Watch: How to give yourself an eye exam in space

Astronaut Chris Hadfield explains why eyesight gets worse in space

Having your eyes dilated under the best of circumstances is not fun, but needing to do it yourself in zero gravity is definitely much harder. However, it is absolutely necessary.

New research is showing that astronauts are returning from space with worse eyesight then they left on their mission with. Since many of the problems are permanent, some of these spacemen are told they cannot go back to work, or worse.

As International Space Station commander Chris Hadfield explains, since they do not know why this happens or how to stop it, each astronaut is responsible for checking their vision, optic nerve and overall eye health to track changes.

Watch Hadfield navigate using eye drops without any gravity, and how he ultrasounds his own eye.