WATCH: 2 American women astronauts venture out for first all-female spacewalk

Almost precisely 35 years after the first American woman walked in space, NASA is setting another milestone Friday morning, with astronauts Christina Koch and Jessica Meir conducting the first-ever all-female spacewalk.

Koch, an engineer, and Meir, a biologist, suited up and entered the "crew lock" that serves as the International Space Station's doorway into space. They switched on the battery power in their suits at 7:38 a.m. ET, signalling the official start of the spacewalk.

The outer hatch began opening as controllers on the ground finished ensuring the astronauts' suits were ready, including visors that are required by the sunlight hitting the station. Then, Koch received the word.

"Christina, you may egress the airlock," said NASA's Stephanie Wilson, the spacewalk's coordinator at Mission Control Center in Houston.

"Copy," Koch answered.

In what the space agency billed "HERstory," the four men aboard the International Space Station remained on the orbital outpost, as Koch and Meir stepped outside to replace a broken battery charger.

After taking a moment to get used to moving around in the vacuum of space, they then set about preparing the tools they'll need to replace a failed power controller in the system that collects and distributes solar power to the space station's systems.

The charger failed after Koch's last extravehicular activity, or EVA, as spacewalks are known in NASA speak, when she and fellow astronaut Andrew Morgan installed new batteries on the outside of the station last week.

The historic spacewalk has been months in the making.

It was originally supposed to happen in March, with astronaut Anne McClain joining Koch. But the station was short one functioning, medium-sized spacewalking suit, the right size for both women. Koch went instead, with colleague Nick Hague. So, the all-female EVA was postponed.

Koch, an electrical engineer, is a veteran of three previous spacewalks since her first with Hague in March. She's seven months into a planned 11-month stay on the station. Friday's EVA is a first for Meir, a marine biologist, who is also on her first spaceflight.

Soviet cosmonaut Alexei Leonov, who died last week, was the first person to walk in space in March 1965. He was followed less than three months later by American Ed White.

Sally Ride became America's first woman in space (aboard Space Shuttle Challenger) in 1983, 20 years after Russian cosmonaut Valentina Tereshkova was launched into orbit on Vostok 6. A year after Ride's first flight, another Russian woman, Svetlana Savitskaya, became the first woman to walk in space, followed a few months later, on Oct. 11, 1984, by America's first female spacewalker, Kathryn Sullivan.

But Koch and Meir's EVA is a first for any nation in space.

Meir is the 15th female spacewalker, compared to 213 men, according to The Associated Press.

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