Gravity Propulsion
Gravity-Assisted
In the early 1960s, a graduate student named Michael Minovitch working at JPL developed a way to use a planet's gravitation as a method of propulsion. Gravity-assisted maneuvers work by transferring angular momentum from an orbiting planet to a passing spacecraft. A spacecraft's trajectory is adjusted so that it will make a fairly close fly-by of a planet. As the spacecraft approaches the planet, the planet's gravitational influence greatly accelerates the spacecraft in its orbit. Since momentum must be conserved, the planet slows in its orbit by a tiny amount.
Gravity assists can be also used to decelerate a spacecraft, by flying in front of a body in its orbit, donating some of the spacecraft's angular momentum to the body. When the Galileo spacecraft arrived at Jupiter, passing close in front of Io in its orbit, Galileo experienced deceleration, helping it achieve Jupiter orbit insertion The spacecraft Voyager 2 toured the Jovian planets (Jupiter, Saturn, Uranus, and Neptune). Its trajectory was carefully planned to utilize gravity assists to accelerate the craft toward its next destination, as shown in this illustration. This chart shows the increase in velocity (relative to the Sun) that the spacecraft gained as a result of each gravity assist.
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