Spacecraft powered by electric propulsion could soon be better protected ... but ion engines require less fuel and therefore less mass, which reduces launch costs, and ion engines don't use ...

“Spacecraft can save significantly on propellant mass by officially maintaining orbit using electric propulsion ... MSU ...

These systems are characterized by the propellant used and emission mode of the ion beam. The most common of these, field emission electric propulsion (FEEP), utilize liquid metals as its ion source ...