Williams, who was making her second spacewalk, will top the women's list with four spacewalks on completion of the work.
CAPE CANAVERAL: Commander of the International Space Station Michael Lopez-Alegria and flight engineer Sunita Williams floated outside the orbital complex on Wednesday to begin the first of three spacewalks to hook up a new cooling system.
The work is the most ambitious NASA has attempted on the half-built station without a space-shuttle crew present.
"See you in a couple of hours," NASA astronaut Sunita Williams told Russian cosmonaut Mikhail Tyurin, who was staying aboard the station during the spacewalk.
"Good luck," Tyurin replied, shutting the door to seal off the outermost segment of the station's Quest air lock.
Once the chamber depressurized, commander Michael Lopez-Alegria slipped open the hatch to venture into space. The spacewalk began at 10:14 a.m. EST (1514 GMT).
Williams, who was making her second spacewalk, and Lopez-Alegria, on his seventh, will resume work started in December during NASA's last shuttle mission to the station.
That flight left the half-built, $100-billion complex with a new electrical grid and the plumbing to replace the station's temporary cooling loops with an integrated system.
The upgrades are needed to prepare the station for additional modules built by the European Space Agency and Japan. The station is a multinational project, headed by the United States and Russia. Partners include Canada, Japan, Brazil and 11 member nations of the European Space Agency.
Wednesday's spacewalk, which was expected to last about 6-12 hours, is the first of a three-part series scheduled over nine days. The next spacewalk is scheduled for Sunday and the final one on Feb. 8.
While Williams will top the women's list with four spacewalks on completion of the work, Lopez-Alegria too will become US space agency's lead spacewalker with 10 after a fourth spacewalk on February 22 with Russian cosmonaut Mikhail Tyurin.
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