The Hubble Space Telescope has lasted for 19 years, because it was designed mostly with parts that astronauts could replace on spacewalks.
Not on this mission.
A key instrument -- a spectrograph that can, among other things, measure the chemical composition of distant objects in the cosmos -- had lost power. NASA decided that instead of replacing the whole thing, it would have the astronauts fix it by hand on the fourth spacewalk of their mission to repair Hubble.
The job fell to spacewalker Mike Massimino, who had his work cut out for him. He had to remove more than 110 small screws.
"It is a repair activity that was not meant to be fixed up in space," said Tomas Gonzalez Torres, NASA's lead spacewalk officer, "with boxing gloves, with big huge gloves, in a big marshmallow suit."
There are 116 tools in the tool kit on Atlantis to fix the Hubble Space Telescope, including a bolt puller vise grip Massimino used to remove a balky bolt from a cover on a panel.
But even with all their tools, Massimino and crewmate Mike Good fell three hours behind.
At one point, Houston sent Massimino back to the space shuttle's airlock to refill the oxygen in his backpack.
In the end, the astronauts wrapped up what was scheduled as a 6½-hour spacewalk after eight hours, putting off other work until Monday.
On Saturday, astronauts John Grunsfeld and Drew Feustel whipped through what was supposed to be the toughest spacewalk of the mission to repair the Hubble Space Telescope with remarkable ease.
The two replaced an old instrument with the new Cosmic Origins Spectrograph, and then repaired a broken camera deep inside the telescope, removing blown circuit boards that were never meant to be taken off in orbit.
Hubble chief scientist Dave Leckrone said Grunsfeld and Feustel were throwing down the gauntlet for the astronauts tasked with the today's spacewalk -- the fourth of the mission.
"Mike and Mike are probably feeling pretty competitive today," Leckrone said. "If I were Mike Massimino tonight I would have my work cut out to show them up. I predict this spacewalk will be successful."
Massimino and Good were working on the telescope's Space Telescope Imaging Spectrograph, or STIS. They had the very challenging task of replacing a low-voltage power supply board, which contains a failed power converter. That meant taking out the 110-plus very small screws -- and not letting any of those screws float away -- to accomplish the repair.
STIS has been in "safe mode" since August 2004, when its power supply failed. Massimino, during training on the ground, managed to perform the task in 40 minutes.
The spacewalkers also were scheduled to install one of two new protective thermal insulation panels to protect Hubble from space junk.
Spacewalking to fix the Hubble Space Telescope is hard work, especially when the shoe doesn't fit. If your feet hurt nothing is fun. Ask any woman who has to smile while wearing 4-inch heels.
Good struggled with an ill-fitting boot on his first spacewalk last week, and a team on the ground at Mission Control sent up suggestions to adjust the fit.
"Once pressurized, you should be able to pull your foot back in the boot away from the pressure point on the top of the foot," wrote astronaut Rex Walheim to Good.