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I was considering Kessler syndrome, however this is not a long-enough duration and I'm unaware of a way to prolong it for the course of several millennia.

I don't want Earth to be destroyed, just isolated so that the self-sufficient colonies can develop further without interference. Because of this, the most extreme methods are out of the question. Both physical travel as well as communication would ideally be blocked, though this isolation could be prolonged by an inside or outside source (e.g. the moon).

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    $\begingroup$ Kessler syndrome doesn't mean the space junk is so dense no rocket can get out, It simply means that given some (/enough) time in orbit, a satellite will get eventually get struck. $\endgroup$ – Adrian Colomitchi Mar 21 at 13:13
  • $\begingroup$ "the most extreme methods are out of the question" What does this mean exactly ? $\endgroup$ – StephenG Mar 21 at 13:54
  • $\begingroup$ @StephenG anything that would destroy the planet itself or remove it completely, such as a repeat of the Theia impact or the Earth being ejected from the solar system. $\endgroup$ – Wax Mar 22 at 0:36
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In the trilogy starting with The Caves of Steel, Asimov explores the same concept and solves it in an original way:

Earthlings have set colonies in outer space, but once the Earth became overpopulated they started living undeground, becoming more and more afraid of open spaces. While the colonies continued to thrive, the Earthlings started being practically self isolated by their agoraphobia.

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  • $\begingroup$ I totally agree; however, I also remember that this did not prevent terrans leaving Earth, it only made it somewhat harder. The actual reason was that the Spacers had lost their immunity to diseases, so a common cold for an earthling would be like COVID-19d if it ever got on a Spacer planet. As a result, they quarantined Earth. $\endgroup$ – Marvin the Paranoid Android Mar 22 at 1:08
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Contagion.

Earth got messed up. Humans cannot visit for fear of contagion - catching it and getting sick, or worse: catching it, not getting sick and bringing it back home. Contagion is of course a timely concern but also a well used trope and there are lots of ways to do it - a regular sickness, an escaped bioweapon, some sort of monster-making plague, nanite grey goo troubles, or what have you. For your purposes it should be something which can contaminate surfaces so even unmanned trade has the potential of bringing the contagion.

One step further would be fear of memetic contagion - an infectious thought or concept. SCP Foundation has a number of these ranging from funny to scary to incomprehensible. This contagion would explain isolation even from radio transmissions or cultural sharing.

Of course the thing about contagion is that fear of contagion might go on long after actual contagion. If you were more the social commentary scifi writer you might have it just as the colonies have cut themselves off from earth for fear of contagion, Earth has cut itself off from the colonies for fear of contagion.

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  • $\begingroup$ It doesn't even have to be a case of Earth getting messed up; colonists would just lose their immunity to Earth diseases. $\endgroup$ – Marvin the Paranoid Android Mar 22 at 1:09
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Complete isolation may not be feasible. You can communicate using something as simple as a beam of light, and you can't completely block those without blocking the sun as well. Your story probably doesn't need true isolation, though. You merely want your colonies to operate without influence from Earth, and there are several ways you can do that.

Unpredictable solar activity

The sun has entered a period of semi-instability where it is prone to severe and unpredictable sunspot activity and coronal mass ejections. This causes waves of highly-charged particles to flood the inner solar system, scrambling any communication signals and potentially disabling the electronics of any spacecraft in transit. Anything planet-side is protected by the magnetosphere, but the CMEs happen so frequently that there's no way to get a spacecraft between planets before the next one hits. Communication satellites don't last very long in orbit, and the streams of highly-charged particles create so much such electromagnetic noise that ground-based interplanetary communication systems aren't effective. Your colonies were established before this instability started, and now they're effectively cut off.

Ignorance/apathy

Earth doesn't interfere or communicate with your colonies because they don't want to. Perhaps the colonists were a group of people that Earth wanted to get rid of. The people of Earth rounded them up, shipped them off to the New Australia colony, and tried to forget they ever existed.

A variation of this is that there was a major incident of some sort as the colonists were on their way to their new planet. Their ship lost communication, and authorities on Earth believe that the ship was destroyed. Your colonists survived, however, but Earth doesn't know that so they never make any attempt to contact them. The apparent "tragedy" elicits such a strong emotional response from around the world that Earth's space programs never attempt to send another colony ship. The colonists are perfectly alive but on their own.

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  • $\begingroup$ Another possible apathetic approach is that Earth has become a Brave New World-style "utopia" where people's needs are trivially seen to and they aren't interested in anything as demanding and unpredictable as space travel. To be sure, there would be exceptions, but the vast majority of the population could be so pampered that they don't really care about the colonies one way or the other... if they even know they exist. $\endgroup$ – Cadence Mar 22 at 4:11
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No more fuel

At some point, there is no more fuel for the rockets. With oil reserves depleted - or so scarce that all of the extracted oil is barely enough to fuel the needs of everyday life - the cost of launching a rocket from Earth becomes unbearable.
The few launches that Earth can perform are only for the really important tasks and only to Erath orbit (like telecom or GPS satellites). Alternative fuels (like nuclear engines) don't prove suitable for a rocket launch from surface (either too low impulse or extreme radioactive pollution of the launch site).

On the other side, the off-planet colonies don't produce enough goods to be of any importance for Earth, so they are left to their destiny and anyway the outer space isn't considered worth the cost to reach it, so that there is no strong technological pressure to research alternative engines/fuel for rockets.

The isolation of Earth would end or when one fo these colonies becomes able to produce enough fuel to be able to sell it to Earth to power its missiles, or when somebody on Earth discovers a way to produce low-cost artificial hydrocarbons or alternative rocket engines.

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  • $\begingroup$ Have you made a list of the rocket fuels which are used at the present and noted what percentages are made from petroleum? One of the main rocket fuels, for example, is hydrogen, produced from splitting water into oxygen and hydrogen. And in a future where there are space colonies with populations of thousands, nobody will be using petroleum based products to fuel everyday life which will be fueled by fission, fusion, and renewable energy in various combinations. $\endgroup$ – M. A. Golding Mar 21 at 16:03
  • $\begingroup$ There's nothing fundamentally impossible with using renewable energy to build new hydrocarbons from appropriate chemical feedstocks. It's energy-intensive, to be sure, but if Earth needs them, it will get them. $\endgroup$ – Cadence Mar 22 at 4:08

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