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Why do orbits of satellites orbiting at sufficient speed, far beyond the Earth's atmosphere, decay if there is no atmospheric drag? What drag is applied on the satellites at that altitude?

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    $\begingroup$ At what point do you think there is no atmospheric drag? :) $\endgroup$
    – Luaan
    Mar 29, 2023 at 7:08
  • $\begingroup$ Can you say what 'sufficient speed' is? That sounds like an inarguable truism… At any altitude, how is gravity not a drag? $\endgroup$ Apr 5, 2023 at 19:12

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If satellites are truly far beyond the atmosphere, their orbits do not decay, except for very small perturbation effects like solar radiation pressure from the Sun or tidal forces from the Moon.

But the atmosphere doesn't sharply end where space begins (by convention 100km). The thermosphere still has a noticeable density up to 600km, causing significant drag and orbital decay for all low Earth orbits.
After that, the yet flimsier exosphere extends for another 10,000km, affecting medium Earth orbits.
The end of the atmosphere is a hundred times farther away than space.

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    $\begingroup$ I am feeling a bit uneasy with the wording: The 100 km line is more or less arbitrary; in a world without SI units it would almost certainly not be say, exactly 62.1371 miles or 328,084 feet. "Space" simply does not "begin" there. If it did, then indeed the atmosphere would end there, because that's basically the definition of "space". The atmosphere extends, increasingly less dense and more ionized, thousands of kilometers into space and kindof blends into the magnetosphere where it mixes and interacts with those particles from the sun and elsewhere which make it through the magnetopause. $\endgroup$ Mar 29, 2023 at 11:56
  • $\begingroup$ I realize you said that much in the last two paragraphs, so I'm mostly uneasy with the "space begins" wording. The 100km mark is mostly of social relevance ;-). $\endgroup$ Mar 29, 2023 at 11:58
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    $\begingroup$ Scott Manley covered the history and arguments on how they arrived at it (from 10 min 28 secs) - 50 miles, rounded to 80 km.". "...SpaceX ... yeah, but you didn't go orbital" $\endgroup$ Mar 29, 2023 at 17:52
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The Wikipedia article on orbital decay lists a number of reasons, in addition to atmospheric drag:

Tidal effects

An orbit can also decay by negative tidal acceleration when the orbiting body is large enough to raise a significant tidal bulge on the body it is orbiting and is either in a retrograde orbit or is below the synchronous orbit. This saps momentum from the orbiting body and transfers it to the primary's rotation, lowering the orbit's altitude.

Gravitational radiation

Gravitational radiation is another mechanism of orbital decay. It is negligible for orbits of planets and planetary satellites (when considering their orbital motion on time scales of centuries, decades, and less), but is noticeable for systems of compact objects, as seen in observations of neutron star orbits. All orbiting bodies radiate gravitational energy, hence no orbit is infinitely stable.

Electromagnetic drag

Satellites using an electrodynamic tether, moving through the Earth's magnetic field, create drag force that could eventually deorbit the satellite.

Mass concentration

While not a direct cause of orbital decay, uneven mass distributions (known as mascons) of the body being orbited can perturb orbits over time, and extreme distributions can cause orbits to be highly unstable. The resulting unstable orbit can mutate into an orbit where one of the direct causes of orbital decay can take place.

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    $\begingroup$ For parts directly quoted from another source, you should mark them with block quotes to distinguish them from your own words. The link to the original source is definitely appreciated though! $\endgroup$ Mar 28, 2023 at 22:45
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    $\begingroup$ @fyrepenguin, Ivan: I just did that, as an example how I would format it. $\endgroup$ Mar 29, 2023 at 11:45
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    $\begingroup$ Are these effects really significant on artificial satellites? $\endgroup$
    – Barmar
    Mar 29, 2023 at 14:36
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    $\begingroup$ @Barmar: Yes. Mass concentrations are what caused the lunar microsatellites released by the later Apollo missions to change into unstable orbits and eventually crash into the moon. $\endgroup$
    – DrSheldon
    Mar 29, 2023 at 16:58
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    $\begingroup$ Their also was an error in the Wikipedia article, which I now have fixed. The words "saps momentum" must be changed to "saps the angular momentum". $\endgroup$ Mar 29, 2023 at 21:36

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