Nasa has grounded future shuttle flights because a big chunk of insulating foam flew off Discovery'sfuel tank during lift-off, as it did with Columbia, but apparently missed the spacecraft.
While there are no signs the piece of insulation damaged the spacecraft, Nasa is grounding future shuttle flights until the hazard can be fixed.
Early signs were that Discoverywas safe for its return home, however.
A large chunk of foam flew off Discovery'sredesigned external fuel tank just two minutes after what initially looked like a perfect lift-off on Tuesday.
Nasa shuttle programme manager Bill Parsons
But in less than an hour Nasa had spotted images of a mysterious object whirling away from the tank.
"Call it luck or whatever, it didn't harm the orbiter," said shuttle programme manager Bill Parsons said last night.
If the foam had broken away earlier in flight, when the atmosphere is thicker increasing the likelihood of impact, it could have caused catastrophic damage to Discovery.
Mission managers did not realise what the object was - or how much havoc it would cause to the shuttle program - until yesterday, after reviewing video and images taken by just a few of the 100-plus cameras in place to watch for such dangers.
Officials do not believe the foam hit the shuttle, posing a threat to the seven astronauts when they return to Earth on August 7th. But they plan a closer inspection of the spacecraft in the next few days to be sure.
"You have to admit when you're wrong. We were wrong," Mr Parsons said. "We need to do some work here, and so we're telling you right now that the . . . foam should not have come off. It came off. We've got to go do something about that."
The loss of a chunk of debris, a problem Nasa thought had been fixed, represents a major setback to a space programme that has spent two-and-a-half years and nearly $1 billion trying to make the 20-year-old shuttles safe to fly.