A former NASA scientist says 'The Martian' movie 'is completely doable.' But Elon Musk's city on Mars is another story.

• A former NASA scientist says "The Martian" sci-fi book and movie "is completely doable."
• The movie depicts a temporary Mars outpost with pre-delivered supplies and habitats.
• But Elon Musk, the founder of SpaceX, rallies his rocket company around the idea of colonizing Mars.
• Building a self-sustaining city on the red planet is a far greater challenge than a stay of a year or two.

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In the 2015 blockbuster sci-fi movie "The Martian," astronaut Mark Watney lands on Mars with five other explorers.

The crew of the Ares III mission, arrives at a pre-assembled outpost, complete with enough food, water, and air to last them about a month. There are also tube-shaped rovers and a nuclear-powered mothership.

There's good amount of fiction in the movie as well as the book it was adapted from, but fans and scientists alike hailed the setup as delightfully realistic - and maybe even inevitable.

"The scenario in 'The Martian' is completely doable," D. Marshall Porterfield, the former director of NASA's Space Life and Physical Sciences Division, told Business Insider.

However, Porterfield and other researchers have a lot of questions for Elon Musk, the billionaire founder of SpaceX, about his ambitious long-term plans for sending people to Mars.

What SpaceX wants to do at Mars

If you apply to work at SpaceX, you'll see the following statement at the top of every job description:

"SpaceX was founded under the belief that a future where humanity is out exploring the stars is fundamentally more exciting than one where we are not. Today SpaceX is actively developing the technologies to make this possible, with the ultimate goal of enabling human life on Mars."

Musk has gone further in some interviews and talks, saying it's crucial that we try to colonize Mars in case something goes seriously wrong on Earth.

During a 2016 presentation, Musk said it's possible to put 1 million people on Mars within 100 years of SpaceX's first launch to the red planet. He envisions the company's Big Falcon Rocket - a nearly 35-story launch system that includes a 16-story spaceship - as the primary tool to get the job done.

Musk's "aspirational" timeline is to launch an uncrewed BFR mission to Mars in 2022, followed by a crewed mission in 2024.

BFR is being designed to be fully reusable, which would make it inexpensive to launch, land, and refuel. (Nearly all rockets today are discarded after one use.) SpaceX re-used a rocket booster with its Falcon 9 system for the first time in March 2017.

"This is a very helpful proof point that it's possible, and I hope people start to think of it as a real goal to which we should aspire, to establish a civilization on Mars," Musk said during a press teleconference after that 2017 launch. "This is not just about humanity, it's about all the life that we care about."

Musk has mentioned the potential to use domed greenhouses to grow food on the red planet and water shielding to fight against radiation. But he has not elaborated much on his plans to sustain the first explorers, who are due to land on Mars in less than a decade, based on his timeline. Building a city of 1 million on a cold, arid, and perhaps lifeless planet 158 million miles from Earth is, of course, an even more far-fetched goal.

What SpaceX might need to colonize Mars - but doesn't exist yet

"His idea about colonizing? That's going to require ... bioregenerative life support capabilities," Porterfield said.

The idea behind bioregenerative life support, which Porterfield worked on at NASA, is to collect a human crew's breath, liquid waste, and solid waste - then use plants and other life forms to recycle it into food, water, and air.

This would reduce the need for resupply missions and help ensure a crew's long-term survival. It would also make a colony vastly more sustainable, affordable, and independent.

"Biological systems are really resilient," Porterfield said. "They tend to be self-healing, self-repairing, so that's one of the advantages of a bioregenerative life support capability."

Mars simulation experiments like Biosphere 2 in the 1990s explored the space-colonization concept on grand scales, but encountered significant problems (including a loss of oxygen).

Specialized greenhouse habitats might enable bioregenerative life support systems to work on smaller scales. However NASA's research in that area was mostly de-funded after 2000.

"NASA basically gutted the entire future of spaceflight exploration in order to finish building the space station, and we really haven't fully invested in supporting the sciences required to use the space station today so that we can be competitive," Porterfield said.

He added that China is poised to overtake the US in bioregeneration with its "Lunar Palace-1" experiment. In July 2017, four students locked themselves inside that structure's airtight confines and subsisted on plants and mealworms for 200 days.

Porterfield said making bioregenerative life support work is anything but easy - and earnest research should start now if we're serious about sending people to Mars for more than a short stay. (SpaceX representatives have so far declined to answer our questions about the company's life-support research.)

"We're really talking about technology that replaces what the Earth does," Porterfield said. "This is our current bioregenerative life support system."

Read more about the challenges facing Mars colonization efforts: "Life in a bubble: How we can fight hunger, loneliness, and radiation on Mars"