NASA on Friday delayed the liftoff of space shuttle Atlantis until Sunday at the earliest and said it might tighten the launch rules, at astronauts' request, to cope with unreliable gauges in the fuel tank.

During a nearly six-hour meeting, representatives of NASA's astronaut office proposed that the launch proceed only if all four of the hydrogen fuel gauges and their associated systems work properly. The launch rule currently calls for only three good gauges.

Shuttle managers said they would meet again Saturday to decide whether to accept that proposal and proceed.

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"We pretty much came to the conclusion that ... we have got a suspect system," shuttle program manager Wayne Hale said.

Each shuttle fuel tank is equipped with four of these gauges, or so-called engine cutoff sensors, that keep track of whether the tank is empty or full of liquid hydrogen.

The sensors are part of a backup system that would kick in if the tank was leaking during the climb to orbit, for example, and safely shut down the engines. The engines could ignite or explode if they kept running without fuel.

Two of these gauges mysteriously failed after Atlantis' fuel tank was filled for a Thursday launch attempt, forcing a delay of the space station mission. A third gauge later acted up.

Hale said even if all four fuel gauges work fine the next time the tank is filled, it's possible they could fail during liftoff. That represents extra risk; how much extra is uncertain, he said.

Shuttle commander Stephen Frick was consulted about the problem and possible solutions via the astronaut office representatives at Friday's meeting, Hale said.

To further ensure a safe launch, NASA will whittle its five-minute window down to a single minute if it approves a Sunday attempt. That would use less fuel and thereby minimize the chance that the tank might run dry early.

NASA has been struggling with fuel gauge problems since 2005, when shuttle flights resumed following the Columbia disaster. Engineers thought they knew what was wrong and made improvements, but clearly something else is going on, Hale said.

It's especially difficult to figure out because the trouble is intermittent: sometimes the fuel gauges work and sometimes they don't for no apparent reason. They are based on a 1960s Apollo-era design.

On Friday, NASA considered loosening longtime launch rules to get around the problem and requiring only two good working fuel gauges. But that idea was dismissed since so many of the gauges in Atlantis' tank were malfunctioning and engineers did not know why, Hale said.

"We would like to have certainty. We would like to know root cause," Hale said Friday night.

Engineers are unsure whether the problem is with the fuel gauges themselves or, quite possibly, with an open circuit somewhere in the extensive wiring of the system. Any repair would take days, if not weeks.

NASA has until next Thursday or Friday to launch Atlantis to the international space station. If it isn't flying by then, the mission will be delayed until January because of computer concerns and unfavorable sun angles for the shuttle when it's docked to the space station.

The day-by-day delays have been especially disappointing for the many visitors from the European Space Agency. Atlantis is supposed to carry a huge European-built science lab, Columbus, to the space station.

"We don't want to get launch fever," Hale said. "Even though the Columbus is out there loaded in the payload bay and everybody is anxious for us to launch that guy, we want to make sure that when we go launch, it is safe or at least as safe as it ever is in this normally risky business."