Two astronauts primed the International Space Station (ISS) for future construction and installed a vital safety sensor to its exterior Thursday during a speedy spacewalk hundreds of miles above Earth.

ISS Expedition 13 flight engineers Jeffrey Williams and Thomas Reiter spent nearly six hours toiling outside the station while their commander - Russian cosmonaut Pavel Vinogradov - worked alone inside the orbital laboratory. They worked so fast that flight controllers at NASA's Johnson Space Center (JSC) in Houston ran out of spacewalking tasks for them.

"Thomas, it feels good when you get to a point where they run out of things for us to do," said Williams, a NASA astronaut, as the ISS flew some 215 miles (346 kilometers) above Earth.

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But despite their speed, laughter and jokes - of which there were more than a few - the Expedition 13 astronauts accomplished several pivotal tasks to ensure spacewalker safety and prepare the ISS for two visiting NASA shuttles, the first of which is set to launch no earlier than Aug. 27.

ISS managers said today's spacewalk was the first of eight to facing station astronauts to build out the orbital laboratory between NASA's space shuttle visits.

A safety sensor, cooling system work

Of all the tasks facing Williams and Reiter, their primary goal was the installation of a so-called Floating Potential Measurement Unit (FPMU) on the tip of the station's starboard - or right side - truss.

The sensor will monitor the electric potential of ISS solar arrays and allow flight controllers to safeguard future spacewalkers from dangerous electric arcs or shocks when working near the area.

"Look at that shiny gold," Williams said as he removed the device's cloth covers. The tool's three swing-up probes were covered in gold paint for thermal protection, NASA said.

The astronauts also spent a good amount of time working with the station's radiator and cooling systems to install a new motor controller, replace a broken computer, and attach ammonia-carrying jumper lines on the starboard truss.

The work will help spacewalking NASA astronauts activate the cooling system during the agency's STS-116 shuttle mission slated to fly in December.

NASA commentator Kylie Clem later said flight controllers verified that all of the newly installed equipment was online and functioning well.

One of Reiter's final planned tasks included recording 17 minutes of infrared video to test a new camera's ability to detect damage to the reinforced carbon carbon panels used in space shuttle heat shields. A similar test was performed during a July 12 spacewalk during NASA's STS-121 shuttle mission to the ISS.

"This is what I call some kind of cool," said Reiter, who wore the flag of his native Germany on his spacesuit, while using the camera. He is the European Space Agency's (ESA) first long-duration ISS astronaut.

Williams and Reiter also set up two materials exposure experiments to complete their planned chores.

Spacewalk bonus round

At one point, Williams and Reiters were more than 90 minutes ahead of schedule, which allowed them to run down several get-ahead tasks originally slated for future spacewalks.

Williams retrieved a broken navigation antenna and installed a new light to illuminate EVA work sites at night while Reiter outfitted NASA's Destiny lab with a new vent to aid vacuum experiments inside the module. They also retrieved equipment that connects hardware to the station's exterior, as well as photographed airlock scratches and themselves during their free time.

One forward-looking task included the relocation of two foot restraints, and repositioning of a third, which will be used in three planned spacewalks for NASA's STS-115 shuttle mission.

That spaceflight, set to launch aboard NASA's Atlantis orbiter no earlier than Aug. 27, will deliver a new set of solar arrays and two truss segments to the ISS.

Today's excursion was the third career spacewalk for both Williams and Reiter. Williams has spent 19 hours and nine minutes working outside a spacecraft, while Reiter clocked out with 14 hours and 15 minutes of orbital work.

The spacewalk was also the 69^th extravehicular activity to support the ISS and the 41^st staged from the station itself.

During the spacewalk, Williams marveled at an orbital sunrise and observed Tropical Storm Chris near the Caribbean, noting that it looked less organized from orbit than it did on Wednesday. At one point, his camera - which he used liberally - appeared to run out of batteries.

"It's nice out here isn't it," Williams asked Reiter as they climbed along the station's exterior.

"Oh yeah," Reiter agreed.

• Heading Out: ISS Astronauts to Make Spacewalk Today
• Complete Coverage: ISS Expedition 13
• Great Space Quizzes: Life in Orbit
• Gallery: Building the International Space Station