The weightlessness of space will be an ally as the spacewalkers struggle with massive instruments, such as a 900-pound wide-field planetary camera.
Even so, NASA says the sheer volume of work to be done over a relatively short period of time will make this the toughest, most tightly scheduled repair mission to date.
The spacewalkers will cut the telescope loose on the ninth day of their mission. Atlantis will then quickly descend to a lower altitude to reduce the risk of being hit by such orbital debris as old rocket parts, lost screwdrivers and castoff nuts and bolts. The region where the telescope orbits is much more congested with debris than the lower altitude, where the International Space Station orbits.
The danger is that even a small piece of debris traveling at 17,000 miles an hour could puncture the skin of the shuttle, rendering it unusable for the return trip to Earth.
NASA engineers have calculated that there is a 1-in-221 chance of a catastrophic accident caused by orbital debris that would destroy the shuttle and kill the seven-member crew. Some assessments have put the risk of danger even higher, at 1 in 167. That would violate NASA's margin of safety, which states that a risk higher than 1 in 200 is unacceptable.
"We've paid close attention to this risk," deputy shuttle program manager LeRoy Cain said in a recent briefing with reporters. "We feel it's acceptable to go fly."
Flying upside down and backward while the crew works on Hubble is one way of curbing risk. That configuration protects the shuttle's nose cone and its vulnerable wing edges.
Rescue plan
If all NASA's efforts to protect Atlantis fail and the orbiter is damaged, either during launch or by orbital debris, the crew will have to be rescued by a second shuttle. That shuttle, Endeavour, will be standing by on a nearby launchpad when Atlantis lifts off. Endeavour could blast off on a rescue mission within a matter of days, officials said.
Atlantis will carry 25 days' worth of provisions, enough to tide the crew over should it need to wait for Endeavour. In a rescue, the Atlantis crew would be transferred one by one to the rescue orbiter. Then, Atlantis would be destroyed.
Massimino said he wasn't worried about his safety. NASA has learned a lot since the Columbia accident, he said.
"If we do have damage, there is a pretty good chance we can go and fix it," he said.
After Columbia was destroyed, NASA engineers developed repair techniques to plug holes, though none has been tried in a real emergency.
"If I was too concerned" about the risk, Massimino said in a phone interview from Johnson Space Center in Houston, "I wouldn't be doing it."
Critics said the willingness of the astronauts to put themselves in harm's way should not be a consideration when deciding whether to permit a mission in space.
"Just because the astronauts are prepared to have a fancy funeral if something goes wrong, that doesn't mean the taxpayers should have to pay for their blaze of glory," said John Pike, a space policy analyst for GlobalSecurity.org.
The mission is just as risky now as it was when O'Keefe canceled it, Pike said, adding that the only thing that has changed is the politics.
Pike's opinion is a minority one. Other observers said they thought NASA had worked hard to minimize risk.
They also say that changes to the shuttle fleet, such as redesigning the external fuel tank to minimize the amount of insulating foam that falls off during a launch, have made the shuttles safer than they were when Columbia was fatally damaged.
"Yes, it's risky to fly in space, period. Yes, it's risky to fly in a place with more orbital debris. But I believe these guys have calculated the danger pretty well," said Roger Launius, a space expert and historian at the Smithsonian Institution in Washington.
"These are not teenagers with a souped-up engine running down the road at 130 miles an hour. The risk is certainly worth it in the context of keeping a world-class scientific instrument operating."
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