Tech entrepreneur Elon Musk has spoken often of colonizing Mars with his company SpaceX.

Many experts believe that setting up a crewed Martian outpost is possible if not inevitable.

However, the technologies required to establish a permanent, thriving colony on the red planet don't yet exist.

Biosphere 2 — a large-scale experiment from the early 1990s — remains one of the best illustrations of the challenges that lie ahead.

One by one, four men and four women wearing dark coveralls stepped up to a lectern to deliver their final remarks.

"I take my last breaths of this atmosphere, knowing that I will take breaths from a different atmosphere from all of you," said Jane Poynter, one of the crew members.

Then they lined up in front of a velvet rope, waved to the cameras, and stepped through an airlock crafted from submarine bulkheads. The doors were sealed. It was September 26, 1991. The group wouldn’t leave until 1993.

The airtight facility they entered, called Biosphere 2, is dug into a hillside of the Sonoran Desert near Oracle, Arizona. It’s a geodesic cocoon made of 6,500 triangular glass panes, and it looks something like a cross between a brilliant jewel and a sprawling terrarium. Inside are acres of lush green plants, millions of cubic feet of air, and an undulating 675,000-gallon saltwater ocean.

Poynter was walking into the largest, longest-running space colony simulation ever built. It would not only pioneer a system to regenerate all the food, air, and water needed to survive on Mars but also test the crew's physical and mental limits. Poynter would also face a daunting emotional trial with another crew member, Taber MacCallum: a relationship they'd hidden for years from public view.

Biosphere 2 is situated in foothills of the Santa Catalina Mountains, in Oracle, Arizona. (Photo: Biosphere 2/University of Arizona) Biosphere 2 is situated in foothills of the Santa Catalina Mountains, in Oracle, Arizona. (Photo: Biosphere 2/University of Arizona)

"It was an incredibly audacious and, in so many ways, incredibly successful attempt at building a prototype space base," Poynter, who now co-runs the high-altitude-balloon company World View, told me nearly 25 years after she emerged from the biosphere.

People no longer get sealed inside Biosphere 2. Today it's a scientific research facility run by the University of Arizona. Yet the original mission of the biospherians has taken on new relevance as threatening changes to the Earth’s climate, and ultimately to humanity, take alarming shape.

The outlook has grown so gloomy that, in the eyes of some, the idea of colonizing Mars as a backup drive for the human race now seems appropriate, if not inevitable. Joining Stephen Hawking and others is billionaire tech mogul Elon Musk, whose grand ambitions have made talk of inhabiting the red planet using his aerospace company, SpaceX, part of casual conversation.

The ultimate goal of his entrepreneurial existence, Musk has said, is to build a permanent, self-sustaining city of 1 million people on Mars — complete with pizza joints — as a sort of insurance policy against all-out catastrophe on Earth.

An illustration of SpaceX's Big Falcon Rocket helping to colonize Mars. (Photo: SpaceX) An illustration of SpaceX's Big Falcon Rocket helping to colonize Mars. (Photo: SpaceX)

SpaceX hopes to launch a "Big F---ing Rocket," a reusable space vehicle Musk is designing to ferry up to 100 people to the red planet, with the first (though uncrewed) missions starting in 2022.

"That's not a typo, although it is aspirational," Musk said during an October 2017 presentation.

There’s a question that looms larger than the months-long trip to Mars: How will anyone survive on a dry, irradiated, and nearly airless world for years on end? Musk has not yet explained how SpaceX plans to sustain a hypothetical Martian metropolis, and he declined to comment for this story through a spokesperson.

Yet Biosphere 2, a now decades-old expedition with troubling results, still offers some of the clearest answers to that question.