BioEd Online

NASA has grounded its space shuttle fleet after seeing foam chunks falling from Discovery's external fuel tank during its launch on 26 July.

The same problem doomed Columbia, which broke up while re-entering Earth's atmosphere on 1 February 2003. Investigations proved that a stray piece of insulating foam tore a hole in the shuttle's wing, allowing superheated gases to burn into the craft on its return.

Although there is no evidence that Discovery has been damaged in the same way, the recurrence of the problem is a crushing blow for the agency, which has spent more than two years attempting to make the shuttle safer. Engineers had changed the way the foam is applied to the external fuel tank, and also removed the insulating foam ramp that detached and struck Columbia.

"We have to do some more work here," said shuttle programme manager Bill Parsons at a briefing yesterday. "Until we fix this, we're not ready to go fly again."

NASA engineers would not speculate on how long it might take to solve the problem. But the next scheduled flight, for the Atlantis shuttle in early September, looks in doubt.

Such a delay would make it increasingly difficult to complete the International Space Station before the shuttle fleet is retired in 2010. Until the shuttles are flying again, the station will have to rely on Russian craft that can deliver only crew and supplies, rather than construction material.

Falling foam

The foam debris that fell off Discovery is estimated to be up to 84 centimetres long, making it only slightly smaller than the suitcase-sized chunk that doomed Columbia. It came from a foam ramp on the tank, which reduces air turbulence around cables and pipes. This had been redesigned after the Columbia accident to prevent debris shedding.

The lump of foam fell off the fuel tank about two minutes after launch, and was spotted by the new camera mounted on the tank, which relayed live pictures back to mission control. They clearly show that the debris did not hit Discovery.

Photographs taken by the crew just after the tank was jettisoned revealed where the foam had come from, and also showed two more patches where smaller bits had been lost.

After the Columbia accident, a review board recommended that NASA stop even small chunks of foam from falling during launch. Engineers found this an impossible task, and settled instead on ensuring that debris would weigh less than 14 grams. (see ' The long road to a safe launch'). However, the largest chunk seen this week was at least ten times that weight.

Turn around

The crew members have used Discovery's 50-metre-long inspection boom to check their craft for damage, and found nothing that could put their mission in jeopardy. Although the craft lost a 4-centimetre-long piece of insulating tile, this is not thought to pose a threat.

They also made Discovery go through a slow backflip so that the two astronauts in the space station could photograph its underside, before docking on 28 July.

Astronauts Steve Robinson and Soichi Noguchi will make their first space walk on 30 July, which will give them the chance to try out techniques for making repairs to the shuttle's tiles while it is in space.