Nasa investigators are slowly building up a picture of how the Columbia space shuttle began to lose stability in the skies over Texas.

That missing link is out there, we just need to be persistent and go find it Ron Dittemore, shuttle programme manager

The latest information indicates the orbiter was firing its control jets to correct its flight - so great was the drag on the left side of the vehicle in the final minute before its loss.

The detail was released by shuttle programme manager Ron Dittemore at a media conference at the Johnson Space Center in Houston, Texas, on Monday.

The drag was increasing faster than the shuttle's autopilot could correct for the problem, he added.

	:	Shuttle disaster
How Columbia broke up over Texas

Mr Dittemore also gave extensive details of the analysis engineers undertook when they discovered the orbiter had been struck by insulation foam peeling away from the vehicle's external fuel tank on launch.

All the modelling suggested the falling foam did no significant damage to shuttle, he said.

Final minute

Investigators are trying to establish what happened to Columbia by studying its final data transmissions.

Already, this has revealed sensors on the left side of the vehicle were picking up temperature rises in the wheel well and on the upper part of the mid section of the main fuselage. Some sensors, it appears, were also failing altogether.

In addition, the data analysis indicates Columbia was experiencing increased drag on its left side, possibly as a result of lost or damaged heat-resistant tiles.

Tile material is designed to withstand more than 1,500 C

And in Monday's technical briefing at the Johnson center, Ron Dittemore was able to add that two yaw jets on Columbia's right side, which help control the attitude of the vehicle, were in use.

These jets fired for one and a half seconds during the final minute before contact was lost with the orbiter.

Mr Dittmore said Columbia's automatic flight systems were not keeping up with the increase in drag.

"It's not the absolute value of the attitude change; what is becoming interesting to us is the rate of change," he said.

"It appears that we were losing ground as far as the rate of attitude excursion was concerned. It was not long after that point that we lost all data and communication with the crew."

Different scenarios

Much speculation has centred on the possible damage Columbia might have sustained on launch when a chunk of insulation foam that surrounds the giant external fuel tank fell away, striking a glancing blow on the orbiter's left wing.

Mr Dittemore said that as soon as film of the event was obtained, engineers began to model the effects of the impact on the shuttle.

The Last Moments

What happened and when At 0858... the vehicle is also experiencing increased drag on its left side

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The engineers estimated the foam block weighed about 2.6 pounds (1.2 kilograms) and measured 20 inches by 16 by 6 (50 centimetres by 40 by 15).

Even when these values were altered and the angle of strike varied, the engineers could find no scenario in which significant damage would be done to the vehicle.

"The analysis predicted that even though you might have structural damage - by that I mean localised heating where you may have some effect on the structure in that area - you would not have sufficient damage to cause a catastrophic event nor impact the flying qualities of the vehicle."

Mr Dittemore said nothing they had found so far could pinpoint the actual cause of the disaster.

"We've made significant progress from Saturday to today," he said.

"That missing link is out there, we just need to be persistent and go find it."

The big breakthrough could come, he added, if crash debris - part of the wing or tiles - was recovered upstream from the primary impact area in New Mexico or Arizona.

Good shape

Earlier on Monday, Nasa officials in Washington said the crew on the International Space Station (ISS) were well able to survive for an extended period on the platform without a visit from a shuttle.

The only issue of real concern would be the tendency of the platform to lose height over time, said deputy associate administrator Mike Kostelnik. Periodically, the ISS needs to be pushed further out into space by an orbiter.

"It turns out, however, that we are in good shape with the altitude and the decay rate and we should not require a re-boost in this calendar year," he said.

A Russian Progress vehicle, carrying new supplies, will dock with the ISS on Tuesday.