An impact event is the collision of a large meteorite, asteroid, comet, or other celestial object with the Earth or another planet. Throughout recorded history, hundreds of minor impact events (and exploding bolides) have been reported, with some occurrences causing deaths, injuries, property damage or other significant localised consequences.
Impact events have been a plot and background element in science fiction since knowledge of real impacts became established in the scientific mainstream.
Sizes and frequencies
Small objects frequently collide with the Earth. There is an inverse relationship between the size of the object and the frequency that such objects hit the earth. The lunar cratering record shows that the frequency of impacts decreases as approximately the cube of the resulting crater’s diameter, which is on average proportional to the diameter of the impactor. Asteroids with a 1 km (0.62 mi) diameter strike the Earth every 500,000 years on average. Large collisions – with 5 km (3 mi) objects – happen approximately once every ten million years. The last known impact of an object of 10 km (6 mi) or more in diameter was at the Cretaceous–Tertiary extinction event 65 million years ago.
Asteroids with diameters of 5 to 10 m (16 to 33 ft) enter the Earth’s atmosphere approximately once per year, with as much energy as Little Boy, the atomic bomb dropped on Hiroshima, approximately 15 kilotonnes of TNT. These ordinarily explode in the upper atmosphere, and most or all of the solids are vaporized. Objects with diameters over 50 m (164 ft) strike the Earth approximately once every thousand years, producing explosions comparable to the one known to have detonated above Tunguska in 1908. At least one known asteroid with a diameter of over 1 km (0.62 mi), (29075) 1950 DA, has a possibility of colliding with Earth on March 16, 2880, but the Torino scale only works for impact possibilities within 100 years, and thus cannot apply to this asteroid.
Objects with diameters smaller than 10 m (33 ft) are called meteoroids (or meteorites if they strike the ground). An estimated 500 meteorites reach the surface each year, but only 5 or 6 of these are typically recovered and made known to scientists.
Geology of Earth-impact events
Earth has gone through periods of abrupt and catastrophic change, some due to the impact of large asteroids and comets on the planet. A few of these impacts may have caused massive climate change and the extinction of large numbers of plant and animal species.
The Moon is widely attributed to a huge impact early in Earth’s history. Impact events earlier in the history of Earth have been credited with creative as well as destructive events; it has been proposed that impacting comets delivered the Earth’s water, and some have suggested that the origins of life may have been influenced by impacting objects by bringing organic chemicals or lifeforms to the Earth’s surface, a theory known as exogenesis.
These modified views of the Earth’s history did not emerge until relatively recently, chiefly due to a lack of direct observations and the difficulty in recognizing the signs of an Earth impact because of erosion and weathering. Large-scale terrestrial impacts of the sort that produced the Barringer Crater, locally known as Meteor Crater, northeast of Flagstaff, Arizona, are rare. Instead, it was widely thought that cratering was the result of volcanism: the Barringer Crater, for example, was ascribed to a prehistoric volcanic explosion (not an unreasonable hypothesis, given that the volcanic San Francisco Peaks stand only 30 miles (48 km) to the west). Similarly, the craters on the surface of the Moon were ascribed to volcanism.
It was not until 1903–1905 that the Barringer Crater was correctly identified as being an impact crater, and it was not until as recently as 1963 that research byEugene Merle Shoemaker conclusively proved this hypothesis. The findings of late 20th-century space exploration and the work of scientists such as Shoemaker demonstrated that impact cratering was by far the most widespread geological process at work on the solar system’s solid bodies. Every surveyed solid body in the solar system was found to be cratered, and there was no reason to believe that the Earth had somehow escaped bombardment from space. In the last few decades of the twentieth century, a large number of highly modified impact craters began to be identified. The largest of these include Vredefort Crater, Sudbury Crater,Chicxulub Crater, and Manicouagan Crater. The first observation of a major impact event occurred in 1994: the collision of the comet Shoemaker-Levy 9 with Jupiter; to date, no such events have been observed on Earth.
Based on crater formation rates determined from the Earth’s closest celestial partner, the Moon, astrogeologists have determined that during the last 600 million years, the Earth has been struck by 60 objects of a diameter of 5 km (3 mi) or more. The smallest of these impactors would release the equivalent of ten millionmegatons of TNT and leave a crater 95 km (60 mi) across. For comparison, the largest nuclear weapon ever detonated, the Tsar Bomba, had a yield of 50 megatons.
Besides direct effect of asteroid impacts on a planet’s surface topography, global climate and life, recent studies have shown that several consecutive impacts can have effect on the dynamo mechanism at a planet’s core responsible for maintaining the magnetic field of the planet, and can eventually shut down the planet’s magnetic field.
While numerous impact craters have been confirmed on land or in the shallow seas over continental shelves, no impact craters in the deep ocean have been widely accepted by the scientific community. Impacts of projectiles as large as 1 km in diameter are generally thought to explode before reaching the sea floor, but it is unknown what would happen if a much larger impactor struck the deep ocean. The lack of a crater, however, does not mean that an ocean impact would not have dangerous implications for humanity. Some scholars have argued that an impact event in an ocean or sea may create a tsunami (a giant wave), which can cause destruction both at sea and on land along the coast, but this is disputed.
Recent prehistoric impact events
In addition to the extremely large impacts that happen every few tens of millions of years, there are many smaller impacts that occur more frequently but which leave correspondingly smaller traces behind. Due to the strong forces of erosion at work on Earth, only relatively recent examples of these smaller impacts are known. A few of the more famous or interesting examples are:
- Barringer Crater in the USA, the first crater to be proven the result of an impact, ~50,000 years old.
- the Rio Cuarto craters in Argentina, produced by an asteroid striking Earth at a very low angle, ~10,000 years old.
- the Lonar crater lake in India, which now has a flourishing semi-tropical jungle around it, ~52,000 years old (though a study published in 2010 gives a much greater age).
- the Henbury craters in Australia (~5,000 years old), and Kaali craters in Estonia (~2,700 years old), apparently produced by objects which broke up before impact.
The Clovis comet hypothesis is a theory that an air burst from a large comet above or even into the Laurentide Ice Sheet north of the Great Lakes set all of the North American continent ablaze around 12,900 years ago. The theory attempts to explain the extinction of most of the large animals in North America and the demise of the North American stone age Clovis culture about at the end of the Pleistocene epoch. Proponents claim the existence of a charred carbon-rich layer of soil found at some 50 Clovis-age sites across the continent. It has been criticized for not being consistent with paleoindian population estimates. Impact specialists have studied the claim and concluded that there never was such an impact, in particular because various physical signs of such an impact cannot be found.
More recent prehistoric impacts are theorized by the Holocene Impact Working Group, including Dallas Abbott of Columbia University’s Lamont-Doherty Earth Observatory. This group points to four enormouschevron sediment deposits at the southern end of Madagascar, containing deep-ocean microfossils fused with metals typically formed by cosmic impacts. All of the chevrons point toward a spot in the middle of the Indian Ocean corresponding with the newly hypothesized Burckle crater proposed to be some 29 km (18 mi) in diameter, or about 25 times larger than Barringer Crater. This group posits that a large asteroid or comet impact c. 2800-3000 BCE produced a mega-tsunami at least 180 m (590 ft) high, a catastrophic event that would have affected humanity’s cradles of civilization. The evidence for the proposed crater has been challenged.
Years 533–534 CE ± 2 impact events have been proposed by the dendrochronologist Mike Baillie as a possible cause of several brief (typically 5-10 year) climatic downturns recorded in ancient tree ring patterns. Bailliel highlights four such events and suggests that these might have been caused by the dust veils thrown up by the impact of cometary debris.
Modern impact events
A Chinese record states that 10,000 people were killed in Shanxi Province in 1490 by a hail of “falling stones”; some astronomers surmise that this may describe the breakup of a large asteroid, although they find the number of deaths implausible.
Kamil Crater, discovered from Google Earth image review in Egypt, 45 meters in diameter, 10 meters deep is thought to have been formed less than 3,500 years ago in a then-unpopulated region of Western Egypt. It was found February 19, 2009 by V. de Michelle on a Google Earth image of the East Uweinat Desert, Egypt.
The Mahuika crater may have resulted from a modern impact event. The crater is located south of the Snares Islands (120 km (70 mi) southwest of Stewart Island) on the southern New Zealand shelf  and is approximately 20 kilometres (12 mi) wide. Material extracted from Siple Dome ice core melt water indicates that the impact occurred around 1443 C.E.
The Wabar craters in Arabia may have been created sometime during the past few hundred years.
The most significant recorded impact in recent times was the Tunguska event, which occurred in Siberia, Russia, in 1908. This incident involved an explosion that was probably caused by the airburst of an asteroid or comet 5 to 10 km (3.1 to 6.2 mi) above the Earth’s surface, felling an estimated 80 million trees over 2,150 km2(830 sq mi).
The late Eugene Shoemaker of the U.S. Geological Survey came up with an estimate of the rate of Earth impacts, and suggested that an event about the size of the nuclear weapon that destroyed Hiroshima occurs about once a year. Such events would seem to be spectacularly obvious, but they generally go unnoticed for a number of reasons: the majority of the Earth’s surface is covered by water; a good portion of the land surface is uninhabited; and the explosions generally occur at relatively high altitude, resulting in a huge flash and thunderclap but no real damage. Some have been observed. Noteworthy examples include the Sikhote-Alin Meteorite fall in Primorye, far eastern Russia, in 1947, and the Revelstoke fireball of 1965, which occurred over the snows of British Columbia, Canada.
A small number of meteor falls have been observed with automated cameras and recovered following calculation of the impact point. The first of these was the Pribram meteorite, which fell in Czechoslovakia (now the Czech Republic) in 1959. In this case, two cameras used to photograph meteors captured images of the fireball. The images were used both to determine the location of the stones on the ground and, more significantly, to calculate for the first time an accurate orbit for a recovered meteorite.
Following the Pribram fall, other nations established automated observing programs aimed at studying infalling meteorites. One of these was the Prairie Network, operated by the Smithsonian Astrophysical Observatory from 1963 to 1975 in the midwestern US. This program also observed a meteorite fall, the Lost City chondrite, allowing its recovery and a calculation of its orbit. Another program in Canada, the Meteorite Observation and Recovery Project, ran from 1971 to 1985. It too recovered a single meteorite, Innisfree, in 1977. Finally, observations by the European Fireball Network, a descendant of the original Czech program that recovered Pribram, led to the discovery and orbit calculations for the Neuschwanstein meteorite in 2002.
The only reported fatality from meteorite impacts is an Egyptian dog that was killed in 1911 by the Nakhla meteorite, although this report is disputed. The meteorites that struck this area were identified in the 1980s as Martian in origin.
The first known modern strike to a human was on the 28’th of April 1927 in Aba, Japan, to the head of the five-year-old daughter of Mrs. Kuriyama; a bean-sized stone was found resting on her headband and now resides in a museum, called the Aba stone.
The first known modern case of a human injured by a space rock occurred on November 30, 1954, in Sylacauga, Alabama. There a 4 kg (8.8 lb) stone chondrite crashed through a roof and hit Ann Hodges in her living room after it bounced off her radio. She was badly bruised. Several persons have since claimed to have been struck by ‘meteorites’ but no verifiable meteorites have resulted.
On August 10, 1972, a meteor which became known as The Great Daylight 1972 Fireball was witnessed by many people moving north over the Rocky Mountains from the U.S. Southwest to Canada. It was filmed by a tourist at the Grand Teton National Park in Wyoming with an 8-millimeter color movie camera. The object was in the range of size from a car to a house and could have ended its life in a Hiroshima-sized blast, but there was never any explosion. Analysis of the trajectory indicated that it never came much lower than 58 km (36 mi) off the ground, and the conclusion was that it had grazed Earth’s atmosphere for about 100 seconds, then skipped back out of the atmosphere to return to its orbit around the Sun.
In the dark morning hours of January 18, 2000, a fireball exploded over the city of Whitehorse in the Canadian Yukon at an altitude of about 26 km (16 mi), lighting up the night like day. The meteor that produced the fireball was estimated to be about 4.6 m (15 ft) in diameter and with a weight of 180 tonnes. This blast was also featured on The Science Channel series Killer Asteroids, with several witness reports from residents in Atlin, British Columbia.
A meteor was observed striking Reisadalen in Nordreisa municipality in Troms County, Norway, on June 7, 2006. Although initial witness reports stated that the resultant fireball was equivalent to the Hiroshima nuclear explosion, scientific analysis places the force of the blast at anywhere from 100-500 tonnes TNT equivalent – around 3% of Hiroshima’s yield.
On September 15, 2007, a chondritic meteor crashed near the village of Carancas in southeastern Peru near Lake Titicaca, leaving a water-filled hole and spewing gases across the surrounding area. Many residents became ill, apparently from the noxious gases shortly after the impact.
On November 21, 2009, a fireball was sighted in South Africa by police and traffic cameras. The probable meteor may have landed in a remote area on the Botswana border, and likely made little impact.
Many impact events occur without being observed by anyone on the ground. Between 1975 and 1992, American missile early warning satellites picked up 136 major explosions in the upper atmosphere. In the November 21, 2002, edition of the journal Nature, Peter Brown of the University of Western Ontario reported on his study of U.S. early warning satellite records for the proceeding 8 years. He identified 300 flashes caused by 1 to 10 m (3 to 33 ft) sized meteors in that time period and estimated the rate of Tunguska-sized events as once in 400 years. Eugene Shoemaker estimated that one of such magnitude occurs about once every 300 years, though more recent analyses have suggested he exaggerated by an order of magnitude.
The 1994 impact of Comet Shoemaker-Levy 9 with Jupiter served as a “wake-up call”, and astronomers responded by starting programs such as Lincoln Near-Earth Asteroid Research (LINEAR), Near-Earth Asteroid Tracking (NEAT), Lowell Observatory Near-Earth Object Search (LONEOS) and several others which have drastically increased the rate of asteroid discovery.
In 1998, two comets were observed plunging into the Sun in close succession. The first of these was on June 1 and the second the next day. A video of this, followed by a dramatic ejection of solar gas (supposedly unrelated to the impacts), can be found at the NASA website. Both of these comets evaporated before coming into contact with the surface of the Sun. According to a theory by NASA Jet Propulsion Laboratory scientist Zdeněk Sekanina, the latest impactor to actually make contact with the Sun was the “supercomet” Howard-Koomen-Michels on August 30, 1979. (See also sungrazer.)
On October 7, 2008, a meteroid labeled 2008 TC3 was tracked for 20 hours as it approached Earth and as it fell through the atmosphere and impacted in Sudan. This was the first time an object was detected before it reached the atmosphere and hundreds of pieces of the meteorite were recovered from the Nubian Desert.
On July 19, 2009, a new black spot about the size of Earth was discovered in Jupiter’s southern hemisphere by an amateur astronomer. Thermal infrared analysis showed it was warm and spectroscopic methods detected ammonia. JPL scientists confirmed that another impact event on Jupiter had occurred, probably a small undiscovered comet or other icy body.
Hubble‘s Wide Field Camera 3 clearly shows the slow evolution of the debris coming from asteroid P/2010 A2, which is thought to be due to a collision with a smaller asteroid.
Mass extinctions and impacts
In the past 540 million years there have been five generally-accepted, major mass extinctions that on average extinguished half of all species. One of the largest mass extinction to have affected life on Earth was in the Permian-Triassic, which ended the Permian period 250 million years ago and killed off 90% of all species; life on Earth took 30 million years to recover. The cause of the Permian-Triassic extinction is still matter of debate with the age and origin of proposed impact craters, i.e. the Bedout High structure, hypothesized to be associated with it are still controversial. The last such mass extinction led to the demise of the dinosaurs and coincided with a large meteorite impact; this is the Cretaceous–Tertiary extinction event (also known as the K–T extinction event); This occurred 65 million years ago. There is no definitive evidence of impacts leading to the three other major mass extinctions.
In 1980, physicist Luis Alvarez; his son, geologist Walter Alvarez; and nuclear chemists Frank Asaro and Helen V. Michael from the University of California, Berkeleydiscovered unusually high concentrations of iridium in a specific layer of rock strata in the Earth’s crust. Iridium is an element that is rare on Earth but relatively abundant in many meteorites. From the amount and distribution of iridium present in the 65-million-year-old “iridium layer”, the Alvarez team later estimated that an asteroid of 10 to 14 km (6 to 9 mi) must have collided with the earth. This iridium layer at the K–T boundary has been found worldwide at 100 different sites. Multidirectionally shocked quartz (coesite), which is only known to form as the result of large impacts or atomic bomb explosions, has also been found in the same layer at more than 30 sites. Soot and ash at levels tens of thousands times normal levels were found with the above.
Anomalies in chromium isotopic ratios found within the K-T boundary layer strongly support the impact theory. Chromium isotopic ratios are homogeneous within the earth, therefore these isotopic anomalies exclude a volcanic origin which was also proposed as a cause for the iridium enrichment. Furthermore the chromium isotopic ratios measured in the K-T boundary are similar to the chromium isotopic ratios found in carbonaceous chondrites. Thus a probable candidate for the impactor is a carbonaceous asteroid but also a comet is possible because comets are assumed to consist of material similar to carbonaceous chondrites.
Probably the most convincing evidence for a worldwide catastrophe was the discovery of the crater which has since been named Chicxulub Crater. This crater is centered on the Yucatán Peninsula of Mexico and was discovered by Tony Camargo and Glen Pentfield while working as geophysicists for the Mexican oil companyPEMEX. What they reported as a circular feature later turned out to be a crater estimated to be 180 km (110 mi) in diameter. Other researchers would later find that the end-Cretaceous extinction event that wiped out the dinosaurs had lasted for thousands of years instead of millions of years as had previously been thought. This convinced the vast majority of scientists that this extinction resulted from a point event that is most probably an extraterrestrial impact and not from increased volcanism and climate change (which would spread its main effect over a much longer time period).
Recently, several proposed craters around the world have been dated to approximately the same age as Chicxulub — for example, the Silverpit crater in the United Kingdom, the Boltysh crater in Ukraine and the Shiva crater near India. This has led to the suggestion that the Chicxulub impact was one of several that occurred almost simultaneously, perhaps due to a disrupted comet impacting the Earth in a similar manner to the collision of Comet Shoemaker-Levy 9 with Jupiter in 1994.
It was the lack of high concentrations of iridium and shocked quartz which has prevented the acceptance of the idea that the Permian extinction was also caused by an impact. During the late Permian all the continents were combined into one supercontinent named Pangaea and all the oceans formed one superocean, Panthalassa. If an impact occurred in the ocean and not on land at all, then there would be little shocked quartz released (since oceanic crust has relatively little silica) and much less material.
Although there is now general agreement that there was a huge impact at the end of the Cretaceous that led to the iridium enrichment of the K-T boundary layer, remnants have been found of other impacts of the same order of magnitude that did not result in any mass extinctions, and there is no clear linkage between an impact and any other incident of mass extinction. Nonetheless it is now widely believed that mass extinctions due to impacts are an occasional event in the history of Earth.
Paleontologists David M. Raup and Jack Sepkoski have proposed that an extinction occurs roughly every 26 million years (though many are relatively minor). This led physicist Richard A. Muller to suggest that these extinctions could be due to a hypothetical companion star to the Sun called Nemesis periodically disrupting the orbits of comets in the Oort cloud, and leading to a large increase in the number of comets reaching the inner solar system where they might hit Earth.
Indeed, in the early history of the Earth (about four billion years ago) bolide impacts were almost certainly common since the solar system contained far more discrete bodies than at present. Such impacts could have included strikes by asteroids hundreds of kilometers in diameter, with explosions so powerful that they vaporized all the Earth’s oceans. It was not until this heavy bombardment slackened that life appears to have begun to evolve on Earth. If such an impact were to occur on Earth today, it is an almost certainty that human civilization would be wiped out. Fortunately, our solar system is much less populated with large objects today and the probability of such an event happening is essentially zero, as the larger asteroids in the asteroid belt and the giant comets in the Kuiper belt that are in this size range are in stable orbits that will not enter the inner solar system, let alone intersect the orbit of Earth, with no observed exceptions.
The leading theory of the Moon’s origin is the giant impact theory, which states that Earth was once hit by a planetoid the size of Mars; if this theory holds then that impact was almost certainly the largest hit Earth ever suffered. Peculiarities in the rotation and inclination of the planets, such as Venus’ retrograde rotation and the extreme tilt of Uranus’ axis, are thought to be the consequence of other such giant impacts, as the current hypotheses of solar system and planetary formation, in absence of impact, could not account for the large transfers of momentum that we clearly see evidence for today, in Venus and Uranus for example.
End of civilization
An impact event is commonly seen as a scenario that would bring about the end of civilization. In 2000, Discover Magazine published a list of 20 possible sudden doomsday scenarios with impact event listed as the No. 1 most likely to occur. Until the 1980s this idea was not taken seriously, but opinions changed following the discovery of the Chicxulub Crater, which was further reinforced by witness to the Comet Shoemaker-Levy 9 event.
Impact events in popular culture
Science fiction novels
Numerous science fiction stories and novels center around an impact event; possibly the best-selling was the novel Lucifer’s Hammer by Larry Niven and Jerry Pournelle. Arthur C. Clarke‘s novel Rendezvous with Rama opens with a significant asteroid impact in northern Italy in the year 2077 which gives rise to the Spaceguard Project, which later discovers the Rama spacecraft. In 1992 a Congressional study in the U.S. led to NASA being directed to undertake a Spaceguard Survey with the novel being named as the inspiration for the name to search for Earth-impacting asteroids. This in turn inspired Clarke’s 1993 novel The Hammer of God. A variation on the traditional impact story was provided by Jack McDevitt‘s 1999 novel Moonfall, in which a very large comet travelling at interstellar velocities collides with and partially destroys the Moon, fragments of which then collide with the Earth. The 1985 Niven and Pournelle novel Footfall features the examination of the effects of planetary warfare conducted by an alien species that culminates in the use of asteroids to bombard the planet, creating very large craters and the species’ near extinction. Jules Verne contributed a story of planetary impact in “Off on a Comet”. In “The Moon is a Harsh Mistress”, Robert A. Heinlein had the moon rebels use rock filled shipping containers as a weapon against their Earthside oppressors.
Similarly in the science fiction television series Babylon 5 war between the Narn and Centauri is brought to a swift end when the Centauri use “Mass Drivers” a weapon system that propels asteroids at the suface of the Narn homeworld causing severe ecological damage.
Several disaster films have also been made: released during the turbulence of World War I, the Danish feature film The End of the World revolves around the near-miss of a comet which causes fire showers and social unrest in Europe. When Worlds Collide (1951) based on a 1933 novel by Philip Wylie, deals with two planets on a collision course with Earth – the smaller planet a “near miss,” causing extensive damage and destruction, followed by a direct hit from the larger planet. Meteor (1979) features small asteroid fragments and a large 8 km (5 mi) wide asteroid heading for Earth. Orbiting U.S. and Soviet nuclear weapons platforms are turned away from their respective earthbound targets, and toward the incoming threat. In 1998, two films were released in the United States on the subject of attempting to stop impact events: Touchstone Pictures’ Armageddon, about an asteroid; and Paramount/DreamWorks’ Deep Impact, about a comet. Both involved using Space Shuttle-derived craft to deliver large amounts ofnuclear weapons to destroy their targets. The 2008 American Broadcasting Company’s miniseries Impact deals about a splinter of a brown dwarf hidden in a meteor shower which strikes the Moon and sends it on a collision course with Earth. The 2011 film Melancholia uses the motif of an impact event incorporated in the aesthetics of romanticism.