Cryogenics in space - A review of the missions and technologies

Since the first liquefaction of He-4 and the discovery of superconductivity by H. Kamerlingh-Onnes (1908 and 1911), cryogenics and its applications ha ve come a long way. The continuous improvement of cryogenic equipment has m ade it easier and easier to achieve temperatures well below the liquefactio n point of nitrogen (77 K), either by means of cryogens (liquid gases such as Xe, H-2, O-2,O- N-2, He-4 and He-3) or by means of mechanical coolers. C ryogenic devices, such as sensors and cold electronics, have taken advantag e of the progress made in materials science, thereby offering a reliable an d effective solution to otherwise unsolvable problems. In the last 15 years, several spacecraft have employed cryogenic equipment, mostly in the context of astrophysics missions, targeting the electromagne tic radiation emitted by celestial objects over a wavelength range that it is difficult to cover from the ground. Such missions include IRAS (infrared Astronomical Satellite, launched in 1983), COBE (Cosmic Background Explore r, launched in 1989) and ISO (infrared Space Observatory launched in 1995). Several new missions are currently in preparation, including Herschel/Plan ck, SIRTF and the Next-Generation Space Telescope (NGST). In the higher tem perature range, between 100 and 10 K, many missions are already operational or under development. They include military reconnaissance satellites (suc h as Helios), Earth-observation satellites (Spot) and meteorological spacec raft (MSG, Meteosat Second Generation), with infrared detectors operating a t about 85 K.