NASA has delayed Friday's launch of Space Shuttle Atlantis on its STS-115 mission to the International Space Station. Lift-off now is set for 11:15 a.m. EDT Saturday.

Shuttle program managers decided to postpone Atlantis' launch at 10:52 a.m. EDT Friday due to 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 Friday morning a sensor inside the external tank failed a routine pre-launch check. The engine cut-off, or ECO sensor, is one of four inside the liquid hydrogen section of the tank.

The tank's liquid oxygen and liquid hydrogen will be drained Friday. While the tank is emptied, engineers will monitor and collect data on the liquid hydrogen sensors. NASA's launch rules have a preplanned procedure that states if a single ECO sensor fails, engineers need to drain the tank and verify all the sensors are working as they go dry. If the sensors and system work as expected, Atlantis will be cleared to launch with three of four working sensors Saturday. The reloading of the external tank is set for 1:15 a.m. EDT Saturday.

During Atlantis' mission astronauts will deliver and install the P3/P4 truss, a girder-like structure, with solar arrays that will double the International Space Station's power capability.

Atlantis' crew consists of Commander Brent Jett, Pilot Chris Ferguson and mission specialists Dan Burbank, Heide Stefanyshyn-Piper, Joe Tanner and Steve MacLean, a Canadian Space Agency astronaut.

For the latest information about the STS-115 mission and its crew and more information about engine cut-off sensors, visit:

http://www.nasa.gov/shuttle