We have already seen that the distances between the planets are quite large, and that the Solar System is mostly empty space. However, by the standards of the distances to the nearest stars, the planets are quite close. They are close enough that the distances can be traveled by interplane

(6.67 X 10-8 dyn cm2/s2)(6 X 1027 g) V/2 (6.4 X 108 cm) )

The escape speed is that with which we must launch an object from the surface, such that it can move infinitely far away with zero kinetic energy. This means that we define potential energies to be zero when objects are infinitely far apart. The total energy of an object with the escape speed is zero. If vesc is the escape speed, then

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