The astronauts came out to do maintenance and prepare a parking spot on the American section of the station on Friday. They spent six hours and 34 minutes in space before returning inside the outpost that orbits more than 400 kilometers (250 miles) above Earth.

For France's Thomas Pesquet, the main task was to find the source of a reported ammonia leak in the outdoor plumbing. He did not find traces of the gas at the presumed location.

"No leaks. No flakes whatsoever," Pesquet reported after tugging at the hoses transporting the coolant. He also greased latching mechanisms on the end of the station's robot arm, known as Dextre, which serves to grab cargo ships and move them inside the station.

Meanwhile, his US colleague, Shane Kimbrough, also replaced a pair of cameras and upgraded a computer relay before turning his attention to a docking port. He unplugged cables on the module to allow it to be moved with Dextre. Once secured in its new location, the port will have better clearance for the future ships.

Passenger flights by the end of next year?

A new docking unit is scheduled to fly up to the ISS within a year and attach to this port.

Eventually, the US side of the station is set to have two docking ports for passenger spaceships and two for cargo vehicles, ready to connect to the new type of commercially developed spaceships. US companies Boeing and SpaceX are each working on vehicles to replace the space shuttle program, which was retired in 2011.

In recent years, the only way to transport astronauts to the ISS and back was with Russia's Soyuz capsules, which costs NASA $81 million (75 million euros) per seat. The Russian section features five spaceports for Russian-made equipment.

ISS astronauts will perform two more spacewalks in the next two weeks to prepare the parking spots. The first of the space taxis is scheduled for an unmanned test flight later this year.

NASA hopes that private companies could begin flying astronauts to the ISS by the end of 2018.

dj/sms (Reuters, AFP, AP)