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Friction comes from two things rubbing together: you can experience friction by rubbing your hands together very fast. This makes movement more difficult, and creates heat. In Earth's atmosphere, the air rubs against airplane wings, creating air resistance, or drag. But in space, there is no air. Without air, there is no air resistance, and no drag. This is why spaceships can keep moving in the same direction, forever. There is no air to slow them to a stop.