Elon Musk, the founder of SpaceX, has unveiled a new design for a 100% reusable rocket and spaceship to colonize Mars.

SpaceX has experience building traditional life support systems for its Crew Dragon capsule, a spaceship built for NASA.

But independent spaceflight experts say technologies that don't yet exist are necessary to keep Martian colonists alive.

Musk and SpaceX have yet to detail exactly how hypothetical Mars explorers and colonists will survive.

Industry experts are excited by Musk's vision to back up the human race by putting 1 million people on a new planet, but many have practical questions for the tech billionaire and founder of SpaceX. Above all, they're eager to know how Musk plans to keep people alive on Mars for months or years on end.

Musk's latest talk at the International Astronautical Congress 2017 in Adelaide updated a roughly one-hour presentation that he gave at the same event in September 2016.

"The future is vastly more interesting and exciting if we're a space-faring civilization, and a multi-planet species, than if we're not," Musk said on Friday. "I can't think of anything more exciting than going out there among the stars."

Musk first unveiled his Mars vehicle design -- which he used to call the "Interplanetary Transport System" but now calls the "Big F---ing Rocket" (BFR for short) -- during IAC 2016.

The original rocket-and-spaceship vehicle was supposed to tower 400 feet, or slightly higher than a Saturn V moon rocket. It was designed to bring up to 300 tons of spaceship cargo -- about two blue whales' worth of mass -- into orbit around Earth.

Musk's latest BFR design is about 50 feet shorter, and its spaceship is supposed to carry about half the payload. SpaceX has already built and tested key pieces of hardware in hopes of launching its first mission to Mars in 2022.

"That's not a typo. Although it is aspirational," Musk said when the date appeared on-screen at the event. "I feel fairly confident that we can complete the ship and be ready or launch in about five years. Five years seems like a long time to me."

Musk hopes to fly four additional ships to Mars in 2024, including two cargo missions and two ships carrying the first Martian explorers. From there, an increasing number of missions could establish and grow a colony.

"You can do it, Elon!" a man shouted from his seat after hearing these details at the conference.

Some spaceflight engineers and experts, however, are wondering whether he can.

"Elon lays out an impossibly large vision, and then revises it slightly downward -- and people say he's being practical. This is no more practical than it was last year," John Logsdon, a space policy expert, author, and spaceflight historian at George Washington University's Space Policy Institute, told Business Insider. "There are so many questions on the viability of this plan."

Big on ambition, thin on details

According to Musk's new plan, two uncrewed missions would pave the way for future human exploration of Mars. The first would locate sources of water in the soil, and the second would set up a chemical factory to turn that water, plus carbon dioxide in the thin Martian air, into oxygen and methane rocket fuel.

After the first people land on Mars, regular cargo launches would resupply them with food and other essentials.

D. Marshall Porterfield, the former director of NASA's Space Life and Physical Sciences Division, said SpaceX's reusable rocket technologies are "a huge game-changer" in terms of lowering steep launch costs and enabling such missions.

"He totally changed the economics," Porterfield, now a professor at Purdue University, told Business Insider during the university's "Dawn or Doom" conference before Musk's latest talk.

A fully reusable rocket and spaceship, as Musk is proposing with the BFR, would compound the savings and open access to space even further by lowering costs 100- or even 1,000-fold, according to Musk.

"We could assemble space exploration capabilities in orbit [around Earth] and launch a mission to Mars from there," Porterfield said, adding that the first crewed Martian missions will be like "vacations" compared to the "camping trips" of the Apollo program.

"The scenario in 'The Martian' is completely doable," he said, referencing the sci-fi book and movie in which a NASA astronaut is accidentally stranded at a Martian outpost.

Indeed, SpaceX may have the expertise it needs to get the first explorers to Mars and back alive.

The company already built the environmental control and life support system, or ECLSS, that keeps astronauts alive and comfortable on its Crew Dragon spacecraft.

NASA is counting on Dragon to ferry its astronauts to and from the International Space Station starting in 2018. (Russia's Soyuz spacecraft is current the only ride to orbit -- and it's recently quadrupled ticket prices.) SpaceX is even pondering a privately funded mission around the moon using Dragon and ECLSS.

In his IAC talks, Reddit chats, and other public statements, however, Musk has not detailed how SpaceX will keep the first Martian explorers alive.

When Business Insider contacted SpaceX for details about its Mars life support plans and responses to expert commentary, a spokesperson declined to comment and instead emailed several of Musk's prior public statements.

One came from his IAC 2016 talk, when Musk compared Mars to California in the early US, and said SpaceX is trying to build the Union Pacific Railroad.

"Once that transport system is built, then there's a tremendous opportunity for anyone who wants to go to Mars and create something new or build the foundations of a new planet," Musk said at the time.

Musk also said he expects SpaceX's Mars effort "to be a huge public-private partnership," and that the company is "trying to make as much progress as we can with the resources that we have available." He added that he hopes his plans spur companies to develop their own competing and innovative approaches, and that colonizing Mars -- no matter who gets the job done -- is his goal.

Porterfield believes the company needs systems that currently do not exist.

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

Using life to support life

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."

Spacecraft today don't rely on bioregeneration to keep people alive. Instead, they use traditional life support systems -- pumps, filters, compressors, chemicals, and the like. SpaceX's ECLSS is one such system.

Research into these traditional systems flourished during the Cold War, and more recent developments have turned out new capabilities. For instance, the space station recently got a new chemical-mechanical system that can recycle -- with 93% efficiency -- an astronaut's urine, evaporated sweat, and breath into drinking water.

Research into biological life support systems also occurred during the Cold War, though to a far lesser extent -- for example, one Soviet experiment used algae to generate oxygen inside sealed nuclear bunkers. Since then, a few experiments in orbit and on the ground have shown it's possible to grow edible plants in space and in Martian soil.

Yet Porterfield said "tremendous challenges" must be overcome before any biology-based technology can fully and reliably keeping people alive in deep space.

"It's not just a module you can stick on to grow some plants in, and have some fresh salads every once in awhile," he said. "We're really talking about technology that replaces what the Earth does. This is our current bioregenerative life support system."

In his talk at the "Dawn or Doom" conference, Porterfield brought up the Biosphere 2 experiment in Arizona as an example of the challenges. Eight people lived in the sealed, three-acre habitat for two years with a bounty of plants and animals -- yet they quickly encountered problems severe enough to require outside help, including pumping in oxygen.

A more compact and successful experiment was NASA's Biomass Production Chamber -- a giant, sealed greenhouse built inside a hyperbaric chamber at the Kennedy Space Center. The chamber broke world records for food production with a variety of crops from the late 1980s through the early 2000s.

But NASA's space greenhouse was scrapped in the 2000s, along with other research into biological life support, due to congressional budget cuts.

"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, which has sealed four students inside a habitat with plants and animals for a year.

'They really don't have a science program'

A few years ago, when Porterfield worked for NASA, he and some colleagues were invited to SpaceX. Musk pitched a flyby mission to Mars that'd cost about $600 million, and Porterfield and his colleagues were interested.

"They were going to launch a payload, orbit Mars, and bring it back to Earth," he said. "For us it would have been a great opportunity to look at radiation environment out beyond the Van Allen Belts, because right now we're at the space station, where we're shielded from that deep-space radiation." (Musk has downplayed the risks of space radiation, though new research suggests it could be twice as dangerous at Mars as previously thought.)

Porterfield thought NASA higher-ups would bite, but they ultimately passed on SpaceX's offer.

"During that time, though, I learned they [SpaceX] really don't have a science program, per se, that would enable them to really consider ... bioregenerative or even just greenhouse-type of technologies in a Mars architecture," Porterfield said.

Logsdon offered a similarly skeptical assessment.

"SpaceX is a very good engineering firm. Certainly they're going to design a system that makes every effort for high level of safety," Logsdon said. "But they haven't said a word about how people will survive once they get to Mars. It just isn't a part of their capabilities."

Despite NASA's budgetary de-emphasis on bioregenerative programs, Porterfield said he hasn't noticed any staff migrations that suggest SpaceX might be acquiring expertise to research bioregenerative capabilities (though he noted he does not keep close tabs on SpaceX's hiring efforts).

If Porterfield had a chance to ask Musk any questions about his Mars plan, he said he'd focus on life support.

"I'd directly ask him, 'What are you doing in terms of countermeasures for crew health? What's your thinking in that area? Are you just going to, in the moment of finalizing your Mars architecture, are you going to adopt the current countermeasures and polices that NASA is doing?'" he said. "Maybe that's enough. But in terms of other aspects of biological foundations for human exploration, it may not be enough."

Disclosure: The author of this post was also a speaker at the "Dawn or Doom" conference.