The loss of a piece of insulation foam from a fuel tank during Tuesday's launch bore a worrying resemblance to Columbia's tragic story and means -- despite the space agency's reassurances -- that Discovery's crew will face a nervous return to Earth on August 7.

While National Aeronautics and Space Administration (NASA) leaders have said Discovery does not appear to have suffered serious damage, they do admit it was the wrong decision let the launch go ahead.

"We decided it was safe to fly as is. Obviously we were wrong," shuttle programme manager Bill Parsons said.

NASA administrator Michael Griffin acknowledged that NASA had been fortunate with Discovery but was unable to say when the next shuttle mission would be. Atlantis was to have staged the next flight in September.

"We will simply never be able to get the amount of debris shed by the tank down to zero," a disappointed Griffin told NBC television. "We are trying to get it down to a level that cannot damage the orbiter."

In Columbia's case, the foam from an external fuel tank pierced the shuttle's protective outer skin and let in super-heated gases that turned the space vessel into an inferno as it reentered the Earth's atmosphere.

On June 28, a group of experts named by NASA warned the agency had not been able to fully meet the conditions for a return to space set by the official inquiry into the Columbia disaster.

The so-called Return To Flight group said "The external tank still sheds debris that could potentially cripple an orbiter. The hard fact of the matter is that the external tank will always shed debris, perhaps even pieces large enough to do significant damage to the orbiter."

The experts faulted NASA's work on the foam. "The extensive work to develop debris models and transport analysis was, until recently, hampered by a lack of rigour in both development and testing," the said.

In Tuesday's launch, at least three pieces of foam came off the shuttle, including one about 80 centimetres (31 inches) by 35cm (14 inches), slightly smaller than the piece which caused Columbia's destruction.

Fortunately for Discovery's crew, most of the debris came off two minutes after the blast off at an altitude where the atmosphere is so thin that there is a reduced risk of damage to the shuttle. The foam just drifted away.

"Call it luck or whatever, it didnt harm the orbiter," commented Parsons.

In Columbia's case, the foam came off about 82 seconds after the launch. The air slowed the debris before it could clear away from the shuttle which was just reaching full speed.

Wayne Hale, deputy manager of the shuttle programme, said that if the foam had fallen off Discovery earlier "we think it would have been really bad."

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