Just before noon on April 29, 2011, Mark Kelly stood in his orange launch suit and waved at the crowd of reporters around him. He was beside the silver Airstream called the Astrovan, which would carry him and his five crewmates to the shuttle Endeavour, boiling away on the nearby pad. If there was such a thing as just another astronaut, he was no longer it. He had been thrust into a different American spotlight four months earlier, when his wife, Representative Gabrielle Giffords of Arizona, was shot in the head outside a grocery store in Tucson. His already extraordinary life had been made even more singular by the introduction of a radical particle named Jared Lee Loughner. Now as much as everything else that Kelly was, he had become the husband of a woman made famous for having somehow survived a bullet's passing through her brain. And right alongside her, he had become the embodiment of American hope against very long odds.

There was a 70 percent chance that Endeavour would lift off that afternoon; at least, there was only a 30 percent chance that thick cloud cover would interfere with the day's big plans. But everybody, the astronauts most of all, knew that weather was only part of the equation. There were still a million possible reasons why the space shuttle might not do what it was expected to do. It was an unpredictable machine.

Kelly smiled and waved one last time before he boarded the Astrovan. A few miles away, his wife was making her own preparations. In the months leading up to that 70-percent-chance-of-sun morning, Kelly and Giffords had been the twin subjects of intense speculation. There were suggestions that Kelly might choose to stay home with her, monitoring her slow and painful recovery, rather than fly into space. The people making those suggestions weren't astronauts.

Somewhere along the way, astronauts diverge from the rest of us. The pilots, especially, become something different. Their standards of "ordinary" change, particular...