CAPE CANAVERAL, Fla. — In an unprecedented step, a space shuttle was moved to the launch pad Friday for a trip NASA hopes it will never make — a rescue mission.
The shuttle Endeavour is on standby in case the seven astronauts who go up on Atlantis next month need a safer ride home.
Atlantis and its crew are headed into space for one last repair job on the 18-year-old Hubble Space Telescope. It’s a venture that was canceled when first proposed a few years ago because it was considered too dangerous.
The risk is this: If Atlantis suffers serious damage during launch or in flight, the astronauts will not be at the international space station, where they could take refuge for weeks while awaiting a ride home. They would be stranded on their spacecraft at the Hubble, where NASA estimates they could stay alive for 25 days. Air would be the first to go.
Endeavour and four more astronauts would need to blast off on a rescue flight as soon as NASA determined Atlantis was too damaged to fly home.
On Friday, Endeavour was parked at its launch pad just a mile from where Atlantis is tentatively set to lift off on Oct. 10.
It is the first time since 2001 — when flights were more closely spaced — that both of NASA’s shuttle pads have been occupied. And it will probably be the last.
The Atlantis astronauts say there’s a slim chance any rescue will be needed, and they say they would fly to Hubble even if there were no such backup plan.
Scott Altman, Atlantis’ commander, said it may seem like overkill, but having a rescue ship on the pad is the right thing to do.
‘‘It’s kind of a belt-and-suspenders approach. But if you need the belt after your suspenders fail, you would be glad you had it,’’ said Altman, a retired Navy captain and former fighter pilot.
On top of the usual launch and landing dangers, the Atlantis crew faces an estimated 1-in-185 chance that a piece of space junk or a micrometeoroid will cause catastrophic damage to their ship. Those are greater odds than for a typical shuttle flight because of Hubble’s extremely high and debris-littered orbit.
Before reaching Hubble and again after leaving it, the Atlantis astronauts will inspect their spacecraft for signs of damage, just as crews always do while in orbit.
Ever since space shuttles resumed flying following the 2003 Columbia tragedy that killed seven astronauts, NASA has had a rescue plan in case of irreparable damage. But all those missions have been to the space station, where astronauts could camp out for two months.
The Hubble mission offers no such safe haven. That’s why the Hubble repair mission was canceled in 2004; NASA’s boss at the time deemed it too dangerous.
A new NASA regime reversed that decision, once space shuttles were flying safely again and repair methods became available to orbiting astronauts. The caveat was that another shuttle be on the launch pad, all prepped and ready to fly — something never before attempted.
NASA took similar steps in 1973 during its first space station program, Skylab. But a rescue was never needed.
Once Atlantis is aloft, ‘‘if it even begins to smell’’ like a rescue might be needed, final preparations for Endeavour will begin, said launch director Mike Leinbach. He said Endeavour could lift off within six days.
The rescue craft would fly to Atlantis and use a 50-foot robot arm to grab the damaged shuttle. The Atlantis astronauts would put on spacesuits and float, a few at a time, to Endeavour over the course of three spacewalks. Endeavour would return home with all 11 astronauts.
The toughest call, officials say, would be deciding that Atlantis indeed had serious enough damage that a rescue should be tried.
‘‘This will be an emotional thing,’’ Leinbach said.
Such a rescue would put four more astronauts at risk and would mean the end of Atlantis, and undoubtedly the space shuttle program, which is set to be phased out in 2010. Atlantis would be sent into the Pacific once its astronauts were aboard Endeavour.
It would rank right up there with the drama of Apollo 13, said Ed Mango, Atlantis’ launch director. For Leinbach, who would head up the rescue launch, it would be the most important thing NASA has ever done, period.
Altman realizes that if pressed into service, Endeavour might not get off in time. Storms or a last-second engine shutdown could keep it grounded.
‘‘There’s no guarantee it would get there,’’ Altman said in an interview with The Associated Press. ‘‘On the other hand, you look at how many things would have to go wrong to make it not possible to pull off. ...
‘‘There’s a scenario out there that doesn’t have a happy ending, and I think we all have to come to grips with that before launch.’’