Today's planned launch of the shuttle Atlantis on a long-awaited mission to carry a European research lab to the international space station has been postponed at least 24 hours because of apparent problems with two of four low-level fuel sensors at the base of the ship's external tank.

The launch team is recycling the countdown for a possible launch attempt Friday at 4:09:13 p.m., but that assumes the problem can be resolved by then. The shuttle's eight-day launch window closes Dec. 13. If Atlantis isn't off the ground by then, the flight will slip into early January.

Today's launch scrub was declared at 9:56 a.m., about three hours after engineers began pumping a half million gallons of supercold liquid oxygen and hydrogen rocket fuel into Atlantis' external tank for a launch attempt at 4:31:45 p.m.

But about two hours into the three-hour fueling process, when the hydrogen section of the tank was about 80 percent full, two of four engine cutoff sensors at the base of the hydrogen tank "failed wet." The sensors are part of a backup system used to ensure the shuttle's three main engines don't run the tank dry if some other problem prevents an on-time shut down.

Hydrogen ECO sensors 3 and 4 dropped off line at the same time, which might indicate some sort of circuitry issue rather than an actual sensor failure. But that remains to be seen. If troubleshooting confirms both sensors have, in fact, failed, launch likely would be delayed beyond the end of the current launch window.

But if a problem can be identified in the circuitry or software associated with the ECO sensor system, it may be possible to press ahead Friday or later in the window if any repairs are required and if the wiring or hardware in question is accessible at the launch pad.