New York: A small piece of insulation that broke off an external fuel tanks while space shuttle Columbia was taking off and hit its thermal tiles is emerging as the most likely culprit in its disintegration on re-entry into Earth but experts are puzzled how it could have done so much damage.

The tiles protect shuttle structure and crew inside, where at times temperature could rise to up to 3,000 degrees, by reflecting heat. Investigators are still unsure whether tiles had broken loose or developed cracks which led to the tragedy.

All seven astronauts on board, including India-born Kalpana Chawla, were killed when the shuttle disintegrated over Texas about 16 minutes before it was due to land.

The investigators are looking at sudden spike in temperature and a rise in wind resistance during Columbia's final minutes which suggest that thermal tiles might have been damaged during the launch. But they are not sure whether it was the result of insulation hitting the tile or there was some other cause.

They are looking for the "missing link," NASA said.

The insulation piece, which broke lose was 20 by 16 by 6 inches and weighed about three pounds. It broke off about 80 seconds after the blast off and some scientists at NASA thought it could have created a hole.

NASA did not realise that insulation had broken off immediately and came to know about it only the next day when the tapes were examined. After repeated evaluation, it came to the conclusion on the 12th day of the 16-day mission that it did not pose any danger to the shuttle.

Scientists now intend to redo the analysis. Officials said the evidence point to shooting up of the temperature on the left side along with greater wind resistance leading to auto pilot to quickly change course to restore balance before the craft broke up.

An astronomer had reported seeing debris shooting out of the craft over California about 15 minutes before the temperature sensors on the left wing stopped sending data.

Shuttle programme manager Ron Dittemore said on February 3 that engineering data shows a rise of 30 degrees to 40 degrees in the left wheel well about eight minutes before the spacecraft's last radio transmission.

The shuttle temperature rose to the normal 15 degrees on the right side over the same period, he said. All the readings came from sensors underneath the thermal tiles, on the aluminium hull of the craft.

But Dittemore said, the temperature rise does necessarily mean there was structural damage to the shuttle.

"You can't draw the conclusion from the left main gear or the wheel that we had a breach there," Dittemore said.

"If you had a breach there, it seems logical that the temperature would be higher than just 30 or 40 degrees from what we normally expect. These relatively small increases in temperature are telling us something. We're just trying to find out exactly what they're trying to tell us," he said.

Meanwhile, the debris were being collected but the area was so large large – more than 500 square mile – and number of parts being found so high, that it would be sometime before all pieces are picked up and examined for possible clues. Remains of the astronauts are also being picked up from several places.

"That missing link is out there. We just to go out there and find it," Dittemore said. "But we may never know the exact root cause and so we're gonna have to use our best judgment as to root cause. But we're gonna work our darnedest to figure that out and fix it."

Dittemore said investigators have recovered tile debris in Forth Worth, Texas, and are hoping to find more in New Mexico and Arizona.

But Bill Readdy, a NASA associate administrator, said the tiles are one theory that investigators are pursuing.

Engineers are also examining 32 seconds of computer data that came just before all communications with Columbia were lost. That data was previously ignored because it was considered flawed.

However, Dittemore said, getting information from that data will take more time than initially expected.

Even if the scientists had come to the conclusion that the tiles had been damage, little could have been done about it," Sally Ride, America's first woman in space and a member of the commission that investigated the Challenger disaster, told ABC Television network.

The Challenger shuttle exploded during lift off 17 years ago.

"There really isn't a way to inspect the bottom of the shuttle and there is no way to do anything about it even if you found there was tile damage," Ride said.

PTI