A common item of sfTerminology derived from a long only theoretical means ofRocket propulsion proposed by KonstantinTsiolkovsky in 1911. 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 (assuming a suitably inexhaustiblePower Source), so that very high terminal velocities could be achieved. The first successful test in space of such a system was in 1964 with NASA'S SERT (Space Electric Rocket Test) probe, which did not enter orbit; the propellant was ionized mercury and the electric power was derived from solar cells. The first successfulorbital test of an ion engine was the ATS-4 flight on 10 August 1968. Practical applications have included NASA's manoeuvrableDeep Space 1 space craft launched in 1998, using ionized xenon as propellant.

There is a surprisingly early sf appearance of the ion drive concept in Donald WHorner'sBy Aeroplane to the Sun: Being the Adventures of a Daring Aviator and his Friends (1910). Ion drives are also mentioned by name in "The Equalizer" (March 1947Astounding) by JackWilliamson, "In the Still Waters" (June 1955Fantastic Universe) by Lesterdel Rey,Trivana I (1966) by R CoxAbel and CharlesBarren, andTau Zero (June-August 1967Galaxy as "To Outlive Eternity"; exp1970) by PoulAnderson. Twenty-first century appearances includePandora's Star (2004) by Peter FHamilton andPolity Agent (2006) by NeilAsher. [PN/DRL]

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