Live At KSC: NASA Shoots For Friday Night Special

NASA is targeting a late Friday night launch for shuttle Discovery to give engineers more time to determine whether it would be safe to fly if an apparent main propulsion system sensor problem crops up again.

Discovery and its seven astronauts are tentatively scheduled to blast off from launch pad 39A at 11:59 p.m. Friday -- the middle of a 10-minute opportunity to propel the shuttle on a two-day trip to the International Space Station.

The weather forecast calls for a 60 percent chance conditions will be good to go at launch time. But there also is a 60 percent chance that seasonal afternoon thunderstorms could prohibit external tank propellant-loading operations set to begin at 2:34 p.m. Friday.

Thunderstorms forced NASA to scrub an initial launch attempt early Tuesday. Then an apparent main propulsion system valve failure resulted in a launch scrub near the end of propellant-loading operations later that day.

NASA engineers think a sensor designed to indicate valve closure might have failed rather than the valve itself. The valve was cycled -- opened and closed -- five times during tests Wednesday evening. Both the valve and the position indicator worked as intended.

Mission managers, however, want to make certain it would be safe to fly if a repeat of the apparent sensor failure cropped up when the valve is exposed to supercold liquid hydrogen -- which is Minus 423 degrees -- during propellant-loading operations. So engineers are off developing a strategy that would enable NASA to ensure the position of the valve even in the event of a sensor failure.

That work will continue through the day today and the early morning hours on Friday. NASA's Mission Management Team is scheduled to meet at noon Friday to determine whether engineers have a concrete strategy for handling a recurrence of the valve trouble.

Assuming a go-ahead, countdown clocks would pick up at the T-Minus 6 hour mark at 2:34 p.m. as engineers started loading super cold liquid hydrogen and liquid oxygen into Discovery's 15-story external tank.

The Discovery astronauts would suit up around 7:39 p.m. and depart crew quarters at the Kennedy Space Center Operations & Checkout Building and arrive at the 195-foot-level of the launch tower at 8:39 p.m.

The Close Out Crew would assist the astronauts as they crawled through the side hatch of the orbiter and strapped into their seats on the flight deck and middeck of the spaceship. The side hatch would be closed for flight at 9:54 p.m.