NASA Resumes Countdown for Possible Friday Shuttle Launch

The clock is again ticking down toward launch for NASA?s space shuttle Discovery as mission managers prepare to discuss whether the spacecraft is indeed ready for its planned Friday blast off.

NASA resumed the countdown today for Discovery?s planned 12:22 a.m. EDT (0422 GMT) launch Friday morning after testing a suspect valve that prevented the shuttle from lifting off earlier this week.

The valve, which is in the shuttle?s main propulsion system, passed five tests to cycle it open and closed late Wednesday. Mission managers will meet this afternoon to discuss those tests and decide whether to press ahead with Discovery?s launch or stand down for lengthy repairs.

?[The valve] didn?t break itself apart. All five times it did exactly as it was expected to do,? NASA spokesperson Allard Beutel told SPACE.com late Wednesday. ?They can take that to the mission management team Thursday for discussion.?

If NASA decides to forge ahead with Discovery?s Friday launch attempt, engineers would have to begin fueling the shuttle?s 15-story external tank at about 3 p.m. EDT (1900 GMT) today.

Mike Moses, head of Discovery?s mission management team, will hold a meeting of shuttle managers at 12 p.m. EDT (1600 GMT) to discuss the valve issue and whether it is safe to fly the spacecraft as-is. If mission managers decide to replace the valve, the repair work could delay Discovery?s flight to October.

Discovery?s six-man, one-woman crew actually has two chances to launch on Friday. In addition to the morning attempt, another opportunity arises at 11:59 p.m. EDT (0359 Aug. 29 GMT). The mission, commanded by veteran spaceflyer Rick Sturckow, is a 13-day spaceflight to deliver a new crewmember to the International Space Station, as well as nearly 8 tons of cargo and a treadmill named after television comedian Stephen Colbert.

Suspect valve under scrutiny

During fueling operations Tuesday for a planned early Wednesday launch, the 8-inch valve - known as a liquid hydrogen fill-and-drain valve - failed to indicate as closed during a test.

The unexpected glitch forced NASA to call off its second attempt to launch Discovery toward the International Space Station. Stormy weather earlier on Tuesday foiled the shuttle?s first launch try.

The drain valve is part of the plumbing system in Discovery?s aft that allows the 526,000 gallons of super-cold liquid hydrogen and liquid oxygen propellant required for launch to be pumped into the shuttle?s huge external tank, and then drained out again if liftoff is delayed.

NASA?s launch rules call for the valve system and its liquid oxygen counterpart to work perfectly in order to attempt a launch. The valves must be closed during launch and are reopened once a shuttle reaches orbit to vent its engine manifold of any remaining propellant.

A positioning sensor may be faulty in Discovery?s hydrogen valve, resulting in the failure to indicate the valve was closed during Tuesday?s prelaunch test, mission managers have said. The tests Wednesday evening were performed at ambient temperature, but the errant position reading came when the valve was immersed in super-chilled liquid hydrogen propellant.

NASA has until Aug. 30 to launch Discovery toward the space station before standing down due to a schedule conflict with other operations on the Air Force?s Eastern Range, which the space agency uses for shuttle and rocket launches.

If the shuttle doesn?t launch by then, NASA would have to wait until Oct. 17 to try again in order to wait out space traffic at the International Space Station. An unmanned Japanese cargo ship is slated to make its debut flight to the station in September. A Russian cargo ship is also due to leave the outpost next month while another Russian spacecraft, a crew-carrying Soyuz, will launch Sept. 30 carrying a new crew and a Canadian space tourist.

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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 New York. Click here for shuttle mission updates and a link to NASA TV.