English: NASA astronaut Rick Mastracchio, STS-131 mission specialist, participates in the mission's first session of extravehicular activity (EVA) as construction and maintenance continue on the International Space Station. During the six-hour, 27-minute spacewalk, Mastracchio and astronaut Clayton Anderson (out of frame), mission specialist, helped move a new 1,700-pound ammonia tank from space shuttle Discovery's cargo bay to a temporary parking place on the station, retrieved an experiment from the Japanese Kibo Laboratory exposed facility and replaced a Rate Gyro Assembly on one of the truss segments.