ROSAT

File:Rosat.jpg

ROSAT

ROSAT (short for Röntgensatellit) was a German X-ray satellite telescope. It was named in honour of Wilhelm Röntgen. It was launched on June 1 1990 with a Delta II rocket from Cape Canaveral, and operated until February 12 1999.

Overview

The Roentgensatellit (ROSAT) was a joint German, US and British X-ray astrophysics project. ROSAT carried a German-built imaging X-ray Telescope (XRT) with three focal plane instruments: two German Position Sensitive Proportional Counters (PSPC) and the US-supplied High Resolution Imager (HRI). The X-ray mirror assembly was a grazing incidence four-fold nested Wolter I telescope with an 84-cm diameter aperture and 240-cm focal length. The angular resolution was <5 arcsec at half energy width. The XRT assembly was sensitive to X-rays between 0.1 to 2 keV. In addition, a British-supplied extreme ultraviolet (XUV) telescope, the Wide Field Camera (WFC), was coaligned with the XRT and covers the wave band between and 6 angstroms (0.042 to 0.21 keV). ROSAT's unique strengths were high spatial resolution, low-background, soft X-ray imaging for the study of the structure of low surface brightness features, and for low-resolution spectroscopy. The ROSAT spacecraft was a three-axis stablized satellite which can be used for pointed observations, for slewing between targets, and for performing scanning observations on great circles perpendicular to the plane of the ecliptic. ROSAT was capable of fast slews (180 deg. in ~15 min.) which makes it possible to observe two targets on opposite hemispheres during each orbit. The pointing accuracy was 1 arcminute with stability <5 arcsec per sec and jitter radius of ~10 arcsec. Two CCD star sensors were used for optical position sensing of guide stars and attitude determination of the spacecraft. The post facto attitude determination accuracy was 6 arcsec.

The ROSAT mission was divided into two phases: (1) After a two-month on-orbit calibration and verification period, an all-sky survey was performed for six months using the PSPC in the focus of XRT, and in two XUV bands using the WFC. The survey was carried out in the scan mode. (2) The second phase consists of the remainder of the mission and was devoted to pointed observations of selected astrophysical sources. In ROSAT's pointed phase, observing time was allocated to Guest Investigators from all three participating countries through peer review of submitted proposals. ROSAT had a design life of 18 months, but was expected to operate beyond its nominal lifetime.
  — NASA

See also: X-ray astronomy

Highlights

• X-ray all-sky survey catalog, more than 150000 objects
• XUV all-sky survey catalog (479 objects)
• Source catalogs from the pointed phase (PSPC and HRI) containing ~ 100000 serendipitous sources
• Detailed morphology of supernova remnants and clusters of galaxies.
• Detection of shadowing of diffuse X-ray emission by molecular clouds.
• Detection of pulsations from Geminga.
• Detection of isolated neutron stars.
• Discovery of X-ray emission from comets.
• Observation of X-ray emission from the collision of Comet Shoemaker-Levy with Jupiter.

Launch

ROSAT was originally planned to be launched on the Space shuttle but the Challenger disaster caused it to be moved to the Delta platform.

Failures and end of life

Originally designed for a 5 year mission, ROSAT continued in its extended mission for a further 4 years before equipment failure forced an end to the mission. For some months after this, ROSAT completed its very last observations before being finally switched off on 12 February 1999.

There is some recent controversy over the precise causes of ROSAT's demise.

On 25 April 1998, failure of the primary star tracker on the X-ray Telescope led to pointing errors that in turn had caused solar overheating. A contingency plan and the necessary software had already been developed to utilise an alternative star tracker attached to the Wide Field Camera.

ROSAT was soon operational again, but with some restrictions to the effectiveness of its tracking and thus its control. It was severely damaged on September 20 1998 when a reaction wheel in the spacecraft's Attitude Measuring and Control System (AMCS) exceeded its maximum rotational speed, losing control of a slew, damaging the High Resolution Imager by exposure to the sun. This failure was initially attributed to the difficulties of controlling the satellite under these difficult circumstances outside its initial design parameters. A reaction wheel operates by changing its rotational velocity, conservation of angular momentum then causing the more massive satellite to rotate in opposition. Their maximum speed is limited by design, which in turns means they are limited in the velocity they can impart to a satellite. "Reaching maximum speed" thus means merely that it cannot impart any more velocity change, not that it's approaching mechanical damage to itself.

As of April 2009, the satellite continues to orbit approximately 390 km above the Earth.

Allegations of cyber-attacks causing the failure

Ten years later in 2008, NASA investigators were reported to have found that the ROSAT failure was linked to a cyber-intrusion at Goddard Space Flight Center. This was also reported through Bruce Schneier's blog, a highly-regarded commentary on IT security issues.

The root of this allegation is a 1999 advisory report by Thomas Talleur, senior investigator for cyber-security at NASA. This advisory is reported to describe a series of attacks from Russia that reached computers in the X-ray Astrophysics Section (i.e. ROSAT's) at Goddard, and took control of computers used for the control of satellies, not just a passive "snooping" attack. The advisory stated:

"Hostile activities compromised [NASA] computer systems that directly and indirectly deal with the design, testing, and transferring of satellite package command-and-control codes."

The advisory is further reported as claiming that the ROSAT incident was "coincident with the intrusion" and that, "Operational characteristics and commanding of the ROSAT were sufficiently similar to other space assets to provide intruders with valuable information about how such platforms are commanded,". Without public access to the advisory, it is obviously impossible to comment in detail. However it does seem to describe a real intrusion, there is a plausible "no attack" explanation for ROSAT's failure, and the report is claimed to link the two incidents as no more than "coincident". IT security remains a significant issue for NASA, other systems including the Earth Observing System having also been attacked.

External links

• "The ROSAT Mission". Max-Planck-Institut für extraterrestrische Physik.
• "UK ROSAT Guest Observer Centre". University of Leicester, Department of Physics & Astronomy. {{cite web}}: External link in |publisher= (help)
• "A live view of where ROSAT is".