By Paul Rincon
BBC News science reporter at Kennedy Space Center
Nasa is hunting the source of a problem that forced it to abort the launch of space shuttle Discovery.
No new launch date has been set
The US space agency had to scrub Wednesday's launch attempt when a fuel tank sensor started to play up.
The orbiter was all set to make the first flight since the loss of Columbia and its crew of seven in February 2003.
Discovery's astronauts had all been strapped into the shuttle and were preparing for blast-off when the order came through to stand down.
Nasa says the launch will now take place no earlier than Saturday.
However, it is considered more likely lift-off will be delayed until Monday.
The agency's administrator, Dr Mike Griffin, told reporters he did not know whether the problem could be fixed at the launch pad or would require the shuttle to be rolled back into its hangar.
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
The latter would mean a lengthy delay to Discovery's mission to the International Space Station (ISS).
"All I can say is aw shucks," said the shuttle's deputy programme manager Wayne Hale.
"It is an unexplained anomaly. We changed every wire, every electronics box. We thought we had a good system."
Nasa officials said that from the troubleshooting done so far, the problem could be down to suspect transistors in the box that supplies the sensors with power.
Mr Hale said the shuttle team would now implement a troubleshooting plan to track down the source of the problem.
The crew were already strapped in preparing to launch
"This system goes from an avionics box in the back of the orbiter to these sensors in the external tank, so we'll start looking from end to end to try to isolate what the problem is," Nasa manager Scott Thurston told the BBC News website.
This type of sensor has caused problems before, during the first "tanking test" of STS-114 in April.
Then, the external fuel tank was rolled back into the Vehicle Assembly Building. But Discovery is now using an entirely different tank.
Earlier, senator and former presidential candidate John Kerry and senator and astronaut John Glenn had turned up at Kennedy Space Center's press complex.
"It's good that they detected this ahead of time. This is a complicated business, this is one of the most complex vehicles ever designed by human beings," said Mr Kerry.
The faulty sensor is one of four in the shuttle's external tank responsible for making sure the spacecraft's engines shut down at the proper point during the ascent.
The sensors ensure the engines are not suddenly starved of propellants and are switched off in a controlled fashion.
The current launch window runs until the end of July, after which Discovery would have to wait until September to get airborne.
The long wait results from the need to have the ISS in the right position in orbit and Nasa's desire to launch during daylight hours so it can photograph all aspects of the ascent.
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.
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.
More cameras than ever before will be trained on Discovery during launch, to watch for any debris that could damage the spacecraft.