Collisions with Comets and Asteroids the Chances of a Celestial Body Colliding with the Earth Are Small, but the Consequences Would Be Catastrophic

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Collisions with Comets and Asteroids The chances of a celestial body colliding with the earth are small, but the consequences would be catastrophic

by Tom Gehrels

re we going to be hit by an aster- Comets and asteroids are, in fact, left- disturbed by events beyond the solar oid? Planetary scientists are di- over planetesimals. Most asteroids in- system. If the sun passes by another A vided on how worrisome the habit the vast belt between the orbits of star or a massive molecular cloud, some danger is. Some refuse to take it seri- Mars and Jupiter. Being quite close to the of these cometary orbits are jarred. The ously; others believe the risk of dying sun, they were formed hot; as on the planetesimal might then swing into a from such an impact might even be early earth, the high temperatures va- narrow elliptical orbit that brings it to- greater than the risk of dying in an air- porized the lighter substances, such as ward the inner solar system. As it nears plane crash. After years of studying the water, leaving mostly silica, carbon and the sun, the heat vaporizes its volatile problem, I have become convinced that metals. (Only recently have astronomers materials, which spew forth as if from the danger is real. Although a major im- found some rare asteroids that contain a geyser. In ancient cultures, this celes- pact is unlikely, the energies released crystalline water embedded in rocks.) tial spectacle was sometimes an omi- could be so horrendous that our fragile Comets, on the other hand, hover at nous event. society would be obliterated. the outer edges of the solar system. As Some visitors from the Oort cloud are Early in our planetÕs history, asteroids the solar system was formed, a good never seen again; others have periods and comets made life possible by accret- deal of matter was thrown outward, be- that get shorter with each successive ing into the earth and then by bringing yond the orbits of Uranus and Neptune. pass. The best known of these comets water to the newborn planet. And they Coalescing far from the sun, the com- are those that return regularly, such as have already destroyed, at least once, ets were born cold, at temperatures as HalleyÕs, with a period of 76 years. The an advanced form of that life. The di- low as Ð260 degrees Celsius. They re- chance that such a comet will collide nosaurs were killed by such an impact, tained their volatile materials, the gas, with the earth is exceedingly small, be- making way for the age of the mam- ice and snow. Sometimes called dirty cause it comes by so infrequently. But mals. Now for the Þrst time, creatures snowballs, these objects are usually ten- the patterns of their orbits suggest that have evolved to a point where they can uous aggregates of carbon and other in the next millennia, comet Halley or wrest control of their fate from the light elements. Swift-Tuttle (with a period of 130 years) heavenly bodies, but humans must will sometimes swing by too close for come to grips with the danger. Fiery Visitors comfort. Some four and a half billion years ago, In 1951 Gerard P. Kuiper, then at the solar system formed out of a swirl- n 1950 Jan H. Oort, professor of as- Yerkes Observatory of the University of ing cloud of gas and dust. Initially the I tronomy at Leiden University in the Chicago, surmised that another belt of planetesimalsÑcoarse collections of Netherlands, was teaching a class that I comets exists, just beyond NeptuneÕs rocky materialsÑcoagulated, merging was allowed to attend as an undergrad- orbit, much nearer than the Oort cloud. with one another to create planets. Be- uate. While reviewing astronomical cal- Working at the University of Hawaii, cause of the energy released by the col- culations for his students, Oort noted David C. Jewitt and Jane Luu discovered liding rocks, the earth began as a molten that a number of known comets reach the Þrst of these objects in 1992 after a globe, so hot that the volatile substanc- their farthest point from the sunÑcalled persistent search; by now some 31 bod- esÑwater, carbon dioxide, ammonia, the aphelionÑat a great distance. He ies belonging to the Kuiper belt have methane and other gasesÑboiled oÝ. As went on to formulate the idea that a been found. In fact, Pluto, with its un- the material of the inner solar nebula cloud of comets exists as a diÝuse usually elliptical orbit, is now consid- was mopped up by the growing planets, spherical shell at about 50,000 or more ered to be the largest of these objects; the bombardment of the earth slowed. astronomical units. (One astronomical Clyde Tombaugh, who discovered Pluto The glowing planet cooled, and a crust unit is the distance from the earth to in 1930, calls it the ÒKing of the Kuiper solidiÞed. Only then did waterÑthe life- the sun.) This distant cloud, containing belt.Ó giving ßuid that covers three quarters perhaps some 1013 objects, envelops Comets belonging to the Kuiper belt of the earthÕs surfaceÑreturn, borne on the solar system. are not directly disturbed by rival stars. cold comets arriving from the solar The Oort cloud reaches a Þfth of the Instead they can stray close to Neptune, systemÕs distant reaches. Fossil records distance to the nearest star, Alpha Cen- which may either help stabilize them or, show that simple life-forms started tauri. Inhabitants of this shell are thus conversely, throw them out of orbit. (An evolving almost right away. loosely bound to the sun and readily as yet unknown 10th planet may also

54 SCIENTIFIC AMERICAN March 1996 Copyright 1996 Scientific American, Inc. be stirring the cometsÕ path, but the ev- impossible to tell where a particular come by only every century or so. A idence for its existence is inconclusive.) cometÑsuch as Tempel-Tuttle, which collision with such a short-period com- The comets may then come very close sweeps by at 72 kilometers per second et might occur once in some three mil- to the sun. Although those from the every 33 yearsÑoriginated from. lion years. Kuiper belt tend to have shorter periods Some comets are bound into small However infrequent a cometary colli- than those from the Oort cloud, both orbits and have short periods, on the sion might be, the consequences would types of comets can be captured in tight order of 10 years. These comets pose be calamitous. The orbits of comets are orbits around the sun. It is therefore more of a concern than the ones that often steeply inclined to the earthÕs; oc-

Explorer . GOHIER F

METEOR CRATER in northern Arizona, a depression 1.2 kilo- wide but, being metallic, was strong enough to penetrate the meters in diameter, was carved out by an asteroid that struck atmosphere without disintegrating. The earth collides with the earth 50,000 years ago. The asteroid was only 30 meters an object of this size or larger once in a century.

casionally, a comet is even going in the if the asteroid goes around the sun glomerate of silicates about 60 meters opposite direction. Thus, comets typi- thrice in the same time that Jupiter or- wide, entered the atmosphere and burst cally pass the earth with a high relative bits once, the planetÕs gravitational in- apart above the Tunguska Valley in Sibe- velocity. For example, Swift-Tuttle, which ßuence on the rock is greatly enhanced. ria. The explosion was heard as far away is about 25 kilometers across, ßies by Just as a child on a swing ßies ever high- as London. Although the fragments did at 60 kilometers per second. It would er if someone pushes her each time the not leave a crater, the area below the impact with cataclysmic eÝect. swing returns, JupiterÕs rhythmic nudg- explosion is still marked by burnt trees Unless it runs into something, a comes ultimately cause the asteroid to veer laid out in a region roughly 50 kilome- et probably remains active, emitting gas- out of its original orbit into an increas- ters across. The identity of the Tungus- es and dust for some 500 passages by ingly eccentric one. ka object inspired a lot of nonsensical the sun. Eventually, the volatile materi- The asteroid may either leave the so- speculation for decades, and some high- als are used up, and the comet fades lar system or move in toward the ter- ly imaginative suggestions were made, away as a dead object, indistinguish- restrial, rocky planets. Eventually, such including that it was a miniÐblack hole able from an asteroid. Up to half of the vagrants collide with Mars, the earth- or an alien spacecraft. Scientists, how- nearest asteroids might in fact be dead, moon system, Venus, Mercury or even ever, have always understood that it short-period comets. the sun. A major fragment enters the in- was a comet or asteroid. ner solar system once in roughly 10 mil- Events such as the Tunguska explo- Falling Rocks lion years and survives for about as long. sion may occur once a century, and it To estimate the chances of such a is most likely that they would occur ndeed, most of the danger to the earth rock hitting the earth, the asteroids have over the oceans or remote land areas. I comes from asteroids. Like comets, Þrst to be sorted according to size. The But they would be devastating if they asteroids have solar orbits that are nor- smallest ones we can observe, which are happened near a populated area. If one mally circular and stable. But there are less than a few tens of meters across, exploded over London, for instance, not so many of them in the asteroid belt rarely make it through the earthÕs at- only the city but also its suburbs would that they can collide with one another. mosphere; friction with air generates be laid waste. The debris from such collisions can enough heat to vaporize them. The as- Of the smaller asteroids, the few me- end up in unstable orbits that resonate teroids that are roughly 100 meters and tallic ones are tough enough to pene- with the orbit of Jupiter. By virtue of its larger in diameter do pose a threat. trate the atmosphere and carve out a immense mass, Jupiter competes with There are 100,000 or so of these that crater. The 1.2-kilometer-wide Meteor the sun for control of the motions of penetrate the inner solar system deeper Crater in northern Arizona is an exam- these fragments, especially if an aster- than the orbit of Mars. They are called ple; it came from a metallic asteroid oidÕs orbit Òbeats,Ó or resonates, with near-earth asteroids. about 30 meters in diameter that fell that of the giant planet. So, for instance, In 1908 one such object, a loose con- some 50,000 years ago.

56 SCIENTIFIC AMERICAN March 1996 Copyright 1996 Scientific American, Inc. The Threat from Asteroids Studies of the eÝects of that explo- he asteroid belt, home of most asteroids, lies sion paint a frightening picture. An T between the orbits of Mars and Jupiter. The enormous Þreball ejected rocks and chart (below), put together from Spacewatch steam into the atmosphere, jarred the observations, shows that smaller asteroids, earthÕs crust and triggered earthquakes produced by fragmentation of the larger ones, are and tsunamis around the globe. Vast more numerous.The rocks normally remain in clouds of dust, from the earth and the circular, stable orbits, but collisions, along with asteroid, erupted into the stratosphere the gravitational inﬂuence of Jupiter, can throw and beyond. There ensued total dark- them into narrow, unstable orbits. Then the ness, which lasted for months. asteroids may enter the inner solar system, where Acid rain began to fall, and slowly they pose a threat to the earth. the dust settled, creating a layer of sed- iment a few centimeters thick over the 3 10 earthÕs surface. Below this thin sheet we see evidence of dinosaurs. Above it they are missing, as are three fourths of the other species. The darkness fol- lowing the explosion must have initially )

h plunged the atmosphere into a freeze.

2 grap Over many centuries, the reverse ef- 10 ( fectÑa slow greenhouse warming, by as much as 15 degrees CelsiusÑhad an equally devastating outcome. The asteroid had struck the earth in a vulnerable place, slicing into a rare region with a University of Arizona ); JOHNNY JOHNSON

NUMBER OF ASTEROIDS deep layer of limestone. (Less than 2 10 percent of the earthÕs crust has so much

illustratio ( T JEDICKE limestone; AustraliaÕs Great Barrier Reef is an example.) The explosion ejected ROBER the carbon dioxide from the limestone

12.8 6.4 3.2 1.6 . KAMAJIAN into the atmosphere, where, along with ASTEROID DIAMETER (KILOMETERS) other gases, it helped to trap the earthÕs

ALFRED T heat. Jan Smit of the Free University, Amsterdam, has proposed that the severe warming, rather than the initial An even greater peril is posed by the be globally disrupted, creating the equiv- freeze, killed the dinosaursÑthere is 1,000 or 2,000 medium near-earth as- alent of a nuclear winter. Large clouds some evidence that they died oÝ slowly. teroids that are roughly one kilometer of dust would explode into the atmo- and larger in size. One of these aster- sphere to obscure the sun, leading to Spacewatch oids is thought to collide with the earth prolonged darkness, subzero tempera- once in about 300,000 years. Note that tures and violent windstorms. oÑare we going to be hit? To begin this estimate is only a statistical aver- Even more dangerous are the largest Swith, the answer lies in the domain age. Such a collision can happen at any near-earth asteroids, which are about of planetary astronomy. The dangerous timeÑa year from now, in 20 years or 10 kilometers in diameter. Fortunately, objects have to be located, as soon as not in a million years. there are only a few such threatening possible, to diminish the chances of objects, perhaps just 10. (Even more our unexpected demise. Furthermore, Frightful Darkness fortunately, they happen to be mere they have to be tracked on the succeed- fragments of the objects in the asteroid ing nights, weeks, months and even he energies liberated by an impact belt, which can be as large as 1,000 kilo- years so that their orbits can be accu- T with such an object would be tre- meters across.) An asteroid of this size rately extrapolated into the future. mendous. The kinetic energy can be cal- collides with the earth only once in 100 In the early 1970s a 0.46-meter pho- culated from 1/2 mv2, where m is the million years or so. tographic camera at the Palomar Obser- mass of the object, and v is the incom- One such event is evident in the fossil vatory in southern California was dedi- ing velocity. Assuming a density of record. The impact of a celestial object cated to the search for near-earth ob- about three grams per cubic centimeter, marks the end of the Cretaceous geo- jects. Eleanor Helin of the Jet Propulsion as known from meteorites, and an av- logic period and the beginning of the Laboratory in Pasadena, Calif., led one erage velocity of 20 kilometers per sec- Tertiary, 65 million years ago. After of the teams of astronomers, and Eu- ond, a one-kilometer-wide object would years of searching, the crater from that gene M. and Carolyn S. Shoemaker of strike with a shock equivalent to tens eventÑa depression about 170 kilome- the U.S. Geological Survey led the other. of billions of tons of TNTÑmillions of ters in diameterÑhas been identiÞed in The scientists photographed the same times the energy released at Hiroshima the Yucat‡n Peninsula of Mexico. Al- large areas of the sky at half-hour inter- in 1945. though the crater cannot be directly vals. As asteroids orbit the sun, they Granted, asteroids do not emit the nu- seen, it has fortuitously been identiÞed move with respect to the background clear radiation that caused the particu- by drillings for oil and in images taken stars. If near to the earth, the asteroid lar horrors of Hiroshima. Still, an explo- from the space shuttle Endeavour. The is seen to travel relatively fast; the mo- sion of millions of Hiroshimas would do depression resulted from the explosive tion is easily recognized from the mul- more than destroy a few cities or some impact of an object perhaps 10 to 20 tiple exposures. countries. The earthÕs atmosphere would kilometers in diameter. Since the pioneering eÝorts at Palo-

NEPTUNE

mar, other observers have become in- oids, was born in Tucson. A 0.9-meter all from Tucson, use this facility regu- terested in near-earth asteroids. At Sid- telescope at the University of ArizonaÕs larly for Þnding comets and asteroids. ing Spring in the mountains of eastern Steward Observatory on Kitt Peak, 70 McMillan, Marcus L. Perry, Toni L. Moore Australia, a dedicated group of scien- kilometers west of Tucson, is now ded- and others, also from Tucson, use it for tists uses a 1.2-meter photographic icated to Spacewatch. Robert Jedicke, Þnding planets around other stars. camera to hunt for these rocks. In 1994 James V. Scotti, several students and I, Instead of photographic plates, our observers in California and Aus- electronic light detectors are tralia, with their photographic charge-coupled devices, or methods, jointly discovered 16 CCDs. These are Þnely divided near-earth asteroids. (At the arrays of semiconductor pic- end of that year, the Palomar ture elements, or pixels. When project closed as more modern light hits a pixel, its energy techniques were developed causes positive and negative elsewhere.) electrical charges to separate. About 15 years ago Robert S. The electrons from all the pix- McMillan, also at the University els provide an image of the of Arizona, and I began to real- light pattern at the focal plane ize that at this rate, it would of the telescope. A computer take more than a century to then compares images of the map the 1,000 or more aster- same patch of sky scanned at oids that are larger than one diÝerent times, marking the kilometer across. By taking ad- objects that have moved. vantage of electronic detection In this manner, Spacewatch devices and fast computers, the observers may Þnd as many as rate of Þnding asteroids could 600 asteroids a night. Most of be greatly increased. Space- these are in the asteroid belt; watch, a project dedicated to only occasionally does an ob- the study of comets and aster- ject move against the star Þeld so fast that it must be close to the earth. (Similarly, an airplane SPACEWATCH telescope on top high above in the sky seems to of Kitt Peak in southern Ari- move slower than one coming zona is dedicated to searching in low for a landing.) In 1994

for comets and asteroids. UNIVERSITY OF ARIZONA Scotti found an asteroid that

omets reside beyond the orbit of Nep- ) once routinely discarded but tal ballistic missiles could blast a small now stored and declassiÞed, object out of the way. (That, incidentally, tune in the Kuiper belt and the Oort graph C o show the continuous shower- would also be a good means of getting

cloud and, like asteroids, come near the phot ( earth only when dislodged from their cir- ing of the planet by small as- rid of these relics from the cold war.) cular paths. The Kuiper belt probably teroids. Because of the atmo- It seems likely, however, that we will merges into the Oort cloud, which ex- sphere, these rocks burn up have more than 100 years to prepare. tends a ﬁfth of the distance to the nearest with little consequence, even Given that much time, a modest chemi- star, Alpha Centauri. Comet Halley (below) though similar ones scar the cal explosion near an asteroid might be is a visitor from the Oort cloud that has airless moon. enough to deßect it. The explosion will swung into a steeply elliptical orbit around The next step for Spacewatch need to change the asteroidÕs trajectory the sun, having a period of 76 years. is to install our new telescope, by only a small amount so that by the which was built with an exist- time the asteroid reaches the earthÕs ing 1.8-meter mirror, so that vicinity, it will have deviated from its

Science Photo Library/Photo Researchers, Inc. we can Þnd fainter and more original course enough to bypass the Y distant objects. This state-of- planet. OR T A

V the-art instrument, the largest Present technology for aiming and in the Þeld of asteroid obser- guiding rockets is close to miraculous. vation, should serve genera- I once overheard two scientists arguing tions of explorers to come. about why Pioneer 11 had arrived 20 Meanwhile, at C™te dÕAzur Ob- seconds late at SaturnÑafter a journey servatory in southern France, of six years. But the detonation will

AL GREENWICH OBSER Alain Maury is about to bring have to be carefully designed. If the as- ; ROY

) a telescope into operation with teroid is made of loosely aggregated n an electronic detection sys- material, it might disintegrate when tem. Duncan Steel and his shaken by an explosion. The pieces illustratio ( colleagues in Australia are could rain down on the earth, causing switching to electronics as even greater damage than the intact as- well, although this project has teroid, as hunters who use buckshot . KAMAJIAN funding problems perhaps know. A ÒstandoÝÓ explosion, at some more severe than ours. Next to distance from the surface, may be the

ALFRED T join the electronic age might most eÝective in that case. Earth-based be Lowell Observatory near radar, telescopes and possibly space FlagstaÝ, Ariz., under the su- missions will be needed to determine passed within 105,000 kilometers of pervision of Edward Bowell. The U.S. Air the composition of an asteroid and how the earth. Also in that year, Spacewatch Force is also planning to use one of its it might break up. reported 77,000 precise measurements one-meter telescopes to this end; Helin Further into the future, laser or micro- of comet and asteroid positions. One and her associates already use the one wave devices might become suitable. gratifying aspect of Spacewatch is that on Maui in Hawaii. And amateur astron- Gentler alternatives, such as solar sails it has private and corporate supporters omers are coming on-line with elec- and reßectors planted on the asteroidÕs (currently 235) in addition to the U.S. tronic detectors on their telescopes. surfaceÑto harness the sunÕs radiation Air Force OÛce of ScientiÞc Research, If there is an asteroid out there with in pushing the asteroid oÝ courseÑ the National Aeronautics and Space Ad- our name on it, we should know by have also been suggested. A few scien- ministration, the Clementine space pro- about the year 2008. tists are studying the feasibility of nu- gram, the National Science Foundation clear devices to deßect very massive and other governmental organizations. Deßecting an Asteroid asteroids that show up at short notice. Spacewatch has discovered an abun- Comets and asteroids remind me of dance of small asteroids, those in the nd what if we Þnd a large object Shiva, the Hindu deity who destroys range of tens of meters. The numbers A headed our way? If we have only and re-creates. These celestial bodies of these objects exceed predictions by a Þve yearsÕ notice, we can say good-bye allowed life to be born, but they also factor of 40, but we do not as yet under- to one another and regret that we did killed our predecessors, the dinosaurs. stand their origins. These asteroids we not start surveying earlier. If we have 10 Now for the Þrst time, the earthÕs in- call the Arjunas, after the legendary In- years or so, our chances are still slim. If habitants have acquired the ability to dian prince who was enjoined to persist we have 50 yearsÕ notice or more, a envision their own extinctionÑand the on his charted course. Military recon- spacecraft could deploy a rocket that power to stop this cycle of destruction naissance satellites have since also ob- would explode near the asteroid. Per- and creation.

The Author Further Reading

TOM GEHRELS was inspired to study celestial objects upon at- THE ORIGINS OF THE ASTEROIDS. Richard P. Binzel, M. Antonietta tending a class given by Jan Oort in the Netherlands, who surmised Barucci and Marcello Fulchignoni in ScientiÞc American, Vol. 265, the existence of a distant shell of comets now called the Oort cloud. No. 4, pages 88Ð94; October 1991. Gehrels is professor of planetary sciences at the University of Ari- HAZARDS DUE TO COMETS AND ASTEROIDS. Edited by T. Gehrels. zona at Tucson, a Sarabhai Professor at the Physical Research Labora- University of Arizona Press, 1994. tory in India and principal investigator of the Spacewatch program ROGUE ASTEROIDS AND DOOMSDAY COMETS. Duncan Steel. John at Kitt Peak, Ariz., where he hunts for comets and asteroids. Wiley & Sons, 1995.