|Faulty heat tiles most likely cause of disaster
Signs of spacecrafts disintegration suggest part of the heat shield was ripped away on re-entry writes Robert Matthews, Sunday Telegraph Science editor
The most likely cause of the break-up of the Columbia is the failure of its heat-resistant tiles.
More than 17,000 silica tiles cover the base of the Shuttle, providing the only protection between the astronauts and the searing heat of re-entry into the Earths atmosphere.
From the earliest days of the Shuttle programme, there has been concern about the ability of engineers to find a fail-safe way to glue each of the tiles in place.
The main fear was that if a few tiles came off, they would expose their neighbours to the heat, melting the glue and causing many others to be dislodged - leaving the base of the Shuttle unprotected.
Inspection of Columbia after it made the first flight in 1981 revealed that hundreds of the tiles had been damaged, mostly by debris during launch and landing. Scientists are investigating reports that at least one tile was seen to fall off the base of the Shuttle during its launch last month, possibly damaging the wing.
Information from Russian space monitoring stations also suggests that there was no ``explosion - rather a disintegration, consistent with the theory that one or more tiles failed, and allowed the others to be ripped off.
Although this theory is considered the most consistent with the early facts, there are many other possibilities.
One is a fuel pipe failure. Last June, Nasa grounded its entire Shuttle fleet after finding 11 hairline cracks in fuel lines in the tail-fin of another Shuttle, Atlantis. Engineers found similar cracks in the other Shuttles, including Columbia.
The likely cause was a manufacturing fault when the Shuttles were first constructed in 1981. This was quickly rectified. Even so, the potential for a catastrophic failure of the fuel lines shocked Nasa, which has since instructed engineers that visual inspections of key components are no longer sufficient.
Although it cannot yet be discounted, it is unlikely that sabotage was involved.
Nasa has played down such a likelihood, although the presence aboard of Ilan Ramon, Israels first astronaut, had raised fears about the trip being a possible target for terrorists. Because of Col Ramons presence, Nasa had imposed unprecedented security measures on all aspects of the flight. None the less, the possibility of an explosive package somehow finding its way onto the Shuttle cannot be completely ruled out.
With disaster striking while the craft was apparently still 200,000 feet - or 38 miles - above the Earth, crash investigators will face an enormous challenge in establishing whether an explosive was used.
It is virtually certain that the Shuttle was not brought down by a missile, given the spacecrafts altitude and its speed of about 12,500mph.
Aerodynamic stress is another possible cause, although it is likely to be ruled out quickly.
While re-entry is second only to launch in terms of the stresses placed on a spacecraft, the high altitude at which disaster struck appears to auggest that air pressure was not to blame; at 200,000 feet, there is very little atmosphere. Potentially dangerous turbulence and deceleration does not start to affect the Shuttle until it is at less than half this altitude.
The only known case of an explosion caused from inside a manned spacecraft in flight happened on the Apollo 13 moon shot in 1970. The cause was traced to faulty electrical circuitry in a fuel cell that overheated. The Shuttle uses three 12-kilowatt fuel cells, which produce all onboard power using hydrogen and oxygen. Their potential for detonation will make them a focus of the investigation.
Another possible cause is an incorrect re-entry angle - making the Shuttle more vulnerable to the extreme temperatures. A computer malfunction could conceivably cause such an error.
Whatever is established as the cause, the disaster is the biggest blow to hit the US space programme since the Challenger Shuttle disaster of January 1986.
The resulting inquiry led to the suspension of all spaceflights by Nasa for the next
two-and-a-half years. So long a break in launches could spell doom for the much-criticised
International Space Station _ at pounds 60 billion, the most expensive object ever built.
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