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One way is if colonies keep failing because of something on the planets. For instance, the colonists could arrive on planet only to find there is low level biological life (like prions) that is a...
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#1: Attribution notice removed
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System
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2021-03-30T23:53:15Z (about 3 years ago)
Source: https://worldbuilding.stackexchange.com/a/33014 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/