The Hubble Rescue Mission: What Could Happen?

CAPECANAVERAL, Fla. - The space shuttle Atlantis is moving ever closer to its Mondaylaunch toward the Hubble Space Telescope, but there?s a secondspaceship standing by for a rescue mission NASA hopes it never needs.

That shipis the space shuttle Endeavour, which sits atop a launch pad here at NASA?sKennedy Space Center poisedfor a mission of mercy to rescue the seven astronauts aboard Atlantis iftheir spacecraft is damaged beyond repair during the Hubble flight.

Atlantis isslated to launch Monday at 2:01 p.m. EDT (1801 GMT) so its astronauts canperform the fifth and final overhaul of the19-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 beoccupied at the same time. But it is also an unprecedented first: It?s the onlytime NASA is launching one shuttle with another on rescue duty.

?Everymission up to this point has been preparing us for this type of rescuescenario,? Mike Moses, head of Atlantis? mission management team for the Hubbleflight, told reporters Saturday. ?We basically kind have been building to thispoint.?

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

Mission:Space rescue

The reasonfor the rescue mission is simple. If Atlantis is irreparably damaged duringflight, its crew won?t be able to reach the safe haven of the International Space Stationto seek refuge for the months requiredto ready an unprepared shuttle for a rescue. Hubble flies higher than the spacestation (about 350 miles up, while the station sits at 220 miles) and in a differentorbital inclination - or tilt with respect to Earth?s equator.

Withoutaccess to the space station, Atlantis would only have enough food and air tokeep its crew alive for 25 days, even less time if serious damage is discoveredlate in the mission.

It?s thatrisk that prompted NASA to cancel the flight outright five years ago after theColumbia accident that killed seven astronauts in 2003. A piece of fuel tankfoam punched a hole in the heat shield on Columbia?s left wing during itsliftoff, leaving the shuttle vulnerable to the searing hot temperatures ofre-entry.

NASAresurrected the Hubble mission in 2006 with the caveat that a rescue ship be atthe ready. By then, the space agency had resumed shuttle flights after theColumbia tragedy and successfully tested heat shield inspection and repairtechniques in space. Those techniques have since become standard features ofshuttle flights, and a rescue mission would only launch if they proveinadequate, NASA officials said.

?I thinkwe?re going into this with open eyes,? Altman told SPACE.com. ?I have alot of confidence that we?re going to be able to pull this off.?

NASA hasalso taken steps to reduce the chances of space junk and micrometeoritesmortally wounding Atlantis. Because Hubble flies in an orbit litteredwith more space trash, there?s a slightly higher risk of damage from theorbital 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 Hubbleback into space near the end of the flight.

How therescue mission works

Accordingto NASA?s plan, when Atlantis blasts off toward Hubble on Monday, Endeavourwill be just seven days away from launching the rescue mission, which NASA haschristened STS-400. While Atlantis is in space, NASA will continue preparingEndeavour until it is just three days from launch-ready status.

Meanwhile,the rescue mission?s small four-man crew commanded by veteran astronaut ChrisFerguson would also be on standby alert. Endeavour and its rescue crew willremain at the ready until Atlantis lands or is cleared for re-entry, missionmanagers said. Ferguson?s crew is a fresher team assigned after a broken parton Hubble delayed this mission by seven months last fall.

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

Meanwhile,Altman and his crew would power down Atlantis to conserve their supplies. If therescue mission launches within the first two or three days of the Hubbleflight, Atlantis could keep its crew alive for nearly a month. But if thedamage is discovered later, during a standard late heat shield inspection, theshuttle will likely only have 16 days of air left, Altman said in an interview.

Doubleshuttle rendezvous

Accordingto NASA?s plan, Endeavour would arrive at Atlantis about 23 hours afterlaunching into space. Endeavour would slowly rise up from below to meet itssister ship, then reach out its robotic arm and grab onto Atlantis? own roboticappendage. The two shuttles would be just 24 feet (7 meters) apart, connectedonly by the bent 50-foot (15-meter) shuttle arm.

A series ofthree tricky spacewalks would follow to move Atlantis astronauts from theirstricken ship and into Endeavour. Once the seven Hubble astronauts are safely transferred - Altman would be among the last to leave - Atlantis would beabandoned and all 11 spaceflyers would return home on Endeavour.

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

Moses said thedecision to launch the rescue flight won?t be easy, especially since itinvolves launching more astronauts into space to save their stranded comrades.

?You?reputting another set of crew at risk to go up and rescue,? Moses ?We?re morethan willing to do it, we?re postured to do it, but again there?s a lot of risktrades we?re going to have to do when that scenario comes.??

SPACE.comis providing continuous coverage of NASA?s last mission to the Hubble SpaceTelescope with senior editor Tariq Malik at Cape Canaveral and reporter ClaraMoskowitz in New York. Clickhere for mission updates and SPACE.com's live NASA TV video feed.

• New Video ? Inside the Hubble Rescue Mission
• New Show: Hubble's Universe: The Final Shuttle Service Call
• Image Gallery ? The Hubble Repair Missions: Part 1, Part 2