By Pallab Ghosh BBC science correspondent

The issue of foam debris remains a thorn in Nasa's side

This mission was supposed to have started the shuttle fleet's return to flight. Instead there is relief that Discovery and crew have returned to Earth safely.

It seems like a journey that was beset with problems.

An ageing fuel sensor system held up the launch for months, pieces of gap filler were shaken loose during launch and, to top it off, the insulating blanket protecting the window was damaged.

The most serious incident, though, was when a suitcase-sized piece of foam was ripped from the fuel tank during launch, narrowly missing the spacecraft.

I think we might be seeing the last flight of the shuttle at the moment because of the problems that they've had Dr Andrew Coates, University College London

It was this very same problem that led to the destruction of the shuttle Columbia.

The first question Nasa's so-called "tiger team", who are analysing the mission, will have to answer is how did this happen again?

A more important question for the future of the shuttle programme, howver, is how quickly can they fix the foam problem?

Quick fix?

There are launch windows in late September and once again in November. But Dr Andrew Coates of University College London thinks it could take longer: "I think we might be seeing the last flight of the shuttle at the moment because of the problems that they've had," he said.

The US space agency faces more problems in the future

"There's not only the problem of with the foam insulation but there's also the problem with the ageing avionics - which was shown up before the launch, delaying take off.

"There was also the problem of thermal insulation around the commander's window. So I think in view of that it may well take some time before they fly again".

If Nasa does fix the foam problem quickly - it may well have the shuttle flying again by the end of this year or early in 2006.

HAVE YOUR SAY There appears to be little point in launching manned missions in space anymore Christian, NS, Canada Send us your comments

But even this short delay will add to the space agency's problems, according to Professor Andre Balogh of Imperial College, London.

"It's bad news for the shuttle and bad news for the International Space Station. The shuttle flights really ought to resume at a rate which can build the space station up to its expected standard," he said.

"However, it looks as though unless something else is done to the shuttle fleet to accelerate the rate at which they can be launched, the International Space Station will be in trouble."

Last year, Nasa said it could complete the space station in 28 missions. Some regarded that as an optimistic schedule. Now it is saying it could meet its commitments to its international partners with as few 15 missions.