The space shuttle Atlantis is moving ever closer to its Monday launch toward the Hubble Space Telescope, but there's a second spaceship standing by for a rescue mission NASA hopes it never needs.

That ship is the space shuttle Endeavour, which sits atop a launch pad here at NASA's Kennedy Space Center poised for a mission of mercy to rescue the seven astronauts aboard Atlantis if their spacecraft is damaged beyond repair during the Hubble flight.

Atlantis is slated to launch Monday at 2:01 p.m. EDT (1801 GMT) so its astronauts can perform the fifth and final overhaul of the 19-year-old Hubble Space Telescope.

With Endeavour also primed for flight, it is the last time ever that both of NASA's shuttle launch pads will be occupied at the same time. But it is also an unprecedented first: It's the only time NASA is launching one shuttle with another on rescue duty.

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"Every mission up to this point has been preparing us for this type of rescue scenario," Mike Moses, head of Atlantis' mission management team for the Hubble flight, told reporters Saturday. "We basically kind have been building to this point."

NASA believes the chances of needing the rescue flight are extremely unlikely, but the agency drew up plans for one just in case.

Mission: Space rescue

The reason for the rescue mission is simple. If Atlantis is irreparably damaged during flight, its crew won't be able to reach the safe haven of the International Space Station to seek refuge for the months required to ready an unprepared shuttle for a rescue.

Hubble flies higher than the space station (about 350 miles up, while the station sits at 220 miles) and in a different orbital inclination — or tilt with respect to Earth's equator.

Without access to the space station, Atlantis would only have enough food and air to keep its crew alive for 25 days, even less time if serious damage is discovered late in the mission.

It's that risk that prompted NASA to cancel the flight outright five years ago after the Columbia accident that killed seven astronauts in 2003.

A piece of fuel tank foam punched a hole in the heat shield on Columbia's left wing during its liftoff, leaving the shuttle vulnerable to the searing hot temperatures of re-entry.

NASA resurrected the Hubble mission in 2006 with the caveat that a rescue ship be at the ready.

By then, the space agency had resumed shuttle flights after the Columbia tragedy and successfully tested heat shield inspection and repair techniques in space.

Those techniques have since become standard features of shuttle flights, and a rescue mission would only launch if they prove inadequate, NASA officials said.

"I think we're going into this with open eyes," Altman told SPACE.com. "I have a lot of confidence that we're going to be able to pull this off."

NASA has also taken steps to reduce the chances of space junk and micrometeorites mortally wounding Atlantis. Because Hubble flies in an orbit littered with more space trash, there's a slightly higher risk of damage from the orbital debris — about a 1-in-229 chance of a critical strike.

To offset that, NASA plans to fly Atlantis down to a safer orbit just after releasing Hubble back into space near the end of the flight.

How the rescue mission works

According to NASA's plan, when Atlantis blasts off toward Hubble on Monday, Endeavour will be just seven days away from launching the rescue mission, which NASA has christened STS-400.

While Atlantis is in space, NASA will continue preparing Endeavour until it is just three days from launch-ready status.

Meanwhile, the rescue mission's small four-man crew, commanded by veteran astronaut Chris Ferguson, would also be on standby alert.

Endeavour and its rescue crew will remain at the ready until Atlantis lands or is cleared for re-entry, mission managers said.

Ferguson's crew is a fresher team assigned after a broken part on Hubble delayed this mission by seven months last fall.

If the rescue flight is required, NASA would begin the three-day countdown toward Endeavour's launch. Ferguson and his rescue crew already plan to be here at the launch site ready to fly, Moses said.

Meanwhile, Altman and his crew would power down Atlantis to conserve their supplies. If the rescue mission launches within the first two or three days of the Hubble flight, Atlantis could keep its crew alive for nearly a month.

But if the damage is discovered later, during a standard late heat-shield inspection, the shuttle will likely only have 16 days of air left, Altman said in an interview.

Double shuttle rendezvous

According to NASA's plan, Endeavour would arrive at Atlantis about 23 hours after launching into space.

Endeavour would slowly rise up from below to meet its sister ship, then reach out its robotic arm and grab onto Atlantis' own robotic appendage.

The two shuttles would be just 24 feet (7 meters) apart, connected only by the bent 50-foot (15-meter) shuttle arm.

A series of three tricky spacewalks would follow to move Atlantis astronauts from their stricken ship and into Endeavour.

Once the seven Hubble astronauts are safely transferred — Altman would be among the last to leave — Atlantis would be abandoned and all 11 spaceflyers would return home on Endeavour.

Atlantis, meanwhile, would be left in space to be remotely ditched in a fiery demise over the Pacific Ocean.

Moses said the decision to launch the rescue flight won't be easy, especially since it involves launching more astronauts into space to save their stranded comrades.

"You're putting another set of crew at risk to go up and rescue," Moses "We're more than willing to do it, we're postured to do it, but again there's a lot of risk trades we're going to have to do when that scenario comes."

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