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Tuesday, 4 February, 2003, 07:16 GMT
Columbia's 'Achilles' heel'
Columbia coming in to land
Attention now focuses on the wheel well door

With reluctance, the US space agency (Nasa) is being drawn to the idea that the piece of foam insulation which broke off the external fuel tank some 80 seconds after launch and struck Columbia's left wing may have been the cause of the disaster.

It may have damaged the shuttle in one of its weakest points - its "Achilles heel" - according to one engineer.

Repair work to Columbia's tiles
In-flight repairs to the tiles would have been hazardous
Nasa says its best engineers initially concluded that the foam fragment's impact on the wing would not have any significant effect on the mission's safety.

Even if a few tiles had been damaged, Nasa concluded that even though there may have been some damage to Columbia during re-entry, there was no question that the space shuttle itself could be lost.

In fact, Nasa's engineers concluded during the flight that it could have caused damage ranging from removal of a single tile to removal of tiles over an area of approximately 20 cm by 80 cm (7 by 30 inches).

But this probably would not have led to a catastrophic burnthrough, they believe.

Vulnerable spot

However, despite this analysis, the foam fragment is now officially the prime suspect in the disaster.

Some engineers think that the foam fragment hit Columbia near the seam of a wheel-well door - one of the most vulnerable spots on the shuttle.

Work on one of Columbia's wheel wells
The wheel well door could be the key to the disaster
The wheel-well door is the only part of Columbia's underside blanket that opens to allow the landing gear to deploy, a few moments before touchdown.

Some are speculating that damage to the tile gap around the wheel-well door caused the fatal damage that was considered statistically unlikely by those engineers.

Nasa investigators are considering a series of reports describing several small fiery objects falling off the shuttle during re-entry, followed by a much bigger object as Columbia passed over California.

Shuttle engineers know that tiles are often damaged by debris - both foam and ice - falling off the external fuel tank.

Speaking on American TV, one Nasa engineer said that there had been flights in which, on return, "half the tiles on the vehicle were damaged".

This speculation makes the recovery of debris from the left wing all-important. Already engineers have begun examining the wheel-well area on one of the intact grounded shuttles.

No return

The possibility that Columbia may have gone into space with unrecognised, and fatal, damage is raising some issues over the way shuttle missions are flown.

The crew of Columbia had no way of assessing any potential damage to the underside of their spacecraft.

Columbia's tiles
The detached tile may have caused a freak accident
They had no TV cameras which could look in the area.

A spacewalk, even with the right equipment, underneath the shuttle is regarded as too risky.

In previous shuttle missions, spy satellites or high-powered telescopes on the ground have peered at the shuttle to look for damage, but they only have a broad view and cannot see individual tiles.

And even if the crew had been aware that Columbia was severely damaged in a way which made re-entry impossible, there was probably nothing they could have done about it.

As things stand at the moment, a rescue shuttle could not have been launched in time unless Nasa was lucky and also threw away the rule book.

In the early days of the space shuttle programme, Nasa considered having a tile repair kit onboard each mission but abandoned the idea because it considered that such repairs in space would not work.

Accidents highlight failures and destroy previous assumptions. The Challenger accident did that in 1986. It remains to be seen if any of these issues will be addressed during the Columbia enquiry.

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