Elon Musk plans to colonize Mars. We have many questions.

Elon Musk has a vision. Within the next decade, humans will blast off from Earth, begin colonizing Mars, and eventually become a "multiplanetary species." It will take a lot of money, a lot more luck, and a really, really, ridiculously large spaceship.

On Tuesday, Musk made his most impassioned and detailed case for space colonization yet at an event with SpaceX, his private spaceflight company. "Someday soon, there will be an extinction event on Earth," Musk said. So we can either sit around and wait for that to hit. "Or, the alternative is to become a spacefaring species."

And the first step is going to Mars, our dusty red neighbor. "We want to make Mars seem possible in our lifetimes," Musk said. To that end, SpaceX released a video describing its plans for the Interplanetary Transport System, a craft that could, in theory, get us there:

The system is composed of what Musk calls a BFS ("big fucking spaceship") sitting atop a BFR ("big fucking rocket"). Put together, they’d stretch 122 meters high — the tallest spaceship ever built, about three-quarters as tall as the Washington Monument.*

How Elon Musk wants to get humans to Mars: a step-by-step guide

So here’s how SpaceX envisions this all working:

(SpaceX)

In the first stage, that big f’ing rocket takes off with a staggering 28.7 million pounds of thrust at liftoff and places the spaceship into orbit (that's three times as much thrust as the Saturn V, the largest rocket used in the Apollo mission). The rocket then returns to Earth, lands safely on a launchpad, and grabs a fueling tank to bring back up to orbit:

The rocket may have to make a few trips until the spaceship in orbit has enough fuel, and the spaceship then sets off on its merry quest for Mars. Suitable launch windows occur every 26 months, when Earth and Mars are close enough for a trip to be viable.

Fueling the spaceship in multiple stages is crucial for bringing costs down, Musk explained, since it’s easier to lift lighter components out of Earth’s gravity. "It makes a lot sense to load the spaceship into orbit, with essentially the [fuel] tanks dry ... and then use the booster and tanker to refill it while it is in orbit, and maximize the payload of the spaceship," he said.

So far, this plan isn’t entirely far-fetched — at least in its broad outlines. SpaceX has already been using its 70-meter-tall Falcon 9 rockets to launch objects into orbit, and the company has successfully landed six of those rockets back onto launchpads here on Earth. The concept is there. Falcon 9s are much smaller rockets, however (generating about 1.7 million pounds of thrust at liftoff), so it remains can be seen if SpaceX can scale things up.

The next key piece here is the spaceship that will actually transport humans and supplies to Mars. According to SpaceX’s vision, this ship will unfurl large, wing-like solar arrays generating 200 kilowatts of power, nearly double the International Space Station's 120 kilowatts.

(SpaceX)

The ship will then travel at 62,000 miles per hour through space en route to its new destination, a trip that could take six months or more.

As the spaceship approaches Mars, the craft will have to descend into the Martian atmosphere, reaching a temperature of 1,700°C, and then use its rockets to lower itself gently onto Mars’s surface.

(SpaceX)

And ... that’s that. We’re on Mars! Musk mused about an era in which dozens of ships launch toward Mars during each launch window, each carrying up to 100 or 200 people a flight. (Musk added that the ships might eventually need to be big enough to contain "everything from iron foundries to pizza joints.")

If humans sent, say, 10,000 flights in all, we could presumably build a self-sustaining civilization on Mars although that would take 20 to 40 years, given the launch window. Musk suggested that we might one day power a Martian civilization with solar panels or geothermal: "If you have energy on Mars, you’re going to have water because there’s massive amounts of ice."

Musk even hinted at the possibility of terraforming Mars — releasing huge amounts of carbon dioxide into the atmosphere to warm the planet and bring back liquid water, making it habitable for humans. "If we could warm Mars up, we would once again have a thick atmosphere and warm oceans," he said.

Musk’s vision for Mars still faces huge, huge hurdles

(NASA)

Now, before anyone gets too excited, there are still incredible challenges to overcome here.

Just on a basic level, SpaceX hasn’t yet built a rocket big enough for this mission. Nobody has. Up until now, SpaceX's success has come from finding ways to repurpose existing rocket technology for its rockets. But now Musk's talking about designing a rocket engine with three times the thrust of anything ever built before, using a novel fuel (methane). It's a huge leap, and we can certainly expect delays.

For that matter, SpaceX has never launched a single human being into orbit before. The company is still reeling from a recent setback this month, when one of its unmanned Falcon 9 rockets exploded before launch while carrying a satellite. The company needs to walk before it can run, let alone leap to Mars.

There’s also the question of getting costs down. As Musk put it in his presentation, "You can’t create a self-sustaining civilization if the ticket price is $10 billion a person." That’s roughly what it would cost if we used the technology we used to get to the moon. "If we can get the cost to moving to Mars roughly equivalent to the median house price in the US, which is around $200,000, then the probability of establishing a self-sustaining civilization is pretty high," he said.

Do the somber math: "We have to figure out how to improve the cost to [traveling to Mars] by 5 million percent," Musk said. "This is not easy."

(SpaceX)

Plus, there’s the human factor. How do you cram all those people together in relatively tight quarters for six months at a time en route to Mars? How do you mitigate the health impacts from radiation exposure midflight, or from zero-gravity environments that will cause bones and muscle to decay? NASA has been studying this stuff, particularly with astronaut Scott Kelly spending 340 days on the International Space Station, but there are lots of unknowns here.

For that matter, how do humans survive on Mars once they've arrived? How do they grow food? How do they ensure that water and oxygen supplies last? How do they protect themselves against incoming radiation given Mars' relatively weak magnetic field? These aren't mere details.

In a Q&A session after his talk, Musk conceded that "the risk of fatality will be high" on the first journey to Mars. "There’s no way around it." He did, however, reassure the crowd that the dangers of radiation exposure midflight were overblown, that the increase in cancer risk would be small, and, hey, we could always create an artificial magnetic field on Mars to deflect radiation if it came to that. Yeah ... the Q&A got a little intense.

Musk was also fairly vague on the question of how humans might get back from Mars once they land there. "Producing propellent on Mars is obviously important," he explained. "It would be pretty absurd to try to build a city on Mars if your spaceships kept staying on Mars. You would have this massive graveyard of ships." He suggested that we might harness methane on Mars to create fuel for return trips, but this needs a lot more fleshing out. (Another possibility is that plenty of people may volunteer for one-way trips at first, though Musk has never been a fan of this approach.)

Finally, there’s financing. SpaceX doesn’t have the money to send even one ship to Mars on its own. So it would either need to partner with NASA or figure out some other path forward. And SpaceX is already reeling from a tough year that could make investors wary. On September 1, a Falcon 9 rocket, carrying a Facebook satellite meant to provided internet access for portions of sub-Saharan Africa, exploded on the launchpad. Last June, another Falcon 9 exploded mid-flight.

"Obviously it’s going to be a challenge to fund this whole endeavor," Musk told the audience. "The main reason I’m personally accumulating assets is in order to fund this." In an ideal world, funding would come from a mix of his current projects, from private investors, and possibly the government. "Ultimately this is going to be a huge public-private partnership," he said wistfully. "Ultimately that’s how the United States was established."

This presentation may have been Musk’s attempt to generate some positive press and put SpaceX back on track. It’s certainly an alluring vision. And Musk is probably further along in sketching out a vision for Mars than anyone else. (NASA has a detailed plan to get to Mars, but it suffers from periodic budget crunches.) But we’re still a long, long ways from making this a reality.

* Note: Many thanks to a reader for suggesting some of the points of comparison I've added to this piece, like the Saturn V and the Washington Monument.

Further reading

— Our full guide to human exploration of Mars

— For NASA, sending a person to Mars is simple. Dealing with Congress is hard.

— Life inside NASA’s plastic dome that simulates life on Mars.

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