They are now seriously being considered for trips to Mars. Such rockets could supply large amounts of thrust at reasonably high exhaust velocities. Of course, any unmanned missions could easily use this technology for quicker trips through the solar system. A great benefit of nuclear engines is their long life-a nuclear reactor can supply power for very long voyages. Such craft need to be put into space before they can actually go anywhere due to the very low thrusts provided by ion drives.
Ion drives work by accelerating charged atoms using an electric field.
In DS1, solar panels supplied the electricity that both ionized, and accelerated the xenon propellant. DS1 carried 81.5 kilograms of xenon, enough to supply 20 months of constant acceleration. During the first test of this system, 11.5 kg of xenon was used, and it accelerated the speed of the spacecraft by 1500 mph. Ion drives are highly efficient, and more powerful systems could easily be developed. They simply need a source of electricity.