the Coulomb force and accelerate the ions in the direction of the electric field. Electromagnetic ion thrusters use the Lorentz force to accelerate the ions in the direction perpendicular to the electric field. Electric power for ion thrusters is usually provided by solar panels. However, for sufficiently large distances from the sun, nuclear power may be used. In each case, the power supply mass is proportional to the peak power that can be supplied, and both provide, for this application, almost no limit to the energy. Electric thrusters tend to produce low thrust, which results in low acceleration. Defining , the standard gravitational acceleration of Earth, and noting that , this can be analyzed. An NSTAR thruster producing a thrust force of 92 mN will accelerate a satellite with a mass of 1ton by 0.092N / 1000 kg = 9.2m/s (or 9.38g). However, this acceleration can be sustained for months or years at a time, in contrast to the very short burns of chemical rockets. Where: F is the thrust force in N, η is the efficiency P is the electrical power used by the thruster in W, and Isp is the specific impulse in seconds. The ion thruster is not the most promising type of electrically powered spacecraft propulsion, but it is the most successful in practice to date. An ion drive would require two days to accelerate a car to highway speed in vacuum. The technical characteristics, especially thrust, are considerably inferior to the prototypes described in literature, technical capabilities are limited by the space charge created by ions. This limits the thrust density (force per cross-sectional area of the engine). Ion thrusters create small thrust levels (the thrust of Deep Space 1 is approximately equal to the weight of one sheet of paper) compared to conventional chemical rockets, but achieve high specific impulse, or propellant mass efficiency, by accelerating the exhaust to high speed. The power imparted to the exhaust increases with the square of exhaust velocity while thrust increase is linear. Conversely, chemical rockets provide high thrust, but are limited in total impulse by the small amount of energy that can be stored chemically in the propellants. Given the practical weight of suitable power sources, the acceleration from an ion thruster is frequently less than one-thousandth of standard gravity. However, since they operate as electric (or electrostatic) motors, they convert a greater fraction of input power