Another Bonestell painting was 
inspired by the hypothesis that the 
huge lunar Mare Imbrium resulted 
from the collision of a large 
meteorite with the moon. 
Frederick I Ordway and the Alabama Space 
and Rocket Center Bonestell Collection 
for example, melting and erosion — 
to such an extent that their original 
identities are almost totally lost. 
Between the orbits of Mars and Ju- 
piter, however, there is a vast expanse 
of solar system space that does not 
contain a major planet to act as a 
vacuum cleaner. In this zone untold 
numbers of lumps, boulders, frag- 
ments, and planetesimals — the aster- 
oids — are still in orbit. This zone ap- 
pears to be a place where the forces 
that conspired to draw lumps and plan- 
etesimals together into planets failed; 
the accretion process here never got 
beyond the planetesimal stage. And 
since the small bodies of the asteroid 
belt were never exposed to the vig- 
orous geologic activity of a full-grown 
planet, many of their earliest prop- 
erties remain imprinted in them. Thus 
the material properties as well as the 
accretional state of the asteroids are 
records of a time long past, preserved 
in “deep freeze” like the carcass of 
a Siberian mammoth. 
Orbits are not immutable. Every as- 
teroid is tugged at by the gravity of 
every planet, as well as that of the 
sun. Most of the planets are so far 
from the asteroids that the tugging 
is insignificant. Sometimes, however, 
an asteroid passes close enough to 
Mars or Jupiter (which are closest to 
the asteroid belt) to be subjected to 
a substantial tug that somewhat re- 
shapes its orbit. Over a period of hun- 
dreds of millions of years, the cumu- 
lative effect of a huge number of close 
passages and gravitational tugs can 
change an asteroid’s orbit so much 
that the asteroid dives deep into the 
solar system, crossing the orbits of 
Mars and the earth in each revolution 
before it swings around the sun and 
loops back out to the asteroid belt. 
In such an orbit, the asteroid stands 
a certain chance of colliding with the 
earth each time it comes around the 
planet (see “A Near Miss,” Natural 
History , March 1981). Actually, it is 
far more likely to be a small fragment 
of an asteroid that collides with the 
earth than one of the kilometers-large 
objects that astronomers see in the 
asteroid belt, because small fragments 
(created by collisions between aster- 
oids) are vastly more numerous than 
large asteroids. In fact, pieces of as- 
teroids actually do encounter the earth 
hundreds of times each year. They 
are slowed down as they penetrate the 
atmosphere and drop to the ground. 
Those we find are called meteorites, 
a term applied to all rocks that fall 
to the earth from space, whether they 
should happen to come from the as- 
teroid belt or anywhere else. 
All the types of planetesimal ma- 
terial mentioned earlier can be seen 
in the meteorite exhibit of a natural 
history museum. The primordial, un- 
fractionated material alluded to is rep- 
resented by chondritic meteorites, a 
broad class of meteorites that falls 
most frequently to Earth and includes 
the so-called ordinary chondrites. The 
lava that erupted to the surfaces of 
melted planetesimals long ago is pres- 
ent in the form of certain meteorites 
known as achondrites. And iron me- 
teorites are fragments of the cores of 
similar melted bodies. Is this infor- 
mation a vindication of the theory of 
planet growth sketched at the begin- 
ning of this article? On the contrary; 
it was principally the study of me- 
teorites that led to the theory. 
Do all meteorites come from the 
asteroid belt? Many meteoriticists are 
convinced that they do. Nevertheless, 
there are nagging doubts about the 
source of the ordinary chondrites, 
stony objects that are the most abun- 
dant group of meteorites. The trouble 
is that objects in Earth-crossing orbit 
do not survive for very long. We have 
two ways of knowing this. First, by 
measuring the amounts of new iso- 
topes that have been created in a me- 
teorite by cosmic rays in space, we 
can learn how much time has elapsed 
since the meteorite was knocked out 
of its parent asteroid. (Cosmic rays 
striking the atoms of a meteoroid alter 
their makeup, creating new isotopes in 
the process.) Meteorites are shielded 
from these cosmic rays as long as they 
remain buried in the parent body. 
“Cosmic ray exposure ages” of about 
ten million years are typically found 
for ordinary chondrites. This is a very 
short time compared with the total 
age of the solar system— four and a 
half billion years. 
Second, we can calculate the odds 
that an object in Earth-crossing orbit 
will collide with any planet during 
each circuit of its orbit, and how long 
the object will last, on the average, 
before this happens. The answer to 
how long is, again, about ten million 
years. 
If ordinary chondrites last such a 
“short” time in Earth-crossing orbits, 
there must be some abundant source 
of supply that keeps feeding new frag- 
ments into Earth-crossing orbits to re- 
place the losses, otherwise no ordinary 
58 
