Even if NASA's 6-tonne UARS satellite does not cause any injury or damage when it re-enters the Earth's atmosphere today, there is more space junk headed our way next month. A defunct German space telescope called ROSAT is set to hit the planet at the end of October – and it even is more likely than UARS to cause injury or damage in populated areas.
No one yet knows where UARS (Upper Atmosphere Research Satellite) will fall to earth. Although most of the craft's mass will be reduced to an incandescent plasma, some 532 kilograms of it in 26 pieces are forecast to survive – including a 150-kilogram instrument mounting.
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.
Broken mirror, bad luck
On its ROSAT website, DLR estimates that "up to 30 individual debris items with a total mass of up to 1.6 tonnes might reach the surface of the Earth. The X-ray optical system, with its mirrors and a mechanical support structure made of carbon-fibre reinforced composite – or at least a part of it – could be the heaviest single component to reach the ground."
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.
ROSAT was deactivated in 1999 and its orbit has been decaying since then. "ROSAT does not have a propulsion system on board which can be used to manoeuvre the satellite to allow a controlled re-entry," says space industry lawyer Joanne Wheeler of London-based legal practice CMS Cameron McKenna. "And the time and position of ROSAT's re-entry cannot be predicted with any precision due to fluctuations in solar activity, which affect atmospheric drag."
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.
But don't expect it to be raining spaceships – what's coming down is partly a legacy of 1990s space-flight activity. "Some of the re-entries we see today [with UARS and ROSAT] are a heritage of years with high launch rates, which were a factor of two higher than they are today," says Klinkrad.
"The trend is towards smaller satellites, with more dedicated payloads," he says, rather than "all-in-one" satellite missions on giant craft like UARS. That means debris from future missions should be smaller.
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