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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. Asteroids with a 1 km diameter strike the Earth every 500,000 years on average.[1] Large collisions—with five kilometer objects—happen approximately once every ten million years. The last known impact of an object of 10 km or more in diameter was at the Cretaceous-Tertiary extinction event 65 million years ago. Asteroids with diameters of 5-10 m 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.[2] Objects of diameters of over 50 meters strike the Earth approximately once every thousand years, producing explosions comparable to the one known to have detonated above Tunguska in 1908.[3] At least one known asteroid with a diameter of over 1 km, (29075) 1950 DA, has a possibility of colliding with Earth in March 2880, with a Torino Scale rating of two.

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.[4]

The geology of Earth-impact events

The 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 even earlier in the history of Earth have been credited with creative as well as destructive events; it has been proposed that the water in the Earth's oceans was delivered by impacting comets, and some have suggested that the origins of life may have been influenced by impacting objects bringing organic chemicals or lifeforms to the Earth's surface, a theory known as exogenesis.

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

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. 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 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. As literally every surveyed solid body in the Solar System was found to be cratered, there was no reason to believe that the Earth had somehow escaped bombardment from space. 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 five kilometers or more. The smallest of these impactors would release the equivalent of ten million megatons of TNT and leave a crater 95 kilometers across. For comparison, the largest nuclear weapon ever detonated, the Tsar Bomba, had a yield of 50 megatons.

Recent pre-historic 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 much 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:

Aerial view of Barringer Crater in Arizona
  • the Rio Cuarto craters in Argentina, produced by an asteroid striking Earth at a very low angle around 10,000 years ago
  • the Henbury crater in Australia, and Kaali crater in Estonia, apparently produced by objects which broke up before impact.
  • the Wabar craters in Arabia, which may have been created sometime during the past few hundred years

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 mysterious charred carbon-rich layer of soil found at some 50 Clovis-age sites across the continent.[5] It has been criticized for not being consistent with paleoindian population estimates.[6]

More recent prehistoric impacts are theorized by the Holocene Impact Working Group, including Dallas Abbott of Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y.[7] This group points to four enormous chevron 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 where newly discovered Burckle crater,[8] 18 miles (29 km) in diameter, or about 25 times larger than Barringer Crater, lies 12,500 feet (3,800 m) below the surface. This group posits that a large asteroid or comet impact c. 2800-3000 BC produced a mega-tsunami at least 600 feet (180 m) high, a catastrophic event that would have affected humanity's cradles of civilization.[9] If this and other recent impacts prove correct, the rate of asteroid impacts is much higher than currently thought.

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. In his book 'Exodus to Arthur: Catastrophic encounters with comets,' he 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.[10]

Trees knocked over by the Tunguska blast.

However, 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 kilometers (3–6 mi) above the Earth's surface, felling an estimated 80 million trees over 2,150 square kilometers (830 sq mi).


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.


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.[12] Another program in Canada, the Meteorite Observation and Recovery Project, ran from 1971 to 1985. It too recovered a single meteorite, Innisfree, in 1977.[13] 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.[14]

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 case of a human hit by a space rock occurred on November 30, 1954 in Sylacauga, Alabama.[15] There a 4 kg 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.[16] 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 kilometers 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.

On 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 kilometers, lighting up the night like day. The meteor that produced the fireball was estimated to be about 4.6 meters in diameter and with a weight of 180 tonnes. This blast was also featured on the The Science Channel series Killer Asteroids, with several witness reports from residents in Atlin, British Columbia.

A picture of the Jupiter 2009 impact event blemish captured by the Keck II telescope and its near-infrared camera at Mauna Kea Observatory, on July 20.

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—at most, around 3% of Hiroshima's yield.[17]


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 Botswan border, and likely made little impact.[18]

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 21-Nov-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 proceeding 8 years. He identified 300 flashes caused by 1 m to 10 m sized meteors in that time period and estimated the rate of Tunguska sized events as once in 400 years.[19] 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 also 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[20] 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.[21] (See also sungrazer.)



Close-approaches and forecasts

On May 19, 1996 a 300–500 m asteroid, 1996 JA1, passed within 450,000 km of Earth; it had been detected a few days before.


On March 31, 2004, a 6 m meteoroid, 2004 FU162 made the second closest approach on record (closest so far was The Great Daylight 1972 Fireball) with a separation of only 1.02 Earth radii from the surface (6,500 km). Because this object is certainly too small to pass through the atmosphere, it is classed as a meteoroid rather than an asteroid.

Path of risk where 99942 Apophis may impact Earth in 2036.

In 2004, a newly discovered 320 m asteroid, 99942 Apophis (previously called 2004 MN4), achieved the highest impact probability of any potentially dangerous object. The probability of collision on April 13, 2029 is estimated to be as high as 1 in 17 by Steve Chesley of NASA's Jet Propulsion Laboratory, though the previously published figure was the slightly lower odds of 1 in 37, calculated in December 2004. Later observations showed that the asteroid will miss the earth by 25,600 km (within the orbits of communications satellites) in 2029, but its orbit will be altered unpredictably in a way which does not rule out a collision on April 13 or 14, 2036 or later in the century. These possible future dates have a cumulative probability of 1 in 45,000 for an impact in the 21st century.

Asteroid 2004 VD17, of 580 m, previously was estimated to have a probability of 1 in 63,000 of striking earth on May 4, 2102 (as of July 2006), with risk 1 on the Torino scale, but further observations lowered the estimate. As of the observation on December 17, 2006, JPL assigns 2004 VD17 a Torino value of 0 and an impact probability of 1 in 41.667 million in the next 100 years.


Asteroid 2007 TU24, with an estimated diameter between 300–500 meters, came very close to earth orbit by 1.4 ld (lunar distance) on January 29, 2008. The orbit of the asteroid is shown on NASA's website.[26]


As of March 2008, the Near-Earth Asteroid with the highest probability of impact within the next 100 years is 2007 VK184, with a Torino scale of 1.[27][28][29]

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. 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[30]; life on Earth took 30 million years to recover.[31]. 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). There is no definitive evidence of impacts leading to the four other major mass extinctions. 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.[32]

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 10–14 kilometers 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.


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 kilometers 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 craters around the world have been dated to approximately the same age as Chicxulub — for example, the Silverpit crater in the United Kingdom and the Boltysh crater in Ukraine. 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 (so-called mother of mass extinctions) was also caused by an impact. However, 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[citation needed].


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 began to slacken that life appears to have begun to evolve on Earth.

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.

End of civilization

An impact event is commonly seen as a scenario[33][34] 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 number one most likely to occur.[35] Until the 1980s this idea was not taken seriously, but all that changed after the discovery of the Chicxulub Crater which was further reinforced by witness to the Comet Shoemaker-Levy 9 event.[citation needed] Since then there has been more interest from the scientific community and greater public awareness of the possibility of impact events.

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 US 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.[36] 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.


Several disaster movies have also been made: When Worlds Collide (1951), 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[37]. Meteor (1979) features small asteroid fragments and a large five-mile-wide asteroid heading for Earth. Orbiting US and Soviet nuclear weapons platforms are turned away from their respective earthbound targets, and towards 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 of nuclear weapons to destroy their targets. The 2008 ABC mini-series 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.

Biblical prophecies

Some effort has also been made to consider biblical predictions of Armageddon to include an impact event. The "Wormwood Star Prophecy" in the Bible alludes to a futurist interpretation suggesting a chemical change in the atmosphere coinciding with a comet or asteroid impact with Earth. An applicable scenario theorizes a chemical change in the atmosphere due to "heat shock" during entry and/or impact of a large asteroid or comet, reacting oxygen and nitrogen in the atmosphere to produce nitric acid rain.[38] The bitterness produced by the Wormwood Star upon a third of the Earth's potable waters could be the Biblical prediction of "acid rain" from the "heat shock" of a large comet or asteroid's impact with Earth.[39]

See also


  1. ^ Bostrom, Nick (March 2002), "Existential Risks: Analyzing Human Extinction Scenarios and Related Hazards", Journal of Evolution and Technology 9, 
  2. ^ Clark R. Chapman & David Morrison (January 6, 1994), "Impacts on the Earth by asteroids and comets: assessing the hazard", Nature 367: 33–40, doi:10.1038/367033a0,, retrieved 2007-10-23 
  3. ^ Richard Monastersky (March 1, 1997), The Call of Catastrophes, Science News Online,, retrieved 2007-10-23 
  4. ^ Lewis, John S. (1996), Rain of Iron and Ice, Helix Books (Addison-Wesley), pp. 236, ISBN 0-201-48950-3 
  6. ^ Buchanan, B.; Collard, M.; Edinborough, K. (2008), "Paleoindian demography and the extraterrestrial impact hypothesis", PNAS 105 (33): 11651–11654, doi:10.1073/pnas.0803762105 
  8. ^ Meteor 'misfits' find proof in sea,, retrieved 2006-11-14 
  11. ^ Ceplecha, Z. (1961), "Multiple fall of Pribram meteorites photographed", Bull. Astron. Inst. Czechoslovakia 12: 21–46, Bibcode1961BAICz..12...21C 
  12. ^ McCrosky, R. E.; Posen, A.; Schwartz, G.; Shao, C. Y. (1971), "Lost City meteorite: Its recovery and a comparison with other fireballs", J. Geophys. Res. 76: 4090–4108, doi:10.1029/JB076i017p04090 
  13. ^ Campbell-Brown, M. D.; Hildebrand, A. (2005), "A new analysis of fireball data from the Meteorite Observation and Recovery Project (MORP)", Earth, Moon, and Planets 95 (1–4): 489–499, doi:10.1007/s11038-005-0664-9 
  14. ^ Oberst, J.; et al. (2004), "The multiple meteorite fall of Neuschwanstein: Circumstances of the event and meteorite search campaigns", Meteoritics & Planetary Science 39: 1627–1641, doi:2004M&PS...39.1627O (inactive 2010-01-09) 
  15. ^ Meteorite Hits Page
  16. ^ Grand Teton Meteor Video at YouTube
  17. ^
  18. ^
  19. ^
  20. ^
  21. ^ A SOHO and Sungrazing Comet FAQ
  23. ^ Mystery impact leaves Earth-sized mark on Jupiter
  24. ^ All Eyepieces on Jupiter After a Big Impac
  26. ^ 2007 TU24 (Java applet), JPL Small-Body Database Browser, Jet Propulsion Laboratory
  27. ^ Current Impact Risk (NEO JPL NASA)
  28. ^ 2007 VK184 Impact Risk Summary, NASA Near Earth Object Program.
  29. ^ 2007 VK184 Orbital Diagram
  30. ^ Permian Extinction
  31. ^ Sahney, S. and Benton, M.J. (2008), "Recovery from the most profound mass extinction of all time" (PDF), Proceedings of the Royal Society: Biological 275: 759, doi:10.1098/rspb.2007.1370, 
  33. ^ Armageddon Online. Possible end-of-civilization scenarios. (Warning: Pop-ups)
  34. ^ Exit Mundi. Possible-end-of-civilization scenarios.(Warning: Pop-ups)
  35. ^ "Twenty ways the world could end suddenly". Discover Magazine.
  36. ^
  38. ^ Hooper Virtual Natural History Museum citing Prinn and Fegley, 1987
  39. ^ The Messianic Literary Corner

Further reading

External links


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