Elon Musk finally unveiled the details behind SpaceX’s long-discussed Interplanetary Transport System (ITS, formerly the Mars Colonial Transporter) — the system and technology that will make it possible for humans to travel to Mars and back. Just as the first transcontinental railroad connected the Eastern United States to the West, Musk hopes the ITS will connect Earth to Mars and establish the company as the Union Pacific Railroad company of space travel.

SpaceX was founded on the express goal to make human spaceflight to Mars possible in order to turn the red planet into a colony for future generations. In Musk’s mind, “One path is we stay on Earth forever, and then there will be some eventual extinction event,” he told the audience during his speech at the International Astronautical Congress in Guadalajara, Mexico, on Tuesday. “The alternative is to become a space faring civilization and a multiplanetary species.”

Musk envisions the future of humanity as hinging on the ability to build a “self-sustaining city” on Mars that can be a home to almost a million people. Such an endeavor would simply be the first in a line of future colonies stretching out into the vast reaches of the solar system.

In most ways, we already possess the technology to send humans to Mars. The major obstacle is to make the journey sustainable. SpaceX is developing a system that it hopes will drastically reduce costs. Traditional methods, he said, currently means it costs about $10 billion to send just one person to Mars.

Musk wants to make it cost about $200,000 — about the price of a modern U.S. home. “Then,” he says, “the probably of establishing a self-sustaining civilization is very high.”

To cut costs by nearly five million percent, Musk’s plan hinges on four major elements: full reusability, refilling in orbit, propellant production on Mars, and using the right kind of propellent.

NASA’s Space Shuttle program showed it was possible to build a spacecraft you could send and bring back over and over. SpaceX and other companies like Blue Origin are already demonstrating you could do the same with rocket boosters that can lift that spacecraft up out of Earth’s gravity.

The ITS calls for sending up a “Mars Vehicle” into space using a reusable booster. Once the vehicle enters a parked orbit, the booster comes back down “within 20 minutes,” Musk said, after which it’s loaded with a propellant tanker, fired back up into space, and delivers propellant to the spacecraft, like a portable gas station on top of a rocket. When depleted, the tanker (and booster) come back down, and repeat the process about 3 to 5 times until the spacecraft is completely fueled.

Musk suggests Mars is pretty well suited as a place for on-the-ground propellant production because of its carbon dioxide atmosphere. He and the company think the most optimal form of propellant to be methane combined with oxygen (produced using water derived from large reserves of water-ice). “Methane is the clear winner,” said Musk. Once humans are on the red planet, Musk says 50 to 60 percent of the energy produced on Mars will go towards propellant production to get ships back home.

The entire system, SpaceX hopes, will be able to fit about 100 - 200 people into a hypothetical Mars Vehicle. The vehicles will make trips during every Mars-Earth rendezvous period (every 26 months), when the two planets are at their closest approach of one-another. Each journey would take about six months on current propulsion methods.

Musk wants to build a fleet of about 1,000 ships that can be launched to Mars at every rendezvous period. He envisions about 10,000 trips to get a population of one million on the red planet, and estimates it will take about 20 to 50 total Mars rendezvous periods to make this happen.

That’s means, if SpaceX ever realizes the full ITS plan, we could see a self-sustaining civilization on Mars within 40 to 100 years.

There are a ton of unanswered questions about nearly every facet of this plan, but the biggest one is pretty simple: what the hell are the people living and working on Mars supposed to do? And for that, Musk has no answer.

SpaceX, he says, is there to “build a transport system.” It’s not in the businesses of establishing the kinds of day-to-day infrastructure, life support, and recreation that will be needed to make life on Mars work. Nevertheless, in order to even plan for all those things, “you need that transport link.”

“Once that transport system is built, he said, then there’s a tremendous opportunity for anyone to go to Mars build something. “That’s really where a tremendous amount of entrepreneurship will flourish.”

He also doesn’t want to this to be simply a one-way journey. “It’s pretty important to give people the option of returning,” said Musk. “Even if they never actually return. Just knowing that if you don’t like it there, you can come back is very important… In any case, we need the spaceship back!”

And of course, the role of SpaceX as the interplanetary Union Pacific Railroad doesn’t stop at Mars. Then, as the “interplanetary” in ITS suggests, the more audacious goal for this system is to take us to worlds beyond the red planet, such as Jupiter’s moon Europa, and Saturn’s moons Titan and Enceladus, or perhaps even Pluto. These kinds of places could act as propellant depots (basically ITS stops for gas). Musk thinks the ITS and its four major elements sets the groundwork for allow someone to “actually go anywhere in the solar system by planet-hopping or moon-hopping.” The ITS’s overall goal is to provide “the freedom to go anywhere you want.”

“This is really about minimizing existential risk,” said Musk, but moreover its about “having a tremendous sense of adventure.” The ITS would accomplish both in spades. The only question is when we’ll actually see it take off.