Flight controllers called it a fact-finding mission.
Meanwhile, some 200 miles below, NASA hoped to recreate fuel gauge problems that grounded shuttle Atlantis earlier this month.
Engineers planned to start filling the spaceship's external tank with liquid hydrogen fuel at daybreak, in a test at the launch pad to pinpoint the nagging trouble. The problem could be anywhere in the 100 feet of circuitry between the shuttle and tank, or in the gauges.
Atlantis' astronauts were supposed to examine the clogged rotary joint at the international space station. But with the shuttle mission delayed until January, NASA moved up the joint inspection and added another chore after a second component in the space station's power system failed 1½ weeks ago.
Commander Peggy Whitson and Daniel Tani made history as they floated outside well before dawn: It marked the 100th spacewalk at the space station.
They quickly headed toward the mechanism that is supposed to tilt the solar wings on the right side of the space station toward the sun. The component experienced circuit breaker trips Dec. 8, possibly after being hit by a piece of space junk or micrometeorite.
Whitson and Tani were instructed to look for any signs of impact.
Afterward, the spacewalkers planned to move over to the bigger solar rotary joint, which is supposed to automatically revolve 360 degrees to keep the solar wings pointed toward the sun. The joint - also on the right side of the space station - has been used sparingly in the past three months because of vibrations and electrical current spikes.
During a spacewalk in October, Tani found steel grit inside the 10-foot-diameter joint. Even though he collected samples of the steel shavings, which were returned to Earth on the last shuttle flight, engineers still do not know what is grinding inside.
Mission Control asked Whitson and Tani to look deeper into the joint, by taking off more covers and using a mirror to peek inside, and to collect more grit samples and remove a set of suspect bearings. No repairs were planned during Tuesday's excursion.
The unrelated predicaments are curtailing power generation at the space station and could delay future shuttle visits.
Atlantis' trip to deliver the European lab, Columbus, is off until Jan. 10, a date that hinges on the results of Tuesday's fueling test. The Japanese lab, Kibo, or Hope, is supposed to follow on multiple shuttle flights.
NASA has been struggling with sporadic fuel gauge problems for two years and wants to resolve them once and for all. The gauges are part of a backup safety system; they prevent the shuttle's main engines from running on an empty tank, which could be catastrophic.