CAPE CANAVERAL, Fla. (AP) – 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 5½-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 3½ 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.

Controllers did not get to see the start of the spacewalk because the cold interfered with a dish antenna on the space station. But the antenna soon warmed up in the sunlight, and live TV images filled Mission Control’s screens, though only sporadically.

The German-made $10 million arm – a 20-inch device featuring two joints, two cameras and a fingerlike pin at the end – is a smaller, simpler version of what engineers hope will lead to satellite-repair robots in perhaps a few years.

For now, the objective is simply to see whether the arm can move along a 28-inch curved track and survive for a year in the vacuum of space.

The arm arrived on Christmas Day aboard a Russian supply ship along with much-needed groceries for the spacemen, whose food and water had been running so low that they had to be put on a diet. They had a month of hearty eating to beef up for the spacewalk, which involved hauling the robotic arm and its platform over 100 feet.

Because of the spacewalk, it was a 20-hour workday for the astronauts, a flight rule violation but a situation that NASA considers occasionally unavoidable with only two people on board. Flight surgeons vouched for the crew’s strength.

Once they were back inside, the spacewalkers said that they were weary and stressed because of glove problems and that they had scraped their fingertips.