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Sending people to live on Mars may sound outlandish, but SpaceX CEO Elon Musk is adamant about his plan. And now, we know a little more about how he sees this all coming together.

Space technology journal New Space published an article by Musk this week outlining his plans, and Musk tweeted Friday night that changes to the plan are coming.

Here’s what the 16-page paper, available for free online from New Space until next month, tells us:

Going to Mars is still too expensive. The people who can afford to go to Mars and the people who actually want to go are not the same people at this point. Musk estimates the cost of getting 12 people to Mars to start a colony is about $10 billion per person.

“If we can get the cost of moving to Mars to be roughly equivalent to a median house price in the United States, which is around $200,000, then I think the probability of establishing a self-sustaining civilization is very high,” he writes. “I think it would almost certainly occur.”

Colonizing Mars (thanks Prof Hubbard for creating this from my talk). Major changes to the plan coming soon. https://t.co/s59qMHUj5O — Elon Musk (@elonmusk) June 17, 2017

He thinks they might be able to get the cost down to less than $100,000. That’s cheaper than a degree from a private university.

Musk has various ideas for bringing down the cost of getting there. If people are going to get to Mars at a price that they are actually willing to pay, the program is going to need various efficiencies.

The ship doesn’t have to be as reusable as a bike or a car since it will be flying only every two years, but it needs to be able to fly to Mars and back more than once. And it should be capable of refueling in orbit.

The colony built on Mars should be capable of producing propellant for ships so they can return to Earth. There are a few options for what that propellant could be. Methane would be best, Musk explains, since it’s cheaper and takes less energy to work with than another option, hydrogen.

You need a really powerful rocket to do this. The rocket will be very big and will need a lot of thrust.

“We are talking about a lift-off thrust of 13,000 tons, so it will be quite tectonic when it takes off,” writes Musk, adding, “This is really intended to carry huge numbers of people, ultimately millions of tons of cargo to Mars. Therefore, you really need something quite large in order to do that.”

Musk is cautiously ambitious about timing. The paper shows a timeline with a window for flights to Mars starting at the year 2023. “If things go super-well, it might be in the 10-year timeframe, but I do not want to say that is when it will occur,” Musk writes.

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