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Current legal and policy efforts to enable adequate defense against potential asteroid or comet collisions with the earth are insufficient because they are indirectly premised upon theories that require verification of a clear and imminent threat before governmental agencies can act. This Article identifies the "precautionary principle" as the preeminent law of planetary defense against asteroid and comet impacts. The precautionary principle requires governments to take action to prevent harm even when it is uncertain if, when, or where the harm will occur. It requires governments to implement specific frameworks for making prompt decisions, directs intergovernmental bodies to plan for the "worst case scenario," and requires planners to develop self-reinforcing standards by scheduling continuing simulations and updates to their technical guidelines. If governments institute these measures and then fail in their efforts to protect earth populations, the principle shields agencies from liability with "Good Samaritan" immunity. Finally, the precautionary principle mandates that governments coordinate the roles of all the different agencies that could foreseeably become involved in planetary protection in advance of any actual threat. The development of the Initial National Response Plan by the Department of Homeland Security and the experience of governments responding to public health issues offer further guidance to planetary defenders.