Figure 7-20. A gravitational slingshot that includes the motion of Jupiter around the Sun. When both, the gravitational interaction and the orbital velocity of Jupiter are taken into account, you can see that the spacecraft changes direction and speed. (Wikipedia)
Figure 7-19. A gravitational slingshot assumingJupiter itself is not moving around the Sun. The gravitational interaction bends the spacecraft’s trajectory, but its outgoing speed is the same as the incoming speed. (Wikipedia)
speed, the spacecraft flies against the movement of the planet. The main practical limit to the use of a gravity assist maneuver is that planets and other large masses are seldom in the right places to enable a voyage to a particular destination. For example the Voyager missions which started in the late 1970s were made possible by the “Grand Tour” alignment of Jupiter, Saturn, Uranus and Neptune. A similar alignment will not occur again until the middle of the 22nd century. That is an extreme case, but even for less ambitious missions there are years when the planets are scattered in unsuitable parts of their orbits.
kinetic energy and momentum gained by the spaceship are equal in magnitude to that lost by the planet. The effects on the planet are so tiny, because planets are so much more massive than spacecraft, that they can be ignored. Gravitational slingshots that accelerate space probes are extremely important to make flights to distant planets possible with chemical rockets. To increase speed, the spacecraft flies with the movement of the planet; to decrease
7.4.3 Free Return Trajectory
Figure 7-21. A diagram of the slingshot trajectories that enabled NASA’s twin Voyager spacecraft to tour the four gas giant planets (NASA)
The free return trajectory is a special case of a gravity assist trajectory: it uses gravity to completely turn the path of a spacecraft around. In a nutshell, the free return trajectory idea is that you aim the spacecraft at the destination, and burn engines only once, when you depart. After that you just coast. When you get to the destination, you approach the body from the front. Gravity will turn and slingshot your vehicle around in such a way that it is sent on a journey back to Earth with-
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