As part of the schedule update, NASA also announced two astronauts, Andrew Morgan and Jessica Meir, will make their first space flights later this year. And Morgan will stay at the ISS longer than originally planned. He'll arrive in July and remain there until spring 2020.

By extending the missions of Koch and Morgan, NASA will be able to gather more data on the effects of spaceflights longer than six months. NASA says there's a significant degree of variability in human response to space travel, and it wants to determine the acceptable degree of change for both men and women. We'll need to better understand those changes in order to plan future explorations to the Moon and Mars.