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Many weather and communications satellites need to remain above a particular point on the Earth. To do so, a satellite’s speed must exactly match the Earth’s rotation—it must orbit once every 24 hours. Such an orbit is called a geostationary or geosynchronous orbit.

Most spacecraft circle the Earth every 90 minutes or so in low Earth orbit (just above the atmosphere). The Moon is 60 times farther from the Earth’s center and orbits about once a month. To place a satellite in a geostationary orbit requires boosting it to an orbit about six times larger than a low Earth orbit.