The Herschel Space Observatory was launched from French Guyana by
an Ariane-5 rocket on May 14, 2009. It shared the rocket with another spacecraft called
which will map the cosmic microwave background radiation that remains from the early
After the launch phase, the two spacecraft separated from the rocket, and traveled
separately toward the location where they carried out their observations.
A Special Orbit
Herschel and Planck spent their first two 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 were
calibrated and checked out to make sure they were in perfect working order.
Each spacecraft then went 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 the Earth
varies from 1.2 to 1.8 million km. Small correction maneuvers need to be performed each
month to compensate for drift.
With its back to the Earth, Moon, and Sun, Herschel's telescope points outward into
the Universe without interference from the strong infrared radiation these bodies emit.
Herschel's telescope focuses light onto three instruments:
a camera and medium-resolution spectrometer sensitive to the wavelength range from
60 to 210 microns.
a camera and spectrometer sensitive to the wavelength range from 200-670 microns.
a very high resolution heterodyne spectrometer sensitive to 480-1250 and 1410-1910 GHz
(which corresponds to about 157-625 microns).
Keeping Its Cool
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
Herschel has been offering 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.