Cape Canaveral, Fla. ? Leaving the international space station unattended while they stepped outside, the two crewmen hooked up a robotic arm Wednesday and found some kind of gunk on the spacecraft’s vents that might explain the frequent breakdowns in its air-supply equipment.

Spacewalkers Leroy Chiao and Salizhan Sharipov did not make all of the electrical connections on the experimental mini-arm tight enough and had to redo part of the job, but managed to get full power flowing with just minutes remaining in their 5 1/2-hour outing.

“Everything’s perfect,” Mission Control radioed.

“Well, thank God, thank God,” came the reply. Before going back inside, they were advised: “Take a breather.”

During their 225-mile-high excursion, the spacewalkers also inspected the station’s vents and found a large patch of dark, oily residue and a white, honeycombed substance. It was not immediately known what the substances were.

The space station’s Russian oxygen generator has broken down repeatedly, and engineers have speculated its vent might be clogged or corroded. The air-cleansing equipment also has a history of malfunctions.

The spacewalk was the first in Chiao and Sharipov’s mission. The American and the Russian are 3 1/2 months into their six-month stay.

“Hello, space, my old friend,” Chiao, a veteran spacewalker, said as he exited.

Because of the grounding of NASA’s shuttle fleet, the space station has been limited to two residents instead of the usual three. As a result, no one was left inside during the spacewalk, but ground controllers kept watch over the spacecraft, an increasingly common practice in the wake of the Columbia disaster two years ago.