Two U.S. space shuttle astronauts have finished the third and final spacewalk of their visit to the space station to complete work on a new section of the outpost's growing backbone.

In three spacewalks since Thursday, astronauts from the visiting shuttle Atlantis, David Wolf and Piers Sellers, have connected wires, optical fibers, and coolant tubes on a segment of aluminum girder mounted on the station last week.

The 15-meter long segment is the second of 11 that will eventually form a 100 meter backbone to hold air conditioning, power, and data relay equipment for future research laboratories.

Monday's spacewalk was devoted mainly to finishing the last coolant tube connections in three heat radiators.

Before the spacewalk began, U.S. ground controllers in Houston remotely unfolded one of the radiators to test the deployment system.

When fully expanded, the three radiators will be the size of a tennis court with enough capacity to cool an area of nearly 800 square meters.