CAPE CANAVERAL, Fla. - Astronauts from the space shuttle Atlantis made the third and final spacewalk of their mission on Monday, putting some finishing touches to a new truss segment they added to the International Space Station.

David Wolf and British-born Piers Sellers began their day on the space station railroad, working their way hand-over-hand to the astro-track that will eventually span a 350-foot truss still under construction.

Their first task was to free a balky bolt on a small trolley used to move the station's 58-foot robot arm along the fledgling rail system. The Big Arm, as it is known, will play a vital role hoisting laboratory modules into place as construction on the orbiting outpost continues.

"Outstanding, Dave! That's wonderful," called out Pamela Melroy, the Atlantis pilot, as Wolf used a pistol-grip power tool to retract the safety bolt.

Had that fix not worked, Wolf and Sellers were prepared to replace an entire safety system on the trolley.

On Saturday, the same pair activated a handcart that will be used by astronauts to move along the rails.

The remainder of their work focused on completing connections to a 45-foot segment of the truss that Atlantis brought to space in its cargo hold.

The truss will eventually support huge solar arrays to power the station and support laboratory modules to be added by NASA's international partners. NASA calls it the station's backbone.

The spacewalkers worked their way quickly through a lengthy to-do list during their six hours and 36 minutes outside the space station, even performing a number of "get-ahead" tasks scheduled for future spacewalks.

The pair spent a total of 19 hours and 41 minutes in the risky vacuum of space during their three sojourns outside.

GETTING HANDEL ON SPACE STATION

A recording of Handel's "Hallelujah Chorus" echoed through the chambers of the space station earlier on Monday, a celebratory note sounded by the astronauts after Mission Control managed to extend a 75-foot radiator panel that is part of the newly installed truss.

Mission Control was supposed to hoist the panel on Sunday but postponed the event because of an electrical problem.

Once a second radiator is in place, they will act like air-conditioning on the station, dissipating heat from the living and science modules into the chill of space.

Monday's was the 46th spacewalk dedicated to space station construction since the first elements were launched in late 1998. U.S., Russian, French and Canadian astronauts have all participated. Sellers, though born in Britain, is considered a U.S. astronaut because he took U.S. citizenship before joining the NASA astronaut corps.

Atlantis and its crew of six leave the station on Wednesday and land back at Kennedy Space Center in Florida on Friday.

The three astronauts now living on the station -- two Russians and an American -- will return to Earth in November aboard another shuttle.