Now the future of space is largely in his and the hands of other free-spending, big-dreaming billionaires like him, including Amazon’s Jeff Bezos.

But what will this future look like.

“I see [guys like Musk] almost like medieval cathedral builders, with this multi-century project that they’re willing to take their time and their livelihood,” Nadis told The Post.

Within a century, Musk envisions reusable rockets blasting off every two years and ferrying some 200 passengers at a time, ultimately establishing an outpost of a million people.

Once you get there, the problem explorers will face is that Mars’ atmosphere is much thinner than Earth’s and the planet generates no electromagnetic field, meaning it gets pounded by cosmic rays and other harmful-to-humans energy.

Musk has offered sketchy details of what life off-world might look like.

Any Mars colony would have to be self-sustaining and not rely on supplies from Earth.

Musk has suggested food be grown on hydroponic farms, either underground or in an enclosed structure to protect the crops from radiation, but because Mars’ surface gets about half the sunlight Earth does, whatever plants that can be grown will likely have to be supplemented with artificial lights — and powering those lights will be no small challenge.

In the same interview, the billionaire suggested Mars’ inhabitants might live under a glass dome with an “outdoorsy, fun atmosphere,” until the planet is terraformed — artificially transforming the planet to make it more Earth-like, with a livable atmosphere.

He has suggested exploding 10,000 nuclear missiles over Mars’ surface in order to melt the planet’s ice reserves, thereby releasing the carbon dioxide locked within.

Meanwhile, Bezos and his company, Blue Origin, are also focused on moving off-world — but onto space colonies.

Bezos draws much of his inspiration from the work of Gerard O’Neill, a Princeton physicist who in the 1970s laid out a grand design for space colonies.

To protect from cosmic radiation, the cylinders would be lined with moon rock.

The colony would likely be parked in a stable orbit between the earth and the moon, first calculated by a mathematician in 1772.

Bezos is a fan of O’Neill’s designs, and has said that he one day envisions “a trillion” of us living on space colonies, though Nadis predicts that’s “hundreds of years” away.

The Amazon founder said it’s his generation’s job to begin laying the groundwork for the colonies so that future generations can actually construct them.

Making it to the moon has long been a dream for many, including Bezos and the Japanese tech billionaire Yusaku Maezawa, who tweeted earlier this year that he was looking for a girlfriend to join him on a trip to the orbiting body.

Nadis said the most likely habitats at first will be simple modular units, built on Earth then flown via rocket to the moon.

Scientists aren’t yet sure how big or deep these tubes are and what they might look like inside.

In his influential magazine, Moon Miners’ Manifesto, sci-fi fan Peter Kokh once described a civilization of thousands of people living on the rocky terrain, almost like setting up camp in an Earth cave.

The radiation in space could “render males temporarily and females permanently sterile,” Nadis writes.

In one Russian experiment, rats were unable to produce babies in space, and when those space rats returned to earth and mated with regular rats, the offspring tended to have “significant abnormalities.”.

On the moon, though, a “day” lasts more than 27 Earth days, severely screwing with human circadian rhythms.

Even with so many potential complications, Nadis appreciates the vision of the billionaire space explorers