and propagated it forward in time to generate a steady state population of ISOs with heliocentric distance. We then simulated the detection of the synthetic ISOs using pointing data for each image and average detection efficiencies for each of three contemporary solar system surveys—Pan-STARRS1, the Mt. Lemmon Survey, and the Catalina Sky Survey. These simulations allowed us to determine the surveys’ combined ISO detection efficiency under several different but realistic modes of identifying ISOs in the survey data. Some of the synthetic detected ISOs had eccentricities as small as 1.01, which is in the range of the largest eccentricities of several known comets. Our best CL of implies that the expectation that extra-solar systems form like our solar system, eject planetesimals in the same way, and then distribute them throughout the Galaxy, is too simplistic, or that the SFD or behavior of ISOs as they pass through our solar system is far from expectation.",
    url = "https://doi.org/10.3847/1538-3881/aa5c8a",
    doi = "10.3847/1538-3881/aa5c8a",
    openalex = "W2591603755",
    references = "doi101111j13652966201118406x"
}

@incollection{crossref2018comets,
    title = "Comets and Asteroids on Earth",
    year = "2018",
    booktitle = "CATCHING STARDUST",
    url = "https://doi.org/10.5040/9781472944023.0007",
    doi = "10.5040/9781472944023.0007",
    openalex = "W4241151238"
}

The debiased absolute-magnitude and orbit distributions as well as source regions for near-Earth objects (NEOs) provide a fundamental frame of reference for studies of individual NEOs and more complex population-level questions. We present a new four-dimensional model of the NEO population that describes debiased steady-state distributions of semimajor axis, eccentricity, inclination, and absolute magnitude H in the range 17 < H < 25. The modeling approach improves upon the methodology originally developed by Bottke et al. (2000, Science 288, 2190–2194) in that it is, for example, based on more realistic orbit distributions and uses source-specific absolute-magnitude distributions that allow for a power-law slope that varies with H. We divide the main asteroid belt into six different entrance routes or regions (ER) to the NEO region: the ν6, 3:1J, 5:2J and 2:1J resonance complexes as well as Hungarias and Phocaeas. In addition we include the Jupiter-family comets as the primary cometary source of NEOs. We calibrate the model against NEO detections by Catalina Sky Surveys’ stations 703 and G96 during 2005–2012, and utilize the complementary nature of these two systems to quantify the systematic uncertainties associated to the resulting model. We find that the (fitted) H distributions have significant differences, although most of them show a minimum power-law slope at H ∼ 20. As a consequence of the differences between the ER-specific H distributions we find significant variations in, for example, the NEO orbit distribution, average lifetime, and the relative contribution of different ERs as a function of H. The most important ERs are the ν6 and 3:1J resonance complexes with JFCs contributing a few percent of NEOs on average. A significant contribution from the Hungaria group leads to notable changes compared to the predictions by Bottke et al. in, for example, the orbit distribution and average lifetime of NEOs. We predict that there are 962−56+52 (802−42+48×103) NEOs with H < 17.75 (H < 25) and these numbers are in agreement with the most recent estimates found in the literature (the uncertainty estimates only account for the random component). Based on our model we find that relative shares between different NEO groups (Amor, Apollo, Aten, Atira, Vatira) are (39.4,54.4,3.5,1.2,0.3)%, respectively, for the considered H range and that these ratios have a negligible dependence on H. Finally, we find an agreement between our estimate for the rate of Earth impacts by NEOs and recent estimates in the literature, but there remains a potentially significant discrepancy in the frequency of Tunguska-sized and Chelyabinsk-sized impacts.

@article{doi101016jicarus201804018,
    author = "Granvik, Mikael and Morbidelli, Alessandro and Jedicke, Robert and Bolin, Bryce and Bottke, W. F. and Beshore, E. C. and Vokrouhlický, David and Nesvorný, David and Michel, Patrick",
    title = "Debiased orbit and absolute-magnitude distributions for near-Earth objects",
    year = "2018",
    journal = "Icarus",
    abstract = "The debiased absolute-magnitude and orbit distributions as well as source regions for near-Earth objects (NEOs) provide a fundamental frame of reference for studies of individual NEOs and more complex population-level questions. We present a new four-dimensional model of the NEO population that describes debiased steady-state distributions of semimajor axis, eccentricity, inclination, and absolute magnitude H in the range 17 < H < 25. The modeling approach improves upon the methodology originally developed by Bottke et al. (2000, Science 288, 2190–2194) in that it is, for example, based on more realistic orbit distributions and uses source-specific absolute-magnitude distributions that allow for a power-law slope that varies with H. We divide the main asteroid belt into six different entrance routes or regions (ER) to the NEO region: the ν6, 3:1J, 5:2J and 2:1J resonance complexes as well as Hungarias and Phocaeas. In addition we include the Jupiter-family comets as the primary cometary source of NEOs. We calibrate the model against NEO detections by Catalina Sky Surveys’ stations 703 and G96 during 2005–2012, and utilize the complementary nature of these two systems to quantify the systematic uncertainties associated to the resulting model. We find that the (fitted) H distributions have significant differences, although most of them show a minimum power-law slope at H ∼ 20. As a consequence of the differences between the ER-specific H distributions we find significant variations in, for example, the NEO orbit distribution, average lifetime, and the relative contribution of different ERs as a function of H. The most important ERs are the ν6 and 3:1J resonance complexes with JFCs contributing a few percent of NEOs on average. A significant contribution from the Hungaria group leads to notable changes compared to the predictions by Bottke et al. in, for example, the orbit distribution and average lifetime of NEOs. We predict that there are 962−56+52 (802−42+48×103) NEOs with H < 17.75 (H < 25) and these numbers are in agreement with the most recent estimates found in the literature (the uncertainty estimates only account for the random component). Based on our model we find that relative shares between different NEO groups (Amor, Apollo, Aten, Atira, Vatira) are (39.4,54.4,3.5,1.2,0.3)\%, respectively, for the considered H range and that these ratios have a negligible dependence on H. Finally, we find an agreement between our estimate for the rate of Earth impacts by NEOs and recent estimates in the literature, but there remains a potentially significant discrepancy in the frequency of Tunguska-sized and Chelyabinsk-sized impacts.",
    url = "https://doi.org/10.1016/j.icarus.2018.04.018",
    doi = "10.1016/j.icarus.2018.04.018",
    openalex = "W2798470423",
    references = "doi101038nature12741, doi101126science28854742190"
}

= 46) sourced from the primary literature. High-precision impact ages can be used to (1) reconstruct and quantify the impact flux in the inner Solar System and, in particular, the Earth-Moon system, thereby placing constraints on the delivery of extraterrestrial mass accreted on Earth through geologic time; (2) utilize impact ejecta as event markers in the stratigraphic record and to refine bio- and magneto-stratigraphy; (3) test models and hypotheses of synchronous double or multiple impact events in the terrestrial record; (4) assess the potential link between large impacts, mass extinctions, and diversification events in the biosphere; and (5) constrain the duration of melt sheet crystallization in large impact basins and the lifetime of hydrothermal systems in cooling impact craters, which may have served as habitats for microbial life on the early Earth and, possibly, Mars.

@article{doi101089ast20192085,
    author = "Schmieder, M. and Kring, D. A.",
    title = "Earth's Impact Events Through Geologic Time: A List of Recommended Ages for Terrestrial Impact Structures and Deposits",
    year = "2019",
    journal = "Astrobiology",
    abstract = "= 46) sourced from the primary literature. High-precision impact ages can be used to (1) reconstruct and quantify the impact flux in the inner Solar System and, in particular, the Earth-Moon system, thereby placing constraints on the delivery of extraterrestrial mass accreted on Earth through geologic time; (2) utilize impact ejecta as event markers in the stratigraphic record and to refine bio- and magneto-stratigraphy; (3) test models and hypotheses of synchronous double or multiple impact events in the terrestrial record; (4) assess the potential link between large impacts, mass extinctions, and diversification events in the biosphere; and (5) constrain the duration of melt sheet crystallization in large impact basins and the lifetime of hydrothermal systems in cooling impact craters, which may have served as habitats for microbial life on the early Earth and, possibly, Mars.",
    url = "https://doi.org/10.1089/ast.2019.2085",
    doi = "10.1089/ast.2019.2085",
    openalex = "W2997502701",
    references = "doi101016jchemgeo201502028, doi101016jgca201306010, doi101016jpalaeo200702037, doi101016jpalaeo201703014, doi101073pnas1319253111, doi101130081372356655, doi101130b310761, doi101130b318901, openalexw1615946943"
}

Abstract Cometary impacts pose a long-term hazard to life on Earth. Impact mitigation techniques have been studied extensively, but they tend to focus on asteroid diversion. Typical asteroid interdiction schemes involve spacecraft physically intercepting the target, a task feasible only for targets identified decades in advance and in a narrow range of orbits—criteria unlikely to be satisfied by a threatening comet. Comets, however, are naturally perturbed from purely gravitational trajectories via solar heating of their surfaces, which activates sublimation-driven jets. Artificial heating of a comet, such as by a laser, may supplement natural heating by the Sun to purposefully manipulate its path and thereby avoid an impact. Deflection effectiveness depends on the comet’s heating response, which varies dramatically depending on factors including nucleus size, orbit, and dynamical history. These factors are incorporated into a numerical orbital model to assess the effectiveness and feasibility of using high-powered laser arrays in Earth orbit and on the ground for comet deflection. Simulation results suggest that a diffraction-limited 500 m orbital or terrestrial laser array operating at 10 GW for 1% of each day over 1 yr is sufficient to fully avert the impact of a typical 500 m diameter comet with primary nongravitational parameter A 1 = 2 × 10 −8 au day −2. Strategies to avoid comet fragmentation during deflection are also discussed.

@article{doi10384715383881ab13a5,
    author = "Zhang, Qicheng and Lubin, Philip M. and Hughes, Gary B.",
    title = "Orbital Deflection of Comets by Directed Energy",
    year = "2019",
    journal = "The Astronomical Journal",
    abstract = "Abstract Cometary impacts pose a long-term hazard to life on Earth. Impact mitigation techniques have been studied extensively, but they tend to focus on asteroid diversion. Typical asteroid interdiction schemes involve spacecraft physically intercepting the target, a task feasible only for targets identified decades in advance and in a narrow range of orbits—criteria unlikely to be satisfied by a threatening comet. Comets, however, are naturally perturbed from purely gravitational trajectories via solar heating of their surfaces, which activates sublimation-driven jets. Artificial heating of a comet, such as by a laser, may supplement natural heating by the Sun to purposefully manipulate its path and thereby avoid an impact. Deflection effectiveness depends on the comet’s heating response, which varies dramatically depending on factors including nucleus size, orbit, and dynamical history. These factors are incorporated into a numerical orbital model to assess the effectiveness and feasibility of using high-powered laser arrays in Earth orbit and on the ground for comet deflection. Simulation results suggest that a diffraction-limited 500 m orbital or terrestrial laser array operating at 10 GW for 1\% of each day over 1 yr is sufficient to fully avert the impact of a typical 500 m diameter comet with primary nongravitational parameter A 1 = 2 × 10 −8 au day −2. Strategies to avoid comet fragmentation during deflection are also discussed.",
    url = "https://doi.org/10.3847/1538-3881/ab13a5",
    doi = "10.3847/1538-3881/ab13a5",
    openalex = "W2942485992",
    references = "doi101007s1056901092773, doi101007s1120700574339, doi1010160032063371901802, doi101016jpss200312010, doi101038438177a, doi10108008929880701319754, doi101086111402, doi101086497684, doi101090s0025571897008739, doi101117122235711, doi10384715383881aaafd2, yeomans2013comparing"
}

Cometary impacts pose a long-term hazard to life on Earth. Impact mitigation techniques have been studied extensively, but they tend to focus on asteroid diversion. Typical asteroid interdiction schemes involve spacecraft physically intercepting the target, a task feasible only for targets identified decades in advance and in a narrow range of orbits---criteria unlikely to be satisfied by a threatening comet. Comets, however, are naturally perturbed from purely gravitational trajectories through solar heating of their surfaces which activates sublimation-driven jets. Artificial heating of a comet, such as by a laser, may supplement natural heating by the Sun to purposefully manipulate its path and thereby avoid an impact. Deflection effectiveness depends on the comet's heating response, which varies dramatically depending on factors including nucleus size, orbit and dynamical history. These factors are incorporated into a numerical orbital model to assess the effectiveness and feasibility of using high-powered laser arrays in Earth orbit and on the ground for comet deflection. Simulation results suggest that a diffraction-limited 500 m orbital or terrestrial laser array operating at 10 GW for 1% of each day over 1 yr is sufficient to fully avert the impact of a typical 500 m diameter comet with primary nongravitational parameter A1 = 2 x 10^-8 au d^-2. Strategies to avoid comet fragmentation during deflection are also discussed.

@article{openalexw3099244276,
    author = "Zhang, Qicheng and Lubin, Philip and Hughes, Gary B.",
    title = "Orbital Deflection of Comets by Directed Energy",
    year = "2019",
    journal = "eScholarship (California Digital Library)",
    abstract = "Cometary impacts pose a long-term hazard to life on Earth. Impact mitigation techniques have been studied extensively, but they tend to focus on asteroid diversion. Typical asteroid interdiction schemes involve spacecraft physically intercepting the target, a task feasible only for targets identified decades in advance and in a narrow range of orbits---criteria unlikely to be satisfied by a threatening comet. Comets, however, are naturally perturbed from purely gravitational trajectories through solar heating of their surfaces which activates sublimation-driven jets. Artificial heating of a comet, such as by a laser, may supplement natural heating by the Sun to purposefully manipulate its path and thereby avoid an impact. Deflection effectiveness depends on the comet's heating response, which varies dramatically depending on factors including nucleus size, orbit and dynamical history. These factors are incorporated into a numerical orbital model to assess the effectiveness and feasibility of using high-powered laser arrays in Earth orbit and on the ground for comet deflection. Simulation results suggest that a diffraction-limited 500 m orbital or terrestrial laser array operating at 10 GW for 1\% of each day over 1 yr is sufficient to fully avert the impact of a typical 500 m diameter comet with primary nongravitational parameter A1 = 2 x 10^-8 au d^-2. Strategies to avoid comet fragmentation during deflection are also discussed.",
    openalex = "W3099244276",
    references = "doi101117122235711, yeomans2013comparing"
}

The Cretaceous/Paleogene mass extinction, 66 Ma, included the demise of non-avian dinosaurs. Intense debate has focused on the relative roles of Deccan volcanism and the Chicxulub asteroid impact as kill mechanisms for this event. Here, we combine fossil-occurrence data with paleoclimate and habitat suitability models to evaluate dinosaur habitability in the wake of various asteroid impact and Deccan volcanism scenarios. Asteroid impact models generate a prolonged cold winter that suppresses potential global dinosaur habitats. Conversely, long-term forcing from Deccan volcanism (carbon dioxide [CO 2]-induced warming) leads to increased habitat suitability. Short-term (aerosol cooling) volcanism still allows equatorial habitability. These results support the asteroid impact as the main driver of the non-avian dinosaur extinction. By contrast, induced warming from volcanism mitigated the most extreme effects of asteroid impact, potentially reducing the extinction severity.

@article{doi101073pnas2006087117,
    author = "Chiarenza, Alfio Alessandro and Farnsworth, Alexander and Mannion, Philip D. and Lunt, Daniel J. and Valdes, Paul J. and Morgan, Joanna and Allison, Peter A.",
    title = "Asteroid impact, not volcanism, caused the end-Cretaceous dinosaur extinction",
    year = "2020",
    journal = "Proceedings of the National Academy of Sciences",
    abstract = "The Cretaceous/Paleogene mass extinction, 66 Ma, included the demise of non-avian dinosaurs. Intense debate has focused on the relative roles of Deccan volcanism and the Chicxulub asteroid impact as kill mechanisms for this event. Here, we combine fossil-occurrence data with paleoclimate and habitat suitability models to evaluate dinosaur habitability in the wake of various asteroid impact and Deccan volcanism scenarios. Asteroid impact models generate a prolonged cold winter that suppresses potential global dinosaur habitats. Conversely, long-term forcing from Deccan volcanism (carbon dioxide [CO 2]-induced warming) leads to increased habitat suitability. Short-term (aerosol cooling) volcanism still allows equatorial habitability. These results support the asteroid impact as the main driver of the non-avian dinosaur extinction. By contrast, induced warming from volcanism mitigated the most extreme effects of asteroid impact, potentially reducing the extinction severity.",
    url = "https://doi.org/10.1073/pnas.2006087117",
    doi = "10.1073/pnas.2006087117",
    openalex = "W3038551147",
    references = "alvarez1980extraterrestrial, doi101007s1091400569434, doi101016jcub201804062, doi101016s0012825200000374, doi10102993jd02553, doi101038s41467019089972, doi101073pnas1211526110, doi101073pnas1319253111, doi101111brv12128, doi101111ecog03049, doi101111j14724642201000725x, doi101111j16000587200805742x, doi101126sciadvaat4858, doi101126science1177265, doi101126science1229237, doi101126science20844481095, doi101126science21545391501, doi101126scienceaau2422, doi101126scienceaay2268, doi1011302014250315, doi1011302014250502, doi101130spe247, doi101144sp35813"
}

The conditions, timing, and setting for the origin of life on Earth and whether life exists elsewhere in our solar system and beyond represent some of the most fundamental scientific questions of our time. Although the bombardment of planets and satellites by asteroids and comets has long been viewed as a destructive process that would have presented a barrier to the emergence of life and frustrated or extinguished life, we provide a comprehensive synthesis of data and observations on the beneficial role of impacts in a wide range of prebiotic and biological processes. In the context of previously proposed environments for the origin of life on Earth, we discuss how meteorite impacts can generate both subaerial and submarine hydrothermal vents, abundant hydrothermal-sedimentary settings, and impact analogues for volcanic pumice rafts and splash pools. Impact events can also deliver and/or generate many of the necessary chemical ingredients for life and catalytic substrates such as clays as well. The role that impact cratering plays in fracturing planetary crusts and its effects on deep subsurface habitats for life are also discussed. In summary, we propose that meteorite impact events are a fundamental geobiological process in planetary evolution that played an important role in the origin of life on Earth. We conclude with the recommendation that impact craters should be considered prime sites in the search for evidence of past life on Mars. Furthermore, unlike other geological processes such as volcanism or plate tectonics, impact cratering is ubiquitous on planetary bodies throughout the Universe and is independent of size, composition, and distance from the host star. Impact events thus provide a mechanism with the potential to generate habitable planets, moons, and asteroids throughout the Solar System and beyond.

@article{doi101089ast20192203,
    author = "Osinski, G. R. and Cockell, Charles S. and Pontefract, A. and Sapers, H. M.",
    title = "The Role of Meteorite Impacts in the Origin of Life",
    year = "2020",
    journal = "Astrobiology",
    abstract = "The conditions, timing, and setting for the origin of life on Earth and whether life exists elsewhere in our solar system and beyond represent some of the most fundamental scientific questions of our time. Although the bombardment of planets and satellites by asteroids and comets has long been viewed as a destructive process that would have presented a barrier to the emergence of life and frustrated or extinguished life, we provide a comprehensive synthesis of data and observations on the beneficial role of impacts in a wide range of prebiotic and biological processes. In the context of previously proposed environments for the origin of life on Earth, we discuss how meteorite impacts can generate both subaerial and submarine hydrothermal vents, abundant hydrothermal-sedimentary settings, and impact analogues for volcanic pumice rafts and splash pools. Impact events can also deliver and/or generate many of the necessary chemical ingredients for life and catalytic substrates such as clays as well. The role that impact cratering plays in fracturing planetary crusts and its effects on deep subsurface habitats for life are also discussed. In summary, we propose that meteorite impact events are a fundamental geobiological process in planetary evolution that played an important role in the origin of life on Earth. We conclude with the recommendation that impact craters should be considered prime sites in the search for evidence of past life on Mars. Furthermore, unlike other geological processes such as volcanism or plate tectonics, impact cratering is ubiquitous on planetary bodies throughout the Universe and is independent of size, composition, and distance from the host star. Impact events thus provide a mechanism with the potential to generate habitable planets, moons, and asteroids throughout the Solar System and beyond.",
    url = "https://doi.org/10.1089/ast.2019.2203",
    doi = "10.1089/ast.2019.2203",
    openalex = "W3082273190",
    references = "doi101016jearscirev200910009"
}

The cause of the end-Cretaceous mass extinction is vigorously debated, owing to the occurrence of a very large bolide impact and flood basalt volcanism near the boundary. Disentangling their relative importance is complicated by uncertainty regarding kill mechanisms and the relative timing of volcanogenic outgassing, impact, and extinction. We used carbon cycle modeling and paleotemperature records to constrain the timing of volcanogenic outgassing. We found support for major outgassing beginning and ending distinctly before the impact, with only the impact coinciding with mass extinction and biologically amplified carbon cycle change. Our models show that these extinction-related carbon cycle changes would have allowed the ocean to absorb massive amounts of carbon dioxide, thus limiting the global warming otherwise expected from postextinction volcanism.

@article{doi101126scienceaay5055,
    author = "Hull, Pincelli M. and Bornemann, André and Penman, Donald E. and Henehan, Michael J. and Norris, Richard D. and Wilson, Paul A. and Blum, Peter and Alegret, Laia and Batenburg, Sietske J. and Bown, Paul R. and Bralower, Timothy J. and Cournède, C. and Deutsch, A. and Donner, Barbara and Friedrich, Oliver and Jehle, Sofie and Kim, Hojung and Kroon, Dick and Lippert, Peter C. and Loroch, Dominik and Moebius, Iris and Moriya, Kazuyoshi and Peppe, Daniel J. and Ravizza, G. and Röhl, Ursula and Schueth, Jonathan D. and Sepúlveda, Julio and Sexton, Philip F. and Sibert, Elizabeth C and Śliwińska, Kasia K. and Summons, Roger E. and Thomas, Ellen and Westerhold, Thomas and Whiteside, Jessica H. and Yamaguchi, Tatsuhiko and Zachos, James C.",
    title = "On impact and volcanism across the Cretaceous-Paleogene boundary",
    year = "2020",
    journal = "Science",
    abstract = "The cause of the end-Cretaceous mass extinction is vigorously debated, owing to the occurrence of a very large bolide impact and flood basalt volcanism near the boundary. Disentangling their relative importance is complicated by uncertainty regarding kill mechanisms and the relative timing of volcanogenic outgassing, impact, and extinction. We used carbon cycle modeling and paleotemperature records to constrain the timing of volcanogenic outgassing. We found support for major outgassing beginning and ending distinctly before the impact, with only the impact coinciding with mass extinction and biologically amplified carbon cycle change. Our models show that these extinction-related carbon cycle changes would have allowed the ocean to absorb massive amounts of carbon dioxide, thus limiting the global warming otherwise expected from postextinction volcanism.",
    url = "https://doi.org/10.1126/science.aay5055",
    doi = "10.1126/science.aay5055",
    openalex = "W2999541819",
    references = "doi101016jepsl200902019, doi1010292008jb005644, doi101073pnas1319253111, doi101098rspb20181194, doi101126science2825387276, doi101126scienceaaa0118, doi101126scienceaau2422, doi1011300091761319980260995adswat23co2, doi101130b318901"
}

The Hayabusa2 spacecraft investigated the small asteroid Ryugu, which has a rubble-pile structure. We describe an impact experiment on Ryugu using Hayabusa2's Small Carry-on Impactor. The impact produced an artificial crater with a diameter >10 meters, which has a semicircular shape, an elevated rim, and a central pit. Images of the impact and resulting ejecta were recorded by the Deployable CAMera 3 for >8 minutes, showing the growth of an ejecta curtain (the outer edge of the ejecta) and deposition of ejecta onto the surface. The ejecta curtain was asymmetric and heterogeneous and it never fully detached from the surface. The crater formed in the gravity-dominated regime; in other words, crater growth was limited by gravity not surface strength. We discuss implications for Ryugu's surface age.

@article{doi101126scienceaaz1701,
    author = "Arakawa, Masahiko and Saiki, Takanao and Wada, Koji and Ogawa, Kazunori and Kadono, Toshihiko and Shirai, K. and Sawada, Hirotaka and Ishibashi, Ko and Honda, Rie and Sakatani, Naoya and Iijima, Y. and Okamoto, Chisato and Yano, Hajime and Takagi, Y. and Hayakawa, Masahiko and Michel, Patrick and Jutzi, Martin and Shimaki, Yuri and Kimura, Shinichi and Mimasu, Yuya and Toda, Tomoaki and IMAMURA, Hiroshi and Nakazawa, Satoru and Hayakawa, H. and Sugita, Seiji and Morota, Tomokatsu and Kameda, Shingo and Tatsumi, Eri and Cho, Yuichiro and Yoshioka, Kazuo and Yokota, Y. and Matsuoka, M. and Yamada, Manabu and Kouyama, Toru and Honda, Chikatoshi and Tsuda, Yuichi and Watanabe, Sei‐ichiro and Yoshikawa, Makoto and Tanaka, Satoshi and Terui, Fuyuto and Kikuchi, Shota and Yamaguchi, Tomohiro and Ogawa, Naoko and Ono, Go and Yoshikawa, Kent and Takahashi, T. and Takei, Yuto and Fujii, Atsushi and Takeuchi, Hiroshi and Yamamoto, Yukio and Okada, Tatsuaki and Hirose, Chikako and Hosoda, S and Mori, Osamu and Shimada, Takanobu and Soldini, Stefania and Tsukizaki, Ryudo and Iwata, Takahiro and Ozaki, Masanobu and Abe, Masanao and Namiki, Noriyuki and Kitazato, K. and Tachibana, Shogo and Ikeda, Hitoshi and Hirata, Naru and Hirata, N. and Noguchi, Rina and Miura, Akira",
    title = "An artificial impact on the asteroid (162173) Ryugu formed a crater in the gravity-dominated regime",
    year = "2020",
    journal = "Science",
    abstract = "The Hayabusa2 spacecraft investigated the small asteroid Ryugu, which has a rubble-pile structure. We describe an impact experiment on Ryugu using Hayabusa2's Small Carry-on Impactor. The impact produced an artificial crater with a diameter >10 meters, which has a semicircular shape, an elevated rim, and a central pit. Images of the impact and resulting ejecta were recorded by the Deployable CAMera 3 for >8 minutes, showing the growth of an ejecta curtain (the outer edge of the ejecta) and deposition of ejecta onto the surface. The ejecta curtain was asymmetric and heterogeneous and it never fully detached from the surface. The crater formed in the gravity-dominated regime; in other words, crater growth was limited by gravity not surface strength. We discuss implications for Ryugu's surface age.",
    url = "https://doi.org/10.1126/science.aaz1701",
    doi = "10.1126/science.aaz1701",
    openalex = "W3011623968",
    references = "doi101126science1118923, doi105194esurf212014"
}

@incollection{bowell2021earthcrossing,
    author = "BOWELL, EDWARD and MUINONEN, KARRI",
    title = "EARTH-CROSSING ASTEROIDS AND COMETS:",
    year = "2021",
    booktitle = "Hazards Due to Comets and Asteroids",
    url = "https://doi.org/10.2307/j.ctv23khmpv.10",
    doi = "10.2307/j.ctv23khmpv.10",
    openalex = "W4205695748",
    pages = "149-198"
}

Over the past few decades, it has become increasingly clear that the impact of interplanetary bodies on other planetary bodies is one of the most ubiquitous and important geological processes in the Solar System. This impact process has played a fundamental role throughout the history of the Earth and other planetary bodies, resulting in both destructive and beneficial effects. The impact cratering record of Earth is critical to our understanding of the processes, products, and effects of impact events. In this contribution, we provide an up-to-date review and synthesis of the impact cratering record on Earth. Following a brief history of the Impact Earth Database (available online at http://www.impactearth.com), the definition of the main categories of impact features listed in the database, and an overview of the impact cratering process, we review and summarize the required evidence to confirm impact events. Based on these definitions and criteria, we list 188 hypervelocity impact craters and 13 impact craters (i.e., impact sites lacking evidence for shock metamorphism). For each crater, we provide details on key attributes, such as location, date confirmed, erosional level, age, target properties, diameter, and an overview of the shock metamorphic effects and impactites that have been described in the literature. We also list a large number of impact deposits, which we have classified into four main categories: tektites, spherule layers, occurrences of other types of glass, and breccias. We discuss the challenges of recognizing and confirming impact events and highlight weaknesses, contradictions, and inconsistencies in the literature. We then address the morphology and morphometry of hypervelocity impact craters. Based on the Impact Earth Database, it is apparent that the transition diameter from simple to complex craters for craters developed in sedimentary versus crystalline target rocks is less pronounced than previously reported, at approximately 3 km for both. Our analysis also yields an estimate for stratigraphic uplift of 0.0945D0.6862, which is lower than previous estimates. We ascribe this to more accurate diameter estimates plus the variable effects of erosion. It is also clear that central topographic peaks in terrestrial complex impact craters are, in general, more subdued than their lunar counterparts. Furthermore, a number of relatively well-preserved terrestrial complex impact structures lack central peaks entirely. The final section of this review provides an overview of impactites preserved in terrestrial hypervelocity impact craters. While approximately three quarters of hypervelocity impact craters on Earth preserve some portion of their crater-fill impactites, ejecta deposits are known from less than 10%. In summary, the Impact Earth Database provides an important new resource for researchers interested in impact craters and the impact cratering process and we welcome input from the community to ensure that the Impact Earth website (http://www.impactearth.com) is a living resource that is as accurate and as up-to-date, as possible.

@article{doi101016jearscirev2022104112,
    author = "Osinski, G. R. and Grieve, R. A. F. and Ferrière, L. and Łosiak, A. and Pickersgill, A. E. and Cavosie, Aaron J. and Hibbard, Shannon M. and Hill, P. J. A. and Bermudez, Juan Jaimes and Marion, C. L. and Newman, J. D. and Simpson, Sarah",
    title = "Impact Earth: A review of the terrestrial impact record",
    year = "2022",
    journal = "Earth-Science Reviews",
    abstract = "Over the past few decades, it has become increasingly clear that the impact of interplanetary bodies on other planetary bodies is one of the most ubiquitous and important geological processes in the Solar System. This impact process has played a fundamental role throughout the history of the Earth and other planetary bodies, resulting in both destructive and beneficial effects. The impact cratering record of Earth is critical to our understanding of the processes, products, and effects of impact events. In this contribution, we provide an up-to-date review and synthesis of the impact cratering record on Earth. Following a brief history of the Impact Earth Database (available online at http://www.impactearth.com), the definition of the main categories of impact features listed in the database, and an overview of the impact cratering process, we review and summarize the required evidence to confirm impact events. Based on these definitions and criteria, we list 188 hypervelocity impact craters and 13 impact craters (i.e., impact sites lacking evidence for shock metamorphism). For each crater, we provide details on key attributes, such as location, date confirmed, erosional level, age, target properties, diameter, and an overview of the shock metamorphic effects and impactites that have been described in the literature. We also list a large number of impact deposits, which we have classified into four main categories: tektites, spherule layers, occurrences of other types of glass, and breccias. We discuss the challenges of recognizing and confirming impact events and highlight weaknesses, contradictions, and inconsistencies in the literature. We then address the morphology and morphometry of hypervelocity impact craters. Based on the Impact Earth Database, it is apparent that the transition diameter from simple to complex craters for craters developed in sedimentary versus crystalline target rocks is less pronounced than previously reported, at approximately 3 km for both. Our analysis also yields an estimate for stratigraphic uplift of 0.0945D0.6862, which is lower than previous estimates. We ascribe this to more accurate diameter estimates plus the variable effects of erosion. It is also clear that central topographic peaks in terrestrial complex impact craters are, in general, more subdued than their lunar counterparts. Furthermore, a number of relatively well-preserved terrestrial complex impact structures lack central peaks entirely. The final section of this review provides an overview of impactites preserved in terrestrial hypervelocity impact craters. While approximately three quarters of hypervelocity impact craters on Earth preserve some portion of their crater-fill impactites, ejecta deposits are known from less than 10\%. In summary, the Impact Earth Database provides an important new resource for researchers interested in impact craters and the impact cratering process and we welcome input from the community to ensure that the Impact Earth website (http://www.impactearth.com) is a living resource that is as accurate and as up-to-date, as possible.",
    url = "https://doi.org/10.1016/j.earscirev.2022.104112",
    doi = "10.1016/j.earscirev.2022.104112",
    openalex = "W4286268600",
    references = "doi101016jearscirev200910009, doi101038202526a0, doi101111j194551002005tb00157x, doi101130b318901"
}

Large meteoroid impacts have punctuated the Phanerozoic Eon, with the Cretaceous–Paleogene (K–Pg) impact as the most prominent example. While some impacts triggered global climate change and mass extinctions, others produced only limited environmental effects. These events injected aerosols into the stratosphere, reducing sunlight, cooling the climate, and decreasing precipitation. Major aerosol types include sulfuric acid (from sulfur-rich rocks), soot (from organic-rich target rocks and wildfires), and dust (from pulverized rock), though their relative roles remain uncertain. Here we quantify the production of each aerosol type and calculate surface temperature anomalies for the nine largest impact craters of the past 250 Myr using target rock lithologies and climate model outputs. Our cross-plot analysis of temperature and extinction magnitude reveals that soot generated from organic carbon in target rocks is likely the primary driver of impact-induced mass extinctions. Additionally, we estimated the frequencies of impact-generated mass extinctions for each aerosol type using two cases: one major extinction (>60 % species loss) in 540 million years (K-Pg) and two major-moderate extinctions (>20 % species loss) in 250 million years (K-Pg and mid Norian). Our findings demonstrate that soot formed from sedimentary rocks most accurately matches the observed frequencies—one major and two minor-major extinctions. These results establish that the severity of cooling and extinction triggered by meteoroid impacts is primarily determined by the abundance of buried organic carbon in the target rocks, underscoring the target sensitivity of impact-induced climate effects. • Soot from ignition of hydrocarbons by impacts is the main cause of mass extinctions. • The degree of cooling depends on amounts of buried hydrocarbons in target rocks. • Hydrocarbon soot has a greater influence on global extinctions than other aerosols. • Small variations in composition of impact sites result in different bio-evolution. • Organisms themselves have prepared the triggers for impact-generated extinctions.

@article{doi101016jpalaeo2025113237,
    author = "Kaiho, Kunio and Oshima, Naga",
    title = "The significance of impact-induced hydrocarbon soot aerosols in global climate change and extinctions",
    year = "2025",
    journal = "Palaeogeography Palaeoclimatology Palaeoecology",
    abstract = "Large meteoroid impacts have punctuated the Phanerozoic Eon, with the Cretaceous–Paleogene (K–Pg) impact as the most prominent example. While some impacts triggered global climate change and mass extinctions, others produced only limited environmental effects. These events injected aerosols into the stratosphere, reducing sunlight, cooling the climate, and decreasing precipitation. Major aerosol types include sulfuric acid (from sulfur-rich rocks), soot (from organic-rich target rocks and wildfires), and dust (from pulverized rock), though their relative roles remain uncertain. Here we quantify the production of each aerosol type and calculate surface temperature anomalies for the nine largest impact craters of the past 250 Myr using target rock lithologies and climate model outputs. Our cross-plot analysis of temperature and extinction magnitude reveals that soot generated from organic carbon in target rocks is likely the primary driver of impact-induced mass extinctions. Additionally, we estimated the frequencies of impact-generated mass extinctions for each aerosol type using two cases: one major extinction (>60 \% species loss) in 540 million years (K-Pg) and two major-moderate extinctions (>20 \% species loss) in 250 million years (K-Pg and mid Norian). Our findings demonstrate that soot formed from sedimentary rocks most accurately matches the observed frequencies—one major and two minor-major extinctions. These results establish that the severity of cooling and extinction triggered by meteoroid impacts is primarily determined by the abundance of buried organic carbon in the target rocks, underscoring the target sensitivity of impact-induced climate effects. • Soot from ignition of hydrocarbons by impacts is the main cause of mass extinctions. • The degree of cooling depends on amounts of buried hydrocarbons in target rocks. • Hydrocarbon soot has a greater influence on global extinctions than other aerosols. • Small variations in composition of impact sites result in different bio-evolution. • Organisms themselves have prepared the triggers for impact-generated extinctions.",
    url = "https://doi.org/10.1016/j.palaeo.2025.113237",
    doi = "10.1016/j.palaeo.2025.113237",
    openalex = "W4413840227",
    references = "doi101016jcub202111061, doi101016jearscirev2024104904"
}

Abstract The near-Earth asteroid (NEA) 2024 PT5 is on an Earth-like orbit that remained in Earth's immediate vicinity for several months at the end of 2024. PT5's orbit is challenging to populate with asteroids originating from the main belt and is more commonly associated with rocket bodies mistakenly identified as natural objects or with debris ejected from impacts on the Moon. We obtained visible and near-infrared reflectance spectra of PT5 with the Lowell Discovery Telescope and NASA Infrared Telescope Facility on 2024 August 16. The combined reflectance spectrum matches lunar samples but does not match any known asteroid types—it is pyroxene-rich, while asteroids of comparable spectral redness are olivine-rich. Moreover, the amount of solar radiation pressure observed on the PT5 trajectory is orders of magnitude lower than what would be expected for an artificial object. We therefore conclude that 2024 PT5 is ejecta from an impact on the Moon, thus making PT5 the second NEA suggested to be sourced from the surface of the Moon. While one object might be an outlier, two suggest that there is an underlying population to be characterized. Long-term predictions of the position of 2024 PT5 are challenging due to the slow Earth encounters characteristic of objects in these orbits. A population of near-Earth objects that are sourced by the Moon would be important to characterize for understanding how impacts work on our nearest neighbor and for identifying the source regions of asteroids and meteorites from this understudied population of objects on very Earth-like orbits.

@article{doi10384720418213ad9ea8,
    author = "Kareta, Theodore and Fuentes-Muñoz, Oscar and Moskovitz, Nicholas and Farnocchia, Davide and Sharkey, Benjamin N. L.",
    title = "On the Lunar Origin of Near-Earth Asteroid 2024 PT5",
    year = "2025",
    journal = "The Astrophysical Journal Letters",
    abstract = "Abstract The near-Earth asteroid (NEA) 2024 PT5 is on an Earth-like orbit that remained in Earth's immediate vicinity for several months at the end of 2024. PT5's orbit is challenging to populate with asteroids originating from the main belt and is more commonly associated with rocket bodies mistakenly identified as natural objects or with debris ejected from impacts on the Moon. We obtained visible and near-infrared reflectance spectra of PT5 with the Lowell Discovery Telescope and NASA Infrared Telescope Facility on 2024 August 16. The combined reflectance spectrum matches lunar samples but does not match any known asteroid types—it is pyroxene-rich, while asteroids of comparable spectral redness are olivine-rich. Moreover, the amount of solar radiation pressure observed on the PT5 trajectory is orders of magnitude lower than what would be expected for an artificial object. We therefore conclude that 2024 PT5 is ejecta from an impact on the Moon, thus making PT5 the second NEA suggested to be sourced from the surface of the Moon. While one object might be an outlier, two suggest that there is an underlying population to be characterized. Long-term predictions of the position of 2024 PT5 are challenging due to the slow Earth encounters characteristic of objects in these orbits. A population of near-Earth objects that are sourced by the Moon would be important to characterize for understanding how impacts work on our nearest neighbor and for identifying the source regions of asteroids and meteorites from this understudied population of objects on very Earth-like orbits.",
    url = "https://doi.org/10.3847/2041-8213/ad9ea8",
    doi = "10.3847/2041-8213/ad9ea8",
    openalex = "W4406373378",
    references = "doi101038s4155002402258z"
}

Abstract On 2032 December 22, the 60 m diameter asteroid 2024 YR 4 has a 4% chance of impacting the Moon. Such an impact would release 6.5 MT TNT equivalent energy and produce a ∼1 km diameter crater. We estimate that up to 10 8 kg of lunar material could be liberated in such an impact by exceeding lunar escape speed. The current overall probability is about 1% that the asteroid will impact the Moon at a location such that more than 10% of the ejected material would accrete to the Earth on timescales of a few days. If this were to occur, the lunar-ejecta-associated particle fluence at 0.1–10 mm sizes could produce up to several years of equivalent background meteoroid impact exposure to satellites in near-Earth space late in 2032. Our results demonstrate that planetary defense considerations should be more broadly extended to cislunar space and not confined solely to near-Earth space.

@article{doi10384720418213adfa8b,
    author = "Wiegert, Paul and Brown, Peter and Lopes, J. A. Peas and Connors, Martin",
    title = "The Potential Danger to Satellites due to Ejecta from a 2032 Lunar Impact by Asteroid 2024 YR 4",
    year = "2025",
    journal = "The Astrophysical Journal Letters",
    abstract = "Abstract On 2032 December 22, the 60 m diameter asteroid 2024 YR 4 has a 4\% chance of impacting the Moon. Such an impact would release 6.5 MT TNT equivalent energy and produce a ∼1 km diameter crater. We estimate that up to 10 8 kg of lunar material could be liberated in such an impact by exceeding lunar escape speed. The current overall probability is about 1\% that the asteroid will impact the Moon at a location such that more than 10\% of the ejected material would accrete to the Earth on timescales of a few days. If this were to occur, the lunar-ejecta-associated particle fluence at 0.1–10 mm sizes could produce up to several years of equivalent background meteoroid impact exposure to satellites in near-Earth space late in 2032. Our results demonstrate that planetary defense considerations should be more broadly extended to cislunar space and not confined solely to near-Earth space.",
    url = "https://doi.org/10.3847/2041-8213/adfa8b",
    doi = "10.3847/2041-8213/adfa8b",
    openalex = "W4413803679",
    references = "doi101038s4155002402258z"
}