The electric propulsion system is an alternative to the nuclear thermal propulsion system. Although its total thrust is lower than that of the nuclear rocket, the electrical engine can provide sufficient thrust to propel a spacecraft during planetary missions such as the manned exploration of Mars. As an alternative for the propulsion system to be taken to Mars, the electric system as well as the other alternatives being seriously considered should all have some basic features that are necessary for the first manned space mission to a celestial body other than the Moon. The basic features for a propulsion system to be employed in the future exploration of Mars are
1. the technology employed should be mature and flight-tested
Out of the four requirements mentioned above, the only current propulsion system that seems to meet all of them is the electric propulsion system. Study and development of these systems has been going on since the late 1940s. By 1990, more than eighty such propulsion systems have been employed in orbital missions. Most of these missions have been carried out by the former Soviet Union. The large number of successful missions that have already employed an electric propulsion system support the feasibility of this type of system for long duration space missions. Within the various systems of electrical propulsion that have been developed, the one that currently best satisfies the requirements of a propulsion system for a long duration space mission is the ion propulsion system. Besides having the best efficiency among the electric propulsion systems, this type of electrical rocket is one of the most developed systems currently available. The main difference between an ion thruster or an another electrical system and a nuclear or chemical system is that the exhaust particles of the rocket are not accelerated by heat energy, but by electrical energy. This accounts for electrical propulsion systems reaching higher exhaust velocities than any of the other available propulsion systems. An electrical engine is also benefited by test results, which show that it is highly efficient in transforming power into thrust.