Question: I heard Tim talk about cosmic rays on his eyes (perhaps when they're sleeping), what are they and how does it effect them?

Kirsty Lindsay answered on 20 Jun 2016:

Cosmic rays are very high energy particles that travel through space really fast. We occasionally get hit my them on Earth, but mostly we are protected by the atmosphere. In space there is less protection so astronauts get hit more often- they where first reported by the Apollo Astronauts who saw bright flashes of light while they were asleep.

Mostly they are harmless in low concentrations, but if you get hit by a lot of them it can damage cells.