The degradation of a shared resource from which each user draws private benefit while imposing on others a cost no one internalizes.
The tragedy of the commons describes a shared, degradable space, collective costs and the absence of any internalizing mechanism. Earth orbit offers a contemporary case, each satellite draws a private benefit, the collision it causes produces a debris cloud harming every present and future user, and that debris belongs to no one and engages no one. The instrument that internalizes precisely this externality has existed for a century, compulsory liability insurance, but nobody imposes it in orbit. Climate offers a variant, where insurers' retreat concentrates the worst risk in the public pool then funded by those who stayed.
The owner of the destroyed satellite is compensated, the party that caused the collision owes nothing, and the debris cloud that harms everyone engages no one, the textbook tragedy of the commons.
tragédie des biens communs