NASA calculates a 1-in-3200 chance of UARS causing injury or damage. But at the end of October or beginning of November, ROSAT – a 2.4-tonne X-ray telescope built by the German aerospace lab DLR and launched by NASA in 1990 – will re-enter the atmosphere, presenting a 1 in 2000 chance of injury.
The higher risk stems from the requirements of imaging X-rays in space, says DLR spokesperson Andreas Schütz. The spacecraft's mirrors had to be heavily shielded from heat that could have wrecked its X-ray sensing operations during its eight-year working life. But this means those mirrors will be far more likely to survive a fiery re-entry.
At the European Space Agency in Darmstadt, Germany, the head of the space debris office, Heiner Klinkrad, agrees that ROSAT's design means more of it will hit the surface. "This is indeed because ROSAT has a large mirror structure that survives high re-entry temperatures," he says
US Strategic Command tracks all space objects and the US-government-run Aerospace Corporation lists both upcoming and recent re-entries on its website. But ROSAT is not yet on the upcoming list because its re-entry time is far from certain.
The moment a craft will re-enter is difficult to predict because it is determined by two main factors.
First, the geometry of the tumbling satellite as it enters the upper atmosphere, which acts as a brake.
Second, the behaviour of the upper atmosphere itself, which grows and shrinks with the amount of solar activity, says Hugh Lewis, a space debris specialist at the University of Southampton, UK.
"Solar activity causes the atmosphere to expand upwards, causing more braking on space objects. The reason UARS is coming back sooner than expected is a sudden increase in solar activity. Indeed, we expect to see a higher rate of re-entries as we approach the solar maximum in 2013," he says.
Sourse newscientist
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