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Since no atmosphere is present, all the reaction (propellant) mass must be carried on-board. The energy required to kick this reaction mass out the back of the rocket can be either in the form of thermal energy ( chemical reactions , nuclear energy , etc.) used to heat a gas ("working fluid") that expands out through a nozzle, or in the form of electrical energy used to accelerate ions (electrically charged atoms or molecules) out of the rocket.