Of NASA's 47 active astronauts, 20 are women. And of the seven astronauts currently living at the space station, McClain and Ayers are the only women. It was the first spacewalk for Ayers, an Air Force major and former fighter pilot, and the third for McClain.

Two NASA astronauts plan to participate in a 6-hour, 35-minute spacewalk Thursday to upgrade the International Space Station's power generation capabilities and relocate a communications antenna.