"There are really two fundamental paths," Musk said. "One path is we stay on Earth forever and there will be some eventual extinction event. The other is to become a ... multiplanet species, which I hope you will agree is the way to go."
Still to be determined: Where the possibly hundreds of billions of dollars will come from to pay for the rockets and spaceships, and how all those people will survive once they're on Mars.
Speaking to the International Astronautical Congress in Guadalajara, Musk described his vision for continuous launches from Earth, with massive rocket boosters returning for reuse after hurling spacecraft into "parking" orbit around Earth, where they would be refueled by propellant tankers.
The spacecraft, each carrying 100 people, and eventually more, would spread their solar wings (for electrical power) and fly on to Mars. They would return using a combination methane and liquid oxygen fuel mined and manufactured on the Red Planet.
That's not the only way Hawthorne-based SpaceX would have to push technology boundaries.
In a tweet before the speech, Musk said the rocket booster for his interplanetary transport system will measure about 39 feet in diameter, and the spaceship will be about 55 feet in diameter.
When the two are stacked, their height will be about 400 feet. That is taller than the Saturn V rocket that carried astronauts to the moon in the 1970s, and the SpaceX rocket would have nearly four times as much thrust. The Saturn V is the most powerful rocket ever used.
Musk gave an aggressive timeline — the spacecraft could have its first test flight in four years, and its first crewed mission might launch in late 2024 with arrival at Mars in 2025. Musk acknowledged the manned mission date was "an aspiration."
SpaceX has defied the odds before. It jump-started the private space industry, challenging traditional aerospace companies that held a monopoly on launching national security satellites. The company has successfully launched its Falcon 9 rocket 27 times and made history when it landed used, first-stage boosters back on land and sea six times.
The company already has completed development work on a massive, carbon-fiber fuel tank for the Mars ship, and it has test fired the Raptor interplanetary transport engine.
But, as with many of Musk's projects — mostly notably his notoriously late delivery of new Tesla electric-car models — industry experts were skeptical he could meet the ambitious Mars schedule.
"I admire Musk's bravado and I certainly think that making humans a multiplanetary species is a worthwhile endeavor. But he has significantly downplayed the technical challenges involved, especially on his projected time scales," said Justin Karl, coordinator of the commercial space operations program at Embry-Riddle Aeronautical University. "A lot of things are predicated on the assumption that it can be done, versus it being in current active development or having flight heritage."
Bill Ostrove, aerospace and defense analyst with market research firm Forecast International, said he thought the time frame seemed "very ambitious."
Musk said rocket and spacecraft reusability and sourcing fuel from Mars were two keys to reducing the per-person cost of getting to Mars. His goal is to cut that cost from $10 billion, the price using "traditional methods," to close to $200,000.
The total cost for SpaceX to develop the interplanetary transport system could be $10 billion. The entire project would cost much more and would ultimately require a public/private partnership, Musk said.
"Obviously it will be a challenge to fund this endeavor," Musk said. "The main reason I'm personally accumulating assets is to fund this. I really don't have any other motivation for accumulating assets other than to make the biggest contribution I can to making life multiplanetary."
Musk said the company expects to generate "pretty decent" revenue from satellite launches and NASA commercial resupply missions to the International Space Station. He added that there are individuals in the private sector interested in funding a Mars base.
Currently, the company is spending less than 5% of its resources on the Mars system. But in a year and a half to two years, Musk said, most of SpaceX could be working on the project and the company could spend about $300 million a year on it.
The financial aspect of the mission will be a major hurdle, said Richard Wirz, director of the plasma and space propulsion laboratory at UCLA.
"None of the things he's mentioning are technological impossibilities," he said. "It's whether or not we can get the money to do it, and if we're willing to accept the risk."
Musk himself acknowledged that the first journeys to Mars will be dangerous, and that travelers must be "prepared to die."
"The risk of fatality will be high," he said.
In April, SpaceX said it was planning to send an unmanned Dragon spacecraft to Mars as early as 2018 to test attributes such as landing capabilities and interplanetary navigation. That mission, known as Red Dragon, will get some technical support from NASA, which has said it is interested in the entry, descent and landing data. Musk said Tuesday that the company wants to send a "steady cadence" of Dragon spacecraft to Mars.
But he gave few details about what a Mars civilization would look like, other than the establishment of fuel farms, saying that SpaceX's goal was to build the transport system to get travelers to Mars.
"It's like building the Union Pacific railroad," Musk said. "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."
Industry experts said there are still a number of hurdles to overcome. For instance, Musk did not describe how the Mars travelers will be sustained on the spaceship on their journey to the Red Planet.
"That is one piece that really needs to be worked on hard before something like this could be achievable," said David Barnhart, director of USC's space engineering research center.
And the journey there might not necessarily be a one-way trip.
"I think it's pretty important to give people the option of returning," Musk said, adding that the spaceships will be returning to Earth from Mars anyways. "The number of people willing to move to Mars is much greater if they know they have the option of returning, even if they never return."
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2:15 p.m.: This article was updated with details from Elon Musk's speech.
Noon: This article was updated to reflect that the speech has begun and to add background information.