With all the excitement surrounding SpaceX, and the success of PayPal and Tesla Motors, one might expect Elon Musk to take SpaceX public. So why hasn't he?

"The reason I haven't taken SpaceX public is the goals of SpaceX are very long-term, which is to establish a city on Mars," Musk said at a press briefing on September 8.

It wasn't the first time Musk has talked about sending people to Mars, but it was the first time he mentioned a Martian metropolis. While this dream seems far fetched, Musk has a history creating game-changing companies that think differently and have revolutionized industries.

Previously, he's said he thinks we'll have people visiting the Red Planet in 10 years. He's even told Steven Colbert his reasoning behind the plan. 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."

Earlier this year, he told Henry Blodget that his plan involves taking a large number of people and a lot of cargo to Mars. He then wants to work on terraforming the planet to make it more habitable, to make it more like earth over time.

It's "a serious fixer upper, but it's possible" Musk said in an interview last year at Ignition. Greenhouse gasses would warm things up, the atmosphere would get dense, the water would melt and form liquid oceans, he said.

Perhaps in the coming weeks, the Mars rover Curiosity will reveal whether the Red Planet had previously been hospitable to life: a big hurdle to making this all happen.