CAPE CANAVERAL, Florida (AP) -- NASA on Tuesday traced fuel gauge failures in shuttle Atlantis' tank to a bad connector, and a top manager said he did not know how long it would take to replace the part or when the spaceship might fly.
Atlantis' external tank contains sensors that cut off the shuttle's engine when the fuel is used up.
The erratic shuttle fuel gauges -- part of a critical safety system -- forced back-to-back launch delays this month. Until Tuesday's tanking test, NASA had been aiming for a January 10 liftoff of Atlantis with a European space station lab.
"We're going to follow this trail where it leads us and we're going to solve this problem, and then we'll go fly ... whether it's January 10 or February 10 or March 10," shuttle program manager Wayne Hale said.
In orbit, meanwhile, spacewalking astronauts helped pinpoint the source of a flawed mechanism in the international space station's power system. But they unearthed few clues involving an even bigger problem with a fouled rotating joint for the solar wings.
As NASA pumped liquid hydrogen fuel into Atlantis' tank at the pad, astronauts Peggy Whitson and Daniel Tani inspected the space station's two crippled power components. The unrelated problems are curtailing power generation and threaten to delay future shuttle flights.
Their first stop was a solar wing-tilting mechanism that experienced circuit breaker trips on December 8 and shut down. Engineers initially suspected a piece of space junk may have damaged it, but Whitson and Tani found no signs of impact. They temporarily disconnected cables for a test that exonerated certain parts, leaving the motor most likely at fault.
NASA's space station program manager, Mike Suffredini, said a spare motor already on board will be installed during Atlantis' visit, a difficult spacewalking job.
Repairs to the damaged solar rotary joint, on the other hand, will be a massive effort requiring as many as four spacewalks and likely will not be attempted until next fall, Suffredini said. That's how long it will take to figure out what's wrong and train a crew on the repairs, he said.
The joint is supposed to automatically rotate 360 degrees to keep the solar wings facing the sun. It's been used sparingly over the past three months, ever since it began vibrating and exhibiting electrical current spikes.
Whitson and Tani spent most of their seven-hour spacewalk inspecting the clogged rotary joint, removing covers and peeking deep inside with a dentist-style mirror on a rod. They found more of the metallic grit that was first detected by Tani during a spacewalk in October, and collected more samples.
All the gears, motors and bearings looked fine, although some were dirtier than others. The spacewalkers removed one bearing for return to Earth on the next shuttle flight, for engineering analysis.
NASA had hoped to learn what was grinding against the rotating ring.
"We didn't find anything that stood out," Suffredini said. The space agency will try to limp along with the joint in its current state until repairs are made, he added.
Suffredini said that if both dilemmas persist, the space station may not be able to generate enough power to support the Japanese lab that's supposed to arrive in three sections beginning in February. There's "a fighting chance" to keep the first Japanese delivery mission on track, but beyond that, it would be "extremely difficult" to continue assembly, he said.
As for Atlantis' woes, two of the four fuel gauges at the bottom of the external tank failed during Tuesday's test, and another did not work right.
Special test equipment indicated open circuits in the connector that passes through the wall of the fuel tank, linking wiring between the gauges in the tank and Atlantis. It was too soon to know whether the shuttle would need to be returned to its hangar for repairs, Hale said.
The space agency has been struggling with sporadic fuel gauge problems for two years, ever since flights resumed following the Columbia tragedy. The gauges prevent the shuttle's main engines from running on an empty tank, which could be catastrophic.
Hale said it's unclear whether the same type of connector caused the previous problems. There could be a manufacturing defect or flawed design, or the part may have been installed improperly, he noted. The connector is less than 10 years old, "pretty new by shuttle standards."
"We are not going to be driven by schedule on this one," Hale said at a late afternoon news conference. "We need to get to the bottom of this, fix it and make sure it's fixed once and for all."
NASA is up against a 2010 presidential deadline for completing the space station and retiring the shuttles.
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