Cryogenic applications: Cooling largest outer-space telescope

In his presentation on cryogenics applications at the IIR-AFF centenary conference in Paris (see Focus), Philippe Lebrun, President of the IIR's Section A, mentioned cryogenic detectors of cosmic signals and cited Herschel probe as an example. The Herschel telescope is a scientific mission launched by the European Space Agency (ESA) dedicated to the observation of the universe in the infrared und submillimetric realms (wavelength from 60-670 µm), an as yet mainly unexplored part of the electromagnetic spectrum. Measuring 9 m x 4 m and weighing over 3 tonnes, it will be launched on October 31, 2008, from Kourou, Guyana. Equipped with a 3.5-m diameter mirror, it will be the largest outer-space telescope ever constructed. The main aims of the mission are related to the origins of the universe as it will first probe molecular clouds which are considered as veritable young star nurseries, in order to understand the first stages of star formation. It will then map the universe in order to discover galaxies on the point of taking shape, in order to gather elements on the history of the universe from the Big Bang until now. In order to do this, it is equipped with HIFI, an ultra-high resolution spectroscope, and two cameras (SPIRE and PACS). Observing theses wavelengths requires instruments cooled to extremely low temperatures, close to absolute zero (-273°C), partly in order to compensate for the instruments' own thermal emissions that are likely to create undesirable pollution. The satellite will thus be equipped with a 2400-litre superfluid helium reserve supply, which can maintain the focal plane at -271°C; the detection planes require even lower temperatures (around 300 millidegrees Kelvin) which are obtained thanks to a very complex system implying several cryorefrigerators. One of the technical challenges involved is that unlike other satellites, the instrumental plane is on the top of the satellite and not submerged.