352 Encyclopedia of the Solar System
FIGURE 2 Radar image of the asteroid 216 Kleopatra. This
irregularly shaped object resembles a 200 m long dog bone.
Although this object is an extreme example, all the asteroids
“seen” so far by either spacecraft or radar are very irregular in
shape. (Photograph courtesy of the Jet Propulsion Laboratory.)
larger than 4 km. The bad news is that there are thou-
sands of small asteroids in Earth-crossing orbits, a few as
large as several kilometers in diameter, that remain undis-
covered and potential threats to Earth. [SeeNear-Earth
Objects.]
Because most asteroids are probably collisionally pro-
duced fragments of larger asteroids it should not be a sur-
prise that they are not perfect spheres. Many asteroids that
have been directly imaged optically or by radar tend to show
very irregular shapes (Fig. 1). The exception is the largest
asteroid (or dwarf planet), 1 Ceres, which is large enough for
hydrodynamic forces to maintain a spherical shape. Other
large asteroids are far from spherical. For example, shown
in Fig. 2 is a radar image of the asteroid 216 Kleopatra,
which has a strong resemblance to a 200 km long bone!
Most asteroid shapes can be approximated as triaxial ellip-
soids, which are objects that have different dimensions on
each of their principle axes. In the case of Kleopatra, the
long dimension in Fig. 2 is over four times greater than the
short dimension.
Star/asteroid occultations provide a direct measurement
of an asteroid’s shape and an opportunity for amateur as-
tronomers to become involved in significant scientific re-
search. The principle is simple: When an asteroid passes
through (or “occults”) the light from a star, the asteroid
creates a “shadow” in the starlight projected on the Earth.
Observers in different locations time the disappearance of
the occulted star and trace out the shape of this shadow
by reconstructing their “chords” or time-tagged observa-
tions of the star disappearing behind the asteroid and reap-
pearing on the other side. When done skillfully with mod-
ern equipment such as CCD detectors, computer-driven
imaging systems, precise time, and the Global Positioning
System, these measurements can be taken with very high
accuracy and provide an excellent “snapshot” of the two-
dimensional shape of the asteroid at the moment of occul-
tation.1.4 Asteroid Density, Porosity, and Rotation Rates
A fundamental physical property of an asteroid is its density.
To first order, asteroid density is related to its composition
and should be similar to the densities of meteorites thought
to be derived from those asteroids. [SeeMeteorites.]
However, as is often the case, such expectations are often
frustrated by unexpected results from direct measurements.
Asteroids in general appear to be significantly under-dense
relative to their meteorite analogs.
The primary complication is porosity. Asteroids appear
to have significant porosity; some may be as much as 50%
empty space, whereas their meteorite analogs have only
small to moderate porosities. The observed power law of
asteroid sizes and studies of the collisional dynamics of the
asteroid belt have suggested a history of intense collisional
evolution and that only the largest asteroids retain their
primordial masses and surfaces. Asteroids below 300 km in
diameter will have been shattered by energetic collisions.
Some objects reaccrete to form gravitationally bound rubble
piles, while the rest are broken into smaller fragments to be
further shattered or fragmented. Thus, most asteroids may
be shattered heaps of loosely bound rubble with significant
porosity in the form of large fractures, vast internal voids,
and loose-fitting joints between major fragments. Thus, it is
not surprising that the average asteroid would have a very
large porosity.
Another line of evidence supporting the rubble pile
model for asteroids are the images of 253 Mathilde. This
object, whose density is only half the density of typical mete-
orite material, has 6 identified impact craters that are larger
than the size necessary to shatter the asteroid. The only way
that Mathilde could have survived these repeated huge im-
pacts is if it were already a shattered rubble pile that dissi-
pates much of the energy of large impacts in the friction of
the pieces of rubble grinding against each other.
In addition to the images of this one asteroid, Mathilde,
and evidence from the densities available only for a few
dozen asteroids to date, data to support this rubble-pile
model of asteroids in general comes from the rotation rates
of asteroids. For objects that have not been catastrophi-
cally disrupted by collisions, rotation rates are probably set
by the accretion conditions of the solar nebula and would
tend to be relatively slow (1 or 2 revolutions per day). For
small asteroids that are fragments of catastrophic impacts,
rotation rates are set by the conditions of angular momen-
tum partitioning during the collision and should be much
more rapid (>5 revolutions per day). However, the faster a