Just how dangerous is it to travel at 20% the speed of light?
Breakthrough Starshot has examined the impact of stray atoms, cosmic dust.
Just how dangerous is it to travel at 20% the speed of light? | Ars Technica
http://arstechnica.com/science/2016/08/could-breakthrough-starshots-ships-survive-the-trip/
The space between here and the nearest star isn't completely empty. Earlier generations of stars have left a sparse collection of tiny dust particles,
and various processes have left individual atoms scattered through the interstellar space. These particles pose very distinct risks, and the team worked
through each separately.
The main problem with running into an atom isn't the physical collision but rather the energy deposited in the spacecraft by the impact, which can cause
local heating that damages the craft in two ways. When high enough, the heating causes the material the craft is made of to evaporate into space. Below
that threshold, it can cause local melting, leading to a change in material properties as the area resolidifies.