What happens to a spacecraft if its speed decreases?

The spacecraft is in near-earth orbit due to the fact that it moves along its trajectory at a certain speed. Even geostationary satellites (which “hang” over a specific point on the Earth’s surface) actually move at very high speeds. This speed is different for each orbital altitude. If the speed of the spacecraft decreases, it will go out of orbit and begin to approach the Earth. Ultimately, it will enter the dense layers of the atmosphere and, if it does not burn out, will fall to Earth.

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