Though the entire system that supplies the shuttle's internal power was working properly, NASA engineers shut down one-third of the orbiter's power supply tanks Monday night.
At that level of power, Pennington said, "we'd have to land one day early," on Friday instead of Saturday morning at Edwards Air Force Base in California. He said a final decision would be made no later than tomorrow morning.
Last night flight director Chuck Shaw said ground controllers hoped to solve the problem by making some adjustments on the power supply tanks and turning them back on.
Discovery took off Monday morning in a picture-perfect launch from the Kennedy Space Center in Florida. The main project of their five-day mission - the first shuttle flight this year and the third since the 1986 Challenger disaster - was to deploy a satellite essential to completing NASA's space communications network.
That went well Monday, hours after the morning launch. NASA officials said yesterday that the Tracking and Data Relay Satellite had been boosted into orbit and was ready to be tested.
"Right now, everything looks great," Pennington said.
But other, minor problems kept creeping up during the second day in space for astronauts Michael Coats, John Blaha, James Buchli, Robert Springer and James Bagian. An experiment testing a new heat pipe for the future space station failed after two hours; it will be tested again. Experts were trying to solve an overheating problem on another experiment, measuring the growth rate of roots in zero-gravity.
The astronauts, who have been much more businesslike than the tortilla- tossing crew aboard Discovery in September, were in "super spirits" as the shuttle sped 187 miles above Earth at 17,000 m.p.h. Pennington said. During the day, television pictures showed them sitting in shirts at their work stations, absorbed in their work monitoring scientific experiments and photographing environmental damage on Earth.
Though Pennington played down any threat that the power system problem might pose to the crew, the talk of a shortened mission illustrated how grave even the slightest power trouble can be.
The system provides water, air and electricity for the crew. The problem with the pressure reading involved one of three sets of tanks, which supply hydrogen to fuel cells where it is combined with oxygen to produce the power and water.
With all tanks working, the shuttle could easily complete a five-day voyage and have two extra days as a safety margin. With one set of tanks now disconnected, the orbiter can count on a minimum of four days in space - with two days in reserve if it is unable to land immediately.
Pennington said the entire power supply system was working properly when engineers turned off one set of tanks. But what worried engineers was an unusual reading from a gauge that measures tank pressure as they are heated to convert their super-cold liquid oxygen and hydrogen to gas. The gas operates the orbiter's three fuel cells, the units that power Discovery.