NASA on Thursday tracked a large piece of space debris set to pass close to the International Space Station as astronauts prepared for the second of three spacewalks outside the orbiter.
Astronauts Danny Olivas and Christer Fuglesang planned to camp out in the Quest airlock starting at 0654 GMT in order to acclimate their bodies for their outing in space more than 12 hours later.
Mission Control was keeping a close watch on the remains of the three-year-old Ariane 5, a European space rocket, that were moving in an oval-shaped orbit.
The piece, which is some 200 square feet (19 square meters) large, was expected to pass almost two miles (three kilometers) from the outpost on Friday, the US space agency said.
NASA Flight Director Office chief John McCullough said it did not appear that the ISS would have to maneuver to avoid the space junk.
A final decision would be made during the last hour of Thursday’s spacewalk, NASA said, but Mission Control was developing a contingency plan to potentially “reboost” the station-shuttle complex that currently holds 13 astronauts.
The linked spacecraft are currently orbiting 220 miles (354 miles) above the Earth.
During their spacewalk, which is set to begin at 2119 GMT, Olivas and Sweden’s Fuglesang were to install a new liquid ammonia tank used to keep the ISS cool.
On Wednesday, Olivas and fellow astronaut Nicole Stott performed the first spacewalk of the space shuttle Discovery’s nine-day mission at the ISS, removing an old tank from the outpost’s truss. The tank will return to Earth with Discovery.
The duo also fetched US and European experiment equipment from the orbiting station’s Columbus laboratory that will be brought back to scientists on Earth.
Discovery’s mission is the fourth of five planned for the shuttle program this year. The last is scheduled for November.
The two crews continued unloading equipment from the Leonardo Multi-Purpose Logistics Module, a huge pressurized chamber carrying 7.5 tons of supplies, including new station crew quarters, a freezer, two research racks and a treadmill named after popular US talkshow comedian Stephen Colbert.
The freezer will store samples of blood, urine and other materials that will eventually be brought back to Earth for study on the effects of zero-gravity.
Discovery, which blasted off from Cape Canaveral, Florida on Friday is due to return to Earth on September 10.
The mission is the 128th for the space shuttle program, and the 30th mission to the ISS.
Once the Discovery mission is complete, just six more shuttle flights remain before NASA’s three shuttles are retired in September next year.
Stott is taking over at the ISS from engineer Tim Kopra, who has been aboard the orbiting laboratory since July and is slated to return to Earth with the shuttle.