Space shuttle Atlantis is preparing for lift off on the final mission of the US shuttle programme.
A small crew of four astronauts including Chris Ferguson, pilot Doug Hurley, flight engineer Rex Walheim and Sandy Magnus, began strapping into reclined seats on Atlantis' top deck shortly after 8 a.m. EDT.
NASA limited the crew size to accommodate small Russian Soyuz capsules serving as escape vessels if Atlantis become too damaged during launch or while in orbit to safely return to Earth.
The U.S. space agency had a second shuttle prepared for any potential rescue but Atlantis has no shuttle backup.
Meteorologists have predicted just a 30 per cent chance of suitable weather for the flight, while dense cloud cover or possible rain and thunderstorms could prompt a delay.
If liftoff is delayed, they may have to wait through the weekend for a glimpse of the final shuttle rocket vaulting into orbit
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.
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, 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, is expected to debut next year.
With the space shuttles retiring the International Space 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 for the $100 billion station, built by NASA over the last 11 years.
The completion of the International Space Station 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.