On the first ever all-female spacewalk today, NASA astronauts Christina Koch and Jessica Meir venture outside the International Space Station to replace faulty equipment on the station’s exterior.

The astronauts will replace a faulty battery charge/discharge unit (BCDU) that failed to activate following the Oct. 11 installation of new lithium-ion batteries on the space station’s exterior structure.

The BCDUs regulate the amount of charge put into the batteries that collect energy from the station’s solar arrays to power station systems during periods when the station orbits during nighttime passes around Earth. Though the BCDU failure has not impacted station operations or crew safety, it does prevent the new batteries from providing increased station power.

Koch is be extravehicular crew member 1 (EV1) wearing the spacesuit with the red stripes, and Meir is the extravehicular crew member 2 (EV2) wearing the suit with no stripes. The spacewalk is the 221st in support of station assembly, maintenance and upgrades and the eighth outside the station this year. This is Koch’s fourth spacewalk and Meir’s first.

It is the first spaceflight for both women, who were selected as astronaut candidates in 2013 as part of the first class, of which 50% were women.

Meir is the 15th woman to spacewalk, and the 14th U.S. woman. It is the 43rd spacewalk to include a woman. Women have been performing spacewalks since 1984, when Russian cosmonaut Svetlana Savitskaya spacewalked in July and NASA astronaut Kathryn Sullivan conducted a spacewalked in October.