  Two technological problems must be solved before daring to interstellar
flight: fuel and propulsion. The highest energy-density fuel is antimatter in
its solid or liquid state and this fuel is likely to be our primary choice for
multi-ton relativistic rockets. High-energy ion thruster powered by
annihilation reactor promises superior performance in comparison with direct
propulsion by annihilation products. However the power generator on board can
significantly increase the rocket dry mass thus limiting the achievable speed.
Two physical factors that stand against our dream of the stars are
thermodynamics and radiation hazard. Heat-disposing radiator also increases the
rocket dry mass. Interstellar gas turns into oncoming flux of hard ionizing
radiation at a relativistic speed of the rocket while the oncoming relativistic
interstellar dust grains can cause mechanical damage. Economy and psychology
will play a decisive role in voting for or against the manned interstellar
flights
