U.S. astronauts lengthened the international space station's new backbone Thursday. They added another section of aluminum girder, work that employed the station's robot arm and two spacewalkers.

The girder is a 15 meter long, 13 metric ton framework that is packed with several kilometers of tubing and wiring to support cooling, power, and data relay equipment for future research laboratories.

Eventually the backbone will span more than 100 meters.

Crewmembers from the station and the visiting space shuttle Atlantis hoisted the unit from the shuttle's cargo bay with the outpost's robot arm and latched it onto the first girder section installed last April.

Then two Atlantis spacewalkers began the process of connecting plumbing, optical fiber, and electrical lines to bring the truss to life. They also unlatched two big radiators from their launch position to put them in the best position to release heat.

They will conduct two more spacewalks Saturday and Monday to complete the work before Atlantis departs the station next Wednesday.