Astronauts spacewalk to fix ammonia leak

Two US astronauts conducted an emergency spacewalk Saturday to inspect and replace a device believed to be the source of an ammonia leak in the orbiting laboratory's power system, US space agency NASA said.

Astronauts Chris Cassidy and Tom Marshburn successfully installed a new pump controller box, which contains the mechanical systems that drive the cooling functions, after removing the old one suspected of leaking flakes of frozen ammonia coolant two days earlier, reported Xinhua.

"Mission Control ran the new pump while the spacewalkers watched for any ammonia snowflakes, but so far there have been no new signs of a leak," NASA said in a statement.

"Long-term monitoring of the pump will be required to determine whether the pump replacement has fixed the leak," it said.

The spacewalk, the third for both Cassidy and Marshburn, who conducted two spacewalks together in July 2009, began at 8:44 a.m. EDT (1244 GMT). It lasted five and a half hours, an hour less than expected.

"Back inside!" tweeted Marshburn after the spacewalk. "Hopeful that our work resolves the leak. What a great way to spend one of my last days in space!"

Marshburn has been in space since December and is set to return to Earth Monday. Cassidy is a new arrival, aboard the outpost for just around 40 days.

The spacewalk was hastily planned after space station crew members alerted NASA about the leak late Thursday morning when they spotted "small white flakes" of frozen ammonia floating near the pump box.