Wait goes on as weather forces delay in return of 'Discovery'

"We're going to wave you off for 24 hours," was how a mission controller broke the news of the delay to the seven-person crew aboard Discovery, as it prepared to complete the first shuttle flight since the Columbia disaster of February 2003.

Technically, the postponement reflects official policy that the shuttle should only land if there is at least five miles (8km) of visibility for the approach to the runway and no rain, lightning or thunderstorms within 35 miles. But it is another sign of how Nasa is taking absolutely no chances with this most scrutinised of shuttle flights.

First, lift-off was held up by problems with a fuel gauge sensor on one of the external tanks. Then crew members carried out unprecedented repairs in space to dislodge fillers between the craft's heat resistant ceramic tiles, vital to protect it in the searing temperatures during re-entry into Earth's atmosphere.

It was damage to these tiles by a piece of insulating foam which fell away from an external tank during lift-off, that doomed Columbia. In an unnerving repeat, a chunk of foam fell off a tank during Discovery's launch on 26 July. This time, however, it missed the shuttle.

Even so, Nasa announced that the fleet has been grounded until the insulating foam problem has been solved once and for all. The next mission, of the shuttle Atlantis, has been pencilled in for 22 September. But Nasa officials admit that date is highly optimistic.

Despite the hitches, officials insist that Discovery is entirely safe to make its expected return today. There are four "windows" for a landing today - at either 5.07am US East Coast time (10.07am British time) or 6.43am (11.43am) in Florida, at 8.12 am (1.12 pm) at the primary backup site in California, followed by a final opportunity an hour and a half later at the last ditch option of White Sands air force base in New Mexico.

"There's no agony," Michael Griffin, Nasa's administrator, said after the postponement. "We're going to land [today] one way or another, one place or another, and all we're talking about is where." For the first time in the 24 years that shuttles have flown some 110 missions, engineers know the exact condition of the tile shield - and have pronounced it excellent.

Nonetheless, in one final precaution, Discovery's commander Eileen Collins was adjusting the shuttle's orbit slightly so that if it had to land in California, it would not overfly Los Angeles. The Columbia accident showered debris over eastern Texas and western Louisiana, and Nasa now ensures that shuttles do not fly over heavily populated areas.

The main purpose of the mission was to service and resupply the International Space Station. The shuttle delivered a new gyroscope to the station to repair its steering system.