Infrared Astronomical Satellite (IRAS)

A highly successful orbiting infrared telescope which operated from launch on the night of 25 January 1983 until the supply of coolant ran out on 23 November 1983. It was a collaborative mission between NASA, which designed and built the telescope, the Netherlands Aerospace Agency, which was responsible for the basic satellite, and the UK, which was responsible for the day-to-day tracking and data reception. IRAS was a Ritchey-Chrétien telescope with mirrors made of beryllium rather than glass in order to withstand the low operating temperature. The diameter of the primary was 57 centimetres (22.5 inches). The telescope was cooled to 2 K by means of liquid helium. The detector consisted of an array of 62 elements, and filters were used for operation in wavebands centred on 12, 25, 60 and 100 micrometres. The different wavebands were used to distinguish between sources at different temperatures. The satellite orbit was oriented in the north-south direction and devised to rotate by about 1° per day so that it was always along the terminator with the telescope pointing away from the Sun. During the ten-month mission, 96 per cent of the sky was scanned twice so that an overall map of the infrared sky could be plotted. In addition, there was a spectrometer and a mapping facility for individual sources. A quarter of a million individual sources were detected, including stars, galaxies, dense interstellar dust clouds and some unidentified objects. Five comets were discovered. The first and brightest, Comet IRAS-Araki-Alcock, discovered in May 1983, passed within five million kilometres (3 million miles) of the Earth - the closest approach of any comet for 200 years.