Defunct German space telescope ROSAT completes death plunge back to Earth

by Chris Bergin

Going out in style, the German X-ray satellite ROSAT has bid farewell to space via a fiery death plunge into the atmosphere in the early hours of Sunday. Interest was high in ROSAT due to its primary mirror, which held the potential of surviving the break up of the spacecraft in the atmosphere during entry. So far, no reports of debris hitting the ground have been reported.


The space telescope – created out of a joint German (DLR), US and British X-ray astrophysics project – may be grabbing the headlines around the world this weekend for its return, but it’ll mainly be remembered for its successful primary mission.

Launched on June 1, 1990, – via a Delta II launch vehicle out of Cape Canaveral – ROSAT was initially tasked with an 18 month mission, which resulted in the discovery that almost all astronomical objects emit X-ray radiation, including some objects where this had not been expected.

Among its badges of honor, ROSAT achieved the first ever full-sky survey with an imaging X-ray telescope, the resolution of the cosmic X-ray background into individual sources, observation and analysis of the hot gas that fills the gravitational potential of galactic clusters, the observation of supernova remnants, the discovering of ‘super-soft sources’ in the Large Magellanic Cloud and, finally, the surprising discovery that comets also emit X-ray radiation.

Although the mission had a provision for up to five years of operations, ROSAT continued its work for over eight years, finally shutting down on February 12, 1999.

Surpassing its mission life was always going to result in the spacecraft suffering major hardware failures, although it continued to valiantly provide scientific results during this period.

Engineers on the ground helped extend ROSAT’s life, such as In 1993 – following the failure of gyros that control the orientation of the satellite in space – a new kind of control system was implemented. This system used the direction of the Sun and the Earth’s magnetic field to determine the spacecraft orientation.

Then, in 1994, the gas supply required for the measurements conducted by the Position Sensitive Proportional Counter (PSPC) ran out, after which observations were performed using only the High Resolution Imager (HRI), an instrument that required no consumables.

Finally, in 1998, the mission came to an end. The failure of a star tracker caused the remaining HRI detector to point directly at the Sun, causing irreversible damage. Since no further scientific use could be made of the satellite, it was shut down on February 12, 1999, more than eight and a half years after its launch.

With ROSAT racing around lifeless on orbit, with no propulsion system on board that could be used to alter its orbit or re-entry trajectory and no longer able to communicate with DLR’s control centre in Oberpfaffenhofen, its eventual re-entry was always going to be at the mercy of physics.

Ever since arriving in space – and as with all orbital objects – friction in Earth’s upper atmosphere had been causing ROSAT to lose altitude continuously.

Such returns usually do not cause problems, as returning satellites usually fully disintegrate during Entry-Interface, leading to most of the fragments burning up in the extreme heat caused by atmospheric friction. However, ROSAT had one major component – namely the primary mirror, made of several barrel shaped mirrors assembled in to one barrel-like construction – which was designed to resist burn up.

Weighing roughly 400 kg, this mirror was made of high-temperature resistant materials, namely glass ceramics, which led to engineers noting it had the potential to survive entry, thus becoming a threat to land. With no way to control ROSAT’s return into an unpopulated disposal corridor – such as the one in the Indian Ocean previously used by the Shuttle’s External Tanks (ETs) – ROSAT’s entry was closely monitored.

Although the precise area where ROSAT finally succumbed to Entry has yet to be accurately plotted, the German Aerospace Center (DLR) have noted that Entry will have occurred between 01:45 GMT and 02:15 GMT on Sunday morning.

Most of the 2,426 Kg spacecraft will have disintegrated, with the primary mirror being the only main concern out of a total of 30 pieces with the potential to survive all the way to the ground. However, in the hours after entry, German space officials noted there had been no confirmation of pieces of debris reaching Earth’s surface.

The focus on ROSAT was raised due to the amount of media attention gained by another recent returnee to Earth, NASA’s defunct Upper Atmospheric Research Satellite (UARS).

UARS – which was launched onboard Space Shuttle Discovery on September 12, 1991, as part of the STS-48 mission – consisted of at least 26 large pieces which were expected to survive re-entry. As with ROSAT, entry was uncontrolled, and held the potential to return almost anywhere in the populated areas of the planet – as much as the risks to humans was always extremely low.

That low risk proved to be correct, as the satellite re-entered the atmosphere on September 24, with surviving hardware – had there been any – splashing down in a remote area of the Pacific Ocean.

The threat is, however, being taken seriously by NASA, as the Agency’s FOIG team held a Special Safety Topic review – just prior to UARS return – to evaluate the hazards posed by space hardware fragmentation during re-entry, with the aim to apply mitigation to any potential risks from hardware breaking up and surviving entry, in turn threatening human life on the ground.

According to the Special Safety Topic meeting’s findings – which reviewed all the major uncontrolled re-entries since the Apollo era – the key to providing additional mitigation against risks to the public will be found via improving computer models on how hardware is expected to disintegrate during entry.

“Improving re-entry fragmentation models can help ensure that planned (or contingency) disposal of on orbit hardware does not pose a hazard to the public,” noted the presentation (available in L2). “For first entry of new program (unproven) hardware, tracking and interception helps to ensure a safe trajectory and provide confirmation of the above models.”

Such mitigation – the review concluded – will provide a safety net for those vehicles which may suffer an issue and an uncontrolled re-entry.

(Images: Via L2, NASA, DLR, ESA)

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