NASA spacewalkers Bob Behnken and Chris Cassidy

NASA spacewalkers Bob Behnken and Chris Cassidy
NASA spacewalkers Bob Behnken and Chris Cassidy NASA spacewalkers (from left) Bob Behnken and Chris Cassidy give a thumbs up during a spacewalk to install hardware and upgrade International Space Station systems. This photograph was taken by an Expedition 63 crew member inside the cupola, the orbiting lab's "window to the world." NASA

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