(Agencies)
Updated: 2007-12-19 08:23

CAPE CANAVERAL, Florida -- A pair of spacewalking astronauts left the International Space Station on Tuesday to inspect damaged parts of the outpost's power system.

Clad in bulky spacesuits, station commander Peggy Whitson and flight engineer Dan Tani floated outside the station for seven hours on the 100th spacewalk devoted to space station assembly and maintenance.

The astronauts' first stop was a mechanism used to tilt a set of solar panels so they can track the sun for power. Engineers suspected a micrometeoroid strike may have damaged the equipment since three circuits failed suddenly and simultaneously on December 8.

Whitson and Tani, both making their fifth spacewalks, found a slightly discolored cable, but no signs of physical damage.

"No smoking gun yet," said spacewalk commentator Rob Navias from NASA's Mission Control Center in Houston.

The pair moved on to the massive rotary joint that spins the wings like a paddle wheel. The joint has been locked in place since an initial inspection in October revealed metal shards in the joint.

Whitson and Tani spent most of their outing pulling off covers to look at the 10-foot (3-meter)-wide joint from different angles. Whitson had one word to describe the mess inside: "Ugly."

The astronauts snared samples of the metal filings on tape to be brought back to Earth for analysis. They also took out one of the mechanism's 12 support bearings for additional inspections.

NASA needs to repair the joint and bring the station to full power before the main part of Japan's laboratory complex arrives next year.

ATLANTIS TANK FUELING

Even with the power shortfalls, NASA expects the station to be able to support Europe's Columbus laboratory module. The laboratory's launch aboard space shuttle Atlantis had been planned for December but was delayed due to problems with the shuttle's fuel sensors.

While Whitson and Tani inspected the station, technicians at the Kennedy Space Center in Florida filled Atlantis' external fuel tank with 500,000 gallons of liquid hydrogen and liquid oxygen in an attempt to replicate sensor glitches that appeared during launch attempts on December 6 and 9.

They weren't disappointed. One sensor, which performed normally during the two launch attempts, failed during Tuesday's test, as did two others that had problems previously.

NASA suspects a faulty circuit is to blame and connected monitoring instruments to about 100 feet of wiring that links the fuel sensors in the shuttle's tank to electronics boxes in the shuttle's engine compartment.

If the problem can be found and fixed while the shuttle is at the launch pad, NASA hopes to launch Atlantis on January 10.

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