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"That's fantastic," Colbert replied. "Why, though?"
It's not an unreasonable question. Anyone who has seen pictures of Mars's barren landscape may wonder why - beyond reasons of futuristic thrill and scientific exploration - private individuals would ever want to go to the Red Planet.
Musk cited two main reasons.
First, he said, if humans become "a multi-planet species, humanity as we know it is likely to propagate into the future much further than if we are a single-planet species." In other words, if and when Earth becomes uninhabitable, human civilization would still be able to continue if we've established settlements on other planets, like Mars. It's "life insurance, collectively," said Musk.
But that practical argument aside, Musk admitted that he's far more motivated by a second reason: "It would be the greatest adventure ever... really exciting and inspiring." It may be a "fixer-upper of a planet," said Musk, but he's confident "it's possible to transform Mars into an Earth-like planet."
Watch the full clip of their Mars discussion below.