Options for space propulsion

Various space propulsion methods exist. These methods usually fall in one of two classes, being:

• Rocket propulsion, wherein the propulsive force is generated by expelling mass (initially stored in the vehicle) from the vehicle at a high velocity.
  
• Non-expulsion or non-rocket propulsion through interaction with e.g. the solar wind, a plasma beam, or the magnetic and/or gravitational field of a celestial body.

The majority of rocket propulsion systems today are (thermo-)chemical or cold gas based rocket systems, wherein the propellant both acts as the matter to be expelled and as the energy source  (energy is derived from a chemical reaction or from the thermal energy stored in the gas) needed to accelerate the expelled matter to a high velocity. Depending on the state of agregation of the propellants under storage conditions, we distinguish liquid, solid, and hybrid rockets and cold gas rockets.

Next to chemical systems also non-chemical or separately powered rocket systems exist which use other power sources, like nuclear and electrical sources. The amount of power needed though for thrust generation is thus high though, that such systems are limited to relatively modest thrust levels.

                            Space propulsion options

A further distinction is after how the propellants are accelerated. In cold gas and chemical systems the propellant is accelerated to a high velocity by  expanding a hot high pressure gas in a nozzle (we refer to these engines as thermal acceleration). Typical such systems include resistojet-, arcjet-, solar/laser-thermal and nuclear-thermal systems. Next to these systems, other ways of accelerating the propellant are used  including electrostatic and electromagnetic acceleration. These allow for much higher exhaust velocities to be reached than using themal expansion as the acceleration principle.