Shuttle launch postponed because of faulty fuel tank sensor

NASA postponed the launch of space shuttle Atlantis onFriday because of a problem with a sensor on the large external fuel tank.

NASA will attempt to launch Atlantis at 11:15 a.m. ET Saturday.

All six astronauts were inside the shuttle and the hatch was sealed when thelaunch was scrubbed45 minutes before it was due to take off from Cape Canaveral, Fla.

The ECO sensor, which stands for "engine cut off," is a sophisticated fuel gauge for the hydrogen tank. It is designed to ensure the main engines run as long as needed during the shuttle ascent into space.

If the engines cut off at the wrong time, it could force Atlantis to make an emergency overseas landing. There are four such sensors on the hydrogen section of the external fuel tank. Under NASA's rules, all four must be working for a launch to proceed.

The ECO sensor was reading full even before the crews started adding the fuel.

Canadian Steve MacLeanand the five others astronauts have left the shuttle. NASA crews will remove the shuttle's fuel and check if the sensors read "dry" when the tank is empty.

Atlantis was originally set to launch on Aug. 27 but was delayed when lightning struck the space centre two days before. The launch was rescheduled, but had to be postponed again because of the approach of tropical storm Ernesto.

It was delayed early Wednesday when a problem with the coolant system on one of the shuttle's fuel cells was found.

Saturday is the last date available for NASA to launch Atlantis before October, unless the agency changes its policy forbidding night-time launches. That's because the Russians are set to launchtheir Soyuz spacecraft in mid-September.

MacLean and five other astronauts have been preparing for more than four years to go into space to perform construction work on the international space station during a mission that is expected to last 11 days.