CAPE CANAVERAL, Fla. -- A pair of spacewalking astronauts disconnected an old empty ammonia tank outside the International Space Station on Friday and got a new one ready to put in its place.
In the first of three spacewalks needed to complete the job, Clayton Anderson had no problem taking apart the ammonia lines on the old tank. But he needed a pry bar to remove the new tank out of space shuttle Discovery's payload bay. The tank got hung up on a bolt.
"Go nice and easy, Clay," spacewalking partner Rick Mastracchio warned as Anderson pushed and prodded with the pry bar. After several tries, the tank finally came free. "We got it!" Anderson called out.
The two men lifted the 1,700-pound tank out of Discovery and handed it off to a robot arm, which maneuvered it to a temporary storage location at the space station.
The actual swap-out of the two tanks will take place during the second spacewalk Sunday, with the entire effort wrapping up on the third and final outing Tuesday. It will be "a big juggle" with the tanks, said David Coan, Mission Control's lead spacewalk officer.
Besides the tank work, Mastracchio and Anderson collected a science experiment from the space station's Japanese lab and replaced a failed station-positioning device.
Then the game plan changed. Instead of tackling battery work, they focused on hoses and clamps. Astronaut Dorothy Metcalf-Lindenburger, directing the spacewalk from inside, urged the men to go slow because of the switch.
Mastracchio and Anderson were originally scheduled to work on old batteries on the far left end of the space station, along the sprawling power truss. But based on new findings, NASA canceled the task this week because of concern the two might get shocked. The job instead will be carried out on the next shuttle flight, once the spacesuits are better protected.
There were a few tense moments early in Friday morning's spacewalk when Mastracchio reported that he bumped a large V-shaped bar in the shuttle payload bay and it was sliding around. He said it did not appear to be off its mooring. Mission Control later said engineers were "pretty convinced" it was normal for the clamp to move around a bit, but as a precaution, warned the spacewalkers to stay away from it.
The V-shaped bar serves as a guide for the cargo carrier that flew up on Discovery and was attached to the space station Thursday. The compartment was unloaded by some of the 11 astronauts inside, as the spacewalk unfolded 215 miles up.
As the 6 1/2-hour spacewalk drew to a successful close, Mastracchio noted there was lot more distance to cover, moving hand over hand, than the last time he worked on the space station in 2007. The orbiting outpost has grown considerably since then.
"Like a marathon with your arms," Metcalf-Lindenburger observed.
After the spacewalkers were back inside, one of them joked, "Of course, she's a former teacher, so we do exactly what she tells us, otherwise she might rap our hands with a ruler." That prompted a big laugh from Metcalf-Lindenburger, one of only a few educator-astronauts.
Discovery and its crew of seven are supposed to remain at the space station until next Friday. But they likely will spend an extra day there because of the failure of the shuttle's main antenna. NASA wants the shuttle astronauts to inspect their ship for any signs of micrometeorite damage before they depart. That way, all the laser images can be transmitted to Mission Control through the space station.
That would stretch the shuttle mission to 14 days, with a landing on April 19.
Late Thursday, Mission Control informed the astronauts that the shuttle seems to have weathered Monday's liftoff well and that there is no need for another inspection to check for launch damage. The survey that's planned before Discovery undocks will be looking for any potentially dangerous impacts from space junk.
Only three shuttle missions remain after this one to wrap up space station construction.