Nasa prepares for mission it hopes never to fly

The space shuttle Endeavour is perched on a launch pad at the Kennedy Space Centre and ready to lift off within days. But this mission is one Nasa hopes will never fly.

The shuttle is poised, if needed, to rescue the seven astronauts on the shuttle Atlantis if they can't make it home on their own ship.

Nasa sees little chance the mission will ever take place. Safety upgrades have greatly reduced the chance of damage from debris hitting the shuttle during launch - the scenario that doomed the shuttle Columbia in 2003.

The destruction of Columbia and death of all the astronauts on board also prompted Nasa to develop new repair techniques and tools. In a worst case scenario, crews also could await a rescue on the International Space Station.

But the Atlantis mission that lifted off on Monday is a special case. Sent to repair the Hubble Space Telescope, which orbits at a different level, the shuttle cannot reach the station, even in an emergency. So Nasa devised a backup plan: Prepare a second shuttle and crew for launch.

"It's been years in the making," Nasa spokesman Rob Navias said. "They specially designed this as the companion to get (the Hubble repair) mission approved to fly."

Under the plan, Endeavour could lift off as early as Monday with a four-person crew and extra seating for Atlantis' seven astronauts, if Atlantis were found to be severely damaged.

The day after launch, Endeavour would meet up with Atlantis. Astronauts would use Endeavour's robot arm to grab hold of Atlantis' boom so that the ships were facing cargo bay-to-cargo bay and oriented at a 90-degree angle to each other.

Over the next two days, the Atlantis crew would make three spacewalks to set up a tether between the ships' airlocks, and then transfer spacesuits and themselves over to Endeavour.

Before leaving Atlantis for the last time, one astronaut would configure the ship to be remotely commanded by the ground. After Endeavour flew away, the empty Atlantis would be guided to re-enter the atmosphere over unpopulated parts of earth to minimise the risk on the ground from falling pieces if it broke apart.

The choreography is similar to what Nasa would do if it needed to rescue shuttle crew members from the space station, which has only a single docking port for a visiting space shuttle.

Under that scenario, however, Nasa would have something it doesn't have for a Hubble crew rescue - time.

Endeavour will remain poised for service until after the Atlantis crew conducts a final inspection next week to check for damage from debris during orbit.

The shuttle, which is flying about 350 miles above earth, is at a slightly higher risk of damage from such debris due to the crash of two satellites in February, which generated a cloud of junk. Moving at 17,500 mph, objects as small as a marble can pack a dangerous wallop.