On Tuesday, thousands of people stampeded into a lecture hall in Guadalajara, Mexico, to hear SpaceX CEO Elon Musk talk about how he wants to colonize Mars. The fervor of the audience gave the room an electric charge as Musk’s speech got underway; people cheered as he avowed to create a self-sustaining Mars colony of 1 million people over the next 40 to 100 years.

Musk dazzled the crowd with concepts for two new types of vehicles: a massive reusable rocket booster and a giant transporter to carry people and cargo to Mars. He also laid out critical details about them — such as how big they would need to be and the materials they would be made from. And we got more insight into the new powerful Raptor engine that SpaceX is building to get this plan off the ground.

Once the electricity of the room died away, questions about his plan started to surface

Above all, Musk explained how aspects of this architecture — such as reusability and propellant choices — could bring down the cost of going to Mars to less than $200,000 a ticket. It was clear that he had been thinking about this minutiae for some time, and experts lauded the idea of trying to make such a massive feat realistic and affordable. "If you drive the costs of transportation down by four orders of magnitude, then everything becomes easy," Chris McKay, a planetary scientist at NASA Ames Research Center, tells The Verge. "You need more advanced life-support systems? Then you just ship them because they’re cheaper to ship. It completely redefines the problem of how you support people on the surface of Mars."

But once the lecture was over and the electricity of the room died away, questions about his plan started to surface. Namely, very few details about human safety were laid out, and some of the engineering claims made by Musk seemed incredibly optimistic and even unattainable.

How exactly are people going to survive the trip?

Musk offered few details about life inside the spaceship that would transport people to Mars. Though he promised that the trip would be "fun" with restaurants and "zero-G" games, there were no concrete descriptions of how the passengers would pass the time and, crucially, stay alive.

The increased levels of radiation that people will experience on a trip to Mars is one of NASA’s biggest concerns. Deep space is filled with tiny energized particles — either from solar flares or deep-space cosmic rays — that have the potential to harm people during the voyage. Astronauts on the International Space Station are largely shielded from this radiation thanks to Earth’s magnetic field, which deflects most of the deep-space particles. But those on missions to Mars will not have the planet’s magnetic shielding, and it’s not known what effects that might have on the human body.

Musk was remarkably casual about this danger, though. "There’s going to be some risk of radiation, but it’s not deadly," said Musk. "There will be some slightly increased risk of cancer, but I think it’s relatively minor." He went on to describe ways that Mars colonists could shield themselves — perhaps by reorienting the spaceship during a solar flare and huddling under a column of water. Still, he made it clear he wasn’t particularly concerned about this problem. "The radiation thing is often brought up, but I think it’s not too big of a deal," said Musk.

"The radiation thing is often brought up, but I think it’s not too big of a deal."

This casual approach to radiation isn’t shared by space researchers. "Radiation — he maybe down played it a little more than I would have," says McKay. "It’s not a show stopper, but you have to worry about solar flares." McKay also says there needs to be a "storm shelter," in which fuel or some other liquid is used to slow down and deflect incoming particles. "He discussed this very quickly. It’s a very sketchy solution, but that's basically the solution to radiation."

While there may be ways to hide from radiation, there’s something that space travelers won’t be able to escape on their way to Mars: microgravity. People’s bodies go through dramatic changes in zero-G. They can lose bone density, their muscles atrophy, and their eyesight gets worse. What exercise regimes will the crew need to do? How will 80 days on a ship affect them? "Zero-G" games may be fun, but the entire mission will be for naught if people’s bodies deteriorate on the way to Mars.

More questions remain once the crew reaches Mars. What will they do all day, and how long are they expected to stay before they take a return trip? Musk already has some ideas about how best to run the planet — he suggests a direct democracy — but it seems unlikely that the very first people on Mars will be tasked with figuring that. Instead, they’ll be more concerned with staying alive.

Where will the colonists live on Mars and how will they survive?

Not once during his talk did Musk mention where people will live once they arrive on Mars. SpaceX’s promo video of the mission conveniently ends right as the colonists get to the planet, providing little insight into what life in the company’s colony would be like. During a Q&A after the event, Musk said that SpaceX only wants to build the "transport system" that will get people to the Red Planet. "It's like building the Union Pacific Railroad, and 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," said Musk.

It’s unclear then who will be responsible for creating the habitats

It’s unclear then who will be responsible for creating the habitats and actually constructing the "self-sustaining civilization" Musk envisions. Such a civilization will have to have key life-support systems to create breathable air, a way to clean and recycle water, and a way to feed the colonists. Once a colony gets big enough, it’s not feasible for all the food to come from Earth, so people will have to eventually live off the land. That means learning to cultivate crops — no easy feat. Martian soil is thought to contain salts known as perchlorates, which are toxic for humans. So if crops are going to be grown on Mars, they either need to be grown in soil from Earth or the Martian soil must be somehow cleaned.

Musk also didn’t address the health challenges people will face on the Red Planet. Radiation levels are still a concern on Mars, though there is some shielding from the planet’s thin atmosphere. An artificial magnetic field could be created on Mars to deflect high-energy particles, Musk said. But he didn't provide any information of how this magnetic field would be created.

Neither did Musk address the problem of reduced gravity. Mars may have more gravity than space, but it still has about one-third the gravity of Earth. The effects of living for so long in this kind of gravity environment still aren’t known. "We do not know at all what will be the human response, or the response of an embryo or a young child, to one-third gravity," says McKay.

What about planetary protection?

Another question is how — and if — Musk plans to prevent Earth microbes from contaminating Mars, and Mars microbes (if there are any) from contaminating Earth. The Outer Space Treaty of 1967 requires that nations exploring other planets take measures against smearing Earth microbes across the Solar System. There are a few reasons for these precautions. One is scientific: containing our microbes means that if we find life on another planet, we can be reasonably sure it’s not from our own world. The second is ethical: what if we introduce Earth life to another planet, where it outcompetes whatever undetected life is already there? And the third is for safety: if we’re contaminating Mars, could it contaminate us?

A mission to colonize Mars with people would necessarily colonize the planet with the microbes

A mission to colonize Mars with people would necessarily colonize the planet with the microbes in our food, our waste, and on our bodies. Moving people back and forth would be an easy way to cross-contaminate the two worlds. But how we’ll protect Mars from our microbes, and us from any on Mars, didn’t factor into any of the plans that Musk discussed yesterday.

Adhering to planetary protection doesn’t seem to be a concern of Musk’s either when it comes to a colony. "So far, we’re really not seeing any sign of surface life on Mars," he said at a press conference after the lecture. "But I think the planet we really need to consider protecting is Earth. That’s where life exists as we know it in abundance." He then went on to say that people are "life’s agents" and that we can help breathe life to the sterile world of Mars.

Things get even more dicey for planetary protection since Musk is a big proponent of "terraforming" Mars — the science fiction term for making the planet like Earth. Last year, Musk talked about nuking Mars to make it inhabitable during an interview on The Late Show with Stephen Colbert. He later explained that he wants to nuke the sky over the Martian poles every couple of seconds, to create two tiny pulsing "suns" over the regions. The tiny suns would then warm up the planet and turn any frozen carbon dioxide into gas. CO2 is a potent greenhouse gas, and its presence in the atmosphere would make Mars warmer.

During his talk, Musk only briefly mentioned terraforming, saying that if we could warm up Mars we could create a thicker atmosphere and have flowing water. But he didn’t present any plan for this and said at the press conference plans to terraform "would be up to the decision of the people on Mars."

How will the ships return to Earth?

Musk said he wants to bring his spaceships back to Earth but didn’t go into detail about how. The SpaceX CEO said he wants to build methane propellant plants that would create fuel for the spaceships on the Red Planet, rather than carrying that fuel from Earth. Methane can be created on Mars with "relative ease," Musk said, because it can be synthesized from subsurface ice and carbon dioxide in the atmosphere.

The first ship to land on Mars would bring with it a "small propellant plant"

The first ship to land on Mars would bring with it a "small propellant plant," which would be expanded as time went on, Musk said. He noted that a large field of solar panels would be used to power the planet, but he didn’t give any details on how this plant would be built or operated, nor how much fuel would be needed to make the return trips possible. "I won't go into detail here, but people can think about it offline," Musk said.

How can Musk make such short trips to Mars?

Musk’s presentation today included some estimates for how long the spaceship’s journeys would take — and they were all quite short, anywhere between 80 and 150 days. NASA’s estimates that this trip would take a lot longer: the shortest estimate would still be more than half a year long, or about 210 days in between the two planets.

"I couldn’t quite follow where that was coming from," Bobby Braun, an associate professor of space technology at the Georgia Institute of Technology, tells The Verge. "I think it’d be very hard to get there in the transfer times that he mentioned. When we send robotic missions to Mars, they tend to take nine months to get there."

Musk explained at the press conference that the estimates are based on how fast you can speed up the spaceship going to Mars. If the spaceship travels at six kilometers per second, then the trip can go from six or seven months to just three months. And potentially it could go even faster than that. But Musk said it would need a substantial amount of energy to get a spaceship to that speed and a substantial amount of energy to brake.

How is the whole Mars colonization plan going to be funded?

A big chunk of Musk’s speech revolved around bringing down the cost of going to Mars. Given his estimates, it would take about $560 million to manufacture one booster, one spaceship, and one propellant tanker. Ultimately, that would mean a cost of less than $140,000 per ton to get to Mars. That seems relatively cheap for such a massive undertaking, but the truth is no one has a solid estimate for how much a human Mars mission would cost. Estimates have ranged from $200 billion and $400 billion to as high as $1.5 trillion.

Musk gave a number of different ways SpaceX could fund this endeavor, such as making higher profits and launching more satellites. But even with his low estimates, Musk still maintained SpaceX can’t do this all on its own. He argued strongly for the need of collaboration with the government and private industry. It makes sense given that SpaceX has relied on partnerships for much of its business. NASA has been a fairly big investor in the company, but it remains to be seen if Musk’s talk will be enough to woo the space agency to pitch in again. "In the future, there may be a NASA contract, there may not be," said Musk at the press conference. "I don’t know."

Thanks to @NASA for putting their faith in us https://t.co/kCtBLPbSg8 — SpaceX (@SpaceX) September 27, 2016

There’s no doubt that Musk knows rockets. He has a clear vision for the types of vehicles he thinks will make a Mars settlement both possible and affordable. And he’s proven that he can deliver capable spacecraft, even if there are some questions surrounding the vehicle's’ reliability.

But a human Mars settlement is more than just hardware. The lives of people will be at stake, and serious thought needs to be given to the safety of the first human settlers. Musk admitted that the first colonists would have to be prepared to die, but killing people either on the way to Mars or once they get there will defeat the entire purpose of creating a colony in the first place. SpaceX may consider itself just a transportation company, but if it wants to get in the business of transporting humans, the company needs to reassure the public it can get them to a destination in one piece.

Alessandra Potenza, Angela Chen, and Rachel Becker contributed to this report.

SpaceX trip to Mars simulation