When another Twitter user bristled at Musk's space ambitions, saying "or we could just solve earth problems first," Musk responded: "Our existence cannot just be about solving one miserable problem after another. There need to be reasons to live."
Indeed, Musk, who also is the CEO of electric car-maker Tesla, says it's about making the future better.
"The thing that drives me is that I want to be able to think about the future and feel good about that," says Musk, speaking to the National Governors Association in July. "We are doing what we can to have the future be as good as possible, to be inspired by what is likely to happen and to look forward to the next day."
For Musk, that means reaching beyond Earth, including getting to Mars and building a city there.
"Fundamentally, the future is vastly more exciting and interesting if we are a space-faring civilization and multi planetary species than if we are not. You want to be inspired by things. You want to wake up in the morning and think the future is going to be great and that is what being a space-faring civilization is all about," says Musk, speaking at the International Astronautical Congress (IAC) conference in Adelaide, Australia, in September.
"It's about believing in the future and thinking that the future will be better than the past. And I can't think of anything more exciting than going out there and being among the stars."