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
A
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
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-
ing cloud of gas and dust. Initially the
planetesimalsÑcoarse collections of
I n 1950 Jan H. Oort, professor of as-
tronomy at Leiden University in the
Netherlands, was teaching a class that I
Yerkes Observatory of the University of
Chicago, surmised that another belt of
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-
F. GOHIER Explorer
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
I ndeed, most of the danger to the earth
comes from asteroids. Like comets,
asteroids have solar orbits that are nor-
rock hitting the earth, the asteroids have
Þrst to be sorted according to size. The
smallest ones we can observe, which are
over the oceans or remote land areas.
But they would be devastating if they
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-
T he asteroid belt, home of most asteroids, lies
between the orbits of Mars and Jupiter. The
chart (below ), put together from Spacewatch
sion paint a frightening picture. An
enormous Þreball ejected rocks and
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
10 3 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-
NUMBER OF ASTEROIDS
lowing the explosion must have initially
plunged the atmosphere into a freeze.
ALFRED T. KAMAJIAN (illustration ); JOHNNY JOHNSON (graph )
10 2 Over many centuries, the reverse ef-
fectÑa slow greenhouse warming, by
ROBERT JEDICKE University of Arizona
as much as 15 degrees CelsiusÑhad an
equally devastating outcome. The aster-
oid had struck the earth in a vulnerable
place, slicing into a rare region with a
deep layer of limestone. ( Less than 2
10 percent of the earthÕs crust has so much
limestone; AustraliaÕs Great Barrier Reef
is an example.) The explosion ejected
the carbon dioxide from the limestone
12.8 6.4 3.2 1.6
into the atmosphere, where, along with
ASTEROID DIAMETER (KILOMETERS) other gases, it helped to trap the earthÕs
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
this estimate is only a statistical aver-
age. Such a collision can happen at any
tures and violent windstorms.
Even more dangerous are the largest
near-earth asteroids, which are about
S oÑare we going to be hit? To begin
with, the answer lies in the domain
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
T he energies liberated by an impact
with such an object would be tre-
mendous. The kinetic energy can be cal-
belt, which can be as large as 1,000 kilo-
meters across.) An asteroid of this size
collides with the earth only once in 100
years so that their orbits can be accu-
rately extrapolated into the future.
In the early 1970s a 0.46-meter pho-
culated from 1/2 mv 2, 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-
Copyright 1996 Scientific American, Inc. SCIENTIFIC AMERICAN March 1996 57
JUPITER
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
UNIVERSITY OF ARIZONA
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. Scotti found an asteroid that
58 SCIENTIFIC AMERICAN March 1996 Copyright 1996 Scientific American, Inc.The Threat from Comets served the Arjunas. The data, haps the most powerful intercontinen-
once routinely discarded but tal ballistic missiles could blast a small
C omets reside beyond the orbit of Nep-
ALFRED T. KAMAJIAN ( illustration ); ROYAL GREENWICH OBSERVATORY Science Photo Library/Photo Researchers, Inc. ( photograph)
tune in the Kuiper belt and the Oort now stored and declassiÞed, object out of the way. (That, incidentally,
cloud and, like asteroids, come near the show the continuous shower- would also be a good means of getting
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
we can Þnd fainter and more original course enough to bypass the
distant objects. This state-of- planet.
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
Alain Maury is about to bring have to be carefully designed. If the as-
a telescope into operation with teroid is made of loosely aggregated
an electronic detection sys- material, it might disintegrate when
tem. Duncan Steel and his shaken by an explosion. The pieces
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
funding problems perhaps know. A ÒstandoÝÓ explosion, at some
more severe than ours. Next to distance from the surface, may be the
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
range of tens of meters. The numbers
of these objects exceed predictions by a
A nd what if we Þnd a large object
headed our way? If we have only
Þve yearsÕ notice, we can say good-bye
Shiva, the Hindu deity who destroys
and re-creates. These celestial bodies
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.
Copyright 1996 Scientific American, Inc. SCIENTIFIC AMERICAN March 1996 59You can also read
