The girder was unloaded using the ISS' robot arm

It is part of almost $1bn (£696m) worth of new parts for the ongoing construction project.

Shuttle astronaut Ellen Ochoa and her ISS counterpart Daniel Bursch used the robot arm on board the station to move the new girder out of the shuttle's payload bay.

Two teams of astronauts will carry out two spacewalks each over the next week to attach the girder to the station.

Space railway

The older team, Jerry Ross and Lee Morin, will be the first pair of grandfathers to walk together in space if they leave the station as planned for a six hour 25 minute spacewalk on Saturday.

Colonel Ross is on his record-breaking seventh trip into space, 22 years after he was first selected as an astronaut.

Atlantis docked with the ISS on Wednesday and its seven crew members joined the three aboard the space station.

The S0 truss which Atlantis carried into space also carries a rail wagon which will help move more new parts of the station as they arrive on future missions.

The wagon has a maximum speed of around an inch (2.54 centimetres) per second.

Launch delay

Atlantis took off from the Kennedy Space Center in Florida, US, on Monday after four days of delays caused by fuelling problems.

As part of a new safety policy, the US space agency kept the new launch time secret until Sunday.

The whereabouts of the seven astronauts was also kept secret until they arrived at the pad.

Only then was a video of the crew's breakfast, suit-up and departure for the pad - normally seen live - broadcast by the space agency.

The shuttle is also carrying 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.