Herschel will be stationed at the second Sun-Earth Lagrange point (L2), 1.5 million km from Earth. This point is theoretically stationary in space with respect to the Earth and Sun, which means that for Herschel, Earth and the Sun will always be in the same general direction.
This provides a stable thermal environment and a good view of the sky. Since the Earth is far away, Herschel is not disturbed by its radiation belts.
At the same time, the spacecraft's position in space and the resulting temperature extremes make the task of optimising the work environment for the instruments a challenge.
The Herschel spacecraft has heritage from the successful ESA Infrared Space Observatory (ISO). It has been improved and optimized for a more distant and more favourable orbit and its complement of instruments.
The Herschel satellite is composed of three sections.
First is the telescope, which has a 3.5 m-diameter primary mirror protected by a sunshade. The telescope focuses light onto three scientific instruments; their detectors are housed in a giant thermos flask, known as a cryostat.
The cryostat provides the interface and cryogenic environment for the instrument focal plane units, and supports the telescope, the solar array and telescope sunshade, and a unit of the Heterodyne Instrument for the Far Infrared.
Inside the cryostat, Herschel's detectors are kept at very low and stable temperatures, necessary for the instruments to operate. The cryostat contains liquid superfluid helium at temperatures lower than –271°C, which makes the instruments as sensitive as possible. The instruments detectors and the cryostat make up the second section, the payload module.
The infrared detectors must be cooled to extremely low temperatures in order to work, in fact close to absolute zero (–273.15°C or 0 K).
All three Herschel instruments will be housed inside and cooled by the cryostat which is filled at launch with more than 2300 litres of superfluid helium kept at 1.65 K, i.e. –271.5°C. Further cooling – down to 0.3 K – is required for the SPIRE and PACS bolometeric detectors. The role of the cryostat is fundamental because it determines the lifetime of the observatory.
The superfluid helium evaporates at a constant rate, gradually emptying the tank. It is expected to evaporate completely about four years after launch.
When it has all gone, the temperature of the instruments will start to rise and Herschel will no longer be able to perform observations. However, the data that Herschel will have supplied will keep astronomers busy for decades.
The third element of the satellite is the service module located below the payload module. It houses the instrument electronics and the components responsible for satellite function, such as the communication hardware. The service module houses the payload electronics that do not need cooling, and provides the necessary subsystems: power, attitude and orbit control, on-board data handling, thermal control and command execution, communication, and safety.