By Paul Rincon
BBC News science reporter at Kennedy Space Center
Space shuttle Discovery will not launch until late next week at the earliest, Nasa officials have announced.
If the shuttle has to be rolled back it may not launch till September
And even then, the flight will only go ahead if engineers are "lucky" in isolating the problem which caused Wednesday's launch to be aborted.
Shuttle engineers are preparing to enter the vehicle's aft (rear) section to check a suspect electronics box.
Discovery had been set to make the first flight since the loss of Columbia and its crew of seven in February 2003.
The launch countdown has been stopped while hundreds of engineers working in 12 teams across the US hunt the source of a problem with one of four low-level fuel sensors, or engine cut-off (Eco) sensors, on the hydrogen fuel tank.
Although they have eliminated some possibilities, the exact nature of the fault remains elusive.
"It's possible that we could be back in the countdown and looking at a launch in the latter half of the week, but that would require a very near-term, lucky find," said shuttle deputy programme manager Wayne Hale.
"As we get into the more detailed testing, we're moving into fixes that might take longer."
But managers were not yet giving up on launching during the July window: "We are giving this the good old college try," Mr Hale said.
Crew staying in place
However, if troubleshooting fails to resolve the issue, managers could discuss launching with three fully functioning Eco sensors.
"We need to know the problem doesn't have implications for more sensors. If we knew absolutely that it was one sensor I think that we could have a really good discussion about do we feel comfortable about three or four," John Muratore, shuttle systems and engineering manager, told reporters.
SHUTTLE RETURN TO FLIGHT
Mission known as STS-114
Discovery's 31st flight
17th orbiter flight to ISS
Payload: Raffaello Multi-Purpose Logistics Module
Lift-off: To be determined
Location: Kennedy Space Center, Launch Pad 39B
Discovery crew: Collins, Kelly, Noguchi, Robinson, Thomas, Lawrence and Camarda
Nasa would be able to launch Discovery four days after fixing the fault. For now, its crew will stay put at Kennedy Space Center and continue training.
Eco sensors warn the shuttle computers if the tank is about to run dry, allowing the computers to shut down the three main engines safely.
The shuttle's engine pumps can force half a tonne of propellant per second out of the tank. And when these run dry, they can overspeed and disintegrate.
Initially, all four sensors on Discovery's liquid hydrogen tank had the correct "wet" reading to show the tank was full.
But when engineers sent a command to the sensor that should have made it read "dry", it continued to show a "wet" reading, an error that continued for about three hours after the tanks were drained.
If the sensors gave a false "dry" reading during ascent, the engines could cut out before the shuttle reached orbit, forcing the vehicle to abort and carry out an emergency landing.
One theory still not eliminated centres on suspect transistors in an electronics box on the aft section of the orbiter. This "point sensor box" contains a separate power supply for each Eco sensor and transmits data on oxygen and hydrogen fuel levels to the orbiter's propulsion system.
The crew were already strapped in when the launch was cancelled
Engineers have put equipment in place on the launch pad to enter the shuttle's rear and examine the box.
"One of the most difficult things with aerospace vehicles is trying to find transient problems that only occur under a certain set of conditions," said Mr Muratore.
"We have been working very diligently to recreate the set of conditions that are causing this problem so we can recreate it, isolate it and eliminate it from the system."
Discovery's mission will be the first for a space shuttle in two and a half years. The agency says it has learnt the lessons of the Columbia accident on 1 February 2003.
Improvements to Discovery include a 50ft-long (15m) robotic arm that will inspect parts of the orbiter for damage once in orbit.
The giant external tank has undergone modifications that should ensure it sheds little of its insulation foam on blast-off.
It was a suitcase-sized chunk of this material that crashed into Columbia's left wing, punching a large hole and leaving the orbiter open to the destructive super-heated gases of re-entry.