Imagine the fuel bill for the Space Shuttle. It’s hard enough to fill your car with 16 gallons of ordinary gas, so imagine the cost to fill a 526,000-gallon tank with rocket fuel. And that’s just to ferry a satellite into orbit. Once in orbit, satellites have to jet themselves around with fuel they’ve brought themselves. Space travel is expensive and there’s a constant search for less costly means of propulsion.

The thrust generated by a propulsion system is proportional to the mass of the propellant multiplied by its velocity. That is, a spacecraft sending 50 grams of propellant away at 3 meters per second has the same thrust as another sending 25 grams away at 6 meters per second. That’s the idea behind electric propulsion: Use less mass than traditional propulsion systems, yet expel it more rapidly. And bringing less mass into Earth’s orbit means the spacecraft is lighter, so it needs less expensive rocket fuel to the begin with.

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Originally published at eHow, SEP 2011