This story was updated at 7:51 p.m. EDT.
NASA has decided to skip its next chance to launch the space shuttle Discovery and is now targeting a late Friday liftoff.
After a 10-minute meeting today, shuttle mission managers opted to forgo the first of two launch opportunities on Friday and aim directly for an 11:59 p.m. EDT (0359 Aug. 29 GMT) liftoff instead, NASA spokesperson Allard Beutel told SPACE.com. It will be NASA?s third try this week to launch Discovery on a construction flight to the International Space Station.
Targeting a late Friday launch, rather than an earlier 12:22 a.m. EDT (0422 GMT) liftoff, allows engineers more time to draw up plans on what to do if a critical liquid hydrogen drain valve in Discovery?s main propulsion system fails another test during fueling, Beutel said. If the plan checks out, NASA would begin fueling Discovery once again tomorrow at 2:34 p.m. EDT (1834 GMT) for the launch attempt.
?To make sure we a truly bulletproof story ? we decided to give them one more day,? said Mike Moses, head of Discovery?s mission management team, in a Thursday briefing.
During fueling operations Tuesday for a Wednesday launch attempt, the so-called fill-and-drain valve failed to indicate that it was properly closed during a standard check. NASA?s launch rules require the valve to function perfectly for liftoff so mission managers called off the attempt.
The valve is part of the plumbing system in Discovery?s aft that allows NASA to pump the 526,000 gallons of super-cold liquid hydrogen and liquid oxygen propellant through the orbiter and into its 15-story external tank. The system is also used to drain the huge tank if launch is delayed.
NASA?s launch rules currently don?t allow engineers to repeatedly cycle the valve during fueling when it is subjected to the cryogenic temperatures from the liquid hydrogen propellant. The worry is that doing so could cause a valve on the cusp of failure to break or jam.
Moses said engineers believe that the position sensor which detects whether the valve is open or closed is most likely at fault. They are nearly complete with new testing protocols to cycle the valve during fueling as a final check if the glitch occurs again.
They will bring those plans to Moses?s mission management team during a Friday meeting at 12 p.m. EDT (1600 GMT) in order to decide whether waive the no-testing during fueling rule and proceed with launch.
It was Moses?s team that met today and opted to stall the shuttle launch to give engineers more time. The move allows the engineers to draw up the valve testing plan and then rest since they would also be involved in performing those tests if the valve glitch reappeared late Friday.
?The better part of valor here is to take a day, let us go polish that off and really make sure we really understand what's going on,? Moses said.
During ambient temperature tests late Wednesday, the hydrogen drain valve performed exactly as expected, opening and closing five times under remote control by NASA engineers.
Discovery?s shuttle mission has been delayed since Tuesday, when stormy weather thwarted its first attempt that morning. NASA had begun fueling Discovery later that day for the Wednesday launch try when the valve glitch occurred.
NASA has until Sunday, Aug. 30, to launch Discovery before it would stand down due to a schedule conflict with another operation on the Air Force?s Eastern Range, which is used for rocket and shuttle launches.
Commanded by veteran spaceflyer Rick Sturckow, Discovery?s seven-astronaut crew plans to fly a 13-day mission to the International Space Station to deliver a new crewmember and nearly 8 tons of science gear and other supplies. The shuttle is also carrying a new space treadmill named after television comedian Stephen Colbert.
If Discovery cannot launch by the end of August, NASA would likely stand down until Oct. 17 due to space traffic conflicts at the International Space Station.
An unmanned Japanese cargo ship is slated to launch on its maiden flight to the space station on Sept. 10. Another cargo ship, this one Russian, is also due to depart the orbiting lab next month and be closely followed by the Sept. 30 launch of a Russian Soyuz spacecraft carrying a new crew and Canadian space tourist to the outpost.
- New Video - Stephen Colbert to NASA: 'No Chubby Astronauts'
- Video - How Astronauts Work Out in Space
- New Video - Discovery's Mission to Boost Station Science
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.