What happens when a ping-pong ball hits an electric fan?
Based in part on JPL’s The Basics of Space Flight by Dave Doody and George Stephan, and on an article by Dave Doody in the May/June 1995 issue of The Planetary Report (courtesy of The Planetary Society). “Gravity assists” make it possible for a spacecraft to reach the distant outer planets without using vast amounts of propellant. Michael Minovitch, a student working at the Jet Propulsion Laboratory in the 1960s, helped develop this marvelous technique. Astronomers had long known that comets’ orbits were altered by encountered with planets, but it was Minovitch who first recognized that the principle could be applied to spacecraft trajectories. Right: This diagram from JPL shows Cassini’s circuitous path to Saturn, featuring gravity assist flybys of Earth, Venus and Jupiter. Click for a larger image. How can gravity assist a spacecraft? Consider the following: At point 4 in the diagram pictured right, the spacecraft flies behind Venus. The planet, of course, pulls Cassini with its grav
