Ion Engine Inspection
An engineer and a technician check out an ion engine in the Electronic Propulsion Research Building at the Lewis Research Center (now NASA's Glenn Research Center) in 1961.
Ion engines use 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.
Image credit: NASA