Ion thrusters
An ion thruster pushes a spacecraft with about the force of a sheet of paper resting on your palm. Kept up for years, that whisper outruns any rocket.
A chemical rocket burns for minutes and is done. An ion thruster does something stranger: it strips electrons from a gas, usually xenon, and hurls the charged atoms out of the back with an electric field, at ten times the exhaust speed of a chemical engine. The thrust is tiny, around 90 millinewtons. But it burns about a hundred grams of gas a day, for years. NASA's Dawn probe used it to orbit two different worlds on one tank, a feat no chemical spacecraft has managed.
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