NASA Targets Friday Launch for Shuttle Discovery
Xenon lights over Launch Pad 39A at NASA's Kennedy Space Center in Florida compete with the lightning strike seen to the left as the space shuttle Discovery is on the pad waiting for a scheduled liftoff on the STS-128 mission on Aug. 25, 2009. Stormy weather delayed the launch by one day.
Credit: NASA/Ben Cooper

CAPE CANAVERAL, Fla. ? NASA is now targeting no earlier than Friday to try to loft the space shuttle Discovery after a critical valve failed a standard check during its second launch attempt earlier today.

Mike Moses, head of Discovery?s mission management team, told reporters here at NASA?s Kennedy Space Center that engineers will begin studying the valve failure on Wednesday and decide Thursday whether to aim for a weekend launch or stand down for lengthy repairs.

The shuttle was slated to launch early Wednesday at 1:10 a.m. EDT (0510 GMT) to send seven astronauts to the International Space Station on a 13-day construction flight. The mission has already been delayed a day due to stormy weather here at NASA?s Kennedy Space Center early Tuesday.

The vital drain valve is part of a system used to fill and drain Discovery?s 15-story external tank with super-cold liquid hydrogen propellant by routing it through the aft section of the orbiter. The shuttle has a similar valve for its liquid oxygen supply and both must function perfectly in order to launch.

While fueling Discovery?s tank today, the hydrogen valve failed to indicate it was closed when launch controllers sent it a test command. NASA engineers are unsure whether the glitch is an instrumentation error or a sign of something more serious - like wear damage to the valve that would require it to be replaced. If it does have to be swapped out, there may not be enough time to make the switch and still try to launch by Aug. 30, when NASA?s window to launch Discovery?s mission closes.

?It?s not a guarantee, but we?re probably not going to make it,? Moses said of the potential repair.

Also muddying Discovery?s chances for launch is a slight hydrogen leak that engineers detected after today?s cancelled launch attempt. The leak was well below NASA?s safety limits but mission management opted to check it out anyway since it is near the location of the faulty drain valve.

?Prudence does dictate that we take a look at it,? said Pete Nickolenko, NASA?s launch director.

Moses said that mission managers plan to meet Thursday afternoon to take stock of the hydrogen valve troubleshooting work and decide whether to push toward a Friday launch.

Discovery actually has two chances to launch on Friday. The first is at 12:22 a.m. EDT (0422 GMT) and would require NASA to begin fueling Discovery?s external tank late Thursday for the attempt. The other is late in the evening at 11:59 p.m. EDT (0359 Aug. 29 GMT), with fueling beginning earlier that day.

NASA has until Aug. 30 to launch Discovery before standing down due to a conflict with another operation on the launch range it shares with the nearby Cape Canaveral Air Force Station.

The space agency wants to launch the shuttle mission before the end of August in order to avoid space traffic conflicts with Japan?s first unmanned space cargo ship and a Russian crew-carrying Soyuz spacecraft, both of which are also set to fly to the International Space Station in September.

If Discovery does not launch by Aug. 30, NASA?s next opportunity will come Oct. 17, mission managers have said.

Commanded by veteran spaceflyer Rick Sturckow, Discovery?s six-man, one-woman crew plans to deliver nearly 8 tons of food, equipment and science gear to the International Space Station. The astronauts plan to replace one member of the station?s six-person crew and deliver a new treadmill named after television comedian Stephen Colbert.

Three spacewalks are also planned for the mission. Discovery?s flight will mark NASA?s fourth shuttle mission of the year and the second to launch at night.

Discovery?s astronaut crew will likely have some time off over the next two days as engineers try to fix what?s wrong with their ailing spaceship.

?Bad luck again, but maybe we can have dinner with the families,? said Swedish astronaut Christer Fuglesang, a Discovery mission specialist, in a Twitter update.

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SPACE.com is providing complete coverage of Discovery's STS-128 mission to the International Space Station with Managing Editor Tariq Malik in Cape Canaveral, Fla., and Staff Writer Clara Moskowitz in New York. Click here for shuttle mission updates and a link to NASA TV.