NASA astronaut Kate Rubins seen in quarantine behind glass during a crew press conference at the Cosmonaut Hotel in Baikonur, Kazakhstan, July 6, 2016. Picture taken July 6, 2016. (NASA/Bill Ingalls/Handout via Reuters)

WASHINGTON, Aug. 19 (Xinhua) -- Two U.S. astronauts on Friday ventured out of the International Space Station and successfully installed the first of two crucial docking ports that will enable future arrivals of American commercial crew spacecraft.

Jeff Williams and Kate Rubins from the U.S. space agency NASA concluded their spacewalk at 2:02 p.m. EDT (1802 GMT) after spending nearly six hours installing the ring-like equipment, known as an international docking adapter (IDA).

It's "a significant milestone in NASA's work to return crew launches to U.S. soil," the agency said in a statement.

Since NASA retired its space shuttle program in 2011, the only vehicle to ferry humans to the space station is Russia's Soyuz spacecraft.

NASA created the commercial crew program in the hope of ending the U.S. reliance on Russia. It has awarded contracts to Boeing and SpaceX to build commercial spaceships, with their first flights slated for 2017.

The IDA arrived at the space station July 20 on a SpaceX Dragon cargo resupply spacecraft. It's a ring weighing more than 1,000 pounds (454 kg) that will provide a standardized connection point for Boeing and SpaceX crew spacecraft to automatically dock with the station.

Another IDA was expected to be launched in 2018.

This is the fourth spacewalk in Williams' career and the first for Rubins. It is the 194th spacewalk for the space station.