http://www.wired.com/wiredscience/2012/12/castles-in-space-1961-1971/
"The gravity-assist concept dates back at least to the early 1950s.
Credit for the first rigorous analysis of the concept after the start of
the Space Age probably belongs to Michael Minovitch. In a 1961 Jet
Propulsion Laboratory internal document, he demonstrated mathematically
that a spacecraft could decelerate or accelerate without using
propellants as it flew past a planet. The planet would accelerate by a
tiny amount if the spacecraft were decelerated or decelerate by a tiny
amount if the spacecraft were accelerated."
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