Space station astronauts Michael Fossum and Ronald Garan Jr. ventured out Tuesday morning.
They are retrieving a broken ammonia pump outside the International Space Station and packing it aboard the docked shuttle Atlantis.
NASA wants the pump brought back so engineers can figure out why it failed last summer. The two will also set up a robotic refueling experiment.
Only four astronauts are flying Atlantis. It's the smallest crew in decades - too small, in fact, to have had time to train for this spacewalk. That's why the job was handed over to the space station crew.
The 13-day flight by Atlantis is the last for the shuttle program.
