Spitzer will study some of the well-known planets and their moons, in addition to
the comets, asteroids and dust scattered throughout our Solar System. The
Observatory will also devote considerable time to studies of the circumstellar
("surrounding a star") disks of dust and gas found around nearby stars, which
are thought to eventually form "extrasolar" planetary systems.
The Outer Planets
Objects like the Sun and Moon are too bright and would saturate the sensitive
solid-state detectors on Spitzer. The primary planetary targets in the Solar
System will be the outer planets, where no robotic exploration is anticipated
in the foreseeable future. Spitzer will barely resolve details on Uranus and
Neptune, but imaging and spectroscopic observations should be able to detect
temperature variations and chemical compositions within their atmospheres.
The diameter of Pluto is smaller than a Spitzer pixel, and hence the planet will be
spatially unresolved. Observations will be limited to global characterizations
of the icy planet's thermal properties.
Spitzer will be able to study and characterize the largest moons of Jupiter, Saturn and the other outer planets. Particular emphasis will be directed towards Titan, the largest natural satellite of Saturn. This intriguing moon has an atmosphere and will be visited by the Cassini spacecraft in
2004, during the Spitzer prime mission. Observations by Spitzer will help to establish the global context for the results from Cassini and its Huygens probe, and will help scientists understand the composition and large-scale changes in the Titan atmosphere. Observations of Neptune's largest moon, Triton, will also be undertaken with Spitzer.
Since comets are reservoirs for primeval material surviving from the time of
Solar System formation, they offer an intriguing target for Spitzer. The
Observatory will examine comets at large distances from the Sun. Studies will
include the structure and composition of cometary dust and ices and their
comparison with similar dust grains and ice particles found in other
astrophysical realms. In 2003, comet Encke will pass within 0.2
AU of Spitzer,
offering an unusual opportunity for the Observatory to conduct infrared
observations that can be integrated with other measurements obtained from Earth
and from the CONTOUR spacecraft.
There are 11,000 named asteroids with known orbits, and four times as many with preliminary orbit determinations. However, only 2000 have measured diameters and albedos, mostly with diameters in excess of 10 km. It is estimated that up to a few million kilometer-sized asteroids exist in the Main Belt between Mars and Jupiter. The large variation in reflected (visible) light from asteroids complicates any attempt to characterize their size distribution. Therefore, Spitzer will rely on measurements of thermal infrared emission to obtain better data for asteroids with diameters smaller than 10 km.
|Kuiper Belt Object 1993 SC
PPARC/NOW, Isaac Newton Telescope,
A. Fitzsimmons et al.
Kuiper Belt Objects
The Kuiper Belt is found beyond the orbit of Neptune and contains perhaps
100,000 faint and icy bodies. This distant reservoir is the source of comets
with short orbital periods and highly eccentric orbits. Astronomers have
discovered only a few hundred of these Kuiper Belt Objects (KBOs) to date.
Due to their faintness, KBOs are difficult to detect through reflected
sunlight. Spitzer will instead directly detect their faint thermal emission and
determine their albedos and thermal/physical properties.
|Zodiacal Dust and comet
Interplanetary dust is found throughout the Solar System and is the result of
comets and asteroid collisions. This zodiacal dust is found within the same
ecliptic plane in which planets orbit the Sun. Studies of this dust are not
only intrinsically interesting, but are necessary in order to "filter out" its
foreground thermal infrared emission when studying faint objects outside the
Solar System. A unique feature of the Spitzer orbit is that the Observatory
will traverse through a dust cloud that is trailing the Earth as it circles
the Sun, providing an unusual opportunity to characterize the structure and
evolution of the dust. Moreover, studying the influence of larger bodies
(such as planets) on the dust cloud morphology is important in interpreting
results from observations of circumstellar disks around nearby stars.
Circumstellar Disks and Extrasolar Planets
The InfraRed Astronomical Satellite (IRAS) discovered the presence of dust disks surrounding a handful of nearby stars. These circumstellar disks are thought to be a common feature of stellar evolution and of planetary system formation. No single telescope currently in operation has adequate spatial resolution (or visual acuity) to directly detect planets around other stars. [JPL is developing concepts for a Terrestrial Planet Finder capable of directly imaging planets within ten years.]
|Disk around Beta Pictoris
ESO/VLT, J.L. Beuzit et al.
|Disk around HD 141569
NASA/HST/Weinberger et al.
|Artist depiction of Spitzer studies of a circumstellar disk around Vega.
Spitzer will be able to detect and characterize circumstellar disks around nearby
stars, providing key information about the formation of 'extrasolar' planetary
systems. Attempting to see the faint disk at visible wavelengths is extremely
difficult because of the bright light from the parent star. However, the
relative difference is reduced in the infrared, where Spitzer will conduct its
measurements. The Observatory will study hundreds of nearby stars in order
determine the frequency of these disks. It will also use imaging and
spectroscopy to search to characterize the spatial structure and composition of
the disks. These data will prove invaluable in addressing the frequency and
nature of planetary systems outside our own.