NASA was keeping a watch Saturday on 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 return home early.
The shuttle'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."
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, the five astronauts tiptoed around the space shuttle so as not to ruin fragile crystals growing on board.
These crystals are called dendrites, branching structures that form inside molten metal alloys during solidification and affect the end product's durability and electrical properties.
Researchers say space-grown dendrites could help improve metal manufacturing on Earth.