CAPE CANAVERAL, Fla. - NASA is trying yet again to launch the space shuttle Endeavour Monday, despite a good chance of rain showers that threaten to delay the mission for a potential fifth time.
Rain poses a special risk to today's liftoff plans because of an exposed thruster on the shuttle's nose. The plastic cover that usually guards the thruster - one of 44 small rocket thrusters on the shuttle used for minor course changes - came loose yesterday during Endeavour's failed fourth launch attempt.
While the loose cover posed no risk for launch yesterday (the seal is designed to drop off during liftoff anyway), if any moisture gets into the thruster it could freeze into ice and render the thruster useless.
That means that if any rain falls on the pad, NASA will likely be forced to call off today's launch, set for 6:51 p.m. EDT (2251 GMT).
"The idea is if you get rain then you start to risk moisture in the thruster, and you don't want it to freeze," said NASA spokesman Allard Beutel. NASA would most likely not launch with a disabled thruster, even though it is not critical for liftoff.
While Sunday's launch attempt was cancelled because of thunderstorms near the launch pad and landing site here at Kennedy Space Center, no precipitation actually fell on Endeavour's Launch Pad 39A.
"If we had had rain at the pad it would have bumped us out for sure," Beutel said of today's liftoff plans.
Rain is a serious threat for Endeavour's flight attempt today, with a 60 percent chance of thunderstorms predicted. Despite the gloomy forecast, NASA decided to begin fueling the shuttle's massive external tank in preparation for launch early this morning, just in case the weather cooperates.
"They thought it was worth at least taking the chance," Beutel said.
If today's launch is cancelled, NASA can try again tomorrow if ground crews have enough time to reattach the thruster cover. This procedure takes three to four hours, because the giant Rotating Service Structure scaffolding on the launch pad must be moved back into place to give crews access to Endeavour's nose.
If the process can't be completed in time, NASA may be able to push the launch window out a few days, though the shuttle is currently scheduled to stand down to let an unmanned Russian cargo ship launch to the space station later this month.
"We can talk to the Russians and maybe we can get some extra time," Beutel said.
Endeavour's oft-delayed mission is a construction visit to the International Space Station to install spare supplies and a new exposed research platform for the Japanese Kibo laboratory. Today is the beleaguered shuttle's fifth attempt to lift off on its STS-127 mission, which has been held at the ground by bad weather and a pesky gas leak, which has since been repaired.
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SPACE.com is providing continuous coverage of STS-127 with reporter Clara Moskowitz at Cape Canaveral and senior editor Tariq Malik in New York. Click here for mission updates and SPACE.com's live NASA TV video feed. Live launch coverage will begin at 1:30 p.m. EDT (1730 GMT).