Atlantis astronauts Dan Burbank and Steve MacLean are expected to venture outside the shuttle Wednesday.
Two of their colleagues ventured outside the shuttle Tuesday and started hooking up a 45-foot addition to the international space station.
That's the first since construction was put on hold following the Columbia disaster in 2003.
Astronauts Joe Tanner and Heidemarie Stefanyshyn-Piper worked for six hours and 26 minutes on a new 17-and-a-half-ton truss section. Their job involved connecting 17 wires or tubes and tightening or loosening 167 bolts on the orbiting space lab.
NASA says a free-flying bolt was the only glitch in an otherwise successful and speedy spacewalk.