U.S. astronauts Christina Koch and Jessica Meir will make history on Oct. 21 as the first women to conduct an all-female spacewalk, or extravehicular activity (EVA).

Koch, who has been aboard the International Space Station (ISS) since March 14, and Meir, who came aboard the ISS only on Sept. 25, will make the historic spacewalk to carry out the same mission the aborted first all-female spacewalk in March 29 should have: replace lithium-ion batteries on the exterior of the ISS. It will be the first spacewalk for Meir.

Koch is on track to break the record for the longest single spaceflight by a woman. She is expected to remain aboard the ISS for 328 days before returning to terra firma. Koch's been aboard the ISS for 205 days as of Oct. 5.

She was originally scheduled to participate in the first all-female EVA back in March with fellow astronaut Anne C. McClain, who has since returned to Earth. The historic mission was scrubbed after the women found there weren't enough "hard upper torsos" for their medium-sized spacesuit components aboard the ISS for both of them. Large-sized spacesuits are used only by men and most of the suits on the ISS were of this size.

The hard upper torso is a key component of any spacesuit. It's a rigid vest made out of fiberglass designed for strength. It's an astronaut's Primary Life Support Subsystem since it houses the backpack containing their air supply, water for the suit's temperature control system and a "thruster," or jetpack, for use in case they detach from the ISS.

McClain, the first openly LGBTQ astronaut, initially volunteered to wear a large-sized spacesuit to get NASA to green light the space walk last March. But she backed-off after completing a spacewalk in the better-fitting medium-sized suit.

Koch holds a master's degree in electrical engineering from North Carolina State University, and is a natural choice for the battery replacement mission. Meir is a marine biologist.

Meir and Koch appeared together in a livestreamed interview from the ISS on Sept. 30. With their ponytails floating behind them, the pair answered questions from fans on Twitter about their life in space, among many other things.

"Well, I think the advice that I give most young people is really to identify what it is you're passionate about," Meir replied when asked about her advice for young girls interested in space.

"You know, that sounds kind of trite -- we say it all the time -- but it really is true. I really don't think you can really excel at something, and more importantly, be happy doing it if it's not something that you're really passionate about. ... I know for both Christina and I, we could never have imagined for both of us that our childhood dream would come true and that both of us would be here ... I think it is proof dreams really can come true."

In cooperation with our partners, the International Space Station will be extended likely to 2020 or beyond enabling this vital orbiting laboratory to reach its full potential. NASA.gov