An ion thruster (or ion drive), one of several types of spacecraft propulsion, uses beams of ions - electrically charged atoms or molecules - for propulsion. The precise method for accelerating the ions may vary, but all designs take advantage of the charge-to-mass ratio of ions to accelerate them to very high velocities using a high electric field. Ion thrusters are therefore able to achieve high specific impulse, reducing the amount of reaction mass required, but increasing the amount of power required compared to chemical rockets. Ion thrusters can deliver one order of magnitude greater propellant efficiency than traditional liquid fuel rocket engines, but are constrained to very low accelerations by the power/weight ratios of available power systems.