This morning, two of NASA’s astronauts will make history when they don their space suits and leave the confines of the International Space Station for a 6.5-hour spacewalk. That’s because the two astronauts who are heading out into space — Christina Koch and Jessica Meir — will be performing the first all-female spacewalk in the history of space travel.

To get a perspective on just how big of a deal this is, look at the numbers. Ever since humans started going into space, 227 people have left their spacecraft and done some sort of spacewalking activity. Of that number, only 14 of those humans have been women. (Meir will become the 15th.) Svetlana Savitskaya, a Soviet cosmonaut, was the first woman to spacewalk in 1984, nearly 20 years after the first spacewalker, cosmonaut Alexei Leonov, floated in the vacuum of space. The US followed a few months later, sending NASA astronaut Kathryn Sullivan outside the Space Shuttle in orbit.

Yet every woman who has ever suited up for space has also had a male counterpart suiting up with her. No two women have spacewalked together, making today’s event a big first for NASA — and the world. Koch and Meir have also acknowledged the impact of their work. “What we’re doing now shows all of the work that went in for the decades prior, all of the women that worked to get us where we are today,” Meir said during an interview on the space station just before receiving news of the assignment.

In actuality, this spacewalk is a bit of a do-over for NASA. The space agency had originally planned to send two women astronauts — Koch and Anne McClain — out on a spacewalk back in March, an event that many space enthusiasts were looking forward to. But a week before the scheduled outing, NASA announced that McClain would be replaced with her male colleague, Nick Hague, instead. The reason? McClain realized she needed a medium-sized space suit for the trip, and only one medium was ready on the space station. So the suit went to Koch, and the already-prepared large-sized space suit went to Hague.

Vitriol descended upon NASA, with many angry that the space agency lacked enough space suits sized for women on the ISS. In fact, NASA did have another medium-sized space suit on the station, but it was a spare and not fully configured for spacewalking. Piecing together the second medium suit would have taken up to 12 hours and introduced risk into the mission, so NASA simply chose to change astronauts. Ultimately, it didn’t matter. The damage had been done, and the world was disappointed. SNL even mocked the decision in a skit.

Now, space suit sizing will not be a problem. NASA launched another medium-sized space suit to the ISS this year that is fully configured for going out into space. Meir and Koch will have the sizes they need, with no assembly required.

Ironically, this spacewalk was actually moved up, rescheduled from Monday, October 21st. The spacewalk was meant to be one in a series of five planned excursions to swap out aging batteries on the outside of the space station with newer lithium-ion versions. Two of those spacewalks already happened this month with no problems. But when flight controllers powered up the newly installed lithium-ion batteries, they noticed that an instrument needed to charge and discharge one of the batteries wasn’t working properly. As a result, the space station is losing power on one of the eight power channels used to provide electricity to the ISS. “It’s manageable, but again, not something that we would want to live with in the long term,” Kenny Todd, the operations integration manager for the International Space Station, said during a press conference. So Koch and Meir are tasked with replacing this charge / discharge unit and getting the power channel fully functioning again.

Koch and Meir are set to turn their suits onto battery power at 7:50AM ET on October 18th, marking the beginning of the trip. As is standard for all of NASA’s spacewalks, the entire excursion should last about 6.5 hours, so there will be plenty of time to watch the event throughout the day. As The Atlantic has noted before, watching a spacewalk can be boring but also beautiful. So if you’re having a slow Friday, take some time out of your day to check out history in action on NASA TV.