ION DRIVE

   A common item of sf TERMINOLOGY derived from a theoretical means of ROCKET propulsion. Chemically fuelled rockets are hampered by the necessity of carrying large burdens of fuel. Other systems, including the ion drive, propose using much lighter fuels, compensating for the decrease in the mass available for propulsion by ejecting it at correspondingly higher velocities. Ions (charged particles) can be accelerated to enormous velocities using a magnetic field, and so would seem an ideal fuel. Also, since all elements can be ionized (albeit with varying degrees of difficulty), ion-drive rockets could theoretically make use of pretty well any substance to hand. Although an ion drive would produce only a small acceleration because of the relatively tiny masses involved, this could be maintained for months or years, so that very high terminal velocities could be achieved. The first tests in space of such a system began in 1971 with the SERT (Space Electric Rocket Test) satellites; the propellant was ionized mercury and the electric power was derived from solar cells.

   PN