R2's assembly also marked the orbital debut of the open-source Robot Operating System, or ROS, which already powers more than 40 different robot models on Earth.
Humanoid robots functioning in space is good for more than just scientists and astronomy enthusiasts. R2 will be able to do tasks that are too dangerous and even just inconvenient for people to do.
Until now, only non-humanoid robots such as the Mars rover have been successfully implemented in space.
R2 has been in production for several years and was developed jointly by NASA and by General Motors. The $2.5 million R2 torso arrived at the space station in 2011 on the last flight of space shuttle Discovery.
Sometimes called "a robot butler for astronauts," R2 will be able to take up some space station chores as soon as it gets its legs.
Roboticists like ROS because it's an open-source language whose community continually works to fix "bugs" and make improvements. It is a required language for several high-profile DARPA (Defense Advanced Research Projects Agency) robotics projects and was the language programmers used in the 2013 DARPA Robotics Challenge.
With ROS, R2 contains improvments over previous prototypes. For instance, its legs are designed to give the robot movement in zero gravity, and R2 is capable of lifting up to 20 pounds.