Radiation Conditions in Relativistic Interstellar Flight

Radiation hazard on board of a relativistic rocket can be of internal and external origin. Due to its highest specific energy density, antimatter is commonly considered the preferred rocket fuel for acceleration of a multi-ton rocket up to relativistic speed. High-energy products of matter-antimatter annihilation (gamma - and meson radiation) can create a severe radiation hazard for crew and electronics without a reliable radiation shield. Two physical factors can stand against our pursuit to the stars: 1) cooling of a multi-GW propulsion engine, which can be done in space by thermal radiation only, and 2) intense nucleonic radiation originated from the oncoming relativistic ``headwind'' of interstellar gas and cosmic rays. When a rocket accelerates to a relativistic speed, the rarefied interstellar gas of neutral and ionized molecules and atoms turns into an oncoming flux of high-energy nucleons irradiating the rocket and creating a severe radiation hazard on board. In addition, the oncoming flux of relativistic dust granules imposes a threat of mechanical damage to the rocket body. Possible protection measures are discussed.