Planetary Flyby

* Türkçesi için

Before the comlexs and chaotic captures and low-energy-transfers in multi-body become a fashion in celestial mechanics, Planetary Flyby scheme was used as Gravity Assist type maneuver. The explanation is given in below and borrowed by Bong (1998)’s book (which have a relativly clear definition), and the figure is mine ^_^.

A spacecraft after its planetary flyby (or swingby) may gain or lose its energy depending on whether it passes behind or ahead of the planet. The gain or loss of energy is caused by the rotation of the spacecraft’s velocity vector with respect to the planet, as illustrated in Below. In this figure, V∞- and V∞+ denote the hyperbolic excess velocity relative to the target planet before and after the planetary flyby, respectively; V- and V+ denote the inertial velocity of the spacecraft with respect to the sun before and after the planetary flyby, respectively; and Vp, is the inertial velocity of the target planet. In a trailing-side flyby of a planet as shown in upper figure, the spacecraft’s inertial velocity is increased. On the other hand, the spacecraft’s inertial velocity is decreased in a leading-side flyby, as shown in below one. Note that the magnitude of relative velocity before and after the planetary flyby is the same; only its direction is changed by the flyby [*].

Planetary Flyby

Such a planetary flyby maneuver is often called a gravity-assist or gravitational slingshot maneuver. One of the space exploration missions that utilized such a gravity-assist maneuver to reach a target planet is the Galileo spacecraft that was sent to Jupiter. In October 1989, after a three-year delay, the Galileo spacecraft was launched from a Space Shuttle parking orbit toward Jupiter, using an inertial upper stage (IUS). Changing from the originally planned use of a liquid Centaur upper stage to a less powerful solid IUS (because of a safety concern after the Shuttle Challenger accident) has resulted in a new trajectory. The Galileo spacecraft took about six years of travel time to reach Jupiter after performing one gravityassist maneuver at Venus and two such maneuvers at Earth [* Bong, 1998].

* Wie, Bong, Space vehicle dynamics and control, American Institute of Aeronautics and Astronautics, 1998.

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