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. Copyright 1996 Scientific American, Inc. SCIENTIFIC AMERICAN March 1996 55 MARS 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 com- es 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.