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Even after the worst imaginable asteroid impact, Earth would still be much more habitable than Mars. Keeping a "backup" on Earth would be far easier than keeping a backup on Mars. At the end of the day, Earth has a thick atmosphere, abundant water, 1g of gravity and topsoil, and Mars lacks all those things.

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We are pretty sure that an impact liquified the entire surface of the Earth (creating the Moon by casting a big chunk of molten material into space). So clearly the “greatest imaginable” impact does not leave the Earth more habitable than the alternatives.

That was an impact by a Mars-sized body early in the history of the solar system. That's not going to happen again. The dangers now come from objects a few kilometers across, at most.

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