For decades, spacecraft have relied on nuclear power as a compact, reliable source of electricity, especially on missions for which solar power is not feasible. 

In spacecraft like the Voyagers, Curiosity and many others, a radioisotope thermoelectric generator (RTG) converts heat energy from passive radioactive decay directly into electricity. Although RTGs are not particularly efficient, they are simple and have no moving parts, making them perfect for applications in which repair is not an option. 

But many future deep space missions, especially the manned ones, would require hundreds and even thousands of megawatts of electric power. Of all known technologies only nuclear reactors are capable of generating this kind of power for prolonged periods of time.

Basically, any modern nuclear power plant is just a good old steam engine with a nuclear fuel inside instead of coal, and an attached electric turbine. Low efficiency thermal conversion, large quantities of coolant are some of the major disadvantages of this power source in its current state.

Heat dissipation in space vacuum is only achievable through thermal emission that requires massive radiators. A 500 MW nuclear power plant aboard of spacecraft would require square miles of radiators. 

Our team is developing a drastically new Direct Energy Conversion Open-Architecture Nuclear Fusion Reactor. The reactor will become a main power source for the Project Space Ark Astrodrive engines.