564 PART 4^ |^ THE SOLAR SYSTEM
collided, fragmented, and been covered with craters. Some aster-
oids were perturbed by the gravity of Jupiter and other planets
into orbits that collided with planets or the sun, caused them to
be captured as planetary satellites, or ejected them from the solar
system. Th e present-day asteroids are understood to be a very
minor remnant of the original mass in that zone.
Collisions among asteroids must have been occurring since
the formation of the solar system (look again at Figure 25-7 and
page 560). Astronomers have found evidence of catastrophic
impacts powerful enough to shatter an asteroid. Early in the 20th
century, astronomer Kiyotsugu Hirayama discovered that some
groups of asteroids share similar orbits. Each group is distinct
from other groups, but asteroids within a given group have the
same average distance from the sun, the same eccentricity, and
the same inclination. Up to 20 of these Hirayama families are
known. Modern observations show that the asteroids in a family
typically share similar spectroscopic characteristics. Apparently,
each family was produced by a catastrophic collision that broke
a single asteroid into fragments that continue traveling along
similar orbits around the sun. Evidence shows that one family
was produced only 5.8 million years ago in a collision between
asteroids estimated to have been 3 km and 16 km in diameter,
traveling at relative speeds of about 5 km/s (11,000 mph), a typi-
cal speed for asteroid collisions. It seems that the fragmentation
of asteroids is a continuing process.
In 1983, the Infrared Astronomy Satellite detected the infra-
red glow of sun-warmed dust scattered in bands throughout the
asteroid belt. Th ese dust bands appear to be the products of past
collisions. Th e dust will eventually be destroyed, but because col-
lisions occur constantly in the asteroid belt, new dust bands will
presumably be produced as the present bands dissipate. Th e
interplanetary dust in our solar system is analogous to dust in
extrasolar planetary debris disks, produced astronomically
recently by collisions of remnant plantesimals (see Chapter 19).
Even though most of the planetesimals originally in the
main belt have been lost or destroyed, the objects left behind
carry clues to their origin in their albedos and spectroscopic col-
ors (p. 561). C-type asteroids have albedos less than 0.06 and
would look very dark to your eyes. Th ey are probably made of
carbon-rich material similar to that in carbonaceous chondrite
meteorites. C-type asteroids are more common in the outer aster-
oid belt. It is cooler there, and the condensation sequence (see
Chapter 19) predicts that carbonaceous material would form
more easily in the outer belt than in the inner belt.
S-type asteroids have albedos of 0.1 to 0.2, so they would
look brighter as well as redder than C-types; S-types may be
composed of rocky material. M-type asteroids are also bright but
not as red as the S-types; they seem to be metal-rich and may be■ Figure 25-11
(a) The LINEAR telescope searches for asteroids every clear night when the
moon is not bright. A diagram showing color codes for the thoroughness
of the LINEAR search over the entire sky for one year. The green (poorly
searched) region is the plane of the Milky Way, where asteroids are diffi cult
to discover against a dense starry background. (MIT/Lincoln Labs)
The LINEAR telescope
is 1 meter in diameter.EclipticMilkyWayM
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