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Spaceships and satellites are often used to orbit celestial bodies, whether that’s the moon, a distant planet, or the Earth itself. But not all orbits are the same. Orbits at low altitudes require different speeds and expenditures of energy than orbits at high altitudes. Once an object is orbiting at a specific altitude, the laws of inertia make it very easy to maintain that orbit. But changing the altitude of orbit is quite complicated. Fortunately, modern physicists have a method to make such a thing possible: the Hohmann transfer.