"Tonight, as our troubleshooting plan comes together, we will probably start to offload the cryogenic reactants for the fuel cells," Wayne Hale, deputy director of NASA's space shuttle program, told a news conference.

"That gives us the safety measure to get into the aft end of the orbiter to try -- start -- the troubleshooting inside if we decide that's the thing to do.

"Given that work, if we were to get extremely lucky, it is theoretically possible that we could still launch on Sunday," said Hale.

A faulty gauge on the hydrogen fuel tanks led NASA Wednesday to halt the countdown on Discovery's voyage, just two hours and 20 minutes before liftoff.

The seven crew members, including Japanese astronaut Soichi Noguchi, were already on board Discovery.

The National Aeronautics and Space Administration must get the shuttle into orbit by July 31 or be forced to wait until September 9 for a new launch "window".

NASA, which is keen to get its space program back on track after the 2003 Columbia disaster, suspended the launch after one of four of the low level sensors in the liquid hydrogen tank had a failed reading.

earlier related report
NASA Postpones Discovery Launch
NASA called off Wednesday's Discovery liftoff after detecting a fuel sensor problem, saying Saturday would be the earliest date for a launch of the first space shuttle since the 2003 Columbia disaster.

The seven crew, including Japanese astronaut Soichi Noguchi, were already aboard the orbiter when NASA managers postponed the flight just two hours and 20 minutes before scheduled liftoff.

NASA managers said they needed time to assess the cause of the problem before setting a new launch date.

"We will not be ready to fly before Saturday," said Wayne Hale, the shuttle program's deputy manager. "That's the very best scenario," he told journalists at the Kennedy Space Center.

But NASA administrator Michael Griffin pointed out that in a worst case scenario, the shuttle would have to be taken back to the assembly building, five kilometers (three miles) from the launch pad.

The decision to scrap Wednesday's launch was taken after launch control detected a problem with a hydrogen fuel sensor that could have prevented the shuttle from reaching its full power during ascent into orbit.

The sensor is one of four designed to transmit data to computers on the levels of hydrogen remaining in the main tank, to determine when engines should be shut off during the ascent into orbit.

The sensor did not respond during tests conducted shortly before the planned launch.

"It is an unexplained anomaly," said Hale, adding that workers had started emptying the massive external fuel tank of its close to two million liters (536,000 gallons) of liquid hydrogen and oxygen.

The tanking operation had been delayed Wednesday morning because a system that helps reduce the chance of an explosion during fueling had to be replaced.

NASA already got a scare Tuesday when a plastic and foam protective cover fell 20 meters (60 feet) off one of Discovery's windows, damaging tiles on one of the two orbital maneuvering systems.

Worsening weather had also jangled nerves at the Florida seashore space center, but NASA officials expressed confidence Tropical Storm Emily, which threatened Caribbean islands, would not head toward central Florida.

NASA has until the end of the month to send the shuttle into orbit so it can hook up with the International Space Station.

During the planned 12-day mission, the seven astronauts, led by Commander Eileen Collins, are scheduled to test changes made since Columbia burst into flames upon reentry into the Earth's atmosphere on February 1, 2003. NASA has grounded its shuttle fleet since then.

Crews on the ground also will be assessing the improvements, and will keep a close eye on the external fuel tank's new insulation system.

At Columbia's launch, a small piece of insulation broke off and struck the orbiter's left wing, causing a crack that allowed superheated gases to penetrate the structure just minutes before the planned landing.

More than 100 cameras are installed on the ground and aboard two airplanes, to capture Discovery's first two minutes of ascent.

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

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

A success of the mission is crucial to the future of the US space program at a time when some lawmakers are showing reluctance to approve new funding for NASA.

But several representatives and senators, including House Majority leader Tom Delay and former presidential candidate Senator John Kerry showed up at the Kennedy Space Center Wednesday, and expressed strong support for NASA.

"NASA needs to rely on the space shuttle, and that's why it's important for us to get back into flying again," said Florida's Democratic Senator Bill Nelson, who flew on the oft-delayed STS-61 shuttle mission that launched in January 1996.

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