NASA astronauts Jessica Meir and Christina Koch made history on Friday when they ventured outside of the International Space Station (ISS) for the first-ever all-women spacewalk.
Meir and Koch were assigned to replace a battery charge/discharge unit that stopped working after Koch and a male astronaut installed lithium-ion batteries on the ISS's exterior structure on Oct. 11, according to NASA.
The assignment is expected to last five to six hours.
It was the first time in more than half a century of space exploration that two women astronauts walked in space together.
Koch and Meir were in the same class of astronaut candidates in 2013, which NASA said was 50% women.
Since then, they've trained closely together, NASA spokesperson Brandi Dean told BuzzFeed News. "And then once they were both assigned for space station missions, their training continued to overlap," she said. "So they are very close by now."
When Meir set off on her first space journey on Sept. 25, Koch tweeted a photo from the ISS of the launch with the caption, "What it looks like from @Space_Station when your best friend achieves her lifelong dream to go to space."
Friday's assignment was Meir's first, and Koch's fourth. Meir is the 15th woman to walk in space.
Koch wore a spacesuit with red stripes, and views from the camera in her helmet have the number 18. Meir's spacesuit had no stripes, and her helmet camera view is number 11.
Both Koch and Meir are on their first spaceflight. Koch is on an 11-month mission in space for researchers to study the effects of long-term spaceflight on a woman that, according to NASA, will help "prepare for human missions to the Moon and Mars."
A previous all-women spacewalk was scrapped earlier this year after NASA determined it didn't have properly-sized spacesuits for two women, leading to backlash online and accusations of sexism.
Koch was set to go on a spacewalk with astronaut Anne McClain on March 29, but McClain ended up ceding her spot to Nick Hague.
NASA explained in an interview with CNN that because the body changes size in space, the spacesuits they had available at the time required some configuration before it would fit McClain.
McClain later went on a spacewalk with Canadian Space Agency astronaut David Saint-Jacques on April 8.