This story was updated at 7:00 p.m. EDT.
CAPE CANAVERAL, Fla. - A broken valve on the space shuttle Discovery forced NASA to call off the spacecraft?s second launch attempt in a row late Tuesday as engineers troubleshoot the glitch.
Discovery was slated to blast off at 1:10 a.m. EDT (0510 GMT) Wednesday morning, but a broken fill-and-drain valve associated with the shuttle?s liquid hydrogen propellant system foiled the attempted launch.
?This particular valve did not open and close the way it was supposed to,? said NASA launch commentator Allard Beutel.
NASA engineers are discussing the malfunction, which cropped up during Discovery?s fueling operations at 5:52 p.m. EDT (2122 GMT), just under seven hours before launch time. Stormy weather delayed Discovery?s first launch attempt earlier today.
NASA has until Aug. 30 to launch the shuttle before standing down due to restrictions on the launch range it shares with the nearby Cape Canaveral Air Force Station. Engineers are discussing how to best solve the valve malfunction and when Discovery may be ready to launch again.
?At this particular time, we don?t know when the next launch attempt might be,? Beutel said.
NASA wants to launch Discovery by the end of August to avoid a traffic conflict with an unmanned Japanese cargo ship also due at the space station this month. If the mission is unable to launch by then, NASA can try again on Oct. 17.
NASA space shuttles use 526,000 gallons of super-cold liquid hydrogen and liquid oxygen propellant to feed their main engines during the 8 1/2-minute launch into space. The orbiters have two 8-inch propellant fill-and-drain valves, one each for hydrogen and oxygen, in their aft compartments that are required to work perfectly in order to lift off.
?When [launch controllers] commanded it to be closed, they didn?t get an indication that it was closed,? NASA spokesperson Candrea Thomas said of Discovery?s hydrogen valve. ?So that?s the issue.?
The valve is currently open, which allows the shuttle?s fuel tank to be drained, but must be closed during launch, she added.
The aft compartment that houses Discovery?s broken valve is accessible to shuttle technicians at the launch pad. But NASA must first drain the shuttle?s 15-story fuel tank and safeguard Discovery?s explosive bolts and other pyrotechnic systems before engineers can study the faulty valve, Thomas said.
Commanded by veteran shuttle flyer Rick Sturckow, Discovery?s STS-128 mission is slated to fly to the International Space Station to deliver nearly 8 tons of cargo, including new science gear and a treadmill named after comedian Stephen Colbert. The 13-day mission was also planned to ferry astronaut Nicole Stott to the station to replace NASA astronaut Tim Kopra, who has lived aboard the orbiting lab since mid-July.
Set to launch aboard Discovery with Sturckow and Stott were STS-128 mission specialists Chris Patrick Forrester, Jose Hernandez, Danny Olivas and Swedish astronaut Christer Fuglesang. Fuglesang represents the European Space Agency, while the rest are NASA astronauts. Three spacewalks are planned for the mission.
NASA plans to hold a press conference late Tuesday to discuss today?s launch scrub and forward plans for Discovery?s mission.
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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.