The launch of the ROentgen SATellite (ROSAT) into space on 1 June 1990 marked the start of a mission that would allow researchers to perform an all-sky survey of X-ray sources with an imaging telescope for the first time. X-rays arise in the Universe by unusually hot, high-energy processes, which often entail extreme states of matter such as black holes or neutron stars.
Determining the energy distribution of X-ray radiation is important to understand these processes. Furthermore, the position of the X-ray source in the sky needs to be deduced as accurately as possible to investigate extended structures, as well as to obtain supplementary information about the X-ray source in other wavelengths. These observations cannot be performed from the ground because our atmosphere absorbs X-ray radiation. To investigate radiation sources in space, a telescope had to be placed in Earth orbit.
The ROSAT project is a collaborative venture between Germany the United States and the United Kingdom, and was developed, built and launched on behalf of and under the leadership of the German Aerospace Center (DLR). The German Space Operations Center (GSOC) later took control of the satellite in close coordination with the collaborating research institutes.
ROSAT, the largest X-ray observatory of its time, was intended to fulfil two objectives during its planned 18-month mission; during the first six months, researchers at the Max-Planck Institute for Extraterrestrial Physics (MPE) in Garching near Munich performed an all-sky survey and compiled a space atlas in the X-ray regime. Then, ROSAT was to carry out detailed observations of selected X-ray sources over the course of one year.
Since the satellite was, not only operational after the end of its official mission period, but still detecting X-ray sources, the mission was extended several times. The satellite detected about 80,000 cosmic X-ray sources and an additional wide-field camera on board added another 6000 sources in the extreme ultraviolet regime. During its eight years of operation, more than 4000 scientists from 24 countries took advantage of the opportunity to commission and analyse measurements. In each case, the data was released into a public domain archive one year after each observation.
As early as January 1991, one of the two Position Sensitive Proportional Counter (PSPC) detectors and one filter of the British wide-field camera were destroyed by overheating. In 1994, the fuel supply ran out on the second PSPC, and observations with this detector had to be terminated. However, observations of X-ray sources continued with the US High Resolution Imager (HRI).
The failure of the primary star tracker in 1998 caused the HRI to point directly at the Sun, causing irreversible damage. Following that incident, on 12 February 1999, more than eight and a half years after its launch, the X-ray satellite telescope ROSAT was abandoned and shut down.
|Launch date||1 June 1990|
|Launch site||Cape Canaveral, Florida, USA|
|Launch vehicle||Delta II rocket|
|End of mission||12 February 1999|
|Control Centre||German Space Operations Center (GSOC), Oberpfaffenhofen|
|Ground station||DLR's ground station in Weilheim|
|Launch mass||2,426 kilograms|
|External dimensions||2.20 metres x 4.70 metres x 8.90 metres|
|Orbit around Earth||Originally at an altitude of approximately 580 kilometres with an inclination of 53°|
|Payload||a) X-ray telescope:|
Four Wolter mirrors: largest diameter 83 centimetres, focal length of 2.4 metres, angular resolution better than five seconds of arc
Sensitivity range of telescope: approx. 0.1 to 2.4 keV
Detectors: Two Position Sensitive Proportional Counters (PSPC); one microchannel plate detector (High Resolution Imager, HRI)
b) Wide-field camera (WFC) for observations in the extreme ultraviolet range