A N S W E R E D B Y :
Tom Gehrel, University of Arizona, Tuscon, Arizona.
A veteran asteroid hunter, he and his colleagues find
roughly 20,000 objects a year—many of them
uncatalogued asteroids—using the Spacewatch
Telescope on Kitt Peak.
some scientists were seriously concerned about the possible
high density of objects in the asteroid belt,
which lies between the orbits of Mars and Jupiter, when
the first robotic spacecraft were scheduled to be sent
through it. The first crossing of the asteroid belt took
place in the early 1970s, when the Pioneer 10 and Pioneer
11 spacecraft journeyed to Jupiter and beyond. The danger
does not lie in the risk of hitting a large object. In fact,
such a risk is minuscule because there is a tremendous
amount of space between Mars and Jupiter and because
the objects there are very small in relation. Even though
there are perhaps a million asteroids larger than one kilometer
in diameter, the chance of a spacecraft not getting
through the asteroid belt is negligible.
Even if there were 100,000 sizable asteroids (more
than a few kilometers in size) in the asteroid belt—and the
real number is quite likely about 10 times less—the average
separation between them would be about five million
kilometers. That is more than 10 times the distance
between the earth and the moon. If you were standing on
one of those asteroids and looked up, you would not see a
sky full of asteroids; your neighbors would appear so small
and dim that you would be quite lucky to even see one, let
alone hundreds.
In some ways, the asteroid belt is actually emptier than
we might like. In the early 1990s, the National Aeronautics
and Space Administration wanted the Galileo space-craft to encounter an asteroid while it was passing through
the asteroid belt on its way to Jupiter. But it took some
effort to find an object that was located even roughly
along Galileo’s path. Special targeting was required to
reach this object, but the result was the first close-up view
of an asteroid, the one called Gaspra.
The number of objects in the asteroid belt increases
steeply with decreasing size, but even at micrometer sizes
the Pioneer spacecraft were hit only a few times during
their passage. That is not to say that asteroids cannot pose
any danger, however. It is worth noting that for a large
planet like Earth, over a long period of time, there is an
appreciable chance of being hit. This hazard comes from
the fragments of mutual collisions in the asteroid belt;
after their break-up, some of these fragments move toward
the earth under the gravitational action of Jupiter.
An asteroid about 12 kilometers in diameter crashed
into the earth 65 million years ago, killing nearly 90 percent
of the animals, including the dinosaurs. Such major
impacts are very rare events, but for smaller objects the
likelihood of impact increases; the chance of the earth
being struck by an object approximately one kilometer in
size is about one in 5,000 in a human lifetime. An object
one kilometer across would still be large enough to cause a
global disaster because of the enormous energy it would
release upon impact: at least a million times the energy of
the bomb dropped on Hiroshima in 1945.