694 Encyclopedia of the Solar System
complement the spatially resolved studies, in reflected light,
of theNew Horizonsmission as it passes by Pluto and
Charon in 2016.
Spitzerhas been trained on trans-Neptunian objects be-
yond Pluto, finding that their albedos vary considerably.
Many have been determined to have high albedos from
the fact that they have not been detected by the telescope,
requiring brighter, hence cooler and thermally fainter sur-
faces. Nominally, an undifferentiated primitive surface ex-
posed to galactic cosmic radiation over the age of the solar
system would be expected to be covered with dark com-
plex organics. Bright surfaces suggest substantial evolution
that allows volatile ices such as nitrogen to migrate to the
surface—perhaps evidence of collisional activity or heating
resulting in differentiation. Even nondetections can provide
essential insights into the nature of things.
Beyond the Kuiper Belt lies the question of what addi-
tional parts of the solar system wait to be discovered. The
sky is filled with cirrus-like structures (Fig. 1) and begs
the question of whether any of them are local to the solar
system.
IRASsurveyed 96% of the sky twice and 75% of the sky
a third time. Images from these surveys were used to con-
duct the first-ever parallactic survey of the sky. Images of
the same location taken weeks to several months apart were
compared to search for reflex motion of extended sources.
There were a couple of exciting possible detections, but in
the end, nothing was identified in a volume extending 100
AU in some directions and 1000 AU in others—within the
sensitivity ofIRAS. With the greater sensitivity of current ra-
diometric detectors and the larger apertures of space-based
telescopes, the question asked withIRAScan be asked again
and again, perhaps one day with a positive result or a defini-
tive answer.7. An Exciting Future
At the time this article is being written,Spitzeris only half
way through its nominal mission, and the initial results are
just being published and digested. Detailed scans are be-
ing made of the zodiacal plane to test models of dust band
origins, search for evidence of additional bands, and make
sample searches for trail structures. Potential noncometary
dust trails, first identified inIRASobservations, have been
recovered, and efforts are underway to determine their spe-
cific origins. It is possible that they are the debris from aster-
oid collisions only tens of thousands of years past.Spitzer
continues its surveys of Kuiper Belt objects, to find the
extent of nonuniformity of this population. It is conduct-
ing surveys of comets, finding interesting variations in their
large dust production. It is conducting surveys of the outer
satellites of Jupiter, Saturn, Uranus, and Neptune.Akari,
successfully launched, is just beginning its mission to con-
duct pointed observations and a new survey of the sky at
thermal wavelengths. An entirely new suite of comets will
be approaching perihelion (for maximum brightness) than
at the time ofIRAS, and the potential for identifying orphan
trails is renewed. There is the potential of a longer baseline
all-sky survey byWISE. We face the prospect of a wealth of
new data and new perspective on the solar system, building
on what we have learned over the past quarter century and
what we are continuing to learn from infrared views of the
solar system.