ROSAT Guest Observer Programme Terminated

November 30, 1998

After eight years of successful operations the guest observer programme of the X-ray Astronomy Satellite ROSAT has been terminated. This decision was announced by the Max-Planck-Institut fŸr extraterrestrische Physik (MPE) in Garching near Munich, the leading scientific institution of the ROSAT project. The reason for this decision is the irreversible damage to the High Resolution Imager (HRI) due to an accident on 20 September, 1998. The HRI was the only operational focal plane instrument left in ROSAT's X-ray telescope after the Position Sensitive Proportional Counters (PSPC) essentially exhausted their gas supply in September 1994. Among all astronomy satellites in a near earth orbit ROSAT has had an extraordinarily long life. It exceeded the design lifetime by more than a factor of four.

The remaining gas in the PSPC tanks will suffice only for a two days of operation. MPE plans the final observation campaign, starting on 7 December 1998. The goal of this campaign will be to observe a few important astrophysical objects. Among them may be the Supernova 1987A, which was the target of ROSAT's first light on 16 June 1990.

The recent problems started on 28 April 1998 with the failure of the star tracker attached to the X-ray telescope; the star tracker was used for navigating the satellite. The remedy was to bring the star tracker of the Wide Field Camera (WFC) into the loop of the attitude control system (AMCS). This activity was initiated immediately and the DASA engineers were supported by experts from the ground station (DLR/GSOC), MPE and University of Leicester/Rutherford Appleton Laboratory. The task soon turned out to be more difficult than anticipated; one of the problems was the fine tuning between the software of the AMCS and that of the star tracker, the latter never designed to be incorporated into an active attitude control system.

As a result, the satellite suffered from many safe mode triggers over the summer, and only a few scientific observations could be performed during this period. At the end of August the situation had improved significantly by fine tuning the parameters of the AMCS software, and almost normal scientific operations were resumed. Shortly before a revised version of the AMCS software was uplinked an accident happened on 20 September: during a slew the pointing direction of the satellite came close to the sun; as a result the HRI was irreversibly damaged.

"It was a shock for all of us when we realized the damage", says Dr. Jacob Englhauser who carried out MPE's mission planning during the last few years. "But all ROSAT instruments have given us much more scientific data than we had ever hoped for."

ROSAT was initiated by MPE in 1975 and became a collaborative project between Germany, the United States and the United Kingdom in 1983. Germany built and operated the spacecraft and built the X-ray telescope including two X-ray cameras, the Position Sensitive Proportional Counters. The United States provided the launch with a Delta rocket and the High Resolution Imager as the third X-ray camera aboard, while the United Kingdom contributed the Wide Field Camera, operating in the extreme ultraviolet range. The data analysis, archiving and distribution was a joint effort of the ROSAT data centers: in Germany these are the Max-Planck-Institut fuer extraterrestrische Physik (MPE) and the Institut fuer Astronomie und Astrophysik at the University Tuebingen (IAAT), in the US the Goddard Space Flight Center (GSFC) and the Smithsonian Astrophysical Observatory (SAO), and in the UK the University of Leicester (UL) and the Rutherford Appleton Laboratory (RAL).

Dr. Ulrich Zimmermann, who organized the ROSAT data effort at MPE, expresses the gratitude of the MPE staff: "It has been a wonderful experience to have so many individuals and institutions working together in a smooth and productive way". ROSAT has enabled to make images of extended astrophysical objects with enprecedented assuracy:

Click on the image for a larger view Figure 1: The Cygnus loop is the remnant of a Supernova explosion which took place about 20.000 years ago. The ROSAT image shows the temperature structure of the explosion cloud in "X-ray colours". The temperatures range from 2 Mio K (red) to 10 Mio K (blue).

Click on the image for a larger view Figure 2: The ROSAT image of the famous Coma Cluster shows clouds of hot (100 Mio K) X-ray emitting gas of the main cluster and of a group of galaxies (bottom right) falling onto it. The X-ray observations allow to determine the mass of the hot gas and of the dark matter needed to hold the cluster gas together.

After the launch on 1 June 1990 ROSAT carried out the first All-Sky Survey with Imaging X-ray and EUV telescopes. This half year of operation led to the discovery of some 80 000 X-ray and 500 EUV sources. In the following 7 years about 9 000 fields in the sky were observed in a guest observer programme involving 650 principal investigators from 26 countries. In total, more than 150 000 X-ray sources have been detected raising the previously known number of sources by more than a factor of 20. Numerous discoveries have been made and more than 3 000 scientific papers with more than 4 000 scientists as co-authors have been published until now. Joachim Trümper, Director of MPE and Scientific Director of the ROSAT observatory comments: "This mission has been extremely rewarding for us and many scientists. It brought forth discoveries in almost all fields of astrophysics ranging from the moon and comets to the most distant quasars, from the tiny neutron stars to the clusters of galaxies as the largest physical objects in the universe."

The impact of ROSAT on astrophysics has been very strong -- and it will continue to be so. Currently, the utilization of the data archives at MPE, GSFC and UL is very high, and more than one ROSAT based publication appears in the scientific literature every day. ROSAT has not only pushed the window of the X-ray sky wide open; it has also paved the way for the large X-ray observatories of the future, i.e. NASA's Advanced Astrophysics Facility (AXAF) to be launched in spring 1999 and ESA's X-ray Multimirror Mission (XMM) to follow in early 2000. There will also be a successor in the German national space programme, ABRIXAS, to be launched in spring 1999 using a novel CCD camera. It will perform an All-Sky Survey in X-rays extending the ROSAT sky survey to higher energies.

International cooperation on ROSAT
Germany:

Satellite
Dornier/MBB, now DASA
Ground station
DLR/GSOC
X-ray mirrors
Carl Zeiss
Focal Plane Assembly
MPE
Two position sensitive proportional counters
MPE
German ROSAT data centre
MPE
German ROSAT EUV data centre
Astron. Institut, Univ. Tübingen
Overall Project Management
DLR/DARA

USA:

High resolution imager
Smithsonian Astrophysical Obs./GSFC
Launch (Delta 2)
NASA
Ground station back-up
NASA
US ROSAT Data Center
GSFC
US project management
NASA/GSFC

United Kingdom:

EUV Wide Field Camera
University of Leicester (UL) and the WFC Consortium
UK ROSAT Data Center
Rutherford Appleton Laboratory (RAL)
UK Guest Observer Centre & Survey Centre
UL
UK project management
RAL/UL

Contact: Joachim E. Trümper
Max-Planck-Institut für extraterrestrische Physik,
Garching/Germany
Phone: 49-89-3299-3559
-end-


Max-Planck-Gesellschaft

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