The decision was taken after NASA managers have reviewed the data on a problem with a fuel cutoff sensor system inside the shuttle and its external fuel tank.

Initially the launch was scheduled Thursday at 4:31 p.m. EST from NASA's Florida launch base, but during tanking, two of four LH2 Engine Cutoff (ECO) sensors failed to respond appropriately, which is a Launch Commit Violation.

On Thursday NASA managers rescheduled the liftoff of mission STS-122 from NASA's Kennedy Space Center for Saturday, Dec. 8 at 3:43 p.m. EST, but because of the length of the Friday meeting, the managers agreed that targeting Sunday would allow the launch and management teams appropriate time to rest and prepare.

The Mission Management Team will meet Saturday at 1 p.m. to decide whether to make a Sunday attempt. A news conference will be held after the meeting's conclusion.

Atlantis' scheduled launch Thursday was delayed after two ECO sensors gave false readings. A third sensor failed after the tank was drained of fuel. 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.

"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."

"We're thinking about our options and whether the risks are acceptable or not," Wayne Hale, NASA's space shuttle program manager, said.

"When we fill the tank up with cryogenic hydrogen again...our past history says they are likely to all work," Hale said of Atlantis' fuel gauge sensors, which are currently functioning perfectly to indicate an empty external tank. "That makes it difficult to troubleshoot."

NASA engineers said analysis indicates the problem does not involve the sensors themselves, and may be caused by an open electrical circuit or another fault in wiring.

Engineers considered flying even if the sensors failed again but managers decided it would be too risky. "As of today, no one has come forward with a good plan to improve our situation," Hale said. If the sensors act up during launch preparations on Sunday, the flight would be postponed again, Hale added.

If Atlantis cannot be launched by December 13 or 14, NASA will delay the mission until January to avoid a period when the Sun would overheat the shuttle while it is berthed at the space station.

The STS-122 mission will be lead by Commander Steve Frick, who is joined by pilot Alan Poindexter and mission specialists Leland Melvin, Rex Walheim, Stanley Love and European Space Agency astronauts Hans Schlegel and Leopold Eyharts. Eyharts will replace current station crew member Dan Tani, who has lived on the outpost since October. Eyharts will return to Earth on shuttle Endeavour's STS-123 mission, currently targeted for launch on Feb. 14, 2008.

The STS-122 includes at least three spacewalks. According to NASA, on flight day 4, the astronauts Walheim and Schlegel’s main task will be to prepare the Columbus module for installation on Harmony. They will install the Power Data Grapple Fixture on Columbus, which will allow the space station’s robotic arm to grab the module and move it from the shuttle’s payload bay to Harmony. The spacewalkers also will begin work to remove the Nitrogen Tank Assembly, a part of the station’s thermal control system, from the P1 truss. The assembly needs to be replaced because the nitrogen is running low.

In the next spacewalk, which is scheduled for the flight day 6, Walheim and Schlegel will remove the old NTA and temporarily store it on an equipment cart. They will then install the new one.

The installation of Columbus will be completed on flight day 8 and Walheim and Love will install two payloads on Columbus’ exterior: SOLAR, an observatory to monitor the sun and the European Technology Exposure Facility (EuTEF) that will carry eight different experiments requiring exposure to the space environment.

NASA officials is also are considering plans for spacewalkers to further inspect the solar array rotary joint, SARJ, on the right side of the station. The goal is to search for and return evidence to help understand and correct the vibration caused by debris in the joint. Understanding the cause for the debris seen in previous spacewalk inspections will provide a path for repair.

In February 2008 the mission STS-123 on Space Shuttle Endeavour will deliver the pressurized section of the Kibo (Hope) Japanese Experiment Logistics Module (ELM-PS) on the twenty-fifth mission to the International Space Station.

For April 2008 NASA plans the STS-124 shuttle mission which will deliver the Pressurized Module and robotic arm of the Kibo Module.

For NASA every mission is critical because the US space agency is rushing double capacity on the space station by 2010, when the ageing shuttle fleet is set to retire.

NASA has 10 shuttle missions left to complete construction of the $100-billion International Space Station. It also would like to fly two resupply flights and a servicing call to the Hubble Space Telescope.