After a three-month delay, Atlantis is ready to blast off Friday on the first space shuttle mission of the year as Nasa presses on with efforts to finish the International Space Station.
Atlantis was originally scheduled to launch in March, but a freak hail storm damaged the shuttle's massive external fuel tank as the orbiter stood on its Florida launch pad, forcing Nasa to bring it back to its hangar for repairs.
The National Aeronautics and Space Administration began Tuesday the countdown for Friday's launch, scheduled for 19:38 pm (2338 GMT) at the Kennedy Space Centre in Cape Canaveral on Florida's east coast.
Nasa officials said the shuttle was in tip-top shape, while forecasts gave a 70 percent chance of favourable weather as winds were expected to nudge clouds away to give Atlantis an open sky for the launch.
The seven shuttle astronauts - all American men - arrived in Cape Canaveral Monday evening from their base at the Johnson Space Centre in Houston, Texas, as they prepare for the 21st shuttle mission to the International Space Station (ISS).