  From the point of view of radiation safety, interstellar space is not an
empty void. Interstellar gas and cosmic rays, which consist of hydrogen and
helium nucleons, present a severe radiation hazard to crew and electronics
aboard a relativistic interstellar ship. Of the two, the oncoming relativistic
flow of interstellar gas produces the most intence radiation. A protection
shield will be needed to block relativistic interstellar gas that can also
absorb most of the cosmic rays which, as a result of relativistic aberration,
form into a beamed flow propagating toward the front of the ship.
