(Adds details on delay)

By Irene Klotz

CAPE CANAVERAL, Fla., Dec 6 (Reuters) - NASA postponed the launch scheduled for Thursday of the U.S. space shuttle Atlantis to resolve a fuel sensor problem but officials expressed hope they could make a second try on Friday.

Launch of the shuttle and Europe's Columbus science laboratory was retargeted for 4:09 p.m. EST (2109 GMT) Friday after two sensors in the shuttle's external tank failed a routine check early on Thursday.

Engineers believe the problem, which cropped up while the shuttle was being filled with fuel for the 8.5-minute sprint to orbit, was due to an open circuit.

Assessments were under way and managers planned to meet in the afternoon to decide about another launch attempt.

"We're keeping all our options open," said launch director Doug Lyons.

NASA has until Dec. 13 for launch attempts this year. This would be the fourth mission in 2007.

The shuttle uses more than 500,000 gallons (2.273 million litres) of cryogenic hydrogen and oxygen to get into orbit.

There are four engine cut-off sensors in the ship's fuel tank that monitor levels of liquid hydrogen and two of them failed.

The sensors are a critical part of a backup safety system to shut down the shuttle's main engines once the ship reaches space or if a problem occurs after liftoff. If the engines continued to run once the fuel supply was gone, an explosion could destroy the ship.

"Engineers need to drain the tank and verify all the sensors are working as they go dry," NASA said in a statement.

NASA wrestled with a series of fuel tank sensor problems as it attempted to resume shuttle flights following the 2003 Columbia accident. The first post-Columbia mission in July 2005 was delayed 13 days due to sensor glitches.

Engineers traced the problem to a bad batch of sensors and replaced them.

Tucked inside Atlantis' cargo bay as it waited for another launch opportunity was the European Space Agency's primary contribution to the $100 billion space station program -- a 27-foot-(8.2-metre)long, 15-foot-(4.6-metre)diameter laboratory named Columbus.

Germany's Hans Schlegel and Leopold Eyharts of France will join five U.S. astronauts on the mission -- commander Stephen Frick, pilot Alan Poindexter, flight engineer Rex Walheim, Leland Melvin and Stanley Love.

ESA has waited for its launch for more than five years. Its initial launch was postponed when NASA grounded the shuttle fleet for safety upgrades after the Columbia disaster.

(Editing by Maggie Fox and David Wiessler)

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