Two space shuttle astronauts will exit the International Space Station early Friday to complete the first of three spacewalks planned for their mission.
The spaceflyers plan to prepare a giant tank of ammonia, used for coolant on the space station, to be installed on the backbone-like truss of the orbiting lab.
STS-131 mission specialists Rick Mastracchio and Clayton Anderson will step out of the spacecraft at 1:41 a.m. EDT (0541 GMT) to begin a six-and-a-half-hour excursion.
Both men are spacewalking veterans, and have even conducted previous spacewalks together during the August 2007 STS-118 mission to the station.
"I'm looking forward to getting out the door again with Clay. He's great to work with," Mastracchio said in a preflight interview.
Mastracchio, the mission's lead sapcewalker, will wear a spacesuit marked with red stripes, while Anderson will wear an all-white spacesuit, to help distinguish them to officers in Mission Control.
"My number one job is to just make sure that we have safe and successful spacewalks," Mastracchio said.
The new ammonia tank was carried up to space by the shuttle Discovery when it launched April 5. The hardware will replace a spent tank that will be returned to Earth at the end of the orbiter's 13-day 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 described in a NASA interview.
To prepare the old tank to be removed, the spacewalkers will disconnect some cables and reconfigure bolts. They will also go to the space shuttle's cargo bay, where the new tank is waiting, and prepare that to be released. Once that is done, pilot James Dutton and mission specialist Stephanie Wilson will work from inside the station to grab the tank with the station's 58-foot (18-meter)-long robotic arm to maneuver the hardware into a temporary storing place.
After their work with the ammonia tank, the spacewalkers will retrieve a science experiment that had been running on the outside platform of the Japanese Kibo laboratory. The project was testing the effects of the space environment on various materials.
Finally, the astronauts plan to remove an old piece of hardware called a rate gyro assembly from the outside of the space station, and replace it with a new one.
NASA had also aimed to have the astronauts prepare some solar array batteries for removal, but mission managers pulled that task from the schedule to avoid a potential electric shock hazard for the astronauts.
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SPACE.com is providing complete coverage of Discovery's STS-131 mission to the International Space Station with Managing Editor Tariq Malik and Staff Writer Clara Moskowitz based in New York. Click here for shuttle mission updates and a link to NASA TV