The International Astronautical Congress (IAC) is one of the most staid and traditional space conferences in the world. Plenary sessions with panels of space agency leaders and aerospace executives are interspersed with tracks of paper presentations on almost every facet of spaceflight. It’s professional, technical and even sometimes a little boring.

None of that applied, though, on Sept. 27, when SpaceX founder Elon Musk was scheduled to give a speech at this year’s IAC in Guadalajara, Mexico. Hours before the talk, people started lining up outside the room, creating a tangled mob of people spilling into the adjacent exhibit hall. When the doors finally opened, the people at the head of the lines sprinted into the room to get the best possible seats. They applauded, cheered and, yes, asked more than a few oddball questions over the next 90 minutes.

They, and perhaps many others, left the room after his talk with a sense of optimism, if not euphoria, about the plan he presented. The giant rocket and spaceship at the core of his plan would open up Mars, and much of the rest of the solar system, to human exploration and settlement. Building and flying them, though, will require SpaceX to deal with a large number of issues that go beyond rocket science.

A “quite big” rocket

The most obvious challenge facing SpaceX is actually building those vehicles. The booster and spacecraft that Musk unveiled in Guadalajara are on a scale never before attempted. “It’s quite big,” Musk said of the overall system, a rare bit of understatement in a speech full of audacious claims.

How big? The booster, with the spacecraft on top, will be 122 meters tall, 10 percent taller than the Saturn 5 rockets that launched Apollo astronauts to the moon. Its liftoff thrust, nearly 29 million pounds-force, will be 3.6 times that of the Saturn 5. It will be able to place 300 metric tons into low Earth orbit, assuming the booster lands and is reused. If the booster is expended, and, thus, doesn’t have to save propellant for a powered landing, it can place up to 550 metric tons into orbit, or about 1.3 times more than the International Space Station would weigh on Earth.

Reusability, though, is a key element of the overall architecture to help make Mars missions affordable. Other factors in lowering costs include the use of methane and liquid-oxygen propellants, production of those propellants on Mars for return trips and use of in-orbit refueling, where “tanker” versions of the SpaceX spaceship will transfer propellants to the crewed spaceship before departing for Mars.

Musk downplayed some of the technical challenges SpaceX faces. The booster, he said, is largely a scaled-up version of the current Falcon 9, albeit one that will have 42 of the company’s new Raptor engines in its first stage. The spaceship will use a version of the heat shield already flying on its Dragon spacecraft, capable of surviving entries into the atmospheres of both Mars and Earth.

And some of those key technical elements are coming together. Just before the IAC, SpaceX test-fired a Raptor engine for the first time. That engine, producing 675,000 pounds-force of thrust, is several times more powerful than the Merlin engine used on the Falcon 9. However, it’s about the same size as the Merlin, thanks to sharply higher chamber pressures within the engine.

“That means we can use most of the production techniques that we’ve honed with Merlin,” Musk said, noting that SpaceX currently produces 300 Merlin engines a year. “We feel really comfortable about being able to make this engine in volume at a price that doesn’t break our budget.”

SpaceX has also produced the first large propellant tank intended for the system, making use of composite materials. That tank, intended to hold liquid oxygen for the spaceship, has gone through some initial tests. “Initial tests with cryogenic propellants actually were quite positive,” Musk said, with no signs of leaks of other issues.

Despite those initial steps, SpaceX still has a lot of work ahead to convince even Mars exploration advocates that it’s able to build this booster and spacecraft, and that this is the best way to get people to Mars.

“I don’t think they are practical in the form he presented them, but with a little modification, they could be made practical and very powerful,” said Robert Zubrin, president of the Mars Society who has been proposing humans-to-Mars plans for more than a quarter of a century.

Zubrin in particular objected to sending the entire spaceship, nearly 50 meters long and capable of carrying 450 tons of cargo and people, all the way to Mars. “Doing that means it can only be used once every four years,” he said. He argued for making it a two-stage vehicle, with most of the system returning to Earth after accelerating the cargo towards Mars, so it could be reused more frequently.

Surviving the trip, and the planet

Building the booster and spacecraft may be the least of SpaceX’s concerns, though. Any long-duration human mission has to deal with significant issues regarding keeping crews alive on the journey, including the deleterious effects of weightlessness and exposure to radiation. NASA has made researching them a key priority for its continued use of the International Space Station through at least 2024.

Musk, in his talk and subsequent press conference, didn’t offer many details about how crews would handle the months of transit to Mars. He emphasized the large internal volume of the spacecraft. “It’s got to be really fun and exciting, and it can’t feel cramped or boring,” he said while showing an animation of the vehicle’s interior. It would be big enough, he said, for “zero-g games” and other creature comforts.

But what about the risks posed by microgravity, radiation, and other health risks? Musk downplayed them. “I think those are essentially solved problems,” he said.

A related issue is keeping people alive once they land on Mars — or, for that matter, figuring out just how they’ll live and what they’ll do after arriving. His plan offered nothing about habitats or other facilities. The only Martian infrastructure Musk discussed was a factory that would create methane and liquid oxygen propellants for the spaceship’s return trip.

Musk suggested that lack of details about life on Mars was, in fact, an opportunity for others. He likened his transportation system to the transcontinental railroads of the 19th century that opened the American West.

“Once that transport system is built, then there’s a tremendous opportunity for whoever wants to go to Mars and create something new, or build the foundations of a new planet,” he said. “That’s really where a tremendous amount of entrepreneurship and talent will flourish.”

Planetary protection

Another issue for Musk’s plans to establish human life on Mars is the question of whether there’s any life already there. NASA robotic missions landing on Mars currently follow planetary protection protocols for cleaning spacecraft of terrestrial microbes while keeping them from “special regions” that could potentially be habitable.

SpaceX will run into those issues long before the first shipload of settlers land on Mars. Part of SpaceX’s Space Act agreement with NASA for its Red Dragon mission, slated for launch in the spring of 2018, includes analysis of planetary protection issues for that Mars lander. SpaceX will likely also have to demonstrate it will abide by treaty requirements for avoiding harmful contamination in order to get a launch license for the mission from the Federal Aviation Administration.

“The U.S. is obligated to make sure its industrial partners pay attention to what the constraints are for planetary protection,” said Michael Meyer, lead scientist for NASA’s Mars Exploration Program, when asked at an Oct. 6 meeting of the Mars Exploration Program Analysis Group about SpaceX’s requirements to adhere to planetary protection requirements.

For the first Red Dragon mission, that should be straightforward: the lander is a technology demonstration mission that can easily avoid any special regions, and can follow existing NASA protocols for limiting contamination. However, it will be followed by later missions — Musk said SpaceX plans to send Red Dragon missions to Mars during every launch window, 26 months apart — that will be more sophisticated.

At the press conference, Musk suggested a later Red Dragon mission would attempt to drill into the Martian surface to look for water. “We want to find out what’s the best way to get water,” he said. “How easy is it?” Accessing subsurface water, though, will raise concerns about contamination, since any water below ground may be among the best places to look for Martian life.

Musk, though, didn’t seem to be too concerned about the prospects of life on Mars, a view that could put him on a collision course with scientists and government agencies. “We really don’t see any sign of surface life on Mars,” he said. “There may be subterranean bacteria. I suspect it will be pretty hardy. There’s not much we could do to it even if we wanted to.”

How much and how soon

The biggest questions, and skepticism, about SpaceX’s Mars plans revolve around schedules and budgets. Many of the other concerns become moot if SpaceX can’t find the funding needed to develop the spaceship and booster, or if it takes far longer than planned.

The current plan calls for having a first spaceship ready for testing around the end of the decade, with the booster shortly following. That would allow an initial human mission to Mars launching in the mid-2020s, if all goes well. “We aspire to launch in late 2024 and arrive in 2025,” he said, acknowledging that schedule is “optimistic.”

“That said, I don’t think it will be significantly beyond that,” he added.

Musk estimates SpaceX will spend $10 billion on this effort before it starts to generate revenue. “Obviously ,it’s going to be a challenge to fund this whole endeavor,” he said. He expected to use profits from SpaceX’s launch business, and, perhaps, a proposed broadband satellite constellation. He also suggested he would tap into his own net worth over time — Forbes magazine estimates his current net worth to be more than $11 billion — to fund the effort.

However, he also thinks governments will pay. “Ultimately, this is going to be a huge public-private partnership,” he said. “That’s probably what occurs.”

For now, SpaceX isn’t putting much money into this effort. Musk said that less than five percent of the 5,000-person company is working on Mars-related projects, and that the company has spent “a few tens of millions of dollars” on it.

That puts SpaceX in something of a conundrum: on the one hand, it’s hard to see how SpaceX will have a massive rocket and spacecraft ready to fly humans to Mars in the mid-2020s at that rate. On the other hand, SpaceX is facing criticism for putting any effort into Mars exploration when it’s dealing with the aftermath of a pad explosion that destroyed a Falcon 9 last month, and also working to develop a commercial crew transportation system NASA is counting on being ready by the end of 2018.

“Right now we’re just trying to make as much progress as we can with the resources that we have available, and just keep moving the ball forward,” Musk said. “As we show that this is possible, that this dream is real — not just a dream but something that can be made real — I think that the support will snowball over time.”

Musk certainly has some support already for his plans, given the enthusiastic reaction some of the IAC attendees gave his talk. Whether he can convert that enthusiasm into something more tangible, like money and spaceflight hardware, will be fodder for discussion at IACs for years to come.