The crux of Musk’s plan for building the BFR is scaling the technology. He wants to use this rocket for every launch, be it sending a legion of tiny research satellites into low Earth orbit or intrepid astronauts to Mars. Musk himself acknowledges that his plan is aspirational. But some elements of it are, honestly, rather ludicrous. Chief among them is the idea of using rockets to fly between cities.

Now I can appreciate that intercontinental travel is something many of us can relate to, and on the face of it, traveling between two cities thousands of miles apart in under an hour sounds appealing. But Musk understates the challenges involved in using rockets to move people.

First, the travel industry itself seems unsure if there is really any profitable market for this type of high-speed travel. After all, Concorde offered quicker flight times between large cities and ultimately it was forced to retire—with no service replacing it.

Second, accelerating to 7,000 miles per hour and decelerating again puts a significant strain on a human body. Astronauts undergo significant training for this. Further, as of now, about one in 20 rocket launches fails, or more precisely, explodes in a massive fireball that rains debris down for miles around. Compare this to commercial aircraft, where about one in 500,000 flights results in a fatal crash.

And you thought you had a fear of flying.

It is conceivable that Musk could overcome many of these problems by choosing less intense flight trajectories and significantly improving rocket launch safety. But there still remains one significant problem that will be much harder to overcome: The technology required to travel very quickly between two places on earth with a rocket already exists—it is called an intercontinental ballistic missile, or an ICBM.

Musk’s plan to fly people in rockets essentially amounts to taking the warheads out of nuclear missiles and putting people in them instead. You then take the passenger-ICBM and fire it toward a large city in another country. This is going to make the other country very nervous, because it is difficult to guarantee that the ICBM only contains passengers. In this sense, Musk’s biggest challenge is not one of engineering but of legislation. How do you normalize nation-states regularly firing rockets full of people at each other without someone starting World War III?

Take a step back and examine Musk’s various ventures: solar power, high-tech tunneling, and cheap rockets. All of these are things you need to effectively build colonies on other worlds. It appears Musk is working hard to develop the infrastructure required to make private space exploration not just affordable, but profitable—and profit is essential for the private sector.

Musk needs to involve the private sector because he needs help paying for space exploration. To give some perspective, U.S. GDP in 2016 was about $19 trillion, enough money to run at least 200 missions to Mars in that year alone. But launching a private rocket is legally gray. It’s not so much that it’s explicitly illegal, but it’s not legal either, and much as with other rapidly expanding markets that sell products of uncertain legal status, the private sector is not keen to invest.

Maybe Musk realizes that his biggest obstacle is not physics but politics, and building hype around his new ventures will help drive legislators to legalize them. Or maybe he’s just a narcissist that likes to stoke popular ideas. It’s always hard to tell.

Leon Vanstone is a researcher at the University of Texas and science communicator in Austin.