How does motion in gravity affect orbits?
Gravity and Orbits
Everyone (almost) knows that the Earth orbits the Sun, and that the Moon orbits the Earth. The force that stops these bodies from flying away from one another is called gravity, and it exists wherever there is mass. Objects with mass always pull on each other. This is what allows objects to remain in stable orbits - their kinetic energy is not high enough to break free of the gravitational pull they feel, but they still have enough to not crash.
Answer and Explanation:
When objects are trapped by the gravity of a bigger object, the gravitational field emanating from the large object is called its "gravity well". To escape, the object must have a high enough kinetic energy to enter the gravity well and still be able to climb out. Objects in orbit do not have enough kinetic energy to do this, hence they stay in the well perpetually. The reason why the Moon doesn't crash into the Earth, nor the Earth into the Sun, is because the orbiting bodies are moving quickly enough to "miss" whatever they're orbiting as they fall, so they perpetually fall and miss and fall again. Objects with more kinetic energy can occupy orbits further from the central body, and objects with less need to stay closer in order to remain stable.
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from Basics of AstronomyChapter 25 / Lesson 6