US President George W Bush has vowed to continue America's space programme, following the Columbia shuttle disaster which claimed the lives of seven astronauts.

"America's journey into space will go on," the president said, after discussions with Nasa's chief administrator on the investigation into what went wrong.

TALKING POINT

This is not a time for arguments, we can learn from our mistakes and go ahead Ibraheem Bayan, India

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Mr Bush's words echoed the sentiments of the astronauts' families, who issued a joint statement urging space exploration to continue.

As authorities continued to retrieve debris from the shuttle, Nasa officials said latest evidence showed a sudden surge in temperature on the shuttle's left side as it re-entered Earth's atmosphere, indicating a possible problem with the spacecraft's protective tiles.

Nasa investigators said the shuttle had to use its jets to correct its flight - so great was the drag on the left side of the vehicle in the minutes before its loss.

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

SHUTTLE BREAK-UP

Re-entered atmosphere at 12,500 mph (20,000 km/h) Disintegrated 40 miles (65 kilometres) above the Earth Debris scattered over Texas and Louisiana

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

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

All the modelling suggested the event presented no safety risk to the shuttle, he said.

Earlier, President Mr Bush paid tribute to Columbia's crew, saying they would be "remembered for their achievements, their heroism and their sense of wonder".

"Their 16-day mission held the promise of answering scientific problems that elude us here on Earth," he said.

In a statement released through Nasa, the astronauts' families said that despite their deep grief "the bold exploration of space must go on."

Nasa said its primary responsibility was to recover the remains of the crew and return them to their relatives.

Remains of some of the astronauts have already been found.

Monumental task

Nasa officials said the sudden rise in temperature and strong winds caused the shuttle's automatic pilot to make the most adjustments ever for a shuttle on re-entry.

First flight: 1981 Orbiting speed: 17,500 mph Landing weight: 105 tonnes Crew (for this mission):7

Columbia: Space pioneer

The shuttle's left wing was hit by a falling piece of insulating foam during Columbia's launch on 16 January, which might have damaged the tiles.

Search teams from about 30 agencies are involved in the hunt for pieces of the craft, which fell into woodland, back gardens and reservoirs after breaking up some 40 miles (65 kilometres) above the Earth.

Pieces landed over at least a 100 mile area of east Texas and Louisiana, from fragments to chunks of twisted metal as big as cars.

Officials plan to eventually assemble the parts and reconstruct the shuttle to try to establish what happened in its final moments.

But collecting the remains is a monumental task: authorities have so far identified more than 1,200 debris sites in Nacogdoches, Texas, alone.

"There is no way to describe how many pieces there are and how spread over the landscape they are," said James Kroll, of the geospacial mapping centre at Stephen F Austin State University in Nacogdoches.

"Ten years from now, folks are going to be walking around the woods and finding stuff," he said.

'Unique disaster'

As speculation over what caused the accident continued, Nasa officials rejected accusations that agency chiefs ignored a series of safety warnings.

SHUTTLE CREW

Commander Rick Husband, US Pilot William McCool, US Michael Anderson, US David Brown, US Kalpana Chawla, US Laurel Clark, US Ilan Ramon, Israel

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"Our focus is painfully clear," said Nasa Deputy Administrator Mike Kostelnik. "It's all about the people who fly and fix and maintain and design these operation vehicles," he said.

"This is a unique disaster that is not comparable," he said.

The disaster has raised questions about the future of the 16-nation International Space Station (ISS).

However, Russia on Sunday went ahead with the launch of an unmanned cargo vessel to serve the ISS.