NASA has put future Shuttle missions on indefinite hold, after two chunks of foam fell from Discovery's external fuel tank during its launch this week.

The falling debris is not thought to have damaged the space-plane, but the fact that NASA has not managed to rectify the problem that led to the loss of Columbia is more worrying for the agency. Engineers have worked for two-and-a-half-years, and spent more than a billion dollars, to improve the safety of the Shuttle, including redesigning the external fuel tanks.

NASA administrator Michael Griffin said that the flight has been designed as a test flight. In a statement, he said: "Among the things we are testing are the integrity of the foam insulation and the performance of new camera equipment installed to detect problems. The cameras worked well. The foam did not."

Meanwhile, Shuttle program manager Bill Parsons was more candid, bluntly acknowledging in a news conference that the agency had got it wrong, and said it was "luck" that the foam chunk didn't damage the orbiter.

The Shuttle Atlantis was due to lift off on its own mission in September. That is now on hold indefinitely. NASA says it will not fly again until it has solved the problem. Parsons said: "I don't know when that might be, so I'll just state that right up front. We're just in the beginning of this process of understanding."

Shuttle is due to retire anyway in 2010, but if the flights don't resume, the International Space Station will have to carry on managing with service visits from Russian spacecraft. The Russian vehicle does not have the same capacity as Shuttle, so building work on the station is likely to be held up further. ®

Related stories

NASA investigates falling debris
Shuttle actually lifts off
Shuttle: no launch this week, engineers still baffled