Two American astronauts ventured outside the International Space Station on Friday to work on upgrading the orbiting outpost's electrical system.
Americans Shane Kimbrough, the 49-year-old commander of the six-person crew aboard the ISS, and flight engineer Peggy Whitson, 56, emerged from an airlock at 1223 GMT.
Their spacewalk is expected to last six-and-a-half hours, during which the pair will install adapter plates and electrical connections for three of six new lithium-ion batteries installed on the station's truss, according to the US space agency NASA.
Japan's uninhabited H-II Transfer Vehicle, or HTV, delivered the batteries last month. The Canadian-built Dextre robotic arm began the installation to pave the way for the astronauts' work.
Kimbrough and Whitson will be assisted by European Space Agency astronaut Thomas Pesquet, 38, and Oleg Novitskiy of Russia from inside the station. The entire orbital ballet is being conducted by the Johnson Space Center in Houston, Texas.
"The major objective of this spacewalk is to complete the replacement of the nickel-hydrogen batteries that are currently on board the station with lithium-ion batteries," extravehicular activity flight director Jud Frieling said in a briefing on NASA TV.
In a NASA TV interview, Kimbrough said: "The batteries that are out there now have been up here for the duration of the space station, some over a decade and the others even more, so it's just time to change them out."
The new batteries are expected to last as long as the ISS is expected to remain in use -- until at least 2024.
Friday's spacewalk marks Whitson's seventh, matching the record previously set by fellow American Suni Williams for the most spacewalks by a woman.
It is also the 196th overall spacewalk for space station assembly and maintenance.
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