At 12:45 GMT, NASA astronauts began the last spacewalk of 2015 leaving Britain's Tim Peake in charge of the US section of the International Space Station.
Both Tim Kopra and Scott Kelly are venturing outside the spacecraft to fix a jammed rail car, which is expected to take around three hours.
Peake will be monitoring both his colleagues while they make the repairs.
Tweeting ahead of the move, he said: "Busy day on #ISS today preparing possible EVA for @astro_tim & @StationCDRKelly tomorrow. My job is to help them get out and back safely."
On Tuesday, Peake made British history, becoming the first UK astronaut to launch into space.
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On arrival, the first person he spoke to back home was his mother, Angela. She told him: "Hello Tim. I think you would call today a spectacular day in the office."