Interstellar Medium Shielding
It might surprise you that you need to shield your ship from the interstellar medium, specifically at velocities greater than 40% of c. This is a result of interstellar space being filled with a diffuse medium of mostly hydrogen, which when relative to a ship at high enough velocities, comes to increasingly resemble ionizing radiation.
The main danger is heating and not erosion – erosion is insignificant enough that a 1 cm thick carbon shield can go 25,000 light-years (at a speed regime of 0.3 c), ignoring that not all particles displaced will be lost to space, instead landing back on the shield.
Interstellar Medium Density
To begin with, the interstellar medium density varies greatly, ranging from 10^-4 particles per cubic centimeter in the coronal gas component of the galactic halo of the Milky Way, to 10^6 particles per cubic centimeter in molecular clouds.
This is important in calculating the flux that the forward portion of the ship will receive at a particular velocity.
Particle Density Table
(In units of particles per cubic centimeter)
| Component | Particle Density |
|---|---|
| Molecular clouds | 102-106 |
| H II regions | 102-104 |
| Cold neutral medium | 20-50 |
| Warm neutral medium | 0.2-0.5 |
| Warm ionized medium | 0.2-0.5 |
| Coronal gas (Hot ionized medium) |
10-4-10-2 |