Commander Michael Bloomfield leads the 13th shuttle mission to the ISS

This is the first occasion that the American space agency (Nasa) has implemented its new security policy of keeping civilian launch times secret until the day before lift-off.

Shuttle Atlantis is due to leave Kennedy Space Center in Florida at 2212 GMT (1712 local time) on Thursday and dock with the International Space Station (ISS) on Saturday.

The crew will fit the first part of the backbone of the station and carry out scientific experiments including one to grow wheat in orbit.

Nasa controllers say that there is a 60% chance of good weather for the launch.

Backbone mission

Atlantis will be powered on the flight by a new set of shuttle engines, built without welds in the turbo-pumps to improve safety and reliability.

The crew of seven will spend 12 days aloft, working with the three members of the ISS to extend the orbiting platform.

The main component they are carrying into orbit is a 12-tonne girder called Truss S0, which will form the first section of the backbone of the station.

They are also carrying a cart that will run along rails on the backbone and allow the station's robotic arm to move across the platform.

Launch worry

The shuttle will also carry several scientific experiments, including one to grow wheat and mustard family plants in space.

This is part of a long-term programme to develop potential life support technologies for long space missions.

There have been some worries that the launch might not go ahead, prompted by a technical hitch in pre-flight testing.

A door that should close on the Atlantis once its external fuel tank is jettisoned closed slightly late.

Nasa controllers say they are confident they will be able to explain the delay and clear Atlantis for launch on time on Thursday.