The Number
My prompt:
I've been thinking about cataclysms.
Say humanity faces the prospect of a civilisation-ending disaster - a planet-killing asteroid hurtling towards Earth, for example - and say that disaster is x years away (from the present day). What, given our current technological and organisational capabilities, is the smallest value of x compatible with our survival?
The disaster will be a discrete event rendering Earth instantly and irreversibly uninhabitable. Survival will necessarily involve leaving the planet.
Criterion for survival: at least 100 million humans in the short-term and the subsequent persistence of human civilisation for at least a few centuries.
Given the uncertainty inherent in projections of technological progress, it may be useful to
1. Outline a number of different scenarios and
2. Assume that the prospect of annihilation focuses minds and leads to several Manhattan Project-like initiatives.
—-
Responses: GPT-5 Thinking, Gemini 2.5 Pro
Also GPT-5 Thinking: “There’s no asteroid size for which moving 50–100 million people off-world in ~100 years becomes the ‘default’ over deflection. For anything small enough that we might plausibly detect and track a century ahead, deflection is vastly easier. For objects so large that deflection becomes non-credible (≈ >50–100 km diameter), a Mars relocation on the scale you describe is still orders of magnitude beyond what humanity could build and supply in a century; the only realistic off-world play is a much smaller ark (≲10⁵–10⁶ people), not tens of millions.”
