Impact event

A major impact event releases the energy of several million nuclear weapons detonating simultaneously, when an asteroid of only a few kilometers in diameter collides with a larger body such as the Earth (image: artist's impression).

An impact event is a collision between celestial objects causing measurable effects. Impact events have physical consequences and have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or meteoroids and have minimal impact. When large objects impact terrestrial planets like the Earth, there can be significant physical and biospheric consequences, though atmospheres mitigate many surface impacts through atmospheric entry. Impact craters and structures are dominant landforms on many of the Solar System's solid objects and present the strongest empirical evidence for their frequency and scale.

Impact events appear to have played a significant role in the evolution of the Solar System since its formation. Major impact events have significantly shaped Earth's history, have been implicated in the formation of the Earth–Moon system, the evolutionary history of life, the origin of water on Earth and several mass extinctions. Notable impact events include the Chicxulub impact, 66 million years ago, believed to be the cause of the Cretaceous–Paleogene extinction event.

Throughout recorded history, hundreds of Earth impacts (and exploding bolides) have been reported, with some occurrences causing deaths, injuries, property damage, or other significant localised consequences. One of the best-known recorded impacts in modern times was the Tunguska event, which occurred in Siberia, Russia, in 1908. The 2013 Chelyabinsk meteor event is the only known such event to result in a large number of injuries, and the Chelyabinsk meteor is the largest recorded object to have encountered the Earth since the Tunguska event.

The Comet Shoemaker–Levy 9 impact provided the first direct observation of an extraterrestrial collision of Solar System objects, when the comet broke apart and collided with Jupiter in July 1994. Most of the observed extrasolar impacts are the slow collision of galaxies; in 2014, one of the first massive terrestrial impacts observed was detected around the star NGC 2547 ID8 by NASA's Spitzer space telescope and confirmed by ground observations. Impact events have been a plot and background element in science fiction.

Impacts and the Earth

Major impact events have significantly shaped Earth's history, having been implicated in the formation of the Earth–Moon system, the evolutionary history of life, the origin of water on Earth, and several mass extinctions. Impact structures are the result of impact events on solid objects and, as the dominant landforms on many of the System's solid objects, present the most solid evidence of prehistoric events. Notable impact events include the Late Heavy Bombardment, which occurred early in history of the Earth–Moon system, and the Chicxulub impact, 66 million years ago, believed to be the cause of the Cretaceous–Paleogene extinction event.

Frequency and risk

Main article: Asteroid-impact avoidance

REP. STEWART: ... are we technologically capable of launching something that could intercept [an asteroid]? ... DR. A'HEARN: No. If we had spacecraft plans on the books already, that would take a year ... I mean a typical small mission ... takes four years from approval to start to launch ...

— Rep. Chris Stewart (R,UT) and Dr. Michael F. A'Hearn, 10 April 2013, United States Congress

Frequency of small asteroids roughly 1 to 20 meters in diameter impacting Earth's atmosphere.

A bolide undergoing atmospheric entry

Small objects frequently collide with Earth. There is an inverse relationship between the size of the object and the frequency of such events. 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 Earth every 500,000 years on average. Large collisions – with 5 km (3 mi) objects – happen approximately once every twenty million years. The last known impact of an object of 10 km (6 mi) or more in diameter was at the Cretaceous–Paleogene extinction event 66 million years ago.

The energy released by an impactor depends on diameter, density, velocity, and angle. The diameter of most near-Earth asteroids that have not been studied by radar or infrared can generally only be estimated within about a factor of 2 based on the asteroid brightness. The density is generally assumed because the diameter and mass are also generally estimates. The minimum impact velocity on Earth is 11 km/s with asteroid impacts averaging around 17 km/s. The most probable impact angle is 45 degrees.

Stony asteroids with a diameter of 4 meters (13 ft) impact Earth approximately once per year. Asteroids with a diameter of 7 meters enter Earth's atmosphere with as much kinetic energy as the atomic bomb dropped on Hiroshima (approximately 16 kilotons of TNT) about every 5 years, but the air burst is reduced to just a third, or about 5 kilotons of TNT. These ordinarily explode in the upper atmosphere, and most or all of the solids are vaporized. Objects with a diameter of roughly 50 m (164 ft) strike Earth approximately once every thousand years, producing explosions comparable to the one known to have detonated roughly 8.5 kilometers (28,000 ft) above Tunguska in 1908.

Stony asteroids that impact sedimentary rock and create a crater

Impactor diameter	Kinetic energy at		Crater diameter	Frequency (years)
	atmospheric entry	impact
100 m (330 ft)	47 Mt	38 Mt	1.2 km (0.75 mi)	5,200
130 m (430 ft)	103 Mt	64.8 Mt	2 km (1.2 mi)	11,000
150 m (490 ft)	159 Mt	71.5 Mt	2.4 km (1.5 mi)	16,000
200 m (660 ft)	376 Mt	261 Mt	3 km (1.9 mi)	36,000
250 m (820 ft)	734 Mt	598 Mt	3.8 km (2.4 mi)	59,000
300 m (980 ft)	1270 Mt	1110 Mt	4.6 km (2.9 mi)	73,000
400 m (1,300 ft)	3010 Mt	2800 Mt	6 km (3.7 mi)	100,000
700 m (2,300 ft)	16100 Mt	15700 Mt	10 km (6.2 mi)	190,000
1,000 m (3,300 ft)	47000 Mt	46300 Mt	13.6 km (8.5 mi)	440,000
Based on density of 2600 kg/m3, speed of 17 km/s, and an impact angle of 45°

Stony asteroid impacts that generate an airburst

Impactor diameter	Kinetic energy at		Airburst altitude	Average frequency (years)
	atmospheric entry	airburst
4 m (13 ft)	3 kt	0.75 kt	42.5 km (139,000 ft)	1.3
7 m (23 ft)	16 kt	5 kt	36.3 km (119,000 ft)	4.6
10 m (33 ft)	47 kt	19 kt	31.9 km (105,000 ft)	10
15 m (49 ft)	159 kt	82 kt	26.4 km (87,000 ft)	27
20 m (66 ft)	376 kt	230 kt	22.4 km (73,000 ft)	60
30 m (98 ft)	1.3 Mt	930 kt	16.5 km (54,000 ft)	185
50 m (160 ft)	5.9 Mt	5.2 Mt	8.7 km (29,000 ft)	764
70 m (230 ft)	16 Mt	15.2 Mt	3.6 km (12,000 ft)	1,900
85 m (279 ft)	29 Mt	28 Mt	0.58 km (1,900 ft)	3,300
Based on ρ = 2600 kg/m3; v = 17 km/s; and an angle of 45°

Objects with a diameter less than 1 m (3.3 ft) are called meteoroids and seldom make it to the ground to become meteorites. An estimated 500 meteorites reach the surface each year, but only 5 or 6 of these typically create a weather radar signature with a strewn field large enough to be recovered and be made known to scientists.

The late Eugene Shoemaker of the U.S. Geological Survey estimated the rate of Earth impacts, concluding 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.

Although no human is known to have been killed directly by an impact, over 1000 people were injured by the Chelyabinsk meteor airburst event over Russia in 2013. In 2005 it was estimated that the chance of a single person born today dying due to an impact is around 1 in 200,000. The four-meter-sized asteroids 2008 TC3 and 2014 AA, and suspected artificial satellite WT1190F are the only known objects to be detected before impacting the Earth.

Geological significance

Impacts have had, during the history of the Earth, a significant geological and climatic influence.

The Moon's existence 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.

Eugene Merle Shoemaker was first to prove that meteorite impacts have affected the Earth.

These modified views of 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 an impact crater, and it was not until as recently as 1963 that research by Eugene 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 20th 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 approximately 40 zettajoules (50 million megatons of TNT) and leave a crater 95 km (60 mi) across. For comparison, annual energy consumption on earth is 0.5 zettajoules; the largest nuclear weapon ever detonated, the Tsar Bomba, had a yield of 40 petajoules (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 an 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 one 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 megatsunami (a giant wave), which can cause destruction both at sea and on land along the coast, but this is disputed.

An impact event may cause a mantle plume (volcanism) at the antipodal point of the impact.

Biospheric effects

The effect of impact events on the biosphere has been the subject of scientific debate. Several theories of impact-related mass extinction have been developed. In the past 500 million years there have been five generally accepted major mass extinctions that on average extinguished half of all species. One of the largest mass extinctions to have affected life on Earth was the Permian-Triassic, which ended the Permian period 250 million years ago and killed off 90 percent of all species; life on Earth took 30 million years to recover. The cause of the Permian-Triassic extinction is still a matter of debate; 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–Paleogene extinction event (also known as the K–T or K–Pg extinction event), which occurred 66 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, Berkeley discovered 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 Cretaceous–Paleogene boundary has been found worldwide at 100 different sites. Multidirectionally shocked quartz (coesite), which is normally associated with large impact events 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, and therefore these isotopic anomalies exclude a volcanic origin, which has also been proposed as a cause for the iridium enrichment. Further, 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 company PEMEX. What they reported as a circular feature later turned out to be a crater estimated to be 180 km (110 mi) in diameter. 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).

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, smaller impacts, some nearing half the size of the Chicxulub crater, which did not result in any mass extinctions, and there is no clear linkage between an impact and any other incident of mass extinction.

Paleontologists David M. Raup and Jack Sepkoski have proposed that an excess of extinction events 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, leading to a large increase in the number of comets reaching the inner Solar System where they might hit Earth. Physicist Adrian Melott and paleontologist Richard Bambach have more recently verified the Raup and Sepkoski finding, but argue that it is not consistent with the characteristics expected of a Nemesis-style periodicity.

Sociological and cultural effects

Main article: 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 an impact event listed as the most likely to occur.

A joint Pew Research Center/Smithsonian survey from April 21–26, 2010 found that 31 percent of Americans believed that an asteroid will collide with Earth by 2050. A majority (61 percent) disagreed.

Earth impacts

Artist's depiction of a collision between two planetary bodies. Such an impact between the Earth and a Mars-sized object likely formed the Moon.

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.

The leading theory of the Moon's origin is the giant impact theory, which postulates that Earth was once hit by a planetoid the size of Mars; such a theory is able to explain the size and composition of the Moon, something not done by other theories of lunar formation.

Evidence of a massive impact in South Africa near a geological formation known as the Barberton Greenstone Belt was uncovered by scientists in April 2014. They estimated the impact occurred about 3.26 billion years ago and that the impactor was approximately 37–58 kilometers (23–36 miles) wide. The crater from this event, if it still exists, has not yet been found.

Two 10-kilometre sized asteroids are now believed to have struck Australia between 360 and 300 million years ago at the Western Warburton and East Warburton Basins creating a 400-kilometre impact zone, according to evidence found in 2015 it is the largest ever recorded.

Pleistocene

Further information: Pleistocene

Aerial view of Barringer Crater in Arizona

Artifacts recovered with tektites from the 803,000-year-old Australasian strewnfield event in Asia link a Homo erectus population to a significant meteorite impact and its aftermath. Significant examples of Pleistocene impacts include the Lonar crater lake in India, approximately 52,000 years old (though a study published in 2010 gives a much greater age), which now has a flourishing semi-tropical jungle around it.

Holocene

Further information: Holocene

The Rio Cuarto craters in Argentina were produced by a very low angle impact event approximately 10,000 years ago, which, if proved correct, would place it at the beginning of the Holocene. The Campo del Cielo ("Field of Heaven") refers to an area bordering Argentina's Chaco Province where a group of iron meteorites were found, estimated as dating to 4,000–5,000 years ago. It first came to attention of Spanish authorities in 1576; in 2015, police arrested four alleged smugglers trying to steal more than a ton of protected meteorites. The Henbury craters in Australia (~5,000 years old) and Kaali craters in Estonia (~2,700 years old) were apparently produced by objects that broke up before impact.

A Chinese record states that 10,000 people were killed in Shanxi Province in 1490 by a hail of "falling stones"; some astronomers hypothesize that this may describe an actual meteorite fall, although they find the number of deaths implausible.

Kamil Crater, discovered from Google Earth image review in Egypt, 45 m (148 ft) in diameter and 10 m (33 ft) 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.

20th-century impacts

Trees knocked over by the Tunguska blast

One of the best-known recorded impacts in modern 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).

A 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.

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 Přibram 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.

On August 10, 1972, a meteor which became known as the 1972 Great Daylight Fireball was witnessed by many people as it moved 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.

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 US early warning satellite records for the preceding eight years. He identified 300 flashes caused by 1 to 10 m (3 to 33 ft) meteors in that time period and estimated the rate of Tunguska-sized events as once in 400 years. Eugene Shoemaker estimated that an event of such magnitude occurs about once every 300 years, though more recent analyses have suggested he exaggerated by an order of magnitude.

In the dark morning hours of January 18, 2000, a fireball exploded over the city of Whitehorse, Yukon Territory 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, 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.

21st-century impacts

On 7 June 2006, a meteor was observed striking Reisadalen in Nordreisa municipality in Troms County, Norway. 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 three percent of Hiroshima's yield.

On 15 September 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 7 October 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.

Trail left by the exploding Chelyabinsk meteor as it passed over the city.

On 15 February 2013, an asteroid entered Earth's atmosphere over Russia as a fireball and exploded above the city of Chelyabinsk during its passage through the Ural Mountains region at 09:13 YEKT (03:13 UTC). The object's air burst occurred at an altitude between 30 and 50 km (19 and 31 mi) above the ground, and about 1,500 people were injured, mainly by broken window glass shattered by the shock wave. Two were reported in serious condition; however, there were no fatalities. Initially some 3,000 buildings in six cities across the region were reported damaged due to the explosion's shock wave, a figure which rose to over 7,200 in the following weeks. The Chelyabinsk meteor was estimated to have caused over $30 million in damages. It is the largest recorded object to have encountered the Earth since the 1908 Tunguska event, by far the best documented, and the only such event known to have resulted in a large number of casualties. The meteor is estimated to have an initial diameter of 17–20 metres and a mass of roughly 10,000 tonnes. On 16 October 2013, a team from Ural Federal University led by Victor Grokhovsky recovered a large fragment of the meteor from the bottom of Russia’s Lake Chebarkul, about 80 km west of the city.

Elsewhere in the Solar System

Evidence of massive past impact events

Main article: List of largest craters in the Solar System

Topographical map of the South Pole–Aitken basin based on Kaguya data provides evidence of a massive impact event on the Moon some 4.3 billion years ago

Impact craters provide evidence of past impacts on other planets in the Solar System, including possible interplanetary terrestrial impacts. Without carbon dating, other points of reference are used to estimate the timing of these impact events. Mars provides some significant evidence of possible interplanetary collisions. The North Polar Basin on Mars is speculated by some to be evidence for a planet-sized impact on the surface of Mars between 3.8 and 3.9 billion years ago, while Utopia Planitia is the largest confirmed impact and Hellas Planitia is the largest visible crater in the Solar System. The Moon provides similar evidence of massive impacts, with the South Pole–Aitken basin being the biggest. Mercury's Caloris Basin is another example of a crater formed by a massive impact event. Rheasilvia on Vesta is an example of a crater formed by an impact capable of, based on ratio of impact to size, severely deforming a planetary-mass object. Impact craters on the moons of Saturn such as Engelier and Gerin on Iapetus, Mamaldi on Rhea and Odysseus on Tethys and Herschel on Mimas form significant surface features.

Observed events

Comet Shoemaker-Levy 9's scar on Jupiter (dark area near Jupiter's limb)

Comet Shoemaker–Levy 9 was a comet that broke apart and collided with Jupiter in July 1994, providing the first direct observation of an extraterrestrial collision of Solar System objects. The event 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 toward 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 (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 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.

Between January and May 2010, Hubble's Wide Field Camera 3 took images of an unusual X shape originated in the aftermath of the collision between asteroid P/2010 A2 with a smaller asteroid.

A 2010 Jupiter impact event occurred on 3 June involving an object estimated at 8–13 meters was recorded and first reported by amateur astronomer Anthony Wesley.

Images from the Mars Reconnaissance Orbiter provide compelling evidence of the largest impact observed to date on Mars in the form of fresh impact craters, the largest measuring 48.5 by 43.5 meters. The impact is estimated to have occurred 27–28 March 2012 and caused by an impactor 3 to 5 meters long.

Lunar Impacts

NASA has actively monitored lunar impacts since 2005, tracking hundreds of candidate events. On March 19, 2013, an impact occurred that was visible from Earth, when a boulder-sized 30 cm meteoroid slammed into the Moon at 56,000 mph creating a 20-meter crater.

Extrasolar impacts

Asteroid collision led to the building of planets near star NGC 2547-ID8 (artist concept).

Collisions between galaxies, or galaxy mergers, have been observed directly by space telescopes such as Hubble and Spitzer. However, collisions in planetary systems including stellar collisions, while long speculated, have only recently begun to be observed directly.

In 2013, an impact between minor planets was detected around the star NGC 2547 by Spitzer and confirmed by ground observations. Computer modelling suggests that the impact involved large asteroids or protoplanets similar to the events believed to have led to the formation of terrestrial planets like the Earth.

Popular culture

Science fiction novels

Main article: Asteroids in fiction § Collisions with planets

Numerous science fiction stories and novels center around an impact event. One of the first and more popular is Off on a Comet (French: Hector Servadac) by Jules Verne, published in 1877, and H. G. Wells wrote about such an event in his short story "The Star." In more modern times, 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 the "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 traveling 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 human species' near-extinction. Robert A. Heinlein used the concept of guided meteors in his novel The Moon is a Harsh Mistress, in which Moon rebels use rock-filled shipping containers as a weapon against their Earth oppressors.

Some science fiction has concerned itself not with the specifics of the impact event and/or its prevention or avoidance but its secondary effects on human society. Ben H. Winters' 2012 novel The Last Policeman is set six months prior to an asteroid collision, following a murder investigation that is complicated by the political and cultural responses to the impending event.

Cinema and television

See also: Asteroid impact avoidance § Fictional representations

Several disaster films center on actual or threatened impact events. 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: Paramount/DreamWorks' Deep Impact, about a comet, and Touchstone Pictures' Armageddon, about an asteroid. Both involved using Space Shuttle-derived craft to deliver nuclear weapons to destroy their targets. The 2008 American Broadcasting Company's miniseries Impact deals with 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.

See also

• Asteroid capture
• Asteroid deflection strategies
• B612 Foundation
• Earth Impact Database
• Global catastrophic risks
• Impact gardening
• Late Heavy Bombardment
• List of impact craters on Earth
• List of meteor air bursts
• Near-Earth asteroids
• Near Earth Object Camera
• Near-Earth objects
• Palermo scale
• Pan-STARRS
• Potentially hazardous asteroid
• Spaceguard
• Torino scale, 0 to 10

External links

• Earth Impact Database
• Earth Impact Effects Program