A lift-off for the US space shuttle Atlantis will not now occur until Tuesday, Nasa has announced.
Atlantis stands ready on its pad
Officials said the delay would give engineers further time to assess the impact of a lightning strike on the orbiter's Florida launch pad on Friday.
But there is concern too that Tropical Storm Ernesto in the Atlantic may bring all launch preparations to a halt.
Atlantis is due to undertake a construction mission to the International Space Station (ISS).
Its six-strong crew is taking giant new power-generating solar arrays to the space station.
The mission marks only the third shuttle flight since the loss of Columbia and seven astronauts in 2003.
The half-built $100bn space station must be completed before 2010, when the shuttle fleet is due to be retired.
Atlantis and its astronauts are now scheduled to lift off at 1543 local time (1942 GMT) on Tuesday from the Kennedy Space Center. However, if Ernesto encroaches too close to the Florida coastline, Nasa managers could decide to move the shuttle back to the protection of its assembly building.
ISS: ORBITING OUTPOST
Construction work has been on hold for four years
16 nations contribute to the ISS, including the US, Russia, Japan, Canada, Brazil and European Space Agency states
The ISS will eventually be the size of a football field
The mission's main objective is to fit the P3/P4 truss, a 17-tonne segment of the ISS's truss backbone that includes a huge set of solar arrays and a giant rotary joint to allow them to track the Sun.
The second of four sets of solar arrays, they span 240ft (73m) when fully extended.
The wings will provide power for three science laboratories, two living chambers and other systems onboard the ISS. They effectively double the station's current ability to generate power from sunlight.
The Atlantis crew arrived at Cape Canaveral on Thursday for final launch preparations.
Engineers close the payload bay on Atlantis
"There's been a lot of talk in the press lately about Nasa being back," commander Brent Jett told reporters. "But we have a saying back in Texas: 'It's time to walk the walk'."
Jett will fly with pilot Chris Ferguson, flight engineer Dan Burbank, and mission specialists Joe Tanner, Heidemarie Stefanyshyn-Piper, and Steve MacLean of the Canadian Space Agency.
During the 11-day mission, the crew will undertake three spacewalks to complete their construction duties.
Some 16 shuttle flights are needed to finish the ISS. Nasa hopes one orbiter flight can also be found to service the Hubble Space Telescope.