358 Encyclopedia of the Solar System
substantial internal fracturing, porosity, and an extremely
rough and irregular surface. Second, small impacts and mi-
crometeorites create a regolith that blankets the asteroid in
a fine soil of debris form the bedrock. Finally, micromete-
orites, radiation, and the solar wind produce chemical and
spectral alteration in the regolith soil and exposed bedrock
that “weathers” the surface of the asteroids.
All the small asteroids viewed by spacecraft show signif-
icant regoliths, and the power of radar waves reflected by
asteroids large and small (especially those passing near the
Earth, and so more easily observed by radar) also shows that
their surfaces are comparable to dry soil or sand. [SeePlan-
etary Radar.] On several of these asteroids the regoliths
appear to have been altered by space-weathering processes,
although just how this alteration affects asteroidal material
is still not completely understood. In the asteroid popu-
lation, there are general spectral trends that appear to be
associated with the age of an asteroid’s surface. The red con-
tinuum slope of S-class asteroids declines in magnitude with
asteroid size. This effect appears to be related to the age
of the asteroidal surface, with younger less altered surfaces
tending to be less red. This effect is seen in the meteorite
population. Meteorites that have evidence of residing on the
surfaces of asteroids have strong spectral differences from
meteorites that were not exposed on asteroid surfaces.
Another major surface effect is the development of small
“ponds” of fine regolith material as shown in Fig. 6. These
ponds have been seen on Eros and Itokawa and consist of
very fine dust that has been somehow mobilized on the
surface and accumulated in local “depressions” or gravita-
tional lows. The actual magnitude and direction of gravity
on a body as small and irregularly shaped as an asteroid
is not at all intuitive, but the effect is still strong enough
to drive surface processes. Ponds appear to develop over
time and appear to bury the boulders and cobbles within
them.
Another process that affects the surfaces of asteroids
is the reaccretion of ejecta debris. Impacts of other small
asteroids produce the abundant craters seen on all these
objects. Although much of the impact debris escapes the
low gravity of an asteroid, a large amount is reaccreted by
the asteroid. The abundance and location of boulders on
objects such as Eros, shown in Fig. 7, and Itokawa (Fig. 6)
has been explained by the low-velocity ejecta debris slowly
“falling” back onto the rotating asteroid.
3.2 Asteroid Satellites
It had been long suspected that some asteroids had satel-
lites; this was spectacularly confirmed when theGalileo
spacecraft flew by asteroid 243 Ida and discovered its moon
Dactyl. As of this writing, 107 asteroid satellites have been
announced in 103 systems including 2 triple systems and
one quadruple. Shown in Fig. 8 is an image of asteroid
22 Kalliope and its satellite Linus.
FIGURE 7 Fractures and boulders on the surface of asteroid
433 Eros. Even with the weak gravity of asteroids, low-velocity
ejecta such as these boulders do reaccrete to the surface.
(Photograph courtesy of APL/JHU.)NEOs tend to have small separation distances from their
satellites, which are probably the result of formation by
“fission.” Almost all NEOs are rubble piles and with a high
enough rotation rate that centrifugal acceleration can throw
boulders from the surface into orbit. Many NEOs have rota-
tion rates close to the fission limit, and additional collisions
or the YORP effect can enhance asteroid spin enough to
cause fission. After fission occurs, the new satellite carriesFIGURE 8 Asteroid 22 Kalliope and its satellite Linus.