Main-Belt Asteroids 359
away some of the primary’s angular momentum, thus drop-
ping the rotation back below the fission limit. The lack of
distant NEO companions may be the result of gravitational
encounters with planets. Distant satellites would be much
more likely to be stripped from their primaries during close
planetary encounters common with NEOs.
Although only about 100 have been discovered, aster-
oid satellites are thought to be fairly common with a few
percent of all asteroids having satellites. With over 137,000
numbered asteroids, a large number of satellites remain
to be discovered. This is another area where amateur as-
tronomers can make a significant contribution to science.
Some satellites have been discovered by direct imaging,
either from spacecraft oradaptive optics(i.e. Fig., 8),
but most satellites are discovered by analysis of asteroid
lightcurves. The principle is that the satellite in its orbit
will periodically add or subtract its illumination from the
brightness of the asteroid. By precisely tracking the change
in brightness, it is possible to identify the satellite and
determine its orbit and period. With CCD imagers avail-
able commercially and modest-sized telescopes, a skilled
amateur can successfully compete in discovering asteroid
satellites.
3.3 Telescopic Observations of Composition
Our understanding of the composition of asteroids rests
on two pillars: the detailed study of meteorite mineralogy
and geochemistry and the use of remote sensing techniques
to analyze asteroids. The meteorites provide, as discussed
in a previous section, an invaluable but limited sample of
asteroidal mineralogy. To extend this sample to what are
effectively unreachable objects, remote sensing uses a vari-
ety of techniques to determine asteroid composition, size,
shape, rotation, and surface properties. The best available
technique for the remote study of asteroid composition is
visible and near-infraredreflectance spectroscopyusing
ground-based and Earth-orbiting telescopes. Reflectance
spectroscopy is fundamentally the analysis of the “color” of
asteroids over the wavelength range 0.2–3.6μm. An expe-
rienced rockhound limited to the three colors of the human
eye can identify a surprisingly wide variety of rock-forming
minerals. For example the silicateolivineis green, and im-
portant copper minerals such asazurite(blue) andmala-
chite (green) are vividly colored. These colors are a fun-
damental diagnostic property of the mineralogy because
the atoms of a mineral’s crystal lattice interact with light
and absorb specific wavelengths depending on its struc-
tural, ionic, and molecular makeup, producing a unique
reflectance spectrum. The reflectance spectrum is essen-
tially a set of colors, but instead of three colors, our remote
sensing instruments “see” very precisely in 8, 52, or even
several thousand colors. What can be seen are very precise
details of the mineralogy of the major rock-forming miner-
als olivine, pyroxene, spinel, the presence of phyllosilicates,
organic compounds, hydrated minerals, and the abundance
of free iron and opaque minerals.
In addition to a spectroscopic inventory of minerals, tele-
scopic measurements yield several other critical pieces of
information. Thealbedoor fundamental reflectivity of the
asteroid can be determined by measurements of the vis-
ible reflected light and the thermal emission radiated at
longer wavelengths. A dark asteroid will absorb much more
sunlight than it reflects, but it will heat up and radiate that
extra absorbed energy at thermal wavelengths. Ratioing the
reflected and emitted flux at critical wavelengths provides
an estimate of an asteroid’s albedo. Reflectance measured
at a series of phase angles can be used to model the pho-
tometric properties of the surface material and estimate
physical properties like the surface roughness, surface soil
compaction, and the light-scattering properties of the as-
teroidal material. Measurements of polarization as a func-
tion of solar phase angle can be used to infer albedo and
also provide insight into the texture and mineralogy of the
surface.3.4 Composition, Taxonomy, and the
Distribution of Classes
The basic knowledge of asteroids is primarily limited to
ground-based telescopic data, usually broadband colors in
the visible and near-infrared wavelengths and albedo that
is indicative of composition; this forms the basis of aster-
oid taxonomy. Asteroids that have similar color and albedo
characteristics are grouped together in a class denoted by
a letter or group of letters. Asteroids in particularly large
classes tend to be broken into subgroups with the first let-
ter denoting the dominant group and the succeeding letters
denoting less prominent spectral affinities or subgroups.
Asteroid taxonomy has developed in tandem with the in-
crease in the range and detail of asteroid observational data
sets. Early observations were often limited in scope to the
larger and brighter asteroids and in wavelength range to fil-
ter sets used for stellar astronomy. As observations widened
in scope and more specialized filter sets and observational
techniques were applied to asteroids, our appreciation of
the variety and complexity of asteroid spectra has also in-
creased. The asteroid classification system has evolved to
reflect this complexity, and the number of spectral classes
has steadily increased. Shown in Table 2 is a listing of the
expanded “Tholen” asteroid classes and the current min-
eralogical interpretation of their reflectance spectra. The
Tholen classification is still widely used, but it is not by any
means the only asteroid classification system. Other widely
accepted classifications include the SMASSII system, the
Barucci system, and the Howell system.
To explain the compositional meaning of asteroid re-
flectance spectra and color data, we can treat the Asteroid
Belt as a series of zoned geologic units, starting at the outer