MIAMI-With more private spaceship traffic expected at the International Space Station in the coming years, two US astronauts embarked on a spacewalk Friday to install a special parking spot for them. The spacewalk began at 8:04 (1204 GMT) when Americans Jeff Williams and Kate Rubins switched their spacesuits to internal battery power.  Shortly afterward, they made their way outside the orbiting laboratory to begin the work of attaching an international docking adaptor, launched aboard a SpaceX Dragon cargo ship last month. NASA describes the gear as “a metaphorical gateway to a future” that will allow a new generation of US spacecraft - the first since the space shuttle program ended in 2011 - to carry astronauts to the space station.

The docking adaptor will be the first of two such additions to the space station. The second is expected to be installed in 2018.

ISS operations integration manager Kenneth Todd called Friday’s installation a “very significant milestone on the path to establishing commercial crew capability.” Built by Boeing, the circular adaptor measures around 42 inches (one meter) tall and about 63 inches wide.

The adaptors will work with Boeing’s CST-100 Starliner and SpaceX’s Crew Dragon, two spaceships under construction that are planned to ferry astronauts to the space station.

The docking adaptor is more sophisticated than past equipment because it will allow automatic parking instead of the current grapple and berthing process, which is managed by astronauts. It also has fittings that will enable the space station to share power and data with the spacecraft.

Williams is making his fourth career spacewalk . He has accumulated 19 hours of spacewalk time in his life. The spacewalk is Rubins’s first. But the work will be hardly brand new for her. She has already practiced the necessary maneuvers, including mating the cables, in NASA’s neutral buoyancy laboratory in Houston. She is the 12th woman to walk in space.

A series of spacewalks last year have helped prepare the groundwork for the adaptor’s arrival.

Late Wednesday, the space station’s robotic arm pulled the docking adaptor from the trunk of the SpaceX Dragon cargo ship, placing it next to the station’s Harmony module for installation.

The last US spacewalk was on January 15, when a problem with American Tim Kopra’s spacesuit allowed a small amount of water to build up inside his helmet by the end of the outing.

It was the latest in a series of spacesuit issues, but was not as severe as an emergency in 2013 when Italian astronaut Luca Parmitano’s helmet flooded, forcing him to end his spacewalk early, Todd said.

NASA commentator Rob Navias said the US space agency has done a thorough review of the spacesuits being worn by Rubins and Williams, and they are in “excellent shape” for Friday’s spacewalk .

NASA is planning another spacewalk on September 1 to retract one of the thermal radiators outside the space station.

Astronauts unsuccessfully tried to push it back into position last year, Todd said. “We will go back here in a couple of weeks and restow that guy for its final time,” he added.