NASA will go ahead with the first space shuttle mission since the 2003 Columbia disaster after fixing damage to a heat-resistant tile on the shuttle Discovery, a spokesman says.

"The issue has been resolved - launch is a go," NASA spokesman Mike Rein said.

The tile was damaged when a window covering fell off Discovery as the spacecraft sat on the launchpad at Cape Canaveral in Florida as the clock was counting down to tomorrow's scheduled launch at 3:51pm local time.

The damage caused by falling debris rang alarm bells because that was precisely the problem that doomed Columbia.

It also came just hours after NASA's administrator, Michael Griffin, had said all issues except possible bad weather had been settled and Discovery was ready for launch.

"Everything is at rest today - yesterday we were working a couple of ... issues and those were amply put to bed, so we're in good shape," Mr Griffin said, adding he hoped "the weather gods are kind for tomorrow".

"Can there be something that we don't know about that can bite us? Yes, this is a tough business, it's a very tough business but everything that we know about has been covered."

Columbia disaster

NASA has not flown a shuttle mission since Columbia disintegrated over Texas on February 1, 2003.

Its wing had been breached on liftoff 16 days earlier by falling foam and superheated gases rushed into the gap as the shuttle re-entered Earth's atmosphere.

All seven crew died.

Discovery's mission will test improvements made to the shuttle to reduce falling debris at liftoff and experimental procedures for repairing damaged heat resistant tiles.

The window covering that fell off would have been removed before launch.

ISS

The shuttle, under the command of veteran astronaut Eileen Collins, will also deliver much-needed supplies and equipment to

the International Space Station (ISS).

The station's construction - a 16-country project, has been on hold since the remaining three-shuttle fleet was grounded.

Weather outlook

NASA weather forecasters said the outlook for launch was good, but they increased the risk of thunderstorms.

"For our launch forecast, we did get a little more pessimistic on this today," weather officer Kathy Winters said as the countdown clock ticked toward the scheduled liftoff.

"There's a 40 per cent chance of weather prohibiting launch," she told a briefing.

Any thunderstorm must be at least 20 nautical miles from the shuttle to allow a launch.

A network of 112 cameras set up to monitor Discovery's surface as it soars will need clear skies to get good images.

Families' support

The families of the seven astronauts killed in Columbia's fatal break-up offered their support.

"We have had two-and-a-half-years to reflect daily on the loss of our loved ones as the shuttle Columbia broke apart over Texas on February 1, 2003," the families said in a statement.

"... we have every confidence that the sacrifice of our loved ones and those that preceded them will be realised for the benefit of mankind - Godspeed Discovery."

Launch window

If Discovery's launch is delayed, NASA can attempt it twice more before having to break for a few days to refuel the craft's onboard power generators.

The current launch window runs from July 13 through July 31.

The next one opens September 9.

Mr Griffin said the launch marked a milestone in US efforts to return to human space flight, but cautioned that space remained a dangerous realm.

"There is no recovery from mistakes we've made, whether it goes back to the Apollo fire, the loss of Challenger or the loss of Columbia," he said.

"Going back even further through 100 years of aviation, the safety lessons that we who fly have learned are written in other people's blood.

"The minute we say we're good enough we start getting bad again."

- Reuters link