The highly productive and long-lived ROSAT X-ray telescope guest observer mission, which detected a previously little-known world of pulsars, supernova remnants and galaxy clusters, has come to an end with the failure of the telescope's last working detector.
The satellite's High Resolution Imager (HRI), an X-ray camera built by the Smithsonian Astrophysical Observatory (SAO), suffered irreversible damage to its collecting plate after accidentally scanning too closely to the sun in September. This was an unavoidable event after ROSAT engineers in April lost control of the satellite's navigational system, which had deteriorated after eight years in space.
"ROSAT has provided us with much more scientific data than we ever hoped for," said Dr. Robert Petre, the U.S. ROSAT Project Scientist based at NASA's Goddard Space Flight Center (GSFC). "Among astronomy satellites in near earth orbit, ROSAT has had an extraordinarily long life. Clever tinkering by ROSAT engineers kept the satellite operational years beyond its expected life span -- even through these past few months."
ROSAT is short for Röntgen Satellite, named after Wilhelm Conrad Röntgen, the discoverer of X-rays. It was an X-ray observatory developed by Germany, Britain and the United States; launched by NASA in 1990; operated by the Max Planck Institute for Extraterrestrial Physics (MPE) near Munich; and utilized by scientists around the world. GSFC, with collaboration from SAO, served as the U.S. center for data analysis, archiving and distribution.
Scientists will make two final days of observations, starting on December 7, by using reserved gas and a second X-ray detector, called the Position Sensitive Proportional Counters (PSPC). The PSPC naturally exhausted its xenon gas supply in 1994 and has been inactive ever since. The two-day reserve gas will allow the PSPC to turn on and make one last observation of a few important astrophysical objects, such as Supernova 1987a, which was ROSAT's very first target in 1990.
ROSAT's lasting legacy will be its impact on X-ray astronomy. The telescope detected more than 150,000 X-ray sources, 20-times more than were previously known. More than 3,000 scientific papers have been published based on ROSAT observations so far. At least one ROSAT-based publication appears in the scientific literature every day.
"The ROSAT mission has been extremely rewarding for many scientists," said Joachim Trümper, MPE Director and ROSAT Scientific Director. "It has brought forth discoveries in almost all fields of astrophysics, ranging from the moon and comets to the most distant quasars -- from the tiny neutron star to clusters of galaxies, the largest physical objects in the universe."
ROSAT was the first observatory to detect X-rays from the moon. In the distant universe, ROSAT resolved virtually all of the cosmic X-ray background into discrete quasars and galaxies. ROSAT observations of supernova remnants and galaxy clusters were detailed enough to determine temperature structures of expanding gas clouds and the mass of hot gas and dark matter, respectively.
Among ROSAT's highlights:
- The detailed exploration of a million-degree, low density halo of gas surrounding the Milky Way galaxy.
- The detection of large halos of gas glowing in X-rays from virtually all comets passing near the sun, produced by the interaction the comet's gas and fast-moving subatomic particles in the solar wind.
- The detection of clusters of galaxies at a much larger distance than expected, leading scientists to question how such massive objects could form so early in the history of the universe.
- The detection of an isolated, nearby neutron star, which, according to previous theories, was a large enough star at one time to collapse into a black hole and has therefore led scientists to question how massive a star can get without its lifecycle ending in a black hole stage.
- Revolutionary discoveries about star formation, including the observation that a large fraction of young stars lie far away from "classical" star-forming regions, indicating that star formation is a more ubiquitous process than thought and that the X-ray emission from young stars plays a key role in the regulation of the star formation rate.
- The measurement of the total amount and distribution of dark matter in assemblages of galaxies, with X-ray emitting gas tracing the effect of gravity and showing that the distribution of dark matter is different from that of the galaxies as seen in visible light.
Analysis of existing ROSAT data is expected to continue for several more years, the results of which will influence the next round of large X-ray observatories: NASA's Advanced X-ray Astrophysics Facility (AXAF), to be launched in spring 1999; the German ABRIXAS (A Broad-band Imaging X-ray All-sky Survey), featuring a CCD camera to be launched in spring 1999; the Japanese/NASA Astro-E, scheduled for a February, 2000, launch; and ESA's X-ray Multimirror Mission (XMM), to be launched in spring 2000.
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