NASA astronaut Mark Vande Hei peers into near-infrared medical-imaging gear, or optical coherence tomography, for detailed views of his retina as part of regularly scheduled eye checks aboard the International Space Station.
Astronauts periodically check their eyes because of a phenomenon known as Space-Associated Neuro-Ocular Syndrome (SANS). Symptoms include swelling in the optic disc, which is where the optic nerve enters the retina, and flattening of the eye shape. When researchers looked back, they found certain aspects of SANS in even the earliest spaceflights.
Image Credit: NASA