This is a post written by the Young Astronomers Editor JosephD
Last month NASA’s Upper Atmosphere Research Satellite (UARS) came crashing down somewhere in the Pacific (no doubt much to the disappointment of the media who seemed obsessed about the chance that it might hit somebody), and as I write this there are reports that the Roentgen Satellite (ROSAT) satellite belonging to DLR, the German space agency, has also fallen out of orbit. Both of these craft had reached fulfilled their objectives and become redundant, and frightening though it may be for us on the ground, re-entry is greatly preferable to leaving the disused craft in orbit.
In 1957 Sputnik became Earth’s first artificial satellite. From that point on we’ve send thousands of craft into orbit between 6,500 (Low Earth) and 42,000 (Geostaitionary) kilometres up.
But like UARS and ROSAT, a great many of those have become redundant and now form part of a blanket of ‘space junk’ which, according to a recent report commissioned by NASA, has now reached critical levels and is posing a very serious threat to functioning satellites as well as the International Space Station.
Space Debris Credit: Wikimedia
This is not a new phenomenon either, in 1995 a US committee on space debris wrote that “The threat that orbital debris poses to international space activities is presently not large, but it may be on the verge of becoming significant. If and when it does, the consequences could be very costly – and extremely difficult to reverse.”
Since all of the craft in orbit were designed to withstand the extreme forces of launch as well as the exotic conditions of space, it may seem odd that a slight bump can do so much damage. Orbiting by its very definition involves travelling so fast that the Earth falls away faster than the satellite falls to Earth (so that it just keeps going round), somewhere in the region of 8km/s. At these immense speeds, even the smallest specks of material can do huge amounts of damage, breaking delicate components or knocking a satellite out of alignment. When inspected after their return to Earth, all of the Space Shuttles showed evidence of tiny impacts which had the potential to damage their ceramic heat-shielding.
Though most craft have some facility for manoeuvring in orbit, the problem is that every movement requires the use of precious fuel, and as the amount of debris increases, more and more movement are required. Obviously there comes a point when the craft no longer has enough can no longer avoid this debris, and for many satellites that point is not far away.
In early 2009 a private American Iridium Communications satellite collided with a defunct Russian Kosmos military satellite at a speed of about 12km/s (42,000 km/h, 26,000mph). Not only did the collision destroy both satellites, but in the process thousands of pieces of debris were created. Though this was the first collision of two intact satellites, it is unlikely to be the last.
Space Fence, a group of very high frequency radar stations operated by the US Space Surveillance Network tracks orbital debris, and similar programs are operated by other agencies across the world. There are estimated to be about half a million objects larger than a centimetre and tens of millions larger than a millimetre, ranging from discarded rocket components to blobs of frozen liquid. Though this tracking data exists, national military agencies that collect the data are rarely keen to share it, and two years after the Iridium/Kosmos collision, the US Air Force is still unsure about how much of its database to make available to other groups.
As the risks posed by this debris have increased, so has the call to do something about it. Part of the problem lies in the fact that each country is currently only allowed to salvage its own objects, since many space projects involve technology that national agencies would prefer not to be public knowledge. In an effort to mitigate the growth of the debris field, space agencies are working to create functionality that allows satellites to de-orbit themselves at the end of their lifespan, either by directly attempting re-entry or by moving into a decaying orbit that will cause the satellite to gradually burn up in the atmosphere. To deal with the existing debris, many proposals have been put forward – catching the debris in nets, collecting it with giant magnets, and blasting it with a ‘laser broom’ – but there is currently no unified clean-up strategy.
At the moment space debris, though prolific, does not pose enough of a threat to significantly affect our current satellites or our plans for the future – but as the 1995 committee said, unless we deal with the problem in the near future then the consequences will not only be very costly but extremely difficult to reverse.
Sigma Orionis 