Two astronauts are spacewalking outside the International Space Station in an attempt to find an ammonia leak in a critical radiator system.

Station commander Sunita Williams and Japanese crewmate Akihiko Hoshide ventured out Thursday morning. They will isolate plumbing to help flight controllers locate the leak and open a spare radiator.

Their spacewalk got under way just hours after the orbiting lab had to dodge a piece of space junk.

On Wednesday evening, thrusters on a docked Russian supply ship were fired to move the space station out of harm's way. But a computer error caused the thrusters to malfunction, and the station did not reach the desired altitude.

NASA says the station and its six residents are safe despite their lower-than-intended orbit. The menacing debris is a satellite fragment.