A few pounds of nuclear fuel can produce many megawatts of power for several years. Provided appropriate safety precautions are taken, NASA believes that space nuclear power systems are fully acceptable. All rockets use a principle called the reaction engine. Chemical rockets already run at temperatures that approach the limits of the materials in the combustion chamber and nozzle. Chemical rockets have a low maximum velocity increment, which means that their exhaust velocities are not high enough to impart very high speeds to the rocket. A nuclear rocket such as this would be inherently safe and environmentally benign: contrary to popular belief, a nuclear rocket need not be strongly radioactive when launched. The spacecraft, with its nuclear thrusters, would be launched as a payload atop a conventional chemical rocket. Then, once the payload was in high-Earth orbit, the nuclear reactor would start up. For a given amount of propellant mass, the nuclear rocket will travel three times faster than the chemical rocket. The intense neutron and gamma ray radiation fields produced by the operating reactor are clearly the main difficulty in using a nuclear thermal reactor on a manned mission. Since hydrogen has one proton and no neutron, its mass is almost identical to the mass of a neutron; therefore hydrogen makes the best moderator. Other molecules, such as water, can compress more hydrogen molecules per unit volume than pure hydrogen gas. Fortunately, the liquid hydrogen can be circulated around the reactor prior to insertion. As the temperature in the core increases, the density of the hydrogen decreases.
             Someday, in exploring the outer planets of our solar system, humankind will want to do more than send probes that merely fly rapidly by them. In time, we will want to send spacecraft that go into orbit around these gaseous giants, land robots on their moons, and even return rock and soil samples back to Earth. Eventually, we will ...