Fuel Sensor Glitch Prevents Space Shuttle Launch

This story was updated at 8:28 p.m. EST.

CAPE CANAVERAL, Fla. -- NASA's shuttle Atlantis and its seven-astronaut crew will have to wait until Saturday to rocket toward the International Space Station (ISS) with a new European laboratory after a sensor glitch thwarted a Thursday launch attempt.

Faulty readings in two of four hydrogen fuel gauge sensors prevented Atlantis from launching from NASA?s Kennedy Space Center here at 4:31 p.m. EST (2131 GMT). NASA flight rules call for three operational sensors in order to launch.

?Preliminary indications are that we have an open circuit there,? said NASA shuttle launch director Doug Lyons after the scrub, adding that more analysis is required to determine if that is the case and the circuit?s location. ?Once we isolate that, we can determine the appropriate corrective action.?

Atlantis and its STS-122 astronaut crew are now set to launch spaceward Friday at 3:43 p.m. EST (2043 GMT), with weather forecasts predict an 60 percent chance favorable conditions at launch time.

Known as Engine Cut-Off (ECO) sensors, the instruments sit on the bottom of Atlantis? 15-story external tank and serve as liquid hydrogen fuel gauges that ensure a shuttle?s three main engines shut down before their hydrogen supply runs dry after liftoff. NASA shuttles consume more than 500,000 gallons (1.9 million liters) of super-chilled liquid hydrogen and liquid oxygen propellant during the 8.5-minute flight into space.

?It is a redundant system,? Lyons said of the fuel sensors, adding that Atlantis? onboard computer should automatically shut down the shuttle?s engines before the cut off sensors are required. ?But it is a system that?s critical to us, so we certainly want them to operate so we can go fly.?

While fueling Atlantis? tank, engineers found that two of the four hydrogen sensors failed a standard test while covered in liquid hydrogen. The test, which commanded the sensors to falsely read dry while awash in propellant, determines whether the sensors are functioning properly. Similar glitches have delayed several shuttle flights over the last two years, beginning with STS-114 - the agency?s first post-Columbia accident flight - in July 2005.

Commanded by veteran shuttle flyer Stephen Frick, Atlantis' planned 11-day mission will deliver the European Space Agency's Columbus laboratory to the International Space Station (ISS) and swap out one member of the outpost's three-astronaut crew. At least three spacewalks are planned during the mission to install Columbus and upgrade the ISS.

Set to launch spaceward aboard Atlantis with Frick are STS-122 pilot Alan Poindexter, mission specialists Rex Walheim, Leland Melvin, Stanley Love and European Space Agency astronauts Hans Schlegel and Leopold Eyharts.

The shuttle mission will mark NASA's fourth ISS construction flight of 2007 and the second to add a new pressurized room to the orbital laboratory this year.

NASA must launch Atlantis by Dec. 13 while the angles between the space station's solar arrays and the sun are favorable to generate enough power while the orbiter is docked. If the shuttle cannot launch by the window?s close, NASA will likely stand down until no earlier than Jan. 2 for another attempt, mission managers have said.

?The team remains in good spirits,? Lyons said. ?And we are really confident that we can work our way through this and get a few launch attempts in this window.?

If Atlantis is unable to launch Friday, a third attempt is possible on Saturday at 3:43 p.m. EST (2043 GMT). The launch time for NASA's STS-122 mission occurs slightly earlier each day as the flight window progresses in order to keep the shuttle on target to meet the ISS in orbit. 

• SPACE.com Video Interplayer: NASA's STS-122: Columbus Sets Sail for ISS
• IMAGES: Discovery's STS-120 Mission in Pictures
• VIDEO: ISS Commander Peggy Whitson Takes Charge