Weather officer Captain Chris Lovett said the weather conditions are slightly less favorable for Saturday's launch with a 40 percent chance of weather prohibiting launch.

The next launch attempt was originally rescheduled for Friday afternoon.

Mission STS-122 was scheduled to launch on 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 the recommendation of Launch Director Doug Lyons the launch was scrubbed at 9:56 a.m. EST.

“Of course, we’re a little disappointed in the events today,” said Launch Director Doug Lyons. “But we’re certainly working to resolve our issues and make an attempt as soon as we possibly can.”

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.

"It is a redundant system," Lyons said of the fuel sensors, adding that Atlantis' onboard computer should automatically shut down the shuttle's engines before the cut off sensors are required. "But it is a system that's critical to us, so we certainly want them to operate so we can go fly."

According to Lyons, a preliminary analysis indicated that the problem may involve an open electrical circuit somewhere in the sensor system and that it might not be in the sensors themselves.

The shuttle uses more than 500,000 gallons of cryogenic hydrogen and oxygen for its 8.5-minute sprint into orbit.

Following the launch postponement, the tank's liquid oxygen and liquid hydrogen were drained. While the tank was being emptied, engineers monitored and collected data on the liquid hydrogen sensors that failed. During that process, another sensor gave a false reading, indicating that the tank was "wet," when it was dry. All ECO sensors are now indicating dry as they should be.

Fuel sensor problems have caused considerable launch delays since the retooling of troublesome tanks after the 2003 Columbia disaster. NASA was draining the fuel from the tanks to inspect the problem.

In addition, NASA extended the launch windows to December 13 and once that period expires, January would be the next target window.

The mission marks he beginning of a new chapter in international space flight that will give Europe its first real foothold in space with the installation of the European Space Agency laboratory. Space travel has been dominated by Russia and the United States for half a century.

Atlantis will carry the European-developed Columbus laboratory and attach it to the International Space Station. Seven astronauts, including two from the European Space Agency, will fly aboard Atlantis.

Columbus is about 23 feet long and 15 feet wide, allowing it to hold 10 "racks" of experiments, each approximately the size of a phone booth. Five NASA racks will be added to the laboratory once it is in orbit. Each rack provides independent controls for power and cooling, as well as communication links to earthbound controllers and researchers. These links will allow scientists all over Europe to participate in their own experiments in space from several user centers and, in some cases, even from their own work locations.

The Columbus laboratory's flexibility provides room for the researchers on the ground, aided by the station's crew, to conduct thousands of experiments in life sciences, materials sciences, fluid physics and other research in a weightless environment not possible on Earth.

In addition, the station crew can conduct experiments outside the module within the vacuum of space, thanks to four exterior mounting platforms that can accommodate external payloads. With a clear view of Earth and the vastness of space, external experiments can run the gamut from the microscopic world of bacteria to the limitlessness of space. The first two experiment packages will fly to the station on the shuttle with the module.

Columbus will be installed by German astronaut Hans Schlegel, a physicist and German army paratrooper. He must make two space walks during the 11-day shuttle mission.

During the first walk, a giant robotic arm will lift Columbus out of the shuttle's payload bay. Schlegel and US astronaut Rex Walheim will then prepare the lab for docking. They will also replace a nitrogen tank assembly used to pressurize the ISS's outside cooling system.

Schlegel's duties also include testing all the seals on the dock and connecting electrical power, water and air cooling systems, air ducts and data cables so that Columbus becomes part of the space station, Schlegel said in an interview posted on the NASA website.

"It's not easy to imagine what kind of provisions you really need to make Columbus an integral part of the space station, interacting as well, and all that is done during the first day," Schlegel said.

Last month ISS astronauts Peggy Whitson and Dan Tani completed a final seven-hour spacewalk to finish hooking up the Harmony module to the International Space Station (ISS).

Harmony, which was delivered by the space shuttle Discovery, will serve as a docking port for the Columbus module.

Next year, a series of space shuttle missions will carry the components of a Japanese laboratory into orbit.

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.

Unfortunately, NASA already is behind schedule on design and development of the International Space Station, though NASA Administrator Michael Griffin said the agency has executed the plan well overall.

"We the United States, as the senior partner in the space station coalition, did not plan it well," Griffin said on the eve of Columbus' launch. "It has taken far too long and I'll just leave it at that."