Why do satellites eventually fall back down to earth?

Question

plz and thanks

Answer 1

Only some satellites fall back. It's because of air friction. Even 100's of miles above the Earth, there's still a *tiny* bit of air.

Answer 2

Atmospheric drag causes low orbiting satellites to lose energy and re-enter the atmosphere.

Satellites in higher orbits (geosynchronous orbits, for example) don't experience atmospheric drag, but they do experience solar radiation pressure. Solar radiation pressure slows the satellite down when it's headed toward the Sun and speed it up when it's headed away from the Sun, so you don't have a net loss of energy. The orbit doesn't decay.

Solar radiation pressure could cause the orbit to become more elliptical and eventually perigee would be low enough to experience atmospheric drag, but it would be a very slow process to circularize the orbit into a low Earth orbit before atmospheric drag would cause the satellite to reenter the atmosphere even if no other forces were at work.

In practice, gravity bulges and valleys around the equator will cause the satellite to sweep back and forth across the gravity valleys. The Sun and Moon will change both the inclination and the right ascension of ascending node (this is important because it keeps the solar radiation pressure from affecting the same part of the orbit over and over and redues the tendency for solar radiation pressure to distort the orbit. Once an object has achieved a resonance with the body it's orbiting, it's not very likely to decay. It can stay in orbit virtually forever (just like the planets and just like the moons orbiting the planets).

Answer 3

Couple of reasons. First is atmospheric drag. In low earh orbit, there is still a very slight amount of gas that slows the satellites down. Second is solar radiation pressure - the light from the sun can slow down a satellite as well. Then there is the nonuniformity of the earth's gravitational field. The perturbations from that can cause a satellite to loose energy and fall back to earth.

Answer 4

Only those at low altitude (Low Earth Orbit, LEO) will fall back in a comparatively short time. Air resistance in LEO is enough to make the orbit "decay": the atmosphere does not end abruptly, but even 500 km above the Earth's surface, there's enough left to slow a satellite.
LEO is popular for reconnaissance (closer view of the scene, more carefully controlled telemetering), for manned satellites (less costly to reach low orbit), and for prevention of lingering "space junk" (parts fall off rockets an satellites, but moving at ~30,000 km/hr, are more dangerous than bullets. This shrapnel in LEO tends to fall and burn up more quickly).

Answer 5

If a satellite is in low earth orbit it will eventually fall back due to atmospheric drag if the orbit is not corrected. If a satellite is in geostationary orbit for example it will not fall back, but the orbit will decay over time due to perturbation.