The mission was halted perilously close to the moment when the solid rocket boosters were due to fire, marking the point of no return. If that had happened, the shuttle would probably have crashed in the ocean off Cape Canaveral.
Observers on the ground were confused to see the countdown clock reach zero. This usually stops as soon as the mission is aborted. They heard the Nasa commentator reach the word 'and. . .' - about to pronounce 'lift off'. Instead he had to announce the main engine failure. Nasa cameras zoomed back from close-up immediately it was clear something was wrong.
The wife of Michael Baker, the shuttle commander, watched the drama unfold from the roof of the control centre three miles from the launch pad. The observers' view was obscured by the huge plume of steam produced immediately before the launch.
'We were quite obviously very concerned. I don't think anyone's happier than the families that the safety systems worked and everything was OK,' she said.
Launch Control spoke to Mr Baker before he crawled out of the shuttle: 'Sorry we didn't do it today . . . We'll give it another try another day.' At a Nasa briefing later, Mr Baker said he and his crew knew they were not going anywhere when they saw red lights flash in the cockpit and felt the engines turn off.
If the engine had failed only a short while later, the commander would have had to attempt a dangerous emergency landing back at the Kennedy Space Center. Such a landing, which would require the twin boosters to burn out and be jettisoned, has never been tried.
During the early ascent, there is no emergency escape system for the crew, but if the shuttle had been carried high enough, the astronauts could possibly have baled out.
Observers saw orange flames beneath the tail of the shuttle and heard a roar of leaking fuel burning one or two seconds after the scheduled lift-off time of 10.54 GMT. Nasa doused the engine with thousands of gallons of water, while the six astronauts remained strapped into their seats aboard the 2,000-ton spacecraft. A whole hour passed before they could escape.
When they finally emerged, their faces showed both relief and disappointment. The flight is expected to be delayed for at least a month.
'I would be very much surprised if there are not some very relieved Nasa launch control people sitting in the centre,' Steven Young, editor of Astronomy Now, said yesterday shortly after the aborted launch. 'If this was a developing problem, then it was really sheer luck that it was aborted before T-0. Once the boosters are lit there is nothing that can turn those off.'
Yesterday's malfunction was the fifth engine shutdown at the pad in 13 years of shuttle flights, and the third since April 1993. None has been so close to launch.
The fault appeared to lie with a high-pressure fuel pump on the third main engine that apparently overheated. These are crucial components, the size of a car engine, which pump the shuttle's super- cooled liquid fuel into the combustion chamber.
Endeavour was to have carried the world's most advanced civilian space radar on an environmental mission. Scientists were to use the dollars 380m ( pounds 250m) radar 'camera' to take snapshots of key spots on the surface of the Earth to help in understanding global climate change.
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