Mechanical Engineering Vol 130 no. 11 November 2008 p. 42 f.

To attain the velocities needeed for exploration, space engines must exhibit high specific impulse,
I= F/Wdot: F is thrust; Wdot is weight of propellant consumed per second.

Space engines use electric and magnetic forces to eject plasma particles, as in current ion engines and Hall effect thrusters.

Ion engines have an electrode that creates an electric field, and works with an electric grid to accelerate the ions. Hall effect thrusters, first developed by the Russians, use ring magnets to create a magnetic field perpendicular to en electric field created by an electrode. The operating life of both these engines is limited because their electrodes erode, as does the grid in the ion engine.

Marshall Spae Flight Center in Alabama is attempting to develope an electromagnetic engine. Electromagnetic engines have magnets that ring a cylinder into which propellant is injected. The propellant is energized by radio frequency waves to a plasma state. Particles in the plasma interact to create more energy , which can be used as thrust. this , the EM engine has no electrodes or grids to wear out.