684 Encyclopedia of the Solar System
TABLE 1 Space-Based Telescopes Operating in the Thermal InfraredSpacecraft Launch Date End of Missiona Aperture (cm) Wavelength Coverage (μm)IRAS January 1983 November 1983 57 12–100
COBE(DIRBE) November 1989 September 1990 19 1.25–240
ISO November 1995 May 1998 60 2.5–240
MSX April 1996 September 1997 33 8.3–21.3
Spitzer August 2003 +5 years 85 3.6–106
Akari April 2006 +18 months 68.5 1.7–180aLoss of cryogen and thermal infrared sensitivity.These telescopes and their detectors were cryogenically
cooled to minimize the noise introduced by the telescope
and detectors themselves. In general, their operating tem-
peratures need to be well below that of the sources they
wish to observe. For solar system objects, this means well
below 20 K to study the Kuiper Belt and beyond (Fig. 3).
This is accomplished by carrying a reservoir of liquid helium
(having a temperature between a fraction of a degree and
several degrees Kelvin), which has a finite lifetime before
it is expended. At that point, the telescope and detectors
warm up and loose their sensitivity.
FIGURE 3 Bodies decrease in temperature with increasing
distance from the Sun as shown by the red curve (which assumes
that a body absorbs all incident sunlight). The corresponding
wavelength at which the thermal emission spectrum peaks is
shown by the black curve. At a given heliocentric distance,
bodies that reflect increasing amounts of sunlight have lower
equilibrium temperatures and will emit an increasing fraction of
their thermal energy at longer wavelengths. For reference,
distances of the planets are denoted by their first letters.
Since the launch ofIRAS, infrared detectors have be-
come increasingly sensitive, thus able to study fainter and
fainter sources. At the same time, the different spacecraft
have operated in different modes in order to focus on dif-
ferent science questions.IRASwas primarily a survey in-
strument, mapping out the complete celestial sphere al-
most three times. Since the sky had not been mapped at
thermal wavelengths, this was a mission of discovery. The
Cosmic Background Experiment(COBE) was also a sur-
vey instrument, with the primary goal of understanding
the distribution of the cosmic background radiation from
the Big Bang. One of its instruments, the Diffuse Infrared
Background Experiment (DIRBE) operated at thermal and
near-infrared wavelengths at lower spatial resolution than
IRAS. TheInfrared Space Observatory(ISO) andMid-
course Space Experiment(MSX) were primarily pointing
instruments, designed to measure specific targets or map
out small regions of sky in detail.Spitzeris also primarily a
pointing and mapping instrument.Akariis planned to spend
a portion of its mission generating the first thermal map of
the sky sinceIRASandCOBEand to use the remainder
of its time conducting pointed observations. Even though
all these missions have been designed to address primarily
astrophysical questions, they have been a great boon to our
understanding of solar system phenomena.2. A New View of the Zodiacal Dust Cloud
and its Sources
When we think of the solar system, the image that often
comes to mind is the textbook picture of planets orbiting
the Sun on concentric orbits, asteroids between Mars and
Jupiter and the occasional comet flying by. However, in the
inner solar system, we are immersed in a cloud of dust
that we see sometimes on the horizon as the zodiacal light
(Fig. 4) and sometimes in the direction opposite the Sun as
the gegenschein. [SeeSolarSystemDust.]
The zodiacal light is caused by the scattering of sun-
light off of small particles near the Earth’s orbit viewed