Explosion threat brings shuttle crew down to earth

Mission control at the US space agency, Nasa, yesterday cut short the 16-day flight of the space shuttle Columbia because of a defective fuel cell and ordered it to return to earth today. It is only the third time in 17 years of shuttle history that a mission has been so curtailed.

After a day of discussions, and vain hopes that the problem with one of the three power generations aboard the craft would resolve itself, officials decided Columbia should land at the Kennedy Space centre in Florida at 2.34 pm. local time today - after just four days in space.

Nasa officials insist that the other two fuel cells are working normally and that the craft and its five-man, two-woman crew are in no danger.

But scientists warn that if the faulty unit was permitted to deteriorate much longer, the risk would increase that the supercold liquid hydrogen and liquid oxygen which combine to produce electricity, could overheat and cause a deadly explosion.

The recall means that most of the scientific experiments scheduled for the next fortnight, aimed at using weightlessness to study various physical processes, will be scrapped.

They included the lighting of controlled fires to study the impact on flames of zero gravity, and the behaviour of protein crystals used in medical research.

"We're certainly disappointed," said Columbia's commander, Jim Halsell, after being told that he and his colleagues would be returning home prematurely.

According to Nasa officials, most of the weakening of the suspect unit occurred during the first 18 hours of the flight.

On Saturday, the unit's condition seemed to stabilise, but even so flight controllers were taking no chances. Nasa's operating procedures require that all three fuel cells on a shuttle must be working properly.

"We have flight rules that are very conservative," said an official at Nasa headquarters in Houston, Texas. "A general explosion would be catastrophic."

The power to conduct the planned experiments aboard Columbia comes from three electricity generating units in the shuttle fuselage, just below the payload bay, where this time a laboratory had been set up for the experiments. The units also provide drinking water for the crew.

Each of them comprises 96 cells arranged in three 32-cell "substacks". Performance is monitored by comparing output of 16 cells in each substack.

Almost immediately after lift off on Friday, engineers noticed lower than expected power from one of the substacks in unit No 2. Even so, just two functioning cells are plenty for normal re-entry into the atmosphere and subsequent landing.

The last time a shuttle mission ended early was in 1991, due to a failed navigational device. In 1981, in only the second flight of the shuttle programme, a defective fuel cell aboard the same craft Columbia caused its mission to be aborted.

These mishaps, however, pale beside the disaster which still haunts Nasa, the explosion of Challenger just 76 seconds after lift- off on 28 January, 1986.