Spacecraft propulsion

Spacecraft propulsion comprises methods that change a vehicle’s velocity in space by exchanging momentum with propellant, radiation, or external fields and particles.

Rocket engine: An engine that produces thrust by ejecting reaction mass at high speed. Chemical and electric rocket engines are the main ways spacecraft generate thrust independently of their surroundings.
Orbital maneuver: A deliberate change to a spacecraft’s orbit, usually produced by a change in velocity. Propulsion performs the burns that raise, lower, or reshape an orbit.
Chemical propulsion: Rocket propulsion powered by energy released in chemical reactions between propellants. It offers high thrust, unlike many alternatives that accelerate propellant slowly.
Nuclear electric propulsion: A propulsion architecture that uses a nuclear power source to run electric thrusters. Its mission value depends on reactor mass, power conversion, and long-duration reliability.
Specific impulse: A measure of rocket-engine efficiency equal to thrust per unit weight flow of propellant. It compares how efficiently propulsion systems turn propellant into impulse.
Orbital transfer: A trajectory that carries a spacecraft from one orbit to another. Transfer paths set the velocity changes and burn timing a propulsion system must supply.
Nuclear thermal propulsion: Rocket propulsion that heats propellant with a nuclear reactor before expelling it through a nozzle. It may combine greater exhaust speed than chemical rockets with substantial thrust.
Fusion propulsion: A proposed propulsion approach that uses energy from nuclear fusion to accelerate exhaust or generate thrust. Practical fusion systems could offer high performance, but remain an unresolved engineering challenge.
Tsiolkovsky rocket equation: An equation relating a rocket’s velocity change to exhaust speed and the ratio of initial to final mass. It quantifies why high exhaust speed or large propellant reserves enable greater velocity changes.
Station-keeping: Small corrections that keep a spacecraft within its assigned orbital or positional limits. Even small disturbances require repeated propulsion to preserve operational positions.

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