Objects in space follow the laws or rules of physics, just like objects on Earth do. Things in space have inertia. That is, they travel in a straight line unless there is a force that makes them stop or change. The movement of things in space is influenced by gravity. Gravity is an important force that can change the course of bodies in space or pull them off of one course, or even cause them to crash together.
While some objects in space travel in irregular paths, most (especially our near neighbors in space) tend to travel in orbits around the Sun or around planets. The orbits are usually close to circular, but are actually slightly flattened ellipses.
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What is an orbit?
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