| CAPE CANAVERAL, Florida
CAPE CANAVERAL, Florida Two of NASA's most experienced spacewalkers floated outside the International Space Station on Sunday for some preventive maintenance to help get the station ready to operate after the shuttle program ends this summer.
Shuttle Endeavour astronauts Drew Feustel and Mike Fincke ventured through the station's Quest airlock at 2:05 a.m. EDT (0605 GMT) for what would be the sixth-longest spacewalk in history, spanning eight hours and seven minutes.
It was the second and most technically demanding of four spacewalks planned during Endeavour's 16-day mission, the next-to-last in the U.S. space shuttle program.
The spacewalking pair topped off a tank of ammonia used in the station's cooling system, and lubricated the massive joint that keeps the station's left-side power-generating solar arrays rotating to track the sun.
NASA had expected some problems with ammonia, a toxic substance that has leaked onto spacesuits on prior spacewalks and required special decontamination procedures.
"This is high-grade industrial ammonia so we have to be super careful not to get it on us or to spill it because it's quite dangerous if we brought it back inside." Fincke said before the flight.
The pair filled the ammonia tank without incident, but Fincke ran into problems with the solar joint.
Bolts securing a cover over the joint unexpectedly floated free after Fincke loosened them, prompting ground controllers to limit the task. Fincke captured three errant bolts in his bulky glove, but one floated out into space.
The astronauts were able to lubricate four of the joints, instead of six as planned.
"I was really careful with these bolts and it didn't help me out," Fincke radioed to Mission Control. "Even ammonia is more fun than taking these guys off."
It was the fifth career spacewalk for Feustel, a former Hubble Space Telescope repairman making his first trip to the space station, and the seventh for Fincke, who has served as a commander of the orbital outpost.
The four spacewalks planned during Endeavour's mission are the last in the 30-year-old shuttle program. NASA has one last flight, a cargo run aboard shuttle Atlantis scheduled to launch July 8, planned before the three-ship fleet is grounded.
NASA wants to use U.S. commercial companies to deliver cargo, and eventually transport crew, to the station. The United States will rely on partner countries' Russia, Japan and in Europe for transport in the meantime.
The shuttles are being retired due to high operating costs and to save funds to develop new spaceships that can travel beyond the station's 220-mile (354-km) high orbit.
The station, a $100 billion project of 16 nations, has been under construction since 1998.
Endeavour, which carried the $2 billion Alpha Magnetic Spectrometer particle detector and spare parts to the station, is due back at the Kennedy Space Center in Florida on June 1.
(Additional reporting by Chris Baltimore in Houston, Editing by Eric Walsh and Eric Beech)