The US space agency set a 10:39 am (1439 GMT) Tuesday launch time for liftoff with seven astronauts aboard.

The countdown started Saturday at noon (1600 GMT) for the the first shuttle launch since the fiery demise of the Columbia shuttle on February 1, 2003.

All seven members of Columbia's crew were killed when the shuttle disintegrated as it was reentering Earth's atmosphere.

The tragedy led to a freeze of the National Aeronautics and Space Administration's shuttle program and forced the agency to adopt new procedures to reduce the risk of equipment failure before a launch can take place.

After more than 10 days of intensive efforts, NASA engineers believed Saturday that they were able to solve the problem that forced to cancel the first Discovery flight.

"We believe our flight systems and ground support systems are ready," said test director Pete Nickolenko. "We are looking forward to a successful launch and mission."

The earlier launch was aborted because one of the four fuel sensors in the Discovery's huge exterior fuel tank was found faulty shortly before liftoff.

The defective sensor sends data on the levels of hydrogen in the spacecraft's massive external fuel tank, to determine when the three engines should be shut off during the ascent into orbit, according to Bill Parsons, an official with the shuttle program.

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

That in turn could either force the crew to attempt an emergency landing or deliver the spaceship to lower orbit, preventing its rendezvous with the International Space Station.

NASA had a similar problem in April but never found the cause. The problem disappeared after technicians replaced cables and electronic boxes attached to the gauges, but it reappeared during the final countdown July 13.

The agency now appears reasonably confident it has the problem under control.

"Discovery is in excellent shape as we complete the troubleshooting from the liquid hydrogen engine sensor cutoff anomaly which caused our first launch attempt to scrub on July 13," Nickolenko said.

"If the sensors work exactly the way we think they will, we'll launch," said Parsons. "We have done everything we possibly could on the vehicle."

The Discovery crew returned to Cape Canaveral from Houston, Texas, on Friday to prepare for the flight.

"We hope that we're able to launch on Tuesday, but regardless of when we launch, what's important to us is that we get through this process and that we do it right," said Discovery commander Eileen Collins, the first woman to ever lead a shuttle crew into space.

Weather could also force another delay of the launch. According to Kathy Winters, shuttle weather officer, the chance of inclement weather conditions blocking the launch is about 40 percent.

If Tuesday's launch is aborted, NASA will have at least four more opportunities before July 31, when the current launch window closes. Then the agency will be forced to wait until the September 9-24 period for a new opportunity.

But if all goes well, Discovery will deliver 12 tons of equipment and other supplies to the international space station, whose completion is dependent upon the shuttle's return to orbit.

During the 12-day mission, the astronauts will also test the spacecraft to see if all the modifications, including those implemented on the external fuel tank, work correctly.

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