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.

27 pages, 10 figures