FAULTY FUEL CELL CUTS SHORT SPACE SHUTTLE FLIGHT.

Byline: Associated Press 

Space shuttle Columbia and its seven astronauts will return to Earth on Tuesday, 12 days earlier than planned, because of a deteriorating and potentially explosive power generator.

It is only the third time in space shuttle history that a mission has been cut short by equipment failure.

While NASA considered Cmdr. James Halsell Jr. and his crew to be in no immediate danger, shuttle managers did not want to take any chances with the generator and, on Sunday, cut short the 16-day science mission.

``The conservative thing to do is land on Tuesday,'' shuttle program manager Tommy Holloway announced at a hastily arranged news conference.

Holloway said a landing today was ruled out in order to allow for an orderly return, and because Columbia's two other electricity-producing fuel cells were working fine.

The fear was that hydrogen fuel and oxygen in the degraded generator could mix, overheat and blow up if voltage in the unit dropped far enough. Flight controllers had the astronauts turn off the unit Sunday to reduce, if not eliminate, that risk. The crew also shut down all noncritical equipment to conserve power.

``Our plans would be to manage this fuel cell in a way that's not a threat to crew safety and, indeed, we are extremely confident in the two remaining fuel cells and their ability to support an orderly and methodical preparation for landing, and landing,'' Holloway said.

To slow the decline in voltage, Mission Control first had the astronauts reduce the power load on the degraded fuel cell. But the voltage continued to dwindle. Engineers debated a few hours before deciding to shut down the unit - once turned off, it cannot be restarted in the event another fuel cell fails.

While a shuttle can land safely with two fuel cells, it's uncertain what would happen if only one fuel cell was available because of all the vital systems that would have to be shut down for lack of power.

Each $5 million fuel cell uses hydrogen and oxygen to generate electricity for the numerous shuttle systems, and produces water as a byproduct. National Aeronautics and Space Administration flight rules stipulate that all three fuel cells be working properly in orbit, otherwise a mission must be cut short.

Mission Control had been monitoring the mysterious decline in voltage in the fuel cell since Friday's liftoff. As it turns out, brief jumps in voltage were detected in the unit several hours before launch, but managers determined it was working properly and cleared Columbia for flight.

Holloway admitted that ``with 20-20 hindsight,'' the countdown should have been halted and the fuel cell replaced. Launch controllers made the best decision at the time with the data available, he said.

Engineers have no idea why the fuel cell slowly was losing voltage. The unit has flown before in space without problems.

A failed fuel cell forced an early landing during NASA's second shuttle flight in 1981; in that case, the unit was flooded with water. The only other time a shuttle mission was cut short for equipment failure was in 1991, because of a failed navigation unit.

Halsell did not sound surprised when Mission Control radioed up the bad news.

``That's certainly a disappointment,'' Halsell said, ``but we know you guys put your best effort forward and you're doing the right thing and we appreciate all the work that's gone into that.''

Mission Control immediately began planning for a Tuesday afternoon landing of Columbia - which wasn't supposed to return until April 20 - at the Kennedy Space Center. And the astronauts rushed to complete as many laboratory experiments in the time remaining.