Start of US EVA no. 21.

ISS035-E-036843 (11 May 2013) --- Expedition 35 Flight Engineer Chris Cassidy is captured in a close-up image in the Quest Airlock of the International Space Station prior to a space walk which he shared with Flight Engineer Tom Marshburn (out of frame). The NASA astronauts completed the space walk at 2:14 p.m. EDT May 11 to inspect and replace a pump controller box on the International Space Station?s far port truss (P6) leaking ammonia coolant. The pair began the 5-hour, 30-minute spacewalk at 8:44 a.m.