US astronauts Christina Koch and Jessica Meir have become the first all-female pairing to carry out a spacewalk, following a spacesuit blunder earlier this year that caused the historic mission to be aborted.
"Christina, you may egress the airlock," spacecraft communicator Stephanie Wilson told the pair shortly after they set out to replace a power controller on the International Space Station at 11.38am.
They began their mission making standard safety checks on their suits and tethers, before making their way to the repair site.
In a call to reporters a few minutes earlier, NASA administrator Jim Bridenstine emphasised its symbolic importance.
"We want to make sure that space is available to all people, and this is another milestone in that evolution.
"I have an 11-year-old daughter, I want her to see herself as having all the same opportunities that I found myself as having when I was growing up."
The first such mission was supposed to take place in March but was cancelled because the space agency had only one medium-sized suit, with a male-female combination performing the required task at a later date.
Traditionally male-dominated NASA's failure to be adequately prepared was denounced in some quarters as evidence of implicit sexism.
LIVE NOW: Tune in to watch the first #AllWomanSpacewalk in human history! 👩🏻🚀— NASA (@NASA) October 18, 2019
Starting at approximately 7:50am ET, @Astro_Christina & @Astro_Jessica venture into the vacuum of space to replace a failed power controller. Watch: https://t.co/2SIb9YXlRh
Ms Koch, an electrical engineer, was leading Ms Meir, who holds a doctorate in marine biology and was making her first ever spacewalk.
The two were working to replace a faulty battery charge/discharge unit, known as a BCDU.
The station relies on solar power but is out of direct sunlight for much of its orbit and therefore needs batteries, and the BCDUs regulate the amount of charge that goes into them.
The current task was announced on Monday and is part of a wider mission of replacing aging nickel-hydrogen batteries with higher capacity lithium-ion units.
The US sent its first female astronaut into space in 1983, when Sally Ride took part in the seventh Space Shuttle mission, and has now had more women astronauts than any other country.
But the first woman in space was Soviet cosmonaut Valentina Tereshkova in 1963, followed by compatriot Svetlana Savitskaya in 1982, who was also the first woman spacewalker two years later.
The spacewalk comes as NASA plans to return to the Moon by 2024 under the Artemis mission, named for the twin sister of Apollo in Greek mythology.
The mission will see the first woman to set foot on the lunar surface, possibly as part of an all-woman team.