Engineer and a technician checking out Ion Engine in the Electronic Propulsion Research Building at Lewis Research Center. The Ion engine uses electrostatic charge, something like pulling hot socks out of a clothes dryer. The electrostatic charge pushes the socks away from each other. The fuel used by this device is Xenon, a gas that is four times heavier than air. Although ion engines have been around for decades, they were not used by NASA to propel spacecraft until the late 1990s. The Lewis Research Center is now the John Glenn Research Center.