The shuttle is scheduled to lift off at 1229 EDT (1629 GMT) from Florida's Kennedy Space Center on a crucial mission to resume construction of the half-finished International Space Station (ISS).

NASA will have two later launch dates, on 7 and 8 September, if bad weather or technical problems force the cancellation of Wednesday's launch.

The announcement came after the agency determined that the shuttle and the space centre successfully weathered Tropical Storm Ernesto, which dumped over 10 centimetres (4 inches) of rain in the area on Wednesday and generated winds of 71 kilometres (44 miles) per hour at the launch pad.

NASA kept alive its hopes of launching Atlantis in September in dramatic fashion this week by abruptly sending the orbiter back to its launch pad to ride out Ernesto. It had started moving Atlantis to its massive hangar on Tuesday. Midway through the trip, the decision was made to return it to its seaside launch pad as new forecasts suggested the storm would veer away from the space centre.

Ernesto and a lightning strike forced NASA to cancel three launch attempts this week.

NASA gained an extra day – 8 September – to lift off after the Russian space agency agreed to push back the launch of a Soyuz rocket to the ISS from 14 September to 18 September. Atlantis will have to undock from the ISS by 17 September to avoid overlapping with the Soyuz's visit to the station.

The shuttle will carry six astronauts, a new 16-tonne truss and two huge solar panels that will double the station's current power. It will be the first ISS construction mission in nearly four years, as the last two shuttle flights focused on improving flight safety following the 2003 Columbia disaster.

Source: New Scientist Space