It was not supposed to be a photo safari. NASA lofted Scott Kelly into orbit aboard the International Space Station in March 2015 for a year so that scientists could learn what happens to the human body during long missions in space. (A lot, as it turned out.)

But between blood draws and equipment repairs, Mr. Kelly aimed a Nikon D4 toward the windows of the space station’s cupola or — his preferred spot — a porthole in the floor of a laboratory module.