Cape Canaveral: Two astronauts ventured outside the International Space Station on Friday on the first of three outings to prepare the orbiting research laboratory for commercial space taxis and to tackle maintenance chores.

US station commander Shane Kimbrough, 49, and French flight engineer Thomas Pesquet, 39, floated outside the station's airlock 402 kilometres above Earth.

The astronauts worked to reconfigure the $US100 billion station, operated by 15 nations, and add docking ports for new spaceships in development by Boeing and Space Exploration Technologies (SpaceX).

Kimbrough, making his fifth spacewalk, headed to the station's central beam to upgrade a computer relay box before turning his attention to a docking system that will be used by future fleets of commercial space taxis.

The US National Aeronautics and Space Administration is making the retrofits in the hope that private companies could begin flying astronauts to the station by the end of 2018. This would break Russia's monopoly on crew transportation, a service that costs NASA more than $US80 million per person.

The first of the space taxis is scheduled for an unmanned debut test flight later this year.

During Friday's spacewalk, Kimbrough was due to disconnect four cables on a docking tunnel to be used by the new commercial space taxis. On Sunday, ground control teams will then use the station's robot arm to move it onto a different module.

A second spacewalk by Pesquet and NASA astronaut Peggy Whitson is planned for Thursday to reconnect cables to the relocated docking tunnel.

Once all the work is finished, the US side of the station will have two docking ports for passenger spaceships and two for cargo ships. Russia, which jointly operates the station with NASA, has five docking ports.

Also during Friday's spacewalk, Pesquet inspected hoses, attachments and other components of the station's ammonia cooling system. Flight controllers had noticed a small leak in the system and wanted to find the source.

A third spacewalk is on hold pending the arrival of a cargo ship that includes items to be installed during the outing.

Engineers are troubleshooting a problem with the cargo ship's launch vehicle, an Atlas 5 rocket made by the Lockheed Martin-Boeing partnership United Launch Alliance. Launch had been targeted for Monday.

Pesquet, flying with the European Space Agency, is a prodigious user of Twitter, his tweets a mix of space science information and space photography.

Reuters