Sorry for the hijack, but I have a question for all you smarty-pants. If a spacecraft traveling at high speeds would be subject to extra bombardment of hydrogen atoms, then it seems to me that there is a universal or privileged velocity which minimizes the average number of impacts. Isn’t this a little odd?
Lucwarm: not really. The hydrogen atoms that fill interstellar space form a very diffuse gas cloud. By necessity, there exists a frame of reference in which this gas cloud is at rest. (That is, a frame in which, at any given point, the average speed of particles near that point is at a minimum). It should not be surprising that this frame is close to at rest with respect to the stars, including our sun, because the stars originally formed from the condensation of particularly dense regions of gas. Therefore if one is traveling somewhere in the same general part of the galaxy as our solar system and is moving very fast with respect to the sun, one will also be moving very fast with respect to most interstellar hydrogen atoms.