Generating electricity from a heat source using no
moving mechanical parts is the ultimate goal of the
Defense Threat Reduction Agency’s thermionics program.
However, developing thermionic energy conversion
devices has proven difficult, although much
progress has been made. In spite of initial success during
the late 1960s and intermittent funding since that
time, for a variety of reasons no thermionic system has
yet been developed in the United States that can be
used today on Earth or in space. The ability of humankind
to reach farther and farther into the solar system
and beyond is determined, in part, by our ability to generate
power in space for spacecraft use.