  Radiation hazard on board of a relativistic rocket can be of internal and
external origin. Because of its highest specific energy density, antimatter is
considered to be the preferred rocket fuel for accelerating a multi-ton rocket
up to relativistic speed. High-energy products of matter-antimatter
annihilation (gamma-photons and meson radiation) can produce a severe radiation
hazard for crew and electronics. Another factor that can stand against our
pursuit to the stars is intense nucleons radiation produced by the oncoming
relativistic headwind of interstellar gas and cosmic rays with their intensity
increasing with rocket velocity. When a rocket accelerates to a relativistic
speed, the rarefied interstellar gas of neutral and ionized atoms and molecules
transforms into the oncoming flux of high-energy nucleons irradiating the
rocket body and creating the severe radiation hazard. The oncoming flux of
relativistic dust granules imposes a threat of mechanical damage to the rocket
body.
