Infrared Views of the Solar System from Space 691
FIGURE 16 The brightest of the “orphan trails” detected by
IRAS, seen against a background of interstellar clouds. Blue
elongated sources are stars distorted by the rectangular shape of
the detectors. Orphan trails are probably associated with comets
never before detected.
the conditions in the early solar system when they formed.
ISO made the first detections of crystalline silicates and CO 2
in its observations of the long-period comet Hale–Bopp and
Jupiter-family comet P/Hartley 2. Crystalline silicates are
formed at high temperatures not associated with the outer
solar system.Spitzermeasured the spectra of pristine ma-
terial excavated by the Deep Impact event on P/Tempel
1 finding materials never before seen in comets such as
carbonates and clay (which form in the presence of liquid
water) as well as metal sulfides, polycyclic aromatic hydro-
carbons, and crystalline silicates. Liquid water is also not
expected to be present in the outer solar system. These
observations suggest perhaps substantial radial mixing of
materials forming in hot and cold environments before the
comets begin to accrete.
5. Asteroid Physical Properties
Over a million asteroids with diameters greater than 1 km
reside primarily in a belt between Mars and Jupiter.
[SeeMain-BeltAsteroids.] The asteroid population ex-
tends interior to the orbit of the Earth and beyond Jupiter.
These objects are the scattered and disrupted remains of an
early population of protoplanets whose continued growth
was interrupted early on by mutual gravitational stirring, the
growth of a massive Jupiter, or both. The size distribution
of asteroids provides insight into their origins and evolution
through collisions and dynamical processes. Asteroid sizes
are also important to determine the hazard of near-Earth
objects. Asteroids are almost all unresolved to telescopes,
and their brightnesses are insufficient for determining their
sizes unless their albedos are known. By combining visible
and thermal infrared observations of an asteroid, the diam-
eter and albedo of an asteroid can be simultaneously deter-
mined. The difficulty of making radiometric (thermal) ob-
servations of asteroids from ground-based telescopes made
for slow growth in the number for which physical proper-
ties could be determined. Space-based surveys in the ther-
mal infrared greatly increased the numbers of asteroids for
which albedos and diameters were available, providing new
information about the composition of the Asteroid Belt.
Because most space-based infrared telescopes have op-
erated in a pointed mode, targeting specific objects or lo-
cations for observations, and few have engaged in surveys,
most asteroid albedos and diameters were derived from the
IRASsurvey in 1983. This survey resulted in 8210 observa-
tions of 2004 asteroids. A more limited survey byMSXin
1996 resulted in observations of 168 asteroids. By and large,
such surveys rely upon the detection of asteroids in known
orbits, since they are unable to provide sufficient astrometry
to determine the orbits of newly discovered asteroids.
During theIRASmission, there were tens of thousands
of asteroids having known orbits. More than two decades
later that number has increased by an order of magnitude.
Coupled with greatly increased detector sensitivity, an all-
sky survey is no longer necessary to produce an even larger
catalog of asteroid diameters and albedos. Over the 5 year
nominal mission ofSpitzer, it is estimated that about 25,000
serendipitous asteroid measurements will be made.
Known asteroid diameters inferred fromIRASare shown
in Fig. 17 as a function of heliocentric distance. The absence
of small asteroids with increasing heliocentric distance is a
consequence of the limits of detector sensitivity. The ab-
sence of larger asteroids beyond 4 AU, however, is real.
This indicates that either the inner and outer belt/JupiterFIGURE 17 Asteroid diameters versus heliocentric distance of
detection. The lower limit of detected asteroid sizes reflects the
sensitivity limits of the IRAS detectors. The outer asteroid belt is
shown to have few large asteroids compared to the inner belt.