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Let's say an extinct alien civilization inhabited an earth-like planet a few million years ago. The had a number of satellites, orbiting the planet, all of which got blown up quite violently (dealer's choice of method to destroy a satellite). Their satellites were in the same orbit radii that we currently use, namely a few hundred km through 40,000 km.

Would millions of years be enough time for virtually all that debris to decay? There can be enough debris left that you could find traces of it if you knew what to look for, there just has to not be enough that it would register as satellite debris in a routine scan of the planet and the things in orbit.

I'm not picky about the method of destruction, so a potential alternate question is: is there a plausible military mechanism of destroying satellites that would have the side-effect of leaving nothing in orbit after several million years?

Research:

It looks like any space debris below a few thousand km will degrade within a few hundred years (source), but there is a lot of atmospheric interaction in those orbits.

It also looks like intact satellites above 6,000 km would stay in orbit for millions of years (source).

But what about the debris from destroyed satellites?

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    $\begingroup$ It looks like you answered your own question. Debris in a 6,000 KM will remain in orbit for millions of years. You even linked to your source for that information. What additional information are you looking for that isn't provided by your (source)? $\endgroup$
    – sphennings
    May 24 at 19:15
  • $\begingroup$ An important note about your question that you may not realize is that the height of the orbit is related to how much energy is used to put it in orbit, so unless the method of destruction changes its orbital energy by a lot, the debris will behave in the same way the original satellite did. $\endgroup$
    – Mathaddict
    May 24 at 21:35

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Let's consider an existing, well-studied case: Saturn's rings.

Due to the recent Cassini mission, we now suspect that they are maybe 100 million years old, and can be expected to survive for roughly 300 million years.

Debris is debris, whether it be from a natural satellite or artificial one. I can't think of a reason why the origin would matter. More important would be the distance to the object they're orbiting and the presence of other moons that might perturb their orbits.

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Any fragment of debris in orbit is a satellite, so if it's in an orbit where a "satellite" will stay up for thousands or millions of years (or billions, if it's well past geosynchronous height), it doesn't matter if it's a Space Shuttle Orbiter, a good sized asteroid, or a paint fleck.

Below some level, the lightest fragments will last less long than larger or denser pieces, because the faint ghost of atmosphere will have more effect on something with larger area relative to mass -- but once you're high enough for an intentional satellite to last a million years, you're high enough for a loose screw to last the same time.

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Consider: there is detectable debris in various orbits that are left over from the formation of the solar system.

I think its safe to say a significant portion will remain. Any debris that has stable orbits that are well outside the host planets atmosphere will remain for a very long time.

The only plausibly practical military method to clear debris would be not cause debris in the first place and to de-orbit the satellites.

Debris would last until red giant.

How long? It would be reasonable to expect that a good portion of debris will remain orbiting until the host star goes red giant. Thus the red giant's atmosphere would pull the last debris orbiting the planet out of orbit.

Alternatively, until it is removed by a civ.

The alternative is a decades long program to clear the debris. Of course if a civilization does do this, then quantity of detectable debris would be radically reduced.

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Use modern artillery on a space-based platform.

The trick is to use large caliber high-explosive shells and/or guided missiles and fire them so that the detonations will strike the satellites head-on, i.e. the explosion wave knocks the debris retrograde.

This establishes an elliptical orbit for the debris with a low periapsis providing gradual deorbiting.

The invaders probably don't really think about covering their tracks for millions of years ahead, what they care about is disrupting enemy communications ASAP and ensuring their victims are unable to replace satellites (area denial in space).

Artificially provoking the Kessler syndrome using shell fragments and satellite debris is one of the most cost-effective solutions.

If the goal of the assault is conquest, not obliteration, the invaders may also opt for low-yield nuclear shells/missiles against higher-orbiting satellites in order to "delete" the debris, or at least ensure thin fragmentation survivable by lightly-armored spacecraft.

Good ol' HE for lower orbits because area denial. Nukes also give off hefty EMP which disrupts ground-to-space communications in case the victim civilization is interplanetary.

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  • $\begingroup$ I like this answer, but it seems like a lot depends on the angle by which the satellites are hit. Wouldn't an intentionally induced Kessler Syndrome cause the remaining satellites to be hit from random directions, leaving a statistically large amount of debris in a near-circular orbit? $\endgroup$
    – Zags
    May 27 at 1:26

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