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Many spaceprobes have been sent to the other planets in the solar system (Pluto being the notable exception) as well as several asteroids and comets. If the timing of the launch is right, a probe can visit several planets on its trip through the solar system. The most notable example of this is the Voyager 2 probe, which, during the period from 1979-1986, visited Jupiter, Saturn, Uranus, and Neptune.




In reality, spaceprobes are not so limited by energy considerations to have to move on Hohmann orbits. Often spacecraft can pick up speed as they pass a planet by a process known as a "gravity assist," where they use the planet's gravitational field to "slingshot" themselves across the solar system. Using this technique, the Voyager 2 probe toured the solar system in eight years -- much faster than would have been possible using Hohmann orbits.

Nonetheless, Hohmann orbits are useful tools for understanding planetary orbits and basic celestial mechanics. With a basic understanding of Kepler's laws, you can chart a path to Mars, and then design a whole "Grand Tour" of the outer planets. Click on the Applet button to the left to get started!