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Figure 4. Marbles circling...Click to Enlarge Image

Scientists in charge of designing the trajectories for interplanetary space probes traditionally did their calculations as if the only objects of any relevance were the spacecraft itself and one other body, usually the planet located nearby. During the departure phase, for example, only Earth mattered to their calculations of how the probe would move. And upon arrival at its destination, only that distant planet counted. In between, the trajectory was calculated as if the spacecraft was traveling alone in a highly elliptical orbit around the Sun. Although this two-body strategy works, the trajectory it produces is very costly in terms of energy. More sophisticated methods take advantage of the competing gravitational tugs of the different planets and their moons, which create a vast network of passageways by which a spacecraft can travel over large distances while expending very little energy. The author describes this new approach and shows how the same principles explain much about the way comets and asteroids are naturally flung about the solar system.

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