That energy is used to heat lightweight hydrogen gas, and shoot it through a nozzle to get thrust. The nuclear reaction heats the hydrogen to much higher velocities than chemical combustion can. Or we can just decide to travel at the same speed as the chemical with the same payload and just require a lot less mass and maybe a smaller vehicle. Advocates of nuclear powered rocket engines point out that at the time of launch, there is almost no radiation released from the nuclear reactors. The nuclear-powered rockets aren't used to get off the ground, just to get to and from Mars, to generate power during the trip, and to brake into Mars and eventually Earth orbit on the return trip.
Plans call for the vehicles to be launched from Earth on traditional chemical rockets.
Thus, at the time of launch, the reactors in a new nuclear rocket are no more dangerous than large pile of dirt, Borowski said. Astronauts are among the most enthusiastic boosters of such a nuclear-powered mission. This reduces the amount of time that astronauts are exposed to the dangerous solar and cosmic radiation that permeates space. Compared to the radiation released from a well-designed, adequately shielded nuclear rocket engine, the radiation environment of space is tremendously more dangerous. The issue with nuclear engines and nuclear power sources is people are afraid of them.