Valkyrie, designated R5, was designed and built by the Engineering Directorate of NASA's Johnson Space Center in a fifteen-month sprint to enter the 2013 DARPA Robotics Challenge Trials. Unlike the hydraulic machines it competed against, Valkyrie is entirely electric: 44 degrees of freedom driven almost exclusively by series elastic actuators, which put a spring between motor and joint so the robot can sense and moderate the force it applies. NASA's stated goal was a humanoid rugged enough to work in human-built environments that have been damaged or degraded — on Earth after a disaster, and eventually on another planet, where a machine could prepare a habitat before astronauts arrive. The design borrows directly from Robonaut 2, the torso-and-arms robot NASA had already flown to the International Space Station, and adds legs, a battery and a perception head. After the 2013 trials the team rebuilt the hands and ankles and upgraded the sensors, then partnered with the Florida Institute for Human and Machine Cognition on walking software. In 2015 NASA lent R5 units to MIT, Northeastern University and the University of Edinburgh, with support funding, to develop control software for the Space Robotics Challenge — a competition whose scenario had the robot repairing a Martian habitat after a dust storm. Valkyrie never became a product and never flew; it remains a research platform, and the direct ancestor of NASA's later humanoid work.