Astronauts Christina Koch and Jessica Meir will now venture out Thursday or Friday, instead of next Monday. A battery power controller failed over the weekend, prompting the change.
The women will replace the broken component, rather than install new batteries, which was their original job. Last week, astronauts conducted two spacewalks to replace old space station batteries.
They have three more spacewalks to go to finish the battery work. A battery-swapping spacewalk scheduled for Wednesday has been called off.
This will be the first spacewalk by only women in more than a half-century of spacewalking. NASA said the six station astronauts remain safe, despite the failure.