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Mission Description
The Herschel Space Observatory is scheduled to be launched from French Guyana by
an Ariane-5 rocket in 2007. It will share the rocket with another spacecraft called
"Planck,"
which will map the cosmic microwave background radiation that remains from the early
universe.
After the launch phase, the two spacecraft will separate from the rocket, and travel
separately toward the location where they will ultimately carry out their observations.
A Special Orbit
Herschel and Planck will spend their first four months traveling about 1.5 million
kilometers (about 931,000 miles, roughly four times the distance of the moon) from
earth, in the opposite direction from the sun. During that journey, they will be
calibrated and checked out to make sure they're in perfect working order.
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Lagrange Points
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Each spacecraft will then go into a separate orbit around the
Earth-Sun L2 point,
a relatively stable location where the gravitational pulls of the Earth and the Sun
combine to keep spacecraft in a uniform position relative to Earth as they orbit the sun.
As it orbits L2 at an amplitude of about 700,000 km, Herschel's distance from Earth will
vary from 1.2 to 1.8 million km. Small correction maneuvers will be performed each month
to compensate for drift.
With its back to the Earth, Moon, and Sun, Herschel's telescope will point outward into
the Universe without interference from the strong infrared radiation these bodies emit.
It will focus light onto three instruments:
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PACS,
a camera and medium-resolution spectrometer sensitive to the wavelength range from
80 to 210 microns.
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SPIRE,
a camera and spectrometer sensitive to the wavelength range from 200-670 microns.
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HIFI,
a very high resolution heterodyne spectrometer sensitive to 480-1250 and 1410-1910 GHz
(which corresponds to about 157-625 microns).
Keeping Its Cool
Keeping
HIFI,
SPIRE,
and
PACS at a temperature near absolute zero is critical
to Herschel's mission for two reasons: the detectors work only at very low temperatures,
and heat from the instruments could drown out the faint far-infrared and submillimeter
light they were designed to detect. Maintaining that ultracold temperature depends on the
superfluid helium that serves as the coolant.
The cryostat that houses the instruments is expected to hang on to enough coolant to enable
Herschel to perform its scientific observations for at least three years. At some point after
that, the helium will evaporate into space, the instruments will warm up, and the mission
will end.
Herschel is expected to offer about 7,000 hours of science time per year.
Communication with the spacecraft, to receive data and convey instructions, will be done via
the ground station in Perth, Australia.
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