Eli5: Why does the ISS require thrusting to maintain orbit, while the moon has been orbiting the earth for a long time without any problems.
In: Physics
The center of gravity between us and the moon is very close to the surface of the earth, and is moving more between us every day as the moon moves away from us.
The center of gravity between the earth and the ISS is basically in the center of the earth. It’s getting pulled down HARD.
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The Earth is 12,742 km wide and the ISS only orbits at an average height of 408 km from its surface. If you shrunk the Earth down to the size of a balloon, the ISS would only be about 0.3 inches [0.8cm] from its surface, which illustrates just how close it is to the planet.
While the atmosphere at this height is very, very, ***very*** thin, it’s still there. Which means that the ISS is always slowing down due to atmospheric drag, eroding its orbit continually. This is why the ISS needs a boost every now and then to push its orbit back up.
Atmospheric drag. The ISS is high, but there’s still enough atmospheric drag to slowly lower its orbit. Without the regular boost, the orbit would drop lower into the atmosphere and drag would keep increasing to the point the orbit gave out.
Just to add, the moon’s orbit is also one that is slowly moving away from the earth. In a few millennia the moon will be far enough so that eclipses will never happen again. The ISS on the other hand is on an orbit that will eventually have it fall back to earth unless it is stabilized.
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