NASA engineers are working to settle fuel tank sensor concerns for the space shuttle Atlantis today in order to clear the orbiter and a European lab for a Saturday launch toward the International Space Station (ISS).

The failure of two fuel gauge sensors during a standard preflight test inside Atlantis' 15-story external tank prevented a planned Thursday afternoon launch for the orbiter and its seven-astronaut crew here at NASA's seaside Kennedy Space Center.

Shuttle commander Stephen Frick and his STS-122 crewmates hope to try again on Saturday, with liftoff currently pegged at 3:43 p.m. EST (2043 GMT) and a 60 percent chance of favorable launch weather, mission managers said late Thursday.

The astronauts will haul the European Space Agency's (ESA) Columbus lab to the ISS during their 11-day mission.

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"We're still hoping, and have reason to believe, that we're going to get off in December," said Doug Lyons, NASA's shuttle launch director, after Thursday's launch delay. "And that's what we're shooting for."

NASA has until Dec. 13 to launch Atlantis toward the International Space Station (ISS), where the shuttle's seven-astronaut crew will deliver the European Space Agency's (ESA) Columbus laboratory during a planned 11-day mission.

If Atlantis cannot launch by the window's close, mission managers have said they would wait until Jan. 2 at the earliest to try again to avoid unfavorable sun angles on the station's solar arrays.

On Friday, engineers are expected to decide whether Atlantis can launch with only two of four critical liquid hydrogen engine cut-off sensors operating properly on Saturday.

The sensors are used as part of a backup system to shut down a shuttle's three main engines before its liquid hydrogen propellant supply runs dry.

NASA fuels its shuttle external tanks with more than 500,000 gallons (1.9 million liters) of super-chilled liquid hydrogen and liquid oxygen propellant to feed an orbiter's three main engines during the 8.5-minute launch into orbit.

Current flight rules require at least three working sensors to support liftoff.

While engine cut-off sensors have afflicted shuttle flight preparations in past — most recently causing a day's delay during a September 2006 launch — those glitches included only one failed sensor, not two.

Engineers suspect that an open circuit, not the sensors themselves, may be the source of Atlantis' current fuel gauge woes, and will take a close look at the risks involved in launching with only two working units.

"The intent would be to have rationale that says, operationally, we can work around this and still drive the crew safety risk down to zero or as close to zero as we can," said LeRoy Cain, head of Atlantis' mission management team, late Thursday.

Shuttle officials will meet this afternoon to study Atlantis' launch options and will hold a press briefing no earlier than 5:00 p.m. EST (2200 GMT) tonight discuss their findings.

Mission managers don't currently plan to call for a hardware repair inside Atlantis to address the glitch.

Replacing the sensors completely, which engineers may be able to perform at the orbiter's Pad 39A launch site, if required, would delay Atlantis' STS-122 mission well into mid-January should engineers believe it is necessary, Lyons said.

Frick and his crew plan to stage at least three spacewalks to install Columbus at the ISS during their spaceflight.

One STS-122 crewmember, French ESA astronaut Leopold Eyharts, will replace U.S. spaceflyer Dan Tani as space station flight engineer during the shuttle flight.

Atlantis' STS-122 mission will mark NASA's fourth shuttle flight of the year when it launches this month.

ESA officials said that while they were disappointed with Thursday's launch scrub, the delay is rooted in maximizing crew safety during liftoff, which is more important than an on-time departure for the 13-ton Columbus.

"This is perfectly normal," said Alan Thirkettle, the ESA's space station program manager, of the scrub. "We want to launch on time, but we want to launch right."

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