With more private spaceship traffic expected at the International Space Station in the coming years, two US astronauts embarked on a spacewalk to install a special parking spot for them.

Key points: Two astronauts will install an adaptor for commercial spacecraft

Two astronauts will install an adaptor for commercial spacecraft Adaptor will allow automatic parking and sharing of power and data

Adaptor will allow automatic parking and sharing of power and data Spacewalk will be first time astronauts have taken equipment from a robot arm on ISS

Americans Jeff Williams and Kate Rubins stepped outside the orbiting laboratory to attach an international docking adaptor launched aboard a SpaceX Dragon cargo ship last month.

NASA has described the equipment 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.

"Commercial crew flights from Florida's Space Coast to the International Space Station will restore America's human spaceflight launch capability and increase the time US crews can dedicate to scientific research, which is helping prepare astronauts for deep space missions, including the journey to Mars," NASA said in a statement.

The International Docking Adaptor, also known as IDA-2, undergoing testing in NASA's Kennedy Space Center in Florida. ( NASA )

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

ISS operations integration manager Kenneth Todd called the 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 (1.6 meters) 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 managed by astronauts.

It also has fittings that enable the space station to share power and data with the spacecraft.

The space walk is expected to take more than six hours. ( NASA: AFP Photo )

A delicate operation over more than six hours

The space agency is streaming live coverage on August 19 from 8:30pm (AEST).

This is the fourth career spacewalk for veteran NASA astronaut Williams, who is about to mark 500 days in space over four missions, and the 194th for the space station.

It is also the first spacewalk for Flight Engineer Rubins, who has practiced the necessary manoeuvres, including mating the cables, in NASA's neutral buoyancy laboratory in Houston.

The two astronauts have ventured outside the space station's Quest airlock in what is expected to be a six-and-a-half-hour task.

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

The space station's robotic arm pulled the docking adaptor from the trunk of the SpaceX Dragon cargo ship on Wednesday, placing it inches away from the station's Harmony module, where it will be installed.

Once the spacewalkers — also known as extravehicular (EV) crew — are outside the space station, an extension of the Canadarm2 robotic arm, called the "Dextre" Special Purpose Dexterous Manipulator (SPDM), will push the bulky adaptor even closer to its installation point.

The plan is for the robotic manipulator to push the adaptor into place so the astronauts only to tether it.

The operation will remain delicate, however, because the SPDM is highly sensitive to external pressure, lead spacewalk officer Glenda Brown said.

"This is the first time that the EV crew members have actually taken a handoff from the SPDM," she told a news conference this week.

"We have to be very careful about putting loads into the SPDM. In space, it has got a lot of capability, but on the ground it can barely support its own weight."

NASA says that a problem that allowed water to build up inside an astronaut's helmet in January has been resolved by the discovery of an apparent flaw in a piece of hardware called a sublimator, which manages condensation in the suit's healing and cooling loops.

ABC/AFP