TWO astronauts will take a space walk today to inspect damage to the heat shield of the shuttle Discovery, hours after Nasa admitted that it had made a mistake in allowing their mission to go ahead.

Although Discovery safely docked with the International Space Station 122 miles above Earth after a spectacular flip manoeuvre to photograph the shield, plans to relaunch its sister shuttles Atlantis and Endeavour were on indefinite hold after Nasa ordered them to be grounded.

In comparison to the euphoria of the seemingly successful launch on Tuesday, an event intended to revive America’s space programme after the Columbia disaster in 2003, managers were once more in crisis mode.

Their three-strong shuttle fleet, they admitted, would never be fully safe to fly.

Questions mounted over whether the programme could weather another crisis of confidence after a near-catastrophic problem with the shuttle’s fuel tank was identified, posing exactly the kind of danger that provoked the burn-up of Columbia with the loss of seven lives.

“We will never be able to get the amount of debris shed by the tank down to zero,” Mike Griffin, head of Nasa, said after it emerged that an object seen tumbling from the tank two minutes into lift-off was a 33in slab of solid foam cladding that only narrowly missed gouging the spacecraft carrying seven astronauts.

Richard Berendzen, a Nasa consultant and professor of physics at American University in Washington DC, said: “At least momentarily, it’s a mammoth blow. It’s a shock, it’s embarrassing for Nasa, it’s a huge disappointment that they have not been able to fix the same problem that brought down Columbia. Two and a half years, close to a billion dollars, enormous effort and apparently it didn’t work.”

Colonel Eileen Collins, the shuttle’s commander, steered Discovery to a smooth rendezvous with the space station, breaking the monotony for its two resident astronauts, John Phillips and Sergei Krikalev, who have been alone in orbit for 105 days.

Engineers were still studying photographs yesterday of its underbelly taken during a 93-second back-flip before docking.

Studies of its nose cone and the leading edge of its wings — sections that will take the brunt of the 3,000C (5,430F) temperatures experienced during re-entry to Earth on August 7 — showed no sign of injury.

But experts were concerned by pictures that showed a fragment of Discovery’s heat shield missing from a point close to the aft landing-gear door — an area that could be susceptible to the super-hot gases of re-entry.

The missing piece appeared to be in an area that underwent repair long before lift-off, after it was accidentally knocked during ground-handling operations. Preliminary assessments were that there was no major cause for concern.

John Shannon, flight operations manager, said that the shuttle fleet had safely endured 15,000 “dings” — meaning small scratches, scrapes and pockmarks — in its 24 years of flight. “But we’re going to look real closely,” he promised.

Andrew Coates, of the Mullard Space Science Laboratory at University College London, said that it would have done more damage, because Discovery was moving faster at the time than Columbia was when its wing was hit by a slightly larger piece. They were “extremely lucky” that the debris that fell from the tank did not do more damage.

He believes that the cameras on board Discovery are sophisticated enough to examine it for damage, and that they have so far found nothing that will stop the safe return of the crew.

“They will be able to detect any damage serious enough to be a risk,” he said. “Unless they find it, I expect the crew to be able to return to Earth safely.”

The big implication of the grounding order, assuming Dr Coates’s assessment is correct, is to the future of the International Space Station.

“It needs at least 20 more flights by shuttles to complete the space station,” he said. “The Russian Soyuz is fine for food and water but the shuttle is needed for bigger items needed to complete the station.”

In the worst-case scenario, if the shuttle is deemed too dangerous for a return journey a relay of Soyuz craft — which can carry only three astronauts — would have to be launched to retrieve the crew.

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