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Systems for propelling a spacecraft from one place to another, such as rockets.

Spacecraft often have the need of changing their velocity, for example when performing an orbital transfer. To do so, propulsion is needed.

While the spacecraft's velocity is changed, momentum has to be conserved. This problem is usually solved by accelerating mass in the opposite direction. That is the case for:

• Chemical rockets, using energy from a chemical reaction to expand the exhaust products through a nozzle.
• Thermal rockets, using some externally provided power (beamed or nuclear) to accelerate a propellant.
• Ion engines, using an electric or magnetic field to accelerate charged particles.

Momentum can also be transferred without sacrificing mass by photons, either by reflecting sunlight or laser light with a sail, or by emitting photons.

The preferred form of propulsion is chosen from the requirements for acceleration and the magnitude of the acceleration change (known as delta-V or $\Delta V$), and by how much propellant the spacecraft can carry or how much energy is available.

### Existing technologies:

• Chemical rockets, high acceleration, low propellant efficiency. Widely used.
• Nuclear rockets, modest acceleration, good propellant efficiency. Prototypes built by the US and the Soviet Union.
• Ion engines, low acceleration, exceptional propellant efficiency. Used by some small probes for interplanetary missions, and by a number of satellites for maintaining orbit (stationkeeping).
• Solar sails, minuscule acceleration, does not require propellant. Some prototypes tested.