After 200,000 years or so of human existence, climate change threatens to make swathes of our planet unlivable by the end of the century. If we do manage to adapt, on a long enough timeline the Earth will become uninhabitable for other reasons: chance events like a comet strike or supervolcano eruption, or ultimately — if we make it that long — the expansion of the sun into a red giant in around five billion years, engulfing the planet completely or at a minimum scorching away all forms of life. Planning for potential escape routes from Earth is, if not exactly pressing, then at least a necessary response to a plausible threat.
The most obvious destination is our neighbor Mars. We’ve already sent multiple probes there, and NASA is planning another moon landing in 2024 with the eventual plan of using it as a waypoint on a mission to Mars. Elon Musk’s Space X claims to be aiming for a crewed trip to Mars in the same year. But Mars is a desert planet, cold and barren, with no atmosphere save for a thin blanket of CO2. Sure, we could survive there, in protective suits and hermetically sealed structures, but it’s not a great place to truly live.
Some scientists have another favorite relocation candidate: Proxima b, a planet that orbits a star called Proxima Centauri, some 4.24 light years distant from our sun. Located in the triple-star Alpha Centauri solar system, Proxima b has a mass 1.3 times that of Earth and a temperature range that allows for liquid water on the surface, raising the possibility that it could support life.
The biggest challenge is getting there. Proxima b is almost unimaginably far away. There is a program underway, Breakthrough Starshot, to send a probe to Alpha Centauri with a journey time of just 20 years, but the entire craft will weigh only a few grams, being propelled by a 100-billion-watt laser fired at it from Earth rather than carrying any of its own fuel or, for that matter, human passengers. Even by generous estimates, traveling one light year in a vessel large enough to transport humans will take centuries…