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24 Oct 2011

A big German spacecraft has made an uncontrolled fall from the sky.

The Roentgen Satellite (Rosat) re-entered the Earth's atmosphere between 01:45 and 02:15 GMT.

Just as for Nasa's UARS satellite, which plunged into the atmosphere in September, there was high uncertainty about the final moments of Rosat.

But if the timings are correct, any wreckage would probably have dived into the Indian Ocean - although no eyewitness reports have yet come in.

If anything did manage to make landfall, the likely areas to be affected would have been Myanmar and China.

What made the redundant German craft's return interesting was that much more debris was expected to survive all the way to the Earth's surface.

Experts had calculated that perhaps as much as 1.6 tonnes of wreckage - more than half the spacecraft's launch mass - could have ridden out the destructive forces of re-entry and hit the planet.

In the case of UARS, the probable mass of surviving material was put at only half a tonne (out of a launch mass of more than six tonnes).

The difference is due to some more robust components on the German space agency (DLR) satellite.

Rosat was an X-ray telescope mission and had a mirror system made of a reinforced carbon composite material. This mirror complex and its support structure were expected to form the largest single fragment in what could have been a shower of some 30 pieces of debris to make it through to the surface.

But again, as was the case with UARS, any Rosat wreckage was strongly tipped to hit the ocean, given that so much of the Earth's surface is covered by water.
Destructive design

UARS' final resting place was tracked to a remote region of the Pacific, north-east of the Samoan islands.

Rosat's operating orbit meant it could have come down anywhere between 53 degrees North and South latitude - a zone that encompasses the UK in the north and the tip of South America in the south.

Future spacecraft sent into orbit may have to meet stricter guidelines that limit the amount of debris likely to fall back on to the planet, but these rules are still some way from being introduced said Prof Richard Crowther, an expert on space debris and adviser to the UK Space Agency.

"Up until now we've designed satellites to survive the harsh environment of space, and we haven't given much thought to designing them for destructive re-entry," he told BBC News.

"But in future, we will have to consider whether we have got this balance right, and perhaps satellites should be designed in such a way that we can ensure more of what comes down is destroyed in the atmosphere and doesn't hit the surface.

"Unfortunately, there is a whole legacy of spacecraft - 50 years of satellites - and we are going to have to put up with this situation for quite some time, I'm afraid."
Science success

Rosat was launched in 1990 to survey the X-ray sky. It mapped more than 100,000 sources of this high-energy radiation. X-rays tend to come from the hottest and most violent parts of the cosmos, such as the regions around exploded stars and the "edges" of black holes.

The spacecraft worked for eight-and-a-half years before its star tracker failed and it could no-longer work out its position and point correctly. It was shut down in February 1999.

Tracking stations will typically witness the uncontrolled return of at least one piece of space debris every day; and on average, one intact defunct spacecraft or old rocket body will come back into the atmosphere every week.

Something the size of Nasa's UARS satellite is seen to enter uncontrolled perhaps once a year.

Much larger objects such as space station cargo ships return from orbit several times a year, but they are equipped with thrusters capable of guiding their dive into a remote part of the Southern Ocean.


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