For the first time since 2001, space shuttles are poised on both launch pads at Kennedy Space Center as NASA simultaneously readies Atlantis for a critical mission to service the Hubble Space Telescope and preps Endeavour for a flight to rescue Atlantis' crew should that orbiter be unable to safely return home because of ascent or on-orbit damage. With Hubble circling Earth in a different orbit than the space station, the shuttle would be unable to seek safe haven at the complex in the event it's unable to re-enter and land safely. Perched atop one of NASA's giant crawler transporters, Endeavour left the Vehicle Assembly Building at 11:15 p.m. EDT. Moving less than one mile per hour, the shuttle reached Launch Complex 39-B and was hard down on the pad at 7 a.m. NASA hopes a rescue mission won't be required, but the agency is taking no chances by having Endeavour ready to mount an emergency mission on short notice if it becomes necessary.The mission to service the Hubble Space Telescope was cancelled by former NASA Administrator Sean O'Keefe in the wake of the Columbia accident. O'Keefe cited the safety concern regarding the lack of the space station to provide safe harbor for a damaged orbiter as the primary reason for canceling the mission. Under pressure from the public and Congress, Administrator Michael Griffin ordered the agency to review the mission and determine if the risk of losing the crew in a re-entry accident could be avoided. The plan devised by the shuttle program calls for a second shuttle to be undergoing prelaunch preparations on the other launch pad when Atlantis lifts off from pad 39-A. If a rescue mission is deemed necessary, the second shuttle, Endeavour will be moved from pad B one and a half miles to pad A launched to retrieve Atlantis' astronauts. Shuttle program managers believe they can launch Endeavour within 5 or 6 days from the time the call is made to launch the rescue flight.If routine inspections of Atlantis on flight day 2 reveal serious damage that can't be repaired on orbit, the crew would be able to power down all but essential equipment on the vehicle and restrict use of other consumables to achieve an on-orbit time of about 25 days, which NASA hopes would provide enough margin to get Endeavour off the ground, given the potential for weather and technical problems to delay launch.However, if Atlantis is damaged by what is termed MMOD, or micrometeoroid orbital debris, that damage probably wouldn't be noticed until the late inspections take place after Hubble is released back into orbit. In that case, NASA would have less than a week in which to determine if the damage could be repaired and, if not, to launch Endeavour on a rescue mission before Atlantis uses up all its power supply.The rescue plan calls for Endeavour and a crew of four astronauts to rendezvous with Atlantis about 23 hours after launch. The two ships lack the ability to dock together, so instead, Endeavour and Atlantis will join using the shuttle's robotic arm to create a firm link between the two spacecraft.Atlantis crew will don spacesuits and, during three spacewalks over two days, will float over to Endeavour's payload bay and climb aboard through its airlock.Once all astronauts have been transferred, Endeavour will leave Atlantis and return home with all eleven astronauts. Mission control in Houston would then command Atlantis to execute an uncontrolled re-entry over the Pacific Ocean, where it would break up and fall harmlessly into the sea.There is, however, no rescue option for Endeavour should it, too, become damaged during flight.The likelihood of such a contingency with Atlantis is considered fairly remote. Most orbital debris that Atlantis might encounter is too small to cause anything more than tiny divots or dings in the shuttles heat shield of the type seen throughout the history of the shuttle program. Even if Atlantis does sustain damage that requires repair or a rescue mission, the orbiter is outfitted with an array of methods for repairing heat shield tile and the reinforced carbon-carbon panels that line the leading edges of its wings. Options include a paint-like heat-reflecting "emittance wash" that can be applied over large but shallow areas of tile damage. For deeper damage, ablative putty can be applied to protect the underlying aluminum skin of the orbiter from the heat of re-entry. For small areas of puncture-type damage to the RCC panels, the agency designed special plugs that can be inserted to cover the hole and protect the interior of the wing.In addition to its potential role as astronaut lifeboat, Endeavour is slated to launch on the next shuttle mission, STS-126, a logistics flight to the International Space Station. After Atlantis is cleared for re-reentry, Endeavour's processing will switch to STS-126 preparations from its role as rescue craft. That flight is targeted for liftoff on November 12, just three weeks after Atlantis return.Today's rollout marks on the 18th time in shuttle program history that NASA has had shuttles on both its launch pads simultaneously. The last time this happened was in July 2001, when Atlantis and Endeavour also occupied the twin launch sites. The previous times that shuttles have occupied both launch pads are listed below:1. STS-104 (Atlantis), STS-105 (Discovery) - July 2, 2001 - July 12, 20012. STS-103 (Discovery), STS-99 (Endeavour) - Dec. 13, 1999 - Dec. 19, 19993. STS-95 (Discovery), STS-88 (Endeavour) - Oct. 21, 1998 - Oct. 29, 19984. STS-73 (Columbia), STS-74 (Atlantis) - Oct. 12, 1995 - -Oct. 20, 1995)5. STS-69 (Endeavour), STS-73 (Columbia) - Aug. 28, 1995 - -Sept. 7, 19956. STS-70 (Discovery), STS-69 (Endeavour) - July 6, 1995 - July 13, 19957. STS-71 (Atlantis), STS-70 (Discovery) - June 15, 1995 - June 27, 19958. STS-71 (Atlantis), STS-70 (Discovery) - May 11, 1995) - June 8, 19959. STS-64 (Discovery), STS-68 (Endeavour) - Aug. 19, 1994 - Aug. 24, 199410. STS-56 (Discovery), STS-55 (Columbia) - Feb. 7, 1993 - April 8, 199311. STS-50 (Columbia), STS-46 (Atlantis) - June 11, 1992 - June 25, 199212. STS-45 (Atlantis), STS-49 (Endeavour) - March 13, 1992 - March 24, 199213. STS-37 (Atlantis), STS-39 (Discovery) - April 1, 1991 - April 5, 199114. STS-38 (Atlantis), STS-35 (Columbia) - Oct. 14, 1990 - Nov. 15, 199015. STS-35 (Columbia), STS-41 (Discovery) - Sept. 4, 1990 - Oct. 6, 199016. STS-31 (Discovery), STS-35 (Columbia) - April 22, 1990 - April 24, 199017. STS-61C (Columbia), STS-51L (Challenger) - Dec. 22, 1985 - Jan. 12, 1986Atlantis is currently targeted for liftoff on October 10 to begin an eleven-day mission to service and upgrade the Hubble Space Telescope for the fifth and final time. At the conclusion of the mission, Hubble will be restored to like new condition, in some respects even better than new, providing Earth-bound scientists with at least 5 more years or groundbreaking observations from NASA's flagship "Great Observatory".