It uses energy from the sun to turn xenon gas into plasma and shoot it into space. This makes it use 10 times less propellant than conventional systems. The xe

It uses energy from the sun to turn xenon gas into plasma and shoot it into space. This makes it use 10 times less propellant than conventional systems. The xenon gas is the propellant. This still comes from the earth so it's far from free. It's just more efficient.