The final mission of the space shuttle Discovery has been postponed again because of a fuel leak.
After 26 years of service, the vehicle is due to make one last flight to the International Space Station (ISS) before being retired to a museum.
Nasa has struggled all week to get Discovery off the ground, frustrated by poor weather and technical problems.
Escaping hydrogen detected midway through fuelling left Nasa no choice but to stand Discovery down once more.
The agency is now planning a launch date on 30 November, to give plenty of time to fix the leak.
Nasa has simply run out of time in the current launch window, which ends on Monday.
There then follows a three-and-a-half-week period of unfavourable sun angles at the orbiting platform that produce strong heating on a docked shuttle.
US politicians have confirmed that the reusable orbiters should give way to a new era of spaceflight, and one by one these remarkable vehicles are taking a final bow.
Discovery is the oldest of the surviving ships. First launched in 1984, it has since completed 38 missions, travelling some 230 million kilometres in the process. Its commander on the final mission, Steve Lindsey, says Discovery is probably the most important of three remaining shuttles.
"It is obviously a very historical vehicle, having flown the 'return to flight' test missions after both the Challenger and Columbia accidents," he said.
"It deployed Hubble (and) it's the fleet leader in terms of number of flights - it'll have flown about a year on orbit by the time we're done with it, which is pretty remarkable for a space shuttle."
After Discovery returns, only the Endeavour shuttle has a firm date to launch, in February next year.
Atlantis could fly in June if the budget allows. Beyond that, American astronauts will use Russian Soyuz rockets to get into space until a range of commercial US launch systems are introduced in the middle of the decade.
The hydrogen leak discovered two hours into the fuelling operation is considered very serious by Nasa managers because of the potential flamability of the gas.
The leak occurred at the ground umbilical carrier plate, an attachment point between the external tank and a 18cm pipe that carries vented hydrogen safely away from Discovery to a flare stack, where it is burned off.
"We always place safety first," said Nasa Associate Administrator for Space Operations Bill Gerstenmaier.
"It is essential we repair this hardware before we fly the mission, and we will take the time to properly understand and fix the failure before we launch."
Discovery's six astronauts had yet to board the spaceplane when the leak was detected.
When the ship does get up, she will deliver a storeroom to be attached to the ISS, along with much needed supplies and spares.