The Solar System and Its Place in the Galaxy 13
FIGURE 5 A sampling of main-belt and near-Earth asteroids: 243 Ida along with its small
satellite Dactyl (top left), 951 Gaspra (top right), 253 Mathilde (bottom left), and 25143 Itokawa
(bottom right). All these asteroids, with the exception of Mathilde, are stony types; Mathilde is a
carbonaceous asteroid. Most of the asteroids exhibit heavily cratered surfaces, but Itokawa is an
exception, appearing to be a complete rubble pile. Ida is 54× 24 ×15 km in diameter, Dactyl is
1.5 km in diameter Gaspra is 18× 10 ×9 km, Mathilde is roughly 53 km in diameter and Itokawa
is only 550× 300 ×260 m. The asteroids were photographed by theGalileospacecraft while it
was en route to Jupiter, in 1993 and 1991, theNEARspacecraft while enroute to Eros in 1997,
and theHayabusaspacecraft while in orbit in 2005, respectively. Ida’s tiny satellite, Dactyl, was
an unexpected discovery of two of Galileo’s remote sensing instruments, the Near Infrared
Mapping Spectrometer and the Solid State Imaging system, during the flyby.
the Earth at higher velocities than asteroidal debris and thus
are more likely to fragment and burn up during atmospheric
entry. However, we may have cometary meteorites in our
sample collections and simply not yet be knowledgeable
enough to recognize them.
Recovered meteorites are roughly equally split between
silicate and carbonaceous types, with a few percent being
iron–nickel meteorites. The most primitive meteorites (i.e.,
the meteorites which appear to show the least processing
in the solar nebula) are the volatile-rich carbonaceous
chondrites. However, even these meteorites show evidence
of some thermal processing and aqueous alteration (i.e.,
processing in the presence of liquid water). Study of
carbonaceous and ordinary (silicate) chondrites provides
significant information on the composition of the original
solar nebula, on the physical and chemical processes
operating in the solar nebula, and on the chronology of the
early solar system.
The other major group of primitive bodies in the solar
system is the comets. Because comets formed farther from