The space shuttle Endeavour rolled out to a Florida launch pad on Friday to stand by for an unprecedented rescue mission NASA hopes it never has to fly.
Endeavour arrived at Launch Pad 39B at NASA's Kennedy Space Center in Cape Canaveral, Fla., at 7:17 a.m. ET, where it will be primed to launch within weeks of any emergency aboard its sister ship Atlantis during next month's risky repair mission to the Hubble Space Telescope. Atlantis is poised atop its own launch pad for a planned May 12 liftoff.
"It's the first time that anyone has had a rescue mission standing by," NASA spokesperson Allard Beutel told SPACE.com from the Florida spaceport. "We thought it gives us that amount of extra insurance."
Endeavour has actually made the trek to the launch pad once before to serve as the upcoming Hubble mission's rescue ship. The shuttle was on standby last September until a failure aboard Hubble delayed a planned October launch for Atlantis.
Altogether, two orbiters have been at their launch pads simultaneously only 18 times in NASA's 28-year history of shuttle flight, Beutel said.
"We really do expect this to be the last time we ever have two space shuttles on the launch pad at the same time," he added.
NASA has said the likelihood of requiring a rescue mission for the seven Hubble-bound astronauts is extremely remote. But the space agency is taking the precaution because unlike recent shuttle flights to the International Space Station, where astronauts could seek shelter if their spacecraft is severely damaged, the crew of Atlantis will have no such safe haven.
Commanded by veteran NASA astronaut Scott Altman, Atlantis is poised to launch toward Hubble on May 12 from Launch Pad 39A, which is near Endeavour's current perch. The much-delayed mission is NASA's final shuttle flight to overhaul Hubble and includes five back-to-back spacewalks to repair broken equipment, add new instruments and extend the iconic space telescope's lifetime through at least 2014.
Because Hubble flies in a higher orbit and different inclination than the international space station, Atlantis would be unable to reach the orbiting laboratory if it suffers a critical failure or severe damage its vital heat shield. The mission also includes a higher risk of damage from orbital debris or micrometeorites, though that risk increase appears to be lower than initially thought, NASA officials have said.
It is because of the lack of a safe haven aboard the station that NASA opted to prepare Endeavour for a potential rescue. The shuttle is already slated to fly a space station construction mission in June, but will be primed to launch with a skeleton crew relatively soon after an emergency is declared, Beutel said.
"Within that timeframe, we would be prepared to launch Endeavour with a crew of four astronauts," Beutel said, adding that the Atlantis astronauts would stage several spacewalks to abandon their ship and return home aboard Endeavour. "We would then splash Atlantis down safely somewhere in the Pacific Ocean."
If Atlantis's Hubble mission goes smoothly as planned, Endeavour would simply be moved to the adjacent launch pad in late May for a planned June 13 launch to the international space station, Beutel said.
NASA plans to launch its remaining shuttle missions from Launch Pad 39A and use the nearby Pad 39B for its new Ares I rocket and Orion capsules, which will replace the agency's space shuttle fleet, once Endeavour is moved, he added.
Over the weekend, NASA will deliver the hardware for Atlantis's Hubble mission to the shuttle's launch pad, offering a relatively rare sight of two shuttles exposed atop their pads with their shroud-like service structures retracted. The two spacecraft are expected to be visible through Monday and will be photographed from the ground and air, Beutel said, adding that NASA expects about 50,000 people to visit the space center during a Saturday employee Family Day.
"I think people are really excited about it," Beutel said.