U.S. astronauts Christina Koch and Jessica Meir on Friday stepped outside the International Space Station (ISS) to conduct the first all-female spacewalk in history.

The much-anticipated milestone for NASA was achieved during a relatively routine mission to swap faulty batteries on the station's exterior.

At 7:38 a.m. ET (1138 GMT), the spacewalk officially started as the two astronauts set their spacesuits to battery power, marking the beginning of their excursion outside of the space station to replace a failed power controller, according to NASA.

The mission was earlier scheduled to be carried out in March. However, it was canceled due in part to spacesuit availability on the station.

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It has been 35 years since Russian Svetlana Savitskaya became the first woman to conduct a spacewalk. The astronaut went outside the USSR's Salyut 7 space station for three hours, 35 minutes on July 25, 1984.

In total, there have been 15 female spacewalkers, including Christina Koch and Jessica Meir. 

Christina Koch, who joined the ISS earlier this year, is slated to set a record for the longest single spaceflight by a woman with an expected total of 328 days in space.

"I think it's important because of the historical nature of what we're doing. In the past women haven't always been at the table. It's wonderful to be contributing to the space program at a time when all contributions are being accepted," said Koch in a recent NASA interview.