Koch and Meir will be replacing a battery charge/discharge unit that failed to activate after new lithium-ion batteries were installed outside the ISS a few days ago. The whole spacewalk will take around five to six hours. You'll be able to tell who's who by checking their suits and camera numbers: Koch's suit will have red stripes, and she will be using a helmet camera with the number 18. Meanwhile, Meir's won't have any stripes, and she will be using a helmet camera with the number 11.

NASA canceled the original all-woman spacewalk a few days before it was supposed to happen, because both Koch and then-partner Anne McClain needed medium-sized extravehicular mobility unit or EMU suits, and only one could be ready on time. The agency stocks the ISS with modular suits with various parts in different sizes that need to be put together. It just so happened that the EMU's medium-sized torso fit both astronauts best, and when it comes to spacesuits, you can't just go a size smaller or bigger -- the fit needs to be perfect for the EMU to be able to protect its wearer from the harsh elements of outer space.

Jessica Bennett, The New York Times' Gender Editor, asked Lady Astronaut book series author Mary Robinette Kowal whether the lack of ready medium-sized suits meant they were "designed by and for men." Kowal said that's true for the generation of spacesuits that's currently on the ISS. They were, after all, made in the late 1970s based on Apollo technology. The EMUs are also time-consuming to assemble, which is probably why the agency decided to develop new spacesuits -- under lead designers Amy Ross and Kristine Davis -- for Project Artemis.