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MAKING PLASMA: The plasma beam of argon propellant can reach temperatures of a million kelvin. Called VASIMR, for variable specific impulse magnetoplasma rocket, the engine is capable of varying how much propellant it uses to generate thrust. In other words, it can run more or less efficiently to give a spaceship an extra boost or conserve propellant during different phases of a mission. A chemical rocket’s efficiency, by contrast, is fixed at a low level.