NASA did not originally intend to launch Atlantis on an actual mission. Using the agency's final set of solid-fuel boosters and its last external tank, Atlantis was designated for stand-by duty as a "launch-on-need" rescue flight for the crew of the shuttle Endeavour, which launched in May.

A Soyuz capsule parachutes to landing. Credit: NASA

This was in keeping with a post-Columbia decision to process the next shuttle in the launch sequence for a quick-response rescue mission if needed.

But as the end of the shuttle program approached, NASA managers decided an additional cargo flight would be good insurance against problems launching the new commercial cargo ships.

But there is no second shuttle to rescue Atlantis' crew if things go wrong after launch. So NASA planners came up with a plan to bring a four-person crew home aboard Russian Soyuz ferry craft during normal space station crew rotations. While it would take a full year for all four Atlantis astronauts to make it home in a rescue scenario, NASA and the Russians believe it could be safely done.

"Because we don't have a rescue vehicle we'll be coming home sort of in a line on the Soyuz, with Rex coming first and then Fergie and then me and Doug," Magnus said. "And so we will slowly become incorporated in the space station life style."

Custom Soyuz seat liners, needed to ease the shock of landing in the cramped Russian capsule, have been made for all four Atlantis astronauts. One of them -- Walheim's -- is stored in the Raffaello cargo module being launched aboard Atlantis. The other three are on standby in Russia and will be launched to the station aboard Progress supply ships if they are needed.

If the crew is stranded aboard the station, Walheim would return to Earth in September aboard the Soyuz TMA-21 spacecraft after a 70-day stay in space, taking Garan's place. Garan, in turn, would extend his stay in orbit to 249 days and return to Earth in December.

Ferguson would be the next to return, taking Fossum's place aboard the Soyuz TMA-02M spacecraft to close out a 131-day stay in orbit. Fossum would return to Earth in April 2012 after an extended 304-day mission.

Magnus would come down next April aboard the Soyuz TMA-03M spacecraft after 273 days in space. Finally, Hurley would return to Earth aboard the Soyuz TMA-04M spacecraft next June after nearly a full year -- 335 days -- aloft.

"When this would happen, if this happens, Rex's (seat liner) would go immediately into the Soyuz of Ron Garan's, I believe, and then Fergie's would go where Mike Fossum's would go. So Mike and Ron would actually extend their stay in order to accommodate Rex and Fergie coming home. And then the next couple of Soyuzes would come up with an empty right seat and then Doug and I would slowly rotate down after those Soyuzes came down after their full six-month mission.

"As far as being ready for it, we've had some basic training on the space station, some of the big-picture emergency-type scenarios and some of the warning-type scenarios where we'd have to react quickly and expeditiously. But the plan is if we would have an extended stay on the space station we would get additional training on the station. After living there for a few weeks, you can very easily get in the routine of the everyday life and how you do operations. ... There's a very good plan in place and we'll be able to, I think sort of assimilate into space station fairly straight forwardly."

Added Ferguson: "This is a very low likelihood case. But the engineering (and) safety arm of NASA has done an extraordinarily thorough job of making sure we have a good plan to get home."

Coming up with a viable rescue plan in the absence of a second space shuttle was just one of the hurdles faced by NASA planners as momentum built to turn the stand-by launch-on-need mission into an actual flight. Another was coming up with refined procedures to get critical tasks done with fewer astronauts than usual. Atlantis' fight is the first since STS-6 in 1983 to launch with just four crew members.

"We've had a chance in training to go through all the various scenarios, contingency deorbits, different kinds of contingency undocking scenarios, and we have actually found procedures that assume you have five or six or seven crew members," Magnus said. "And so these have been very good exercises for us to go through and figure out, OK, so now we only have four people, how are we going to do it?

"The overall workload is pretty high, we've had to do a lot more cross training than normal for a shuttle crew. It actually feels like a station mission because you end up being a little bit more of a jack of all trades and not so much just a pipeline specialist. And that's been a lot of fun, because we get to dabble in each other's areas.

"So I think between the cross training and the fact that we've nosed out these scenarios where we need to be a little bit smarter, we're going to be in pretty good shape. But we are going to be working very hard. But it'll be a lot of fun at the same time, and very challenging."

PART 6: Tight timeline for Atlantis' astronaut crew -->