Star Travel and the Interstellar Medium particle effects

The interstellar medium - the coronal gas - forms a column between locations in space. The sun is 4 light years from one edge of a 60 light-year-across cloud with a density of 0.3 hydrogens per cubic centimeter, or 3e5 H/m³, encountering the sun as it moves towards Scorpio at 25 km/s. Around that cloud is the Local Bubble, 300 LY across with a density of 50 H/m³. A light year is 9.5e15 meters, so for the first 4 light years in one direction, and more than 50 LY in the opposite direction, we encounter a column of hydrogen atoms with a density of about 1e21 H/m²-LY. That is the density of a layer of liquid water 30 nm thick per light year. 100 light years, 3 micrometers.

These protons will implant themselves in the surface of any object passing through them. At a mere 25 km/s, the energy per proton is 3.6 electron volts - many will bounce off. Approaching the speed of light, the energy approaches multiples of the mass energy of the proton, 938 MeV/c². That will damage solid structure to the implant depth.

I don't know how much. A particle physicist can tell you how deep.