Setback for 'Mission Earth': Nasa's shuttle failure has compromised a crucial project to investigate ecological threats, reports Susan Watts, Technology Correspondent

NASA'S shuttle near-miss yesterday was an uncomfortable reminder of the Challenger disaster in 1986, when the vehicle exploded 72 seconds after lift-off, killing its crew of seven.

The six astronauts on board Endeavour had to sit for an hour on the launch pad, before blinking in the light they had not expected to see for another 10 days. This is the latest in a series of embarrassments for Nasa, at a time when it faces budgetary cuts and its workforce is threatened with far- reaching redundancy. The public, too, is becoming increasingly frustrated at the cost of Nasa's ambitious projects.

The failure will be particularly galling for Nasa, since the mission was part of the agency's attempt to win back its detractors. The flight was part of its 'Mission to Planet Earth', a long-term project to determine how natural systems work and what is needed to reverse potentially dangerous trends, such as the depletion of Earth's protective ozone layer.

The mission involved 49 scientists from 13 nations, including the UK. From its privileged vantage point 140 miles above the surface of the Earth, a camera the size of a bus was to take some of the most eye-catching snapshots the world has seen.

The camera, nestling in Endeavour's payload, was designed to bounce radar beams to Earth and back, catching reflected beams carrying an imprint of features of the planet that are usually hidden. Its beams can peer through clouds and deep into the oceans. Rock formations hidden by vegetation would have revealed themselves to its powerful eye, even after nightfall. The dollars 380m ( pounds 250m) camera, known as the Space Radar Laboratory, is the most powerful civilian radar flown in space.

The mission was the second of two flights, the first of which took place in April. The shuttle was to fly along exactly the same orbit as that taken in April, capturing shots of what Nasa calls the Spring and Summer Earth (at least in the northern hemisphere) so scientists could compare the two. Timing is crucial now. A protracted delay would devalue these comparisons.

There have been four previous last-minute engine failures on the shuttle launch pad, but these were all within a few seconds of launch time. They were all eventually established as sensor problems, with faulty sensors indicating that valves had not closed, when in fact they had.

Preliminary reports suggest that yesterday saw the first accurate sensor response. Two sensors separately gave readings of very high temperatures.

The turbo pump that failed was in one of the shuttle's three liquid engines. The turbo pumps sit one on each side of the system. They are among the most crucial components on board, and are usually changed after only two flights, but overhauled each time the shuttle flies. 'Every time an engineer maintains these, the risks are higher of someone making a mistake,' Steven Young, editor of Astronomy Now, said yesterday.

The pumps are designed to send super-chilled liquid hydrogen and oxygen into the combustion chamber, acting like a fuel injector in a car engine.

The turbo pumps spin at 28,000 revolutions per minute. This creates the potentially explosive scenario of hot chunks of metal spinning in a liquid oxygen environment.

Nasa's best option now is to swap Endeavour's engines for a fresh set. This will take several weeks. If the set that is taken off has a generic fault, the agency's engineers will also have to ensure this is corrected in the new set.

(Photograph omitted)