Main-Belt Asteroids 357
observations of a half-dozen recovered chondrites that were
photographed falling by camera networks, or whose fireballs
were recorded by many well-separated video images. These
data show that each of the meteorites had its orbital origins
in the Main Asteroid Belt. Other evidence includes the sim-
ilarity of meteorite reflectance spectra to several classes of
asteroids; the existence of xenoliths (pieces of other mete-
orite types included in meteorite breccias) in meteorites,
which requires that the source region have the mineralogi-
cal diversity found in the Asteroid Belt; and the solar-wind
implanted gases found inregolithmeteorites indicating
that implantation took place in regions consistent with the
location of the Asteroid Belt.
Meteorites do not automatically provide the location and
taxonomic class of their particular parent bodies. The very
fact that a meteorite is “in our hands” suggests the occur-
rence of some violent event that may have fragmented
and perhaps destroyed the parent body. The best that
can be done is to link individual asteroid spectral classes
with meteorite compositional groups. This task is somewhat
speculative because most meteorites were originally buried
beneath the surface of an asteroid, asteroid surface con-
ditions are unknown, and the effects of space weathering
on asteroids are poorly understood. All spectral matches be-
tween asteroids and meteorites, including the ones detailed
here, should be viewed with healthy skepticism.
There are several factors that bias the population of me-
teorites arriving on Earth and therefore limit our sample of
the Asteroid Belt. First, the dynamical processes that de-
liver meteorites from the Asteroid Belt to Earth are proba-
bly strongly biased toward sampling relatively narrow zones
in the Asteroid Belt. Calculations demonstrate that the
vast majority of meteorites and planet crossing asteroids
originate from just two resonances in the belt, the 1:3
Kirkwood gap andν 6 resonance. Both of these zones are
in the inner Asteroid Belt where the asteroid population
is dominated by S-type asteroids. However, the Yarkovsky
effect significantly increases the chances of fragments from
anywhere in the Asteroid Belt working their way into Earth-
crossing orbits. A second factor is the relative strength of
the meteorites. To survive the stress of impact, accelera-
tion, and then deceleration when hitting the Earth’s atmo-
sphere, without being crushed into dust, the meteorite must
have substantial cohesive strength. Large iron meteorites
are more likely to survive until they hit the surface of the
Earth; they may form a crater (like Meteor Crater in Ari-
zona) when they hit, but in that process most of the iron
is vaporized and lost. The Earth’s atmosphere is probably
the most potent filter for meteorites. The relatively weak,
volatile-rich meteorites from the outer Asteroid Belt stand
little chance of surviving the stress and heating of atmo-
spheric entry. It is very likely that the meteorites available
to us represent only a small fraction of the asteroids, and it
is possible that most asteroids either cannot or only rarely
contribute to the meteorite collections.
FIGURE 5 The surface of asteroid 243 Ida. (Photograph
courtesy of the Jet Propulsion Laboratory.)3. Physical Characteristics and Composition
3.1 The Surfaces of Asteroids
As shown in Figs. 1, 5, and 6, the surfaces of asteroids appear
cratered, lined with fractures, and covered with regolith.
These surfaces are dominated by impact processes. As dis-
cussed in earlier sections, asteroids are strongly affected
by collisional disruption and have a complex history of im-
pact fracturing and fragmentation. Objects in the size range
shown in the figures are probably formed as disrupted frag-
ments from larger objects, and some are likely rubble piles
themselves. Because asteroids are far too small to retain an
atmosphere that could offer some protection from the expo-
sure to space, the surfaces of asteroids are exposed to an ex-
tremely harsh environment. There are a range of processes
associated with exposure to the space environment; high
levels of hard radiation, high-energy cosmic rays, ions and
charged particles from the solar wind, impacts by microm-
eteorites, impacts by crater-forming objects, and finally im-
pacts by other asteroids large enough to destroy the parent
asteroid. The overall result of these processes is threefold:
First, large impacts shatter the parent asteroid creatingFIGURE 6 Asteroid 25143 Itokawa. The asteroid is
approximately 700 m in its longest dimension. The smooth areas
in the center and on the lower left center are examples of
ponding of fine regolith.