NASA ran more than 160 tests but has failed to explain what exactly caused the failure of one of the four hydrogen level sensors that prompted the last-minute cancelation of the July 13 launch.

The space agency hopes to detect the root of the problem during last-minute testing while the shuttle's massive external tank is being filled.

If the problem reoccurs on the same sensor or another similar one and engineers have a clear understanding of the problem, the launch would go ahead as planned, said Wayne Hale, deputy manager of the space shuttle program.

But he insisted that if any other low fuel level sensors malfunctioned, the launch would be called off.

The sensors send data on the levels of hydrogen in the tank to determine when the three engines should be shut off during the ascent into orbit.

Failure of the sensors can result in premature shutdown of the orbiter's engines during the shuttle's ascent.

But Hale was optimistic. "We are ready to go launch Tuesday morning," he said at a news conference at the Kennedy Space Center, near Cape Canaveral, Florida.

NASA appeared far more concerned weather could delay the mission.

Weather forecasters said there was a 40 percent chance that cloud cover or rain could scrub Tuesday's planned launch, as NASA wants a clear view of the shuttle as it ascends into orbit.

"We do have some concern for launch," said NASA weather officer Kathy Winters.

The launch also would be called off if there is thunderstorm activity within 20 nautical miles (37 kilometers) of the Kennedy Space Center launch pad, since it could affect an eventual emergency landing. There were also concerns about the weather in alternate emergency landing sites in France and Spain.

The Space Shuttle Discovery, formally known as STS-114, is scheduled to liftoff from its Florida seashore launch pad at 10:39 am (1439 GMT) Tuesday, taking the seven crew -- including a Japanese national -- to a rendezvous with the International Space Station (ISS).

Should Tuesday's launch be called off, NASA will have until the end of the month to send Discovery into orbit so it can hook up with the orbiting ISS. After that, the next window for a daytime launch will be in September.

NASA's decision not to launch at night is among the new safety measures the space agency has adopted after Columbia burst into flames upon re-entering the Earth's atmosphere on February 1, 2003, killing all seven astronauts.

The tragedy was blamed on a small piece of insulation that broke off just after liftoff and struck the orbiter's left wing, causing a crack that allowed superheated gases to penetrate the structure just minutes before the planned landing.

Now, more than 100 cameras will be installed on the ground and aboard two airplanes to capture the shuttle's first two minutes of ascent.

Should something go wrong in the first few minutes, the shuttle could return to the Kennedy Space Center, or head to a base on the other side of the Atlantic, either in France or in Spain.

During one of three planned spacewalks, Japanese astronaut Soichi Noguchi and his US counterpart Stephen Robinson will test repair techniques.

The spacecraft will also deliver vital supplies and equipment after docking with the International Space Station, whose crew will photograph Discovery's underside.

If the images show Discovery suffered significant damage during ascent, the seven astronauts will wait for another shuttle to bring them back to Earth from the ISS.

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