7 Things You Need To Know About SpaceX's Plan To Put A Million People On Mars

3. The Planet

This is all very science fiction, but why leave Earth in the first place, and why go to Mars?

Basically, it's a bit like Earth, but with a thinner atmosphere. Its days are a roughly similar length to Earth days (24 hours 40 mins), it's only slightly further away from the sun and, crucially, it has lots and lots of water, albeit mostly locked up in an enormous ice cap at the moment. Compared to our other closest neighbour, Venus, which is essentially a "hot acid bath", Mars is a cosy home-from-home.

The other part of the equation, though, is why go at all? Earth is by far and away the most habitable space rock in the solar system, why would we leave to battle it out with a barren, airless, frozen wasteland?

Well, Elon Musk, and many others like him, think that the future of the human race could depend on it. He called it "back up the biosphere", and it would essentially be humanity's version of not keeping all our eggs in one basket. For many, our two options are either to become a space-faring species, or perish on Earth.