The astronauts left the station shortly after 8am EST, with a mission is to install the International Docking Adapter so that the next generation of US space vehicles can safely carry future astronauts to the station.
The new dock will be used in 2017 by private companies Boeing and SpaceX to bring their crews to the International Space Station.
NASA started using private U.S. companies to carry out cargo shipments a few years back, but commercial crew launches will be an even bigger step.
Having private compaines do this has provided NASA time to focus on other outer-space exploration; the space agency is working to get astronauts to Mars in the 2030s.
“We have a new port of call for the new U.S. commercial crew vehicles,” announced Mission Control commentator Rob Navias.
The space station is currently home to two Americans, one Japanese and three Russians. Up there for five months, Williams and two of the Russians will return to Earth in a couple weeks.