The radiator will be a key part of the space station's solar energy arrays, dissipating the heat generated by the solar panels' electronics.
Heidemarie Stefanyshyn-Piper and Joe Tanner had started the installation of the $372 million solar panel addition to the space station in their first spacewalk, and on Friday were wrapping up the third and final outing of Atlantis' 11-day mission. It was the first construction mission to the space station since the 2003 Columbia disaster.
A minor glitch delayed the spacewalkers Friday morning, but it was fixed after about 45 minutes.
With launch restraints removed, flight controllers sent commands to unfold a 44-foot-long radiator panel on the space station that will help keep the electronics inside a new solar array module cool once it comes on line, reported CBS News space consultant Bill Harwood.
"You just can't imagine a flight going as well as this one has gone," said lead space station flight director John McCullough. "I couldn't ask for a better start — a re-start — to assembly."
The astronauts were already well ahead of schedule by Friday. A couple of hours into their spacewalk, Tanner and Stefanyshyn-Piper picked up a science experiment that tested how various materials fare in space and set to work on some installation tasks. They will spend the rest of the roughly 6 1/2 hours on tasks like replacing an antenna.