Main-Belt Asteroids 363
Our first guess is to assume that the asteroids, like the
planets, formed in a solar nebula of gas and dust that
smoothly varied in density and temperature from the hot,
dense center where the Sun was forming to the thin, cold
outer edges where the nebula bordered interstellar space.
[SeeThe Origin of the Solar System.] It is possible
to calculate the rate at which dust in this cloud would en-
counter and stick to other bits of dust. These calculations
indicate that it is possible in the early solar nebula for very
loose balls of dust (more than 90% empty space) as large as
a kilometer across to be formed. Relatively low-speed col-
lisions between such dust balls would lead to further com-
pression and accretion into objects big enough to not be car-
ried away with the gas when the last of the solar nebula was
pulled into the Sun or ejected in a massive early solar
wind.
But it seems probable that these proto-asteroids looked
very different from the asteroids we see today. When Jupiter
and the other major planets formed, their concentrated
gravity would have begun to stir up the asteroidal mate-
rial. This stirring, and the absence of a nebula gas to damp
down their motions, would have added enough energy to
the orbits in the Asteroid Belt that further collisions be-
tween the asteroids would lead to asteroids breaking apart
instead of sticking together. If one takes the present-day
masses of the planets, adding a solar proportion of hydro-
gen and helium to the rocky planets’ compositions, and then
imagines spreading this material in a disk around the Sun
to simulate the smallest possible nebula capable of making
planets, one can see that the amount of material in such a
nebula varies smoothly from the center to the outer reaches
of the solar system, with three notable exceptions. In-
side Mercury and outside Neptune the nebula had distinct
boundaries. And in the region of Mars and the Asteroid Belt,
there appears to be a significant amount of mass missing
today.
We saw in Section 2 how Jupiter and Saturn perturb
asteroids out of the Asteroid Belt. But modeling the early
solar nebula allows us to estimate just how much material
was so perturbed. It suggests that Mars is made up of less
than 10% of the material originally available in its region
of the solar nebula, while the mass of the Asteroid Belt is
less than 0.1% of the inferred original material present. The
perturbations of asteroidal material by Jupiter and Saturn
must have been extremely efficient, at least in the earliest
stages of the solar system’s history.
One inevitable result of having 99.9% of the mass of the
Asteroid Belt excited into such orbits is that there must
have been a very high collision rate among asteroids in the
early solar system. These collisions would break larger aster-
oids into smaller pieces and destroy the smaller pieces en-
tirely. But for the largest asteroids—many tens of kilometers
in radius—impacts energetic enough to shatter them may
not have enough energy to disperse the pieces completely.
Instead, the fragments were likely to reaccrete into piles
of rubble, consistent with the structure that asteroids are
inferred to have today.
As the Asteroid Belt is dissipated, the rate of collision
likewise would have dropped. Given the present-day pop-
ulation of the Asteroid Belt, collisions that are capable of
breaking pieces of an asteroid into earth-crossing orbits or
creating families of asteroids where one asteroid once or-
bited still do occur. We do see young families of asteroids
today. Likewise, by measuring short-lived radioactive iso-
topes formed in meteorites by cosmic rays, we can see peaks
in the ages of certain meteorite classes that imply they were
broken off a parent body at a specific moment some tens
to hundreds of millions of years ago. But these events must
be many, many times less frequent today than when the
Asteroid Belt was much more heavily populated.
One result of this scattering of asteroids by Jupiter and
Saturn may have been that a few rare bodies originally from
the Asteroid Belt may have been captured into orbits around
other planets. Among the moons suspected of being cap-
tured asteroids are the Martian moons Phobos and Deimos,
and the irregular moons of the gas giant planets.4.3 Spacecraft Missions to Asteroids
Although telescopic studies are by far the most prolific
source of data on asteroids, critical science questions on
asteroid composition, structure, and surface processes can
only be addressed by spacecraft missions getting close to
these objects. The range of spacecraft encounters includes
flybys, rendezvous, and sample return missions, which pro-
vide information of ever-increasing detail and reliability. We
have now seen the results of a number of flybys, starting with
two by theGalileospacecraft (243 Ida and 951 Gaspra) on
its way to Jupiter.
TheNEAR(Near Earth Asteroid Rendezvous) space-
craft, the first dedicated asteroid mission, flew past asteroid
253 Mathilde and arrived in orbit around 433 Eros in 2001.
After orbiting Eros for one year and mapping its mor-
phology, elemental abundances, and mineralogy with an
X-ray/gamma ray spectrometer (XGRS), imaging camera,
near-infrared reflectance spectrometer, laser rangefinder,
and magnetometer, the spacecraft ended its mission
by landing on the surface of Eros. [SeeNear-Earth
Objects.]
The next mission to fly past an asteroid wasDeep Space 1
(DS1). Primarily a technology demonstration to test the new
solar-electric propulsion ion drive system, it flew past aster-
oid 9969 Braille on its way to comet Borrelly, but unfortu-
nately a camera-pointing error during the Braille encounter
limited the amount of useful data from that mission.
In late 2005, the Japanese space agency’s ambitious
Hayabusaasteroid sample return mission rendezvoused
with asteroid 25143 Itokawa. This NEA turned out to have