Spacewalk On Tap At The International Space Station

Two American astronauts will venture outside the International Space Station early Sunday, aiming to perform some maintenance work on a cooling system that plays a critical role in keeping the outpost humming along in an orbit 220 miles above planet Earth.

Discovery mission specialists Rick Mastracchio and Clayton Anderson will switch their spacesuits to battery power around 2:16 a.m., marking the official start of the 142nd spacewalk performed in the assembly and maintenance of the station.

The first two building blocks of the station were linked in low Earth orbit during the STS-88 mission in December 1998 -- a flight commanded by NASA astronaut Robert Cabana, who now is director of Kennedy Space Center.

Mastracchio and Anderson first will team up to retrieve a spent ammonia coolant tank that weighs about 1,800 pounds and is bolted to a starboard segment of the station's central truss, which is 335 feet long and cost about $2 billion to construct.

The first section of the 11-segment truss was launched in 2000 and the final piece was put in place in early 2009. The skeletal backbone of the station is tipped with four massive U.S. solar wings that generate enough power to light up a neighborhood of about 55 average American homes.

The two astronauts ultimately aim to haul the spent coolant tank into shuttle Discovery's cargo bay for a return trip to Earth. A new, fully filled tank will be installed by Mastracchio and Anderson.

You can watch the 6.5-hour excursion live here in The Flame Trench. Click the NASA TV box on the right side of the page to launch our NASA TV viewer and refresh this page for periodic updates.

The spacewalk will be the fifth performed so far this year at the station. Added up, astronauts for at least a half-dozen countries have already spent 879 hours and 43 minutes assembling the station. The construction and outfitting of the outpost is expected to be complete after three more shuttle missions to be launched on May 14, July 29 and Sept. 16. The shuttle fleet is to be retired after that last flight.