Astronauts take 3rd, final spacewalk

A pair of astronauts finished installing a fresh storage tank outside the International Space Station on Tuesday, accomplishing a main mission objective that required three spacewalks.

Rick Mastracchio hooked up the fluid lines for the ammonia tank that feeds the station's cooling system, as soon as he and Clayton Anderson floated out early Tuesday morning on the final spacewalk of shuttle Discovery's mission. He was done within minutes.

But as flight controllers began activating the tank to join it with the space station's cooling system, a valve appeared to be stuck. The problem was on the nitrogen side of the tank, which is needed to provide pressure to the loops. Engineers were figuring out their next step.

At the same time, 350 kilometres above Earth, the spacewalkers struggled with a stubborn bolt on the old ammonia tank, which had been removed on the previous spacewalk.

In what almost seems to be a common occurrence on this flight, one of the four bolts on the tank would not engage. Mastracchio and Anderson were trying to secure the boxy, 590-kilogram tank in Discovery's cargo bay when the problem cropped up. It appeared to be a misalignment, and they pulled out a pry bar to try to fix it. The pry bar wasn't needed, but a torque-increasing device was.

Mission control told the astronauts that they had to drive in all four bolts all the way, one way or another.

"How you guys feeling?" asked shuttle pilot James Dutton Jr. "Just take it easy. No rush here."

"I'll feel better when we get this thing bolted in," Mastracchio said. "No kidding," Anderson added.

'Good job'

When Mastracchio finally drove in the bolt, astronaut Dorothy Metcalf-Lindenburger shouted, "Now I can finally say good job, we have the ammonia tank in the payload bay." Earlier, she was too quick in offering congratulations.

NASA wants to return the old tank to Earth next week, in order to fill it and fly it back up this summer as a spare.

The space agency is trying to stockpile as many big parts up there as possible. Only three shuttle missions are left after this one, and there will be limited room on the much smaller Russian, European and Japanese cargo ships that will be supplying the station until its projected end in 2020.

President Barack Obama will outline his objectives for NASA's human spaceflight program Thursday during a visit to Kennedy Space Center. He's already axed his predecessor's effort to return astronauts to the moon.

The ammonia and nitrogen hoses for the new tank should have been connected during Sunday's spacewalk. But the astronauts had trouble attaching the new tank to the space station because of a stubborn bolt, and some chores had to be put off. At least one chore was scuttled Tuesday because of all the time spent on the latest troublesome bolt.

Return to Earth coming

At the start of Tuesday's spacewalk, Mastracchio had a little trouble opening the hatch. Mission control asked them to check the handle before they left the air lock. Everything seemed to be in place.

Two hours later, Anderson noticed a possible flaw in his left glove.

"I've got 51-year-old eyes, but it doesn't look like it's cut," Anderson said. Mission control confirmed through video pictures that the glove was fine.

Discovery will depart the space station Saturday and return to Earth on Monday.

NASA is keeping the shuttle docked an extra day to move up a survey of the wings and nose, a routine search for micrometeorite damage. The shuttle's main antenna is broken, and so the laser images must be sent down from the station. Normally, the inspection is conducted after a shuttle leaves.