[Illustration: 56. A 21st-century space colony in orbit between
Earth and the Moon, as suggested by Dr. Gerard O’Neill of Princeton
University. This colony could accommodate 200,000 persons, using
solar energy for power and lunar or asteroid materials for
construction. The teacup-shaped containers ringing the cylinder are
agricultural stations, and the mirrors would direct sunlight into
the interior, regulate the seasons, and control the day-night
cycle.]
During the first twenty years of the space age, all launch vehicles were
propelled by solid or liquid chemical rockets; however, nuclear and
electric rocket motors are needed to provide the higher thrusts and
velocities required for possible future manned journeys to other
planets. Robert H. Goddard, the American rocket pioneer, was the first
to suggest the possibility of electric rocket motors, but it was not
until 1964 that electric rockets were actually tested in space.
Two types of ion engines represent the most fully developed electric
propulsion systems. In contact ion engines, a propellant gas (mercury or
cesium, for example) is ionized, or given an electrical charge, by
passage through a hot porous metal. The resulting ions are accelerated
out of the engine by an electrical field. The charged ions are
neutralized as they approach the nozzle to form an exhaust beam that
imparts the thrust. Bombardment ion engines rely on the bombardment of
the propellant gas by electrons from a cathode, or negative electrode,
to create ions. The ions are accelerated from the engine in the same
manner as in the contact ion engine.