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Shuttle's risky mission to repair Hubble

With custom-made tools and a rescue plan, the seven-member crew of the Atlantis aim to upgrade the iconic telescope.

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"There won't be any next chance to do things that we don't get accomplished," says Preston Burch, Hubble program manager at the Goddard Space Flight Center. With five back-to-back days of spacewalks and a tightly packed timeline for finishing tasks, this trip is the most challenging servicing mission that shuttle astronauts have ever faced, he adds.

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Riskier than a space-station mission

The mission also hovers near the boundary of acceptable risk to the orbiter and crew.

A mile and a half away from Atlantis's Pad 39-A, the shuttle Endeavour sits poised on Pad 39-B. If needed, it will carry a four-member crew to bring the Hubble repair crew home in case space junk or micrometeroids inflict irreparable damage to Atlantis's thermal protection system. Not since the Skylab missions in the mid 1970s has NASA kept rockets at the ready for a rescue.

The thermal-protection system safeguards the shuttle and its crew from the heat of reentry. The system consists of heat-shedding tiles on the orbiter's underside and heat-resistant material covering the nose and the leading edges of the wings and tail. Damage to the system from debris strikes at liftoff led to the Columbia disaster in 2003 as the craft reentered the atmosphere.

Hubble's orbit won't allow Atlantis to reach the safe haven of the space station if a debris strike proves too serious to patch.

Leroy Cain, NASA's deputy shuttle program manager, puts some numbers to the risk. The agency uses one chance in 200 of catastrophic damage as a benchmark, he says. Higher risks are classified as "probable" catastrophic loss of orbiter or crew. Lower risks get dubbed "remote."

A typical flight to the space station carries a risk of between one chance in the low 300s to one chance in the high 200s of catastrophic damage, he explains. As late as mid April, however, mission planners were looking at an average risk of about one in 185 for the Hubble mission, putting it in the probable column.

But planners nudged the risk into the "remote" column by having Atlantis scurry to a lower, less debris-strewn orbit once the crew finishes with Hubble. And while Atlantis has Hubble locked to a servicing frame in its cargo bay, the shuttle will orbit tail first and cargo bay down. Planners say that will help protect the nose and leading edges of wings and tail from potential collision damage.

Toss those changes into the mix, and the latest risk number stands at one chance in 229 of catastrophic damage, according to John Shannon, the shuttle's program director. That's still greater than a trip to the space station. But it also falls just on the upbeat side of the border between probable and remote.

Plans for a rescue, just in case

If a rescue becomes necessary, Endeavour will launch and rendezvous with Atlantis in its lower, post-repair orbit. Oriented nose to tail and payload bay over payload bay, Endeavour's crew will reach out to Atlantis with the shuttle's robotic arm and grapple to a fixture on Atlantis's arm.

The rescue would involve stringing a tether along the robotic arm, then moving the crew across in three spacewalks over two days. It also would involve a transfer of all seven crew reentry suits from Atlantis to Endeavour. All the spacewalking would be done by Atlantis's crew, with Dr. Grunsfeld acting as choreographer for the first two sets of evacuations.

The day after Endeavour arrives, Grunsfeld would transfer suits and shepherd two crew members across the gap.

On the second day, Grunsfeld would shepherd two more crew members across. After a 4-1/2-hour break, one of those, mission specialist Mike Massimino, would return to Atlantis and act as guide for the last two crew members, including the commander. Before leaving, the commander would ensure enough systems remain operating on Atlantis to allow controllers on the ground to guide it to a reentry and splash down in the Pacific Ocean.