For a manned mission to a distant planet like Mars a nuclear propulsion system or nuclear thermal system seems to be more advantageous in terms of propulsive power than a chemical system as a result of some basic differences. The two main features that lead to the advantages of a nuclear thermal rocket over a chemical one are the enormous energy available per unit mass of fission (or fusion) fuel, and that in a nuclear thermal system the energy producing medium is separate from the thrust-producing propellant.
The first difference between both systems provides nuclear thermal systems with a greater specific impulse (Isp) than chemical ones. The greater specific impulse of the nuclear rocket allows it to carry a larger payload into space, and to accomplish its missions in a reduced time span. The other advantage of a high specific impulse is that the spacecraft can attain higher transfer orbits that result in a transfer orbit that minimizes travel time to the destination.
The second fundamental difference allows nuclear systems to use propellants of low molecular weight, which increase the propulsive force per unit propellant flow. The low molecular weight of the propellant permits for the use of a greater proportion of the total weight placed in space to be composed of the actual payload and not of the propellant. Low molecular weight propellants give mission designers a degree of flexibility for mission design that is not permitted by the chemical propulsion system. With the use of a nuclear propulsion system, mission designers can design missions that are more scientifically complex in nature, because more equipment can be taken up into orbit.
A consequence of these advantages is that nuclear propulsion thus allows the planification of manned missions to distant planets such as Mars. This is not possible with the use of a chemical system, because the crew would not survive the prolonged travel time between Earth and Mars that would be necessary with a rocket propelled by conventional chemical reactions.
Although the interest in high-thrust nuclear thermal propulsion systems (NTP) has grown after President Bush's Space Exploration Initiative in 1991, research into NTP systems has been going on for approximately 50 years. Among the many research programs that have focused on this type of propulsion for the past 50 years is the Rover/NERVA program. The Rover/NERVA program, which lasted for 17 years, proved the feasibility of, and built full-scale operating versions of fission-driven rocket reactors. Also, this program developed an NTP engine system. The NTP engine developed by the Rover/NERVA program was never fully tested, because the program was canceled before flight-testing was achieved.