SpaceX delivered a four-person crew to the International Space Station early Sunday, setting the stage for Barry Wilmore and Sunita Williams to depart.

NASA astronauts Sunita Williams and Butch Wilmore face significant physiological and psychological challenges upon returning from microgravity. They may experience muscle atrophy, bone loss, dizziness,

The return was initially planned for no earlier than Wednesday but was moved up due to unfavorable weather expected later in the week.