The U.S. space shuttle Atlantis undocks from the International Space Station Wednesday, leaving the outpost a little bigger than when it arrived one week ago.

Atlantis will depart following Tuesday's spacewalk by astronauts Jerry Ross and Lee Morin to complete work on the space station's latest addition.

It is a 13-meter long girder mounted last week on the back of the U.S. laboratory. The beam will eventually be lengthened to more than 100 meters to hold massive solar panels and cooling radiators for future Japanese and European labs.

In the fourth and last spacewalk of the shuttle mission, the two astronauts performed minor tasks, such as installing handrails on the girder, relocating tools for spacewalks on the next shuttle visit in June, adjusting a thermal blanket partly obstructing an antenna.

The next shuttle will arrive with more girder parts and a new three-man U.S. and Russian crew to replace the one that has been there since December.