SPACE CENTER, Houston--A problem with a hydrogen tank aboard the Space Shuttle Discovery forced its astronauts to conserve energy yesterday and threatened to shorten the mission, but Mission Control said there was no threat to the crew.

The five crew members monitored scientific experiments and photographed environmental damage on the Earth on the day after they roared into orbit and launched a key NASA communications satellite.

The problem was an erratic pressure reading on one of three liquid hydrogen tanks. The tank was taken out of service while engineers studied the problem.

The hydrogen is combined with oxygen in fuel cells to produce electricity for the shuttle systems, with water as a byproduct.

"There are no safety problems associated with it and no electrical problems," Mike Baker, the ground control capsule communicator, told the astronauts. "We're now looking at other flights to see if we've seen this before."

If the problem can't be resolved and Discovery has to continue to use only two hydrogen tanks, the shuttle may land Friday instead of Saturday, Flight Director Granvil Pennington said. But he stressed that no decision has been made.

"Right now, we're still planning to land on Saturday," Pennington said. "There's nothing to say we're going to come down early."

Without that third tank, the shuttle might not have electricity to support a five-day flight plus two days for any contingencies.

Pennington said NASA could wait as late as tomorrow before deciding to end the mission a day early. Discovery currently is scheduled to land Saturday at Edwards Air Force Base in California at 6:34 a.m., PST.

NASA spokesperson Jeff Vincent said late yesterday afternoon that engineers had not yet determined the cause of the erratic reading.

The other two hydrogen tanks continued feeding the shuttle's fuel cells and the crew took steps to save electricity. Unnecessary lights were turned off as well as some redundant computers and two data display screens.

Pennington said the crew's power conservation efforts would not hurt the experiments.

On Monday, several hours after Discovery's liftoff, the crew completed its primary task--deployment of a $100 million Tracking Data and Relay Satellite to complete a communications network that will allow shuttle astronauts to have nearly constant contact with Mission Control.