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Ideally, once in orbit, a satellite remains the same altitude and angular velocity forever. There are two common ways in which a satellite can slow down.

Atmospheric Drag


Low earth orbit (LEO) satellites - below 2,000 Km (1,200 mile) altitude - suffer orbital decay due to atmospheric drag, especially below 500 Km altitude. While extremely thin, the atmosphere is still present at such altitudes, and as a result, such low earth orbit satellites lose some of their orbital kinetic energy (speed) to friction with the thin atmosphere, called drag.

The International Space Station (ISS), for example, is a LEO satellite, orbiting the earth at an altitude of approximately 330 Km (210 miles). At this altitude, the ISS encounters a tiny amount of atmospheric drag, and as a result, it slows down infinitesimally in each orbit. The tiny orbital decay accumulates orbit after orbit, and after a long while, the ISS will have slowed down noticeably. Kepler's law of orbital dynamics tell us that, as an orbiting object slows down, it cannot sustain its existing orbit, and without an external force to push the object into a higher orbit, the object will also lose altitude. So not only does the ISS slow down, but it ever so gradually descends toward earth as well. As it loses altitude, the ISS encounters slightly thicker atmosphere, creating an even stronger drag, causing the ISS to slow down even faster, and lose altitude more quickly, and thus a vicious cycle begins.

The visiting space shuttles periodically use their rocket thrusters to boost the ISS back up to its optimal orbit and speed. Without these regular repositioning missions, the ISS is doomed to a fiery re-entry and plummet to the earth's surface.

Tidal Dynamics


Tidal Dynamics occur when a secondary body orbits a primary body, and the primary body has oceans that give rise to tides due to the secondary body's gravitational pull.


Consider the Earth (primary body) and the moon (secondary body). The moon orbits the Earth, and as it does, it causes tides to bulge in the Earth's oceans. The angular gravitational torque between the moon and this tidal bulge acts as an external force upon the moon, thus causing the moon to ascend in its orbit, and in accordance with Kepler's law, the velocity of the moon decreases at the higher orbit.


Therefore the moon is imperceptibly slowing down every year due to tidal forces. At the same time, tidal friction here on Earth is causing our planet to reduce the speed of its rotation ever so slightly. These changes in the moon's orbit and Earth's rotation are so infinitesimally small that they almost go unnoticed - almost. The slowing of the Earth's orbit due to tidal friction does give rise to the odd leap second from time to time.


In theory, this phenomenon will continue for about 2 billion years until the earth's rotation and moon's orbit were in perfect lock, and the moon would forevermore hover over the exact same point on Earth. This tidal-locked geosynchronous orbit already exists with Pluto and its moon, Charon. In the case of Earth, there is a good chance our sun will have expanded to such a size and intensity that that the oceans, and all living things on Earth, will have been vaporized long before the moon locks over a single position above Earth.

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15y ago

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