A 20-year-old Nasa satellite the size of a schoolbus is expected to come crashing back down to Earth at some point in the next few weeks. Should we be running for cover?
Mind the space junk as you head for the final frontier
The news that a 20-year-old Nasa satellite the size of a school bus is expected to come crashing back down to Earth at some point in the next few weeks will come as no surprise to Robert Dunn, a 57-year-old resident of Dixon, Wyoming.
On March 21 this year, Mr Dunn was hiking when he heard "an odd noise" on the otherwise tranquil prairie and stumbled on a helium tank from Zenit-3 SLBF, a Russian spacecraft that had been launched the previous month. Still hot to the touch, the 32kg titanium sphere, virtually intact, had created a 30cm crater in the snow-covered ground.
Not every piece of space junk, in other words, is neatly and safely incinerated upon re-entry to Earth's atmosphere - a fact made even more alarming by a timely report this week by the US National Research Council, which concludes that "the current orbital debris environment has already reached a 'tipping point'".
Perhaps, given that the US Space Surveillance Network is currently tracking more than 22,000 orbiting objects larger than 10cm, of which only 1,000 are operational spacecraft and the rest is junk, that should have been "fly-tipping point".
The NRC report, Limiting Future Collision Risk To Spacecraft, estimates there are a further half a million bits of junk smaller than 10cm, while "the number of particles smaller than 1cm probably exceeds tens of millions".
The problem, says the NRC, is that the cloud of space junk girdling the globe is now self-multiplying, continually colliding with itself and the occasional intact spacecraft - in 2005, for instance, a live Iridium communications satellite was destroyed by a defunct piece of runaway Russian hardware - creating more debris and leading to "corresponding increases in spacecraft failures".
Accidents are one thing, but Nasa and others pelt us deliberately with their junk. It was, for example, always part of the plan that the $750m Upper Atmosphere Research Satellite, launched by the Space Shuttle Discovery in 1991 to study the Earth's weather patterns, would return to sender as a flaming chunk of junk. And when it does, at some point towards the end of this month, it is expected to scatter debris along a 800km path, with a one in 3,200 chance of seriously ruining somebody's day.
Should we duck? Hard to say. Unhelpfully, for anyone who might think odds of 1 in 3,200 warrant taking cover of some sort, Nasa says UARS could impact anywhere in an area between 57 degrees north and south of the Equator - somewhere between northern Scotland, in other words, and the edge of Antarctica.
Space junk has been raining down on us since the space race began and redundant satellites and rocket parts make it back through the atmosphere more often than we realise. On its website, Nasa is breezily reassuring on the subject. Sure, "On average, one non-functional spacecraft, launch vehicle orbital stage, or other piece of catalogued debris has fallen back to Earth every day for more than 40 years," but don't panic. The majority of these objects, it says, "do not survive the intense re-entry environment".
Quite a lot do, however. In 1977, for instance, Kosmos 954, a nuclear-powered Russian spy satellite, fell out of orbit and onto Canada, where it strewed sizeable chunks of radioactive material along a fortunately sparsely populated 600km corridor. And two years later it was Nasa's turn, when its Skylab lost the will to orbit and scattered large pieces of itself over Western Australia (much of it on the town of Esperance, which tried unsuccessfully to fine Nasa Dh1,500 for littering).
Now, the NRC has told Nasa to appreciate the gravity of the situation, as it were, and clean up its space-debris act, chiefly by tightening up its "post-mission disposal standards" and, who knows, maybe even finding a way to take its rubbish home safely.
What goes up, after all, must come down. It isn't rocket science.