thrust that can be produced to a small value. Power generation adds significant mass to the spacecraft, and ultimately the weight of the power source limits the performance of the vehicle. Current nuclear power generators are approximately half the weight of solar panels per watt of energy supplied, at terrestrial distances from the Sun. Chemical power generators are not used due to the far lower total available energy. Beamed power to the spacecraft shows some potential. Some electromagnetic methods: Ion thrusters (accelerate ions first and later neutralize the ion beam with an electron stream emitted from a cathode called a neutralizer) Electrostatic ion thruster Gridded ion thruster Field-emission electric propulsion MagBeam Hall-effect thruster Colloid thruster Electrothermal thrusters (electromagnetic fields are used to generate a plasma to increase the heat of the bulk propellant, the thermal energy imparted to the propellant gas is then converted into kinetic energy by a nozzle of either physical material construction or by magnetic means) DC arcjet Microwave arcjet Helicon double-layer thruster Electromagnetic thrusters (ions are accelerated either by the Lorentz Force or by the effect of electromagnetic fields where the electric field is not in the direction of the acceleration) Plasma propulsion engine Magnetoplasmadynamic thruster Electrodeless plasma thruster Pulsed inductive thruster Pulsed plasma thruster Variable specific impulse magnetoplasma rocket (VASIMR) Vacuum arc thruster Mass drivers (for propulsion) In electrothermal and electromagnetic thrusters, both ions and electrons are accelerated simultaneously, no neutralizer is required. Without internal reaction mass The law of conservation of momentum is usually taken to imply that any engine which uses no reaction mass cannot accelerate the center of mass of a spaceship (changing orientation, on the other hand, is possible). But space is not empty, especially space inside the Solar System; there are gravitation fields, magnetic fields, electromagnetic waves, solar wind and solar radiation. Electromagnetic waves in particular are known to contain momentum, despite being massless; specifically the momentum flux density P of an EM wave is quantitatively 1/c2 times the Poynting vector S, i.e. P = S/c2, where c is the velocity of light. Field propulsion methods which do not rely on reaction mass thus must try to take advantage of this fact by coupling to a momentum-bearing field such as an EM wave that exists in the vicinity of the craft. However, because many of these phenomena are diffuse in nature, corresponding propulsion structures need to be proportionately large. There