Spacewalkers upgrade station's coolant system

CAPE CANAVERAL, Florida (Reuters) - Two NASA astronauts left the International Space Station on Sunday to hook up a new cooling system that will pave the way for installation of European and Japanese modules beginning this year.

Station commander Michael Lopez-Alegria and flight engineer Sunita Williams, who began their slightly longer-than-planned seven-hour spacewalk at 8:38 a.m. EST (1338 GMT), also photographed a solar panel that is scheduled to be folded up during a station assembly mission next month.

Problems retracting an identical panel during the last shuttle mission in December prompted NASA to extend that crew's stay at the station for an unplanned spacewalk to help fold up the balky wing. The extra day in orbit ate into supplies normally reserved for weather-related landing delays.

Engineers are tweaking plans for folding up the second panel in hopes of avoiding similar problems.

The spacewalk was the second of three planned over nine days, the most ambitious station assembly work ever attempted without a U.S. space shuttle crew present.

Lopez-Alegria and Williams, both U.S. astronauts, breezed through their main goal -- attaching a second pair of ammonia cooling lines to the station. The first pair was hooked up during a spacewalk on Wednesday.

The third outing is planned for Thursday to remove and discard shrouds no longer needed to keep equipment warm.

Managers shifted tasks normally handled by visiting shuttle crews to the resident station astronauts to free as much time as possible for the shuttles to haul and install the station's remaining laboratories, modules and trusses before the fleet is retired in 3-1/2 years.

AMMONIA LEAK

Lopez-Alegria and Williams encountered a few problems during Sunday's spacewalk, including minor ammonia leaks as they hooked up the new cooling lines.

Once exposed to the frigid cold of space, ammonia freezes, creating snowflake-like particles that might have touched the astronauts' spacesuits.

"It was a very, very slow leak," said space station flight director Derek Hassmann.

NASA wanted to prevent ammonia from getting into the station's air, where it could pose a respiratory risk, so in addition to using the sun to gently bake off any hazardous particles on the suits, Lopez-Alegria and Williams waited a few minutes inside the station's airlock while ground control teams monitored sensors for signs of contamination.

Finding none, the crew's extended spacewalk ended shortly before 4 p.m. EST.

Lopez-Alegria and Williams did not have enough time to finish hooking up a new cable that will allow specially equipped shuttles to tap into the station's electrical system rather than use their own limited supplies of chemicals to generate power.

Once operational, visiting shuttles will be able to stay about three days longer at the outpost. NASA intends to try out the new gear during shuttle Endeavour's mission, now scheduled for June.

The cable installation will be finished on a future spacewalk, possibly Thursday's, Hassmann said.