Four veteran astronauts scrambled aboard space shuttle Atlantis on Friday, hoping the weather would clear for liftoff on the final mission of the US shuttle program.
Launch was targeted for 11:26am EDT from the Kennedy Space Center. Meteorologists, however, predicted just a 30% chance of suitable weather for the flight.
Dense cloud cover or possible rain and thunderstorms could prompt a delay.
Up to 1 million spectators lined beaches and causeways around the shuttle's central Florida launch pad. If liftoff is delayed, they may have to wait through the weekend for a glimpse of the final shuttle rocket vaulting into orbit.
An abridged crew of four -- Chris Ferguson, pilot Doug Hurley, flight engineer Rex Walheim and Sandy Magnus -- began strapping into reclined seats on Atlantis' top deck shortly after 8am EDT.
Typically, two or three other astronauts would be seated in the windowless middeck during launch. But Nasa limited the crew to accommodate small Russian Soyuz capsules serving as escape vessels, should Atlantis too damaged during launch or while in orbit to safely return to Earth.
Previously, the US space agency had a second shuttle prepared for any potential rescue but Atlantis, which will be making the 135th and final flight of the program, has no shuttle backup.
Atlantis, which was set to be retired last year, is laden with food and other supplies critical to the International Space Station, a recently completed orbital research outpost 220 miles above Earth.
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Nasa added the final flight to buy time in case the commercial delivery firms hired to resupply the station starting next year run into problems with their new rockets.
Space Exploration Technologies, or SpaceX, which is owned by Internet entrepreneur Elon Musk, successfully tested its Dragon capsule in orbit last December and hopes to make it to the space station in a second test flight later this year.
The other cargo hauler being developed by aerospace company Orbital Sciences Corp, is expected to debut next year.
With the space shuttles retiring, the station and its six-member crew will need regular supply runs from both companies, in addition to deliveries from Russian, European and Japanese spacecraft.
All have just a fraction of the shuttle's 25,000-tonne lift capacity.
Nasa has been steadily building the $100 billion station over the last 11 years. Completing it was the primary reason the United States decided to fix the shuttles and resume flying after the loss of Columbia and her crew in 2003.
With the space station assembly complete, the United States wants to use the $4 billion or so it has spent each year to maintain and operate Nasa's three space shuttles to develop new spacecraft that can travel beyond the station's near-Earth orbit, where shuttles cannot go.