HERSCHEL: AN OVERVIEW OF THE SPACECRAFT

Since its launch on May 14^th 2009 Herschel is the largest telescope of its kind in space. Herschel’s 3.5‐metre diameter mirror collects long‐wavelength infrared radiation from some of the coldest and most distant objects in the Universe. Herschel is the only space observatory to cover the spectral range from far‐infrared to sub‐millimetre wavelengths. The Herschel satellite is 7.5 metres high and 4 metres wide, with a launch mass of approximately 3.3 tonnes. The bulk of the spacecraft consists of a superfluid liquid helium cryostat inside which three scientific instruments (PACS, SPIRE & HIFI) are operated. Via various cooling stages the instruments are cooled to between a few tens and a few tenths of a degree above absolute zero. This paper gives a brief overview of the spacecraft and the on‐ground testing, with an emphasis on the cryogenic issues encountered.