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1.1 Mission Overview


[Aschenbach et al.1985], [ROSAT AO-21991, NRA 91-OSSA-31991]

ROSAT, an acronym for the German word Röntgensatellit, resulted from a proposal made by the Max-Planck-Institut für extraterrestrische Physik (MPE)  to the Bundesministerium für Forschung und Technologie (BMFT) in 1975.  The original version of the project entailed an all-sky X-ray survey to be carried out with a moderate angular resolution ( tex2html_wrap_inline16335 ) imaging telescope.     Between 1977 and 1982, extensive studies of the project were carried out by German space companies in prephase-A and phase-A studies.   Following the regulations of the ESA convention, BMFT offered collaboration on ROSAT to the ESA member states in 1979.   The University of Leicester   proposed a wide-field XUV camera (WFC)  to be flown alongside the main X-ray telescope (XRT) in order to extend the spectral bandpass to lower energies. The XRT covers the tex2html_wrap_inline16337 Å ( tex2html_wrap_inline16339 keV) band, and the WFC covers the tex2html_wrap_inline16341 Å ( tex2html_wrap_inline16343 keV) band. In 1982, an agreement between BMFT and NASA concerning U.S. participation in the ROSAT project was reached:    in return for 50% of the pointed observing time being made available to U.S. principle investigators (PIs),   NASA agreed to provide a high-resolution imager (HRI)  for the focal plane of the XRT as well as to launch the satellite on board the Space Shuttle.  Agreement was reached between BMFT and SERC in 1983:   in return for 12% of the pointed observing time being made available to U.K. PIs, SERC would supply the WFC and associated sub-systems. The Challenger accident on 1986 January 28 brought significant changes to the spacecraft design and a delay in the launch. In 1987 December, the decision was made to launch ROSAT with an expendable Delta-II launch vehicle,  rather than the Space Shuttle.   ROSAT was finally launched into low Earth orbit (Tab. 1.1) from Cape Canaveral on 1990 June 1.    



Launch 1990 June 1
Nominal Lifetime 18 months
Initial Altitude 580 km
Inclination tex2html_wrap_inline16345
Period 96 minutes
Precession 66 days
Table 1.1: Orbital characteristics of ROSAT

Spacecraft operations  are controlled from the German Space Operations Center (GSOC)  ground station at Oberpfaffenhofen, Germany.    Contact occurs for tex2html_wrap_inline16347 minutes on five or six contiguous orbits out of 15 orbits per day. Thus there are groups of contacts over an tex2html_wrap_inline16349 hour period followed by a tex2html_wrap_inline16351 hour period of no contact. 

The main aim of the ROSAT mission    was the first all-sky survey (1990 August to 1991 January)    with imaging X-ray and XUV telescopes, possessing an X-ray sensitivity of about a factor of 1000 higher than that of UHURU [Voges1992, Snowden and Schmitt1990].    During that mission phase,  the satellite scanned the sky continuously along great circles roughly perpendicular to the earth-sun direction. Thus sky strips of 2 tex2html_wrap_inline16353 and 5 tex2html_wrap_inline16355 width (for the XRT/PSPC and the WFC instruments, respectively) and 360 tex2html_wrap_inline16357 length were observed during each orbit.  Due to the applied scan law, these sky strips lay perpendicular to the ecliptic plane and their intersection with the ecliptic plane precessed with a rate of about 1 tex2html_wrap_inline16359 per day (roughly following the solar motion).     In this way the ecliptic poles received up to 50ks exposure while sky fields near to the ecliptic plane were seen for some hundreds of seconds during the half year survey period. About 60,000 sources have been detected in the survey,   an order of magnitude greater than were known before the ROSAT mission. No further all-sky survey is planned in the foreseeable future, thus the ROSAT results will strongly influence the observations of the next generation of large X-ray missions like XMM,    AXAF,    and ASTRO-E,    which will fly near the end of this decade.

The survey was followed by pointed observation phases,  where the 3-axis stabilized observatory investigated selected targets over the entire sky. Usually the observation times during these mission phases are appreciably longer than during the survey. In very deep exposures this resulted in point source sensitivities  up to a factor 10 higher than achieved by Einstein     with appreciably better spatial and spectral solution.

On the whole, ROSAT has had few significant operational difficulties.   By far the most severe problem occurred in early 1991 when there was an almost fatal on-board computer glitch which sent the spacecraft (S/C) tumbling out of control, destroying one of the PSPCs and severely damaging the WFC\ (see Sect. 1.2.2).  A number of subsystems have also failed: the STC-2 star tracker (Sect. 1.2.5) and the Y- and Z-gyros (Sect. 1.2.3). For a full mission history see App. A.

Further information on the ROSAT mission, its history, and payload, can be obtained from Trümper [Trümper1982] and the ROSAT Mission Description [ROSAT AO-21991, NRA 91-OSSA-31991].

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