ROSAT

ROSAT

File:Rosat.jpg
COSPAR ID	1990-049A
SATCAT no.	20638
Website	[1]
Start of mission
Launch date	1 June 1990

ROSAT (short for Röntgensatellit, in German X-rays are called Röntgenstrahlen, in honour of Wilhelm Röntgen) was a German Aerospace Center-led satellite X-ray telescope, with instruments built by Germany, the UK and the US. It was launched on 1 June 1990, on a Delta II rocket from Cape Canaveral, on what was initially designed as an 18 month mission, with provision for up to 5 years of operation. ROSAT actually operated for over 8 years, finally shutting down on 12 February 1999.

In February 2011, it was reported that the 2,400 kg (5,291 lb) satellite was unlikely to burn up entirely while re-entering the Earth's atmosphere due to the large amount of ceramics and glass used in construction. Parts as heavy as 400 kg (882 lb) could impact the surface intact. ROSAT eventually re-entered the Earth's atmosphere on 23 October 2011.

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

Instruments

X-ray Telescope (XRT)

The main assembly was 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 (33 in) diameter aperture and 240 cm (94 in) 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.

Position Sensitive Proportional Counters (two) (PSPC)

Each Position Sensitive Proportional Counter (PSPC) is a thin-window gas counter. Each incoming X-ray photon produces a electron cloud whose position and charge are detected using two wire grids. The photon position is determined with an accuracy of about 120 micrometers. The electron cloud's charge corresponds to the photon energy.

High Resolution Imager (HRI)

The US supplied was also used a crossed grid detector with a position accuracy to 25 micrometers. The instrument was samaged by solar exposure on 20 September 1998.

Wide Field Camera (WFC)

The Wide Field Camera (WFC) was a British-supplied extreme ultraviolet (XUV) telescope co-aligned with the XRT and covered the wave band between and 6 angstroms (0.042 to 0.21 keV).

Highlights

Earth's Moon on 29 June 1990 by ROSAT

• X-ray all-sky survey catalog, more than 150,000 objects
• XUV all-sky survey catalog (479 objects)
• Source catalogs from the pointed phase (PSPC and HRI) containing ~ 100,000 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.

Catalogues

• 1RXS - an acronym which is the prefix used for the First ROSAT X-ray Survey (1st ROSAT X-ray Survey), a catalogue of astronomical objects visible for ROSAT in the X-ray spectrum.

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. This move made it impossible to recapture ROSAT with a Shuttle and bring it back to Earth.

Failure 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 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 20 September 1998 when a reaction wheel in the spacecraft's Attitude Measuring and Control System (AMCS) reached 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.

Re-entry into the Earth's atmosphere

In 1990, the satellite was put in an orbit at an altitude of 580 km (360 mi) and inclination of 53°. Since that time, due to atmospheric drag, the satellite slowly lost height until, in September 2011, the satellite was orbiting approximately 270 km (168 mi) above the Earth, and was expected to re-enter the Earth's atmosphere on or around 23 October 2011.

On 23 October 2011 the German Aerospace Center released a statement confirming that ROSAT had re-entered the Earth's atmosphere sometime between 1:45 UTC and 2:15 UTC. At that time there was no confirmation if pieces of debris had reached the Earth's surface.

Allegations of cyber-attacks causing the failure

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 satellites, 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

• "ROSAT". German Aerospace Center DLR.
• "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)
• "ROSAT reentry twitter feed".
• 1RXS Catalog site