earth do so at very great speeds, be- 
tween ten and thirty kilometers per 
second. (For comparison, a .30 caliber 
rifle bullet moves at about 0.7 kilo- 
meter, or less than half a mile, per 
second.) They would wreak great 
havoc if it were not for the shield- 
ing effect of the atmosphere we live 
in. 
A meteorite has to punch its way 
through a column of air having the 
same cross-sectional area as the me- 
teorite itself, and which is as long as 
the meteorite’s path through the at- 
mosphere. If the meteorite is small, 
the mass of air in this column will 
be greater than the mass of the me- 
teorite. In colliding with such an 
amount of air, the meteorite is gradu- 
ally slowed until it falls to the earth 
at a velocity no greater than if it had 
been dropped out of an airplane, less 
than fifteen meters per second. 
If, on the other hand, the meteorite 
is large, the results are different. The 
mass of the meteorite increases as the 
cube of its diameter, while its cross- 
sectional area — and therefore the mass 
of air it must shove aside — increases 
only as the square of its diameter. 
Meteorites larger than a few meters 
in diameter are heavier than the air 
they must displace, so they are not 
much slowed by it. Very large me- 
teorites strike the ground at velocities 
greater than ten kilometers per second 
and explode with greater energy than 
a comparable weight of TNT. 
Fortunately for us, large meteorites 
are very rare. The objects that come 
to the earth from space are fragments 
from collisions between asteroids (and 
perhaps comets), and collisions tend 
to create more small pieces of debris 
than large ones. Thus, for every thou- 
sand-kilogram fragment, a collision 
would result in about ten hundred- 
kilogram pieces, roughly a thousand 
ten-kilogram chunks, and so forth. The 
vast majority of extraterrestrial frag- 
ments that collide with the earth are 
in the small-size, slowable range. But 
not all the incoming fragments are 
in this range. Occasionally the earth 
is struck by meteorites large enough 
and fast enough to blast huge craters 
in it. The best example of such a struc- 
ture is the 'Arizona Meteor Crater, 
a pit more than a kilometer wide and 
a hundred meters deep. The desert 
around the crater is strewn with iron 
meteorite fragments. Cosmic rays had 
generated radioactive isotopes in these 
meteorites while they were in space; 
the amounts of these isotopes found 
to be still present indicate that the 
meteorite struck the earth at least 
2,700 years ago. 
Even larger craters can be found 
elsewhere. They are older and less well 
preserved than the Arizona crater, 
having been worn down by geologic 
erosion. Many are still preserved in 
the terrain of the Canadian Shield, 
an expanse of ancient granitic rock 
extending from the province of Que- 
bec to the Northwest Territories. 
Craters are abundant in this region. 
The Shield is geologically stable, ero- 
sion is slower there than elsewhere, 
so craters are preserved longer and 
tend to accumulate. Luckily no me- 
teorites large enough to form explosion 
craters have fallen in historical times. 
(A titanic explosion that occurred in 
an unpopulated area of Siberia in 1 908 
left no crater or meteorite fragments 
and is thought to have resulted from 
the impact of an icy comet nucleus.) 
Nevertheless, the law of averages 
dictates that extremely large objects 
must strike the earth at infrequent 
intervals. These impacts could affect 
the earth profoundly and on a global 
scale. Recently it has been proposed 
that an object roughly ten kilometers 
in diameter struck the earth 65 million 
years ago (see “The Belt of an As- 
teroid,” Natural History , June 1980). 
This is not unreasonable, given the 
present rate of fall of small meteorites 
and the relative abundances of small 
and large meteorites. The huge impact 
postulated is believed to have thrown 
such a colossal mass of pulverized rock 
into the air that the atmosphere of 
the whole earth was clogged with a 
pall of dust for several years. The dust 
cut off sunlight from the earth’s sur- 
face to such an extent that plant life 
was decimated and as a consequence 
many forms of animal life became ex- 
tinct, including the dinosaurs. (The 
depletion of plant life starved the 
herbivores, and their extinction, in 
turn, starved the carnivores.) As evi- 
dence of this event, a thin layer of 
clay — presumably dust deposited 
from the atmospheric pall — is found 
worldwide in sedimentary rock strata 
at the level corresponding to the 
boundary between the Cretaceous and 
Tertiary periods, the time when the 
dinosaurs vanished. The clay every- 
where contains trace elements in pro- 
portions characteristic of meteorites. 
Of course, meteorites also bombard 
the moon. Since there is no atmos- 
phere there to slow them down, small 
meteorites as well as large ones blast 
and crater the lunar surface. Because 
geologic erosion does not operate on 
the moon as it does on the earth, lunar 
craters have persisted and accumu- 
lated through the ages, resulting in 
the pockmarked landscape visible to- 
day. The pulverizing effect of mete- 
orite impacts, large and small, created 
the deep layer of lunar dust that we 
saw the astronauts walking in during 
the Apollo missions to the moon. Bits 
of meteorites can be found in the lunar 
soil samples they brought back. 
There is a possibility that a major 
meteorite impact on the moon has 
been witnessed from the earth. In the 
chronicles of Gervase of Canterbury, 
a.d. 1178 (translated from the Latin 
by R.Y. Hathorn), an account is given 
by five men who had sat outdoors 
on a July night: 
Now there was a bright new moon, and 
as usual in that phase its horns were 
tilted toward the east; and suddenly the 
upper horn split in two. From the mid- 
point of this division a flaming torch 
sprang up, spewing out, over a consid- 
erable distance, fire, hot coals, and sparks. 
The place on the lunar surface 
where this would have occurred can 
be pinpointed, and when photographs 
of this area taken from the Apollo 
spacecraft are examined, a fresh, 
twenty-kilometer-wide crater is found. 
The formation of this crater (named 
Giordano Bruno for the sixteenth-cen- 
tury Italian astronomer) may have 
caused the dramatic spectacle de- 
scribed above, although the calculated 
probability of such a large impact oc- 
curring on the moon during the entire 
period of recorded history is only 
about one chance in a thousand. 
There is no question, however, that 
asteroids are out there, though, and 
must occasionally collide with the 
earth. Impacts as large as the ones 
that may have destroyed the dinosaurs 
and created the lunar crater Giordano 
Bruno are estimated to occur every 
thirty million years or less. When will 
the next one fall? □ 
Comets are believed to be the 
main source of the dust particles 
that produce meteor showers and 
so-called shooting stars in the 
night sky. Shown here is Comet 
Kohoutek, which got brilliant 
advance notices but did not live 
up to expectations in 1974. 
NASA 
60 
