Schemeatic of an early fission rocket, using a fission reactor to heat up hydrogen reaction mass and eject it at high velocity

Reaction Mass is material which is ejected from an ion engine or rocket thruster to provide thrust.  In the vacuum and zero-G of deep space, the only way to accelerate, decelerate, or change direction (unless you cheat with a gravity drive) is to throw something in the opposite direction from how you want to move.  Reaction mass is what gets thrown, be it hydrogen gas or rocket exhaust.  The best possible reaction mass is monatomic hydrogen (single atoms of hydrogen instead of two-atom hydrogen gas molecules), since it is more efficient to throw out many tiny particles of very low mass at very high speed than it is to throw away fewer big particles at lower speeds. Other common types of reaction mass are water (cheap and has other uses on board a ship), xenon gas (easy to store and great for ion engines), and Hydrazine (a hypergolic fuel used mostly in maneuvering thrusters).

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