Fault Forces Nasa to Call Off Shuttle Launch

America's plans to get back into space suffered a humiliating blow last night when the launch of the shuttle Discovery was cancelled two hours before its scheduled lift-off.

With most of the crew already strapped in to seats on the flight deck, Nasa managers aborted the mid-afternoon launch, blaming defective sensors on an external fuel tank for the delay to the agency's first manned mission since the Columbia disaster in 2003.

"For some reason they did not behave today and so we're going to have to scrub this attempt," said launch director Mike Leinbach.

He told his launch control team to "appreciate all we've been through together, but this one is not going to result in a launch attempt today".

The delay is a considerable disappointment for Nasa, which is determined to restore a reputation tarnished by the Columbia accident that killed seven astronauts and which has spent $1.4bn on safety improvements to its ageing shuttle fleet.

It is also the latest in a series of setbacks for Mission STS-114, which was originally scheduled for launch in May but was delayed when a hairline crack was discovered on the main fuel tank.

Yesterday Nasa said the sensor appeared to be showing a low fuel level, although the tank was filled with more than 530,000 gallons of liquid oxygen and liquid hydrogen earlier in the day.

Technicians had already overcome two other earlier hitches to get Discovery ready for launch.

Nasa managers were meeting last night to assess the extent of the problem and how much time would be needed to fix it, although any issue with the shuttle's fuel supply is considered serious and will require a thorough system inspection.

No new target date was announced immediately, although a Nasa commentator said a launch attempt today was extremely unlikely.

Nasa has until July 31 to launch Discovery. After that it will have to delay until September 9, when the International Space Station again comes into the right position for a shuttle rendezvous after a daylight liftoff.

All but two of Discovery's crew of seven were fully strapped in to their seats on the shuttle's flight deck when the decision to abort was made. They wore resigned smiles and shook hands with flight deck assistants as they left the spacecraft and headed back to their crew quarters.

"We are very disappointed but we'll fly again another day," said David Wolf, an astronaut speaking from the launch control room.

The mission, like many of the remaining 28 due to take place before the ageing shuttle fleet is retired in 2010, is crucial to the completion of the International Space Station, pivotal to Nasa's plans for manned exploration beyond earth's lower orbit.

But the launch has been plagued by a succession of problems in recent days. Late on Tuesday a window panel fell 65ft from Discovery's crew cabin and damaged several of the shuttle's heat resistant tiles. Engineers quickly fitted a replacement panel and tiles and the countdown resumed.

Yesterday morning, just hours before the scheduled lift-off, a heater unit designed to prevent ice forming on the external fuel tank failed and had to be replaced.

Discovery's crew are Commander Eileen Collins, 48, pilot Jim Kelly, 41, Japanese astronaut Soichi Naguchi, 40, and mission specialists Steve Robinson, 49, Andy Thomas, 53, Charlie Camarda, 53, and Wendy Lawrence, 46.

Collins, Lawrence and Thomas are embarking on their fourth space flights. It is the third shuttle mission for Robinson, the second for Kelly, and first for Camarda and Naguchi. Robinson and Naguchi are due to conduct three space walks.

Amid the anticipation of Discovery's launch, memories of the day Columbia broke apart over Texas were still raw, as is the stinging criticism of an independent accident investigation panel that concluded that the agency's arrogant management culture was largely to blame.

The panel made 15 recommendations for a safe return to flight, but only 12 were met. The other three, according to Nasa administrator Mike Griffin, including the elimination of risk from all falling debris, were impossible to follow within the limits of human knowledge.

Columbia was doomed when a block of insulating foam broke away and struck a wing shortly after take-off, allowing deadly hot gasses into the spacecraft on re-entry.

Discovery has travelled more than 98 million miles in 31 previous flights, and also led Nasa back into space in September 1988, more than two years after the Challenger disaster.

The families of the lost Columbia crew, many of whom planned to watch the launch, issued a statement of support for the Discovery mission.

"As the families of Apollo 1 and Challenger before us, we grieve deeply but know the exploration of space must go on. We hope we have learned, and will continue to learn, from each of these accidents, so that we will be as safe as we can be in this high-risk endeavour."