NASA struggled Saturday to understand unusually high pressure readings in a fuel line for one of space shuttle Columbia's three auxiliary power units.
The units, called APUs, generate power to drive hydraulic pumps that supply pressure for Columbia's critical hydraulic systems, including landing gear and nose-wheel steering.
Mission operations director Lee Briscoe said the problem could be a bad pressure sensor, contamination on that sensor, instrument trouble or a blocked fuel line. He stressed that it was way too soon to say whether the affected APU might be in danger of shutting down _ if it did, Columbia would have to come home early.
Columbia's 14-day research mission just began Friday.
NASA flight rules require three working APUs. As of Saturday, all three of Columbia's APUs were working. The pressure readings returned to normal after the crew switched to another set of heaters, Briscoe said.
"We really don't know what we have," Briscoe said. "The system appears to be working nominally right now. We'll go off and understand that and any speculation . . . of shortening the flight or something like that is really premature at this time."
Late Saturday afternoon, Mission Control had the crew open a valve in the APU fuel line to see how that would affect the pressure readings.
Throughout the day, Columbia's five astronauts tiptoed around the space shuttle like cooks protecting their souffles, so as not to ruin fragile crystals growing on board.
The astronauts took turns pedaling on a stationary cycle equipped with a vibration absorber. Muscles become flabby in weightlessness, a liability if an emergency required speed and strength.
Pilot Andrew Allen and Charles "Sam" Gemar also slipped one at a time into a waist-high sack that, via reduced pressure, forced blood from the top of their bodies into their legs to simulate gravity.
Doctors believe repeated sessions in the vacuum sack can help prevent dizziness at the end of a flight, especially a long one like this. Many astronauts become faint once they're back on Earth because of the blood rushing from their heads and chests.
Motion sensors on Columbia measured every bump and quiver, some of it unavoidable, so scientists on the ground could gauge the effects of vibration on experiments and alter the tests if necessary.
To reduce jostling, Columbia's robot arm and its new magnetic grappler rested idle in the cargo bay. The electromagnetic system is to be tested next week after the sensitive crystal, fluid and metal-melting experiments are completed.