The commercial space industry has boomed in recent years, with companies like SpaceX and Blue Origin leading the pack.

Their respective founders, Elon Musk and Jeff Bezos, essentially share the same dream: to expand humanity's reach and build human settlements beyond Earth.

But they each have different ideas of how to get us there.

Musk has his sights on Mars, saying he wants to establish a permanent, self-sustaining human colony of one million people on the red planet by 2050. SpaceX is helping him toward that goal.

Meanwhile, Bezos wants to build enormous space stations that would allow a trillion humans to live throughout the solar system. Though he's said he won't see them built in his lifetime, Bezos founded Blue Origin for the purpose of making his vision come to life.

Both billionaires' aspirations come with major technical, scientific, and ethical hurdles. But according to experts in architecture, astrobiology, artificial gravity, and reproductive health, one vision is more realistic.

Construction

Establishing habitats where humans can not only live, but thrive, is the biggest challenge in both billionaires' visions.

If you ask independent architect Anthony Longman which world is more feasible to build, Bezos' space stations are the way to go in the long-run because you can make them Earth-like more easily than an entire planet.

Musk's long-term vision is to terraform Mars by giving it a thicker atmosphere with breathable oxygen and a protective magnetic field to shield from harmful space radiation, so that one day humans could live on and explore Mars's surface like on Earth.

Longman thinks transforming an entire planet is too challenging, which is why he says colonizing Mars is "not a good idea." Bezos' space stations could be built to resemble Earth more easily — no massive terraforming necessary.

The scale Bezos envisions for these space stations, however, is unlike anything ever built. They would resemble a '70s concept called O'Neill colonies — enormous cylinders measuring 20 miles long and four miles wide that could hold a million people.

Longman has his own concept for space habitats designed to house just 8,000 people. While he thinks that building a massive station like what Bezos imagines is more realistic, it's too big to be feasible near-term.

"I'm not saying they won't be built, but I think it will probably be some hundreds of years before we're able to build anything at that scale," Longman said.

With that said, Bezos is still the winner of this round.

Score: Bezos 1, Musk 0.

Food and agriculture

Aside from shelter, having enough food is crucial. But growing food off-world is challenging since crops have evolved in Earth's conditions.

Scientists have already grown a few crops on the International Space Station, including tomatoes and lettuce. But scaling up production to feed millions requires more research, astrobiologist Rebeca Gonçalves told BI.

"We need to develop these very safe, closed-loop, self-sufficient agricultural systems," Gonçalves said. She thinks this will be easier on Mars than in space.

That's because a planet already has a surface where humans can grow food, she said. In space, you'd have to build farms from scratch, and likely need to venture out to mine water and minerals from asteroids. Mars, on the other hand, has soil and water ice on its surface.

The challenge on Mars will be developing an agricultural system that can sustain crops in Martian conditions.

Researchers have already made progress toward that goal, successfully growing food in lunar regolith, and Gonçalves is working to develop a closed-loop, self-sustaining agricultural system for Martian colonies.

"If I had to pick a billionaire's vision of the future, I would definitely go with Elon Musk's Martian colony," Gonçalves told BI.

Score: Bezos 1, Musk 1.

Artificial gravity

The human body has evolved to exist in Earth gravity and doesn't adapt too well to low or zero gravity. Researchers have found that spending weeks to months in space can lead to muscle and bone loss, vision problems, and even kidney stones.

That means a space or Mars colony would need artificial gravity to keep residents healthy. Mars has some built-in gravity already — about 38% of what we experience on Earth. But a space station would need to start from scratch.

That's why Rachael Seidler thinks Musk's Martian cities are a better bet than Bezos' space stations. Reaching Earth-like gravity on Mars might be easier since it's already over a third of the way there.

"Artificial gravity is thought to be very difficult to implement in space," Seidler, a professor of applied physiology and kinesiology at the University of Florida who studies astronauts, told BI.

Bezos' enormous 20-mile-long stations would rotate to simulate Earth's gravity. But getting that sort of goliath up and running is too far-fetched for Seidler.

Scientists are already looking into building artificial gravity on places like Mars.

So, Musk wins this round.

Score: Bezos 1, Musk 2.

Human reproduction

If we're going to establish a longstanding colony off world, reproduction will be necessary.

A Russian experiment in 2007 showed us that giving birth and conceiving in space is at least possible after a cockroach named Hope birthed 33 baby roaches and one of those roaches later conceived during the mission.

However, cockroaches aren't humans, and a lot of questions remain around how space radiation and low gravity would affect a developing fetus since we've never sent a pregnant person to space, and likely won't for a long time given the potential risks.

Even here on Earth in fully-equipped hospitals, giving birth can be dangerous.

So, both Musk's Martian colony and Bezos' space stations would have to be equipped with healthcare systems identical to those on Earth, if not better, said Adam Watkins, a professor of reproductive biology at the University of Nottingham.

It's not clear to Watkins whether space stations or Mars cities would be a better place for giving birth — they may both come with equal risk. But Watkins thinks it's likely that Musk would achieve off-world human reproduction before Bezos since Musk is already working on establishing his space colony before the end of this century.

"I think establishment of human colonies on Mars is more likely to occur before we have significant human colonies established on large structures in space," he said later in an email to BI.

Score: Bezos 1, Musk 3.

The winner is: Musk's Martian Colony

The majority of experts BI spoke with agreed that Musk's Martian colony is more feasible than Bezos' enormous space stations.

But neither of these billionaires are anywhere close to making these dreams a reality. It will take decades — if not centuries — of innovation to build the technology that would allow humans to live safely beyond Earth.

Should we instead focus on caring for our home planet? Is it worth the billions of dollars we'll spend to get there? Will humans really want to leave Earth? The experts BI spoke with raised all these questions and more.

But if you ask Musk and Bezos, colonizing space is humanity's destiny. Through SpaceX and Blue Origin, they're laying the groundwork for our great expansion.

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