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KENNEDY SPACE CENTER, FLA. -- Shadows spill across space shuttle Atlantis, still poised on the pad after its launch on mission STS-122 was postponed Thursday. In the background is the Atlantic Ocean. Shuttle program managers decided at 9:56 a.m. to postpone the launch because of an issue with a fuel cut-off sensor system inside the external fuel tank. This is one of several systems that protect the shuttle's main engines by triggering their shut down if fuel runs unexpectedly low. During countdown activities this morning, two sensors failed a routine prelaunch check. There are four engine cut-off, or ECO, sensors inside the liquid hydrogen section of the tank, and Launch Commit Criteria require three of the four sensor systems to be functioning properly. The tank's liquid oxygen and liquid hydrogen was drained from the tank, and preparations will begin for a possible launch attempt Friday. NASA's launch rules have a preplanned procedure that states in the case of ECO sensor system failure, engineers need to drain the tank and verify all the sensors are working as they go dry. Atlantis carries the Columbus Laboratory, the European Space Agency's largest contribution to the construction of the space station. When permanently attached to Node 2, the laboratory will carry out experiments in materials science, fluid physics and biosciences, as well as perform a number of technological applications, in a microgravity environment. Photo credit: NASA/George Shelton