CAPE CANAVERAL, Fla. - Rick Mastracchio and Clayton Anderson, two members of the space shuttle Discovery's crew, took the first spacewalk of their current mission on Friday.

Both astronauts performed spacewalks together in the August 2007 mission to the international space station.

To help Mission Control identify them, lead spacewalker Mastracchio's spacesuit is marked with red stripes, while Anderson's suit is all white.

Their task, which will be done in three sessions, is to remove the old ammonia tank, which supplies the station's cooling system, and replace it with the new tank they brought with them on this mission.

"It's probably the size of a double refrigerator-freezer component and it lives on the backside toward the center of the station," Anderson said during a NASA interview.

The two astronauts were originally assigned to perform work on the batteries at the far left end of the station, but NASA postponed this work for the next shuttle flight out of concern they could be shocked working along the power truss.

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AP contributed to this report.