CAPE CANAVERAL, Fla. -- 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.
Venturing out on the final spacewalk of shuttle Discovery's flight, Rick Mastracchio quickly hooked up the fluid lines for the ammonia tank that feeds the station's cooling system. 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 trouble was on the nitrogen side of the tank, which is needed to provide pressure to the loops.
Engineers tried without success to get the valve working, then stood down to decide on the next step. A NASA spokesman said there was no immediate need to fix the problem; the ammonia in the tank is essentially a backup in the event of a leak in the cooling system.
At the same time, 215 miles above Earth, Mastracchio and Clayton Anderson 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, 1,300-pound 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.
"I'll feel better when we get this thing bolted in," Mastracchio said. "No kidding," Anderson added.
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. A couple other tasks were scuttled Tuesday because of all the time spent on the latest troublesome bolt.
Mastracchio and Anderson wrapped up the 6 1/2-hour spacewalk with some prep work for the next shuttle visit, scheduled for just a few weeks from now. Down at the Florida launch site, Atlantis was moved out of its hangar for the short trip to the Vehicle Assembly Building, the last stop before the pad. NASA temporarily parked the shuttle outside in the sunshine so workers could snap pictures.
It will be the last flight of Atlantis. Liftoff is targeted for May 14.
The spacewalkers indulged in some picture-taking of their own.
"Look over here," Anderson told Mastracchio. "Oh, baby, you're going to want to take this one to the grandkids."
A few hours later, Anderson commented: "The Earth is a beautiful place. Too bad more people can't have that view. Maybe one day."
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.