Elon Musk wants SpaceX to send people to Mars and then bring them back to Earth on the same rocket they came in on. Nothing has ever been launched from Mars before, and Musk didn't go into all that much detail about how SpaceX would accomplish it. But he did says that it would all hang on one thing: methane.

Rather than carrying fuel from Earth for use on Mars, Musk wants SpaceX to create fuel when it gets there. Musk says SpaceX would rely on methane — "deep cryo-methalox," to be specific — because it can be created on Mars with "relative ease." SpaceX says it would "build a propellant farm" used to create new fuel and send its spaceships back.

"I won't go into detail here, but people can think about it offline."

Methane can be synthesized on the Red Planet from subsurface ice and carbon dioxide in the atmosphere. Creating this fuel on Mars will reduce the total amount of fuel that the spaceship needs to bring from Earth, lowering the cost of the trip.

The ship won't need a booster to take off from Mars, either. Because Mars has less gravity than Earth, the spacecraft can produce enough thrust to escape the planet’s gravity well on its own.

Launching anything from Mars is viewed as a technological hurdle. The planet may have less gravity, but any vehicle departing Mars still needs rocket engines and a lot of propellant to take off. Just getting such complicated machinery to the surface of Mars intact is a challenge in the first place. If SpaceX is able to land and then launch from the Red Planet, it would be a major feat of engineering.

But, perhaps unsurprisingly, Musk didn't present a fully detailed vision of how to get there and back during his hour-long presentation. In particular, he almost completely skimmed over the specifics of what would happen once a ship landed on Mars.

Most of the big questions are still unanswered

"I won't go into detail here, but people can think about it offline," Musk said while presenting plans for how fuel would be created on Mars.

He spoke optimistically about SpaceX's ability to create propellant plants. The first ship to land on Mars would bring with it a "small propellant plan," which would be expanded as time went on, he said. And further down the road, SpaceX hopes to build plants on various moons to send other ships even deeper into the Solar System.

That still leaves many questions unanswered, including how these plants would be built, how they'd work, and how they'd be operated. Nor did Musk detail how much fuel would be needed for a single return trip and how long he expected the plant to take to supply enough fuel. Basic logistics were skipped over also, like how often SpaceX would give people the opportunity to return to Earth, the cost of the return flight, and what — if anything — the ship would bring back from Mars, be it trash or samples for scientists.

Check out our SpaceX Mars Colonization liveblog for the latest updates and our storystream for all the news!

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