Written by an expert panel convened by the US National Research Council, Protecting the Space Shuttle from Meteoroids and Orbital Debris, warns that debris impacts that penetrate the leading edge or underside of a shuttle wing or fuselage might not be immediately critical or detectable.
But it goes on: "The consequent thermal heating on re-entry could have a 'blowtorch' effect inside the wing that causes loss of flight control or failure of the primary structure resulting in the loss of the vehicle." In the moments before Columbia disintegrated over Texas, its left wing appeared to became abnormally hot, for as yet unexplained reasons. Also, the shuttle's flight computer was struggling to maintain control as it entered the upper atmosphere.
NASA commissioned the NRC report because shuttle managers noticed an increase in debris damage to windshields, payload bay doors and rudder parts in some missions up to 1996.
The components most vulnerable to impacts, the NRC reported, are the reinforced carbon-composite leading edges of the wings, the rest of the wing and the wing's "elevon"control surfaces. It cites a study by Boeing that says the wing as a whole accounts for at least 40 per cent of the risk of a catastrophic failure.
It wouldn't take a particularly large piece of debris to fatally damage a shuttle. With 1997 shuttle technology, an object 1 millimetre across travelling at 10 kilometres per second would have been able to penetrate the leading edge of the wing. In an overhaul of Columbia two years ago, NASA added an insulation layer designed to allow the wing's leading edge to survive impacts of 2 to 3-millimetre objects.
The smallest objects that can be tracked reliably measure 10 centimetres across. The US Air Force Space Command tracks more than 9000 such objects, and can tell shuttle pilots when to get out of their way. But there are 20 times as many particles too small to track but still big enough to endanger the shuttle.
Most space debris in the 1-millimetre to 10-centimetre range is junk from human activity, including fragments of rockets and satellites, shrapnel from tests of anti-satellite weapons and aluminium oxide chunks from solid fuel boosters.
A shuttle crew would have no warning of smaller objects, and would not even notice a small but potentially lethal impact unless they heard it, says William Schonberg, a member of the NRC panel and an aerospace engineer at the University of Missouri-Rolla.
NASA told the NRC that it was aiming to keep the risk of a critical failure on any mission due to space debris impact below 1 in 200. It also presented results from a computer model that estimated the actual chances of a critical failure due to debris bigger than 5 millimetres at 0.1 in 200. But the NRC panel felt NASA did not fully understand the debris problem and the magnitude of any errors in its risk calculations (see the report at www.nap.edu/books/0309059887/html).
NASA has investigated how best to position the shuttle in orbit to shield its most vulnerable areas. Nevertheless, the agency's orbital debris programme was almost closed last year. It survived, but its budget was halved.
New Scientist issue: 15th February 2003
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