The tech billionaires are wrong. We are never going to have cities on Mars.

But it goes further: cities do not exist in a vacuum. The greatest cities in history were always nodes in a larger network of cities. Athens had a harbor (Piraeus), and so did Venice and New York. In the Roman Empire, all the roads literally lead to Rome. The Milliarium Aureum, the golden milestone, stood in the Forum and served as the starting point to count and measure the distance to other imperial cities along the main Roman viaes.

The existence of tight urban networks probably explains why cities do not die. Drop an atomic bomb on Hiroshima or Nagasaki, and twenty years later they are back up and running because of these network effects. That is true of most cities, regardless of the calamities that befall them, whether it’s war, earthquakes, or hurricanes.

With that in mind, one can begin to grasp the absurdity of interplanetary urbanization. Sending people on a one-way trip to Mars in order to build a base and then a city simply ignores the single most basic fact of economic history: cities, and networks of cities, grow by their proximity and their ability to feed off of each other’s dynamism. Fast, easy and safe trade between the great centers of knowledge and industry is what allowed us to thrive as a civilization. Not forsaken outposts on barren and distant islands. As economist Robin Hanson demonstrates, dreams of autarky are just dreams.

But let’s speculate for the sake of the argument. Let’s say a sizeable cult of techno-libertarians band together and get on a rocketship called the ‘Galactic Mayflower.’ Even if an outpost on Mars (or on the Moon, or in Earth orbit) could somehow become self-sustaining – and The Martian amply demonstrates the incredible dangers and difficulties of this proposition – it is hard to imagine a scenario in which such outpost would become viable, let alone flourish. None of these places can boast of a fertile and plentiful hinterland, like the North American colonies.

Colonial endeavours are always in part motivated by economic prospects. Space colonization takes money. A lot of money. You will not attract investors, able-bodied people and government funds without a reasonable expectation of financial return. The problem is: what is there in space or on other planets that would warrant the investment and the risk? Space is very empty.  There is nothing on Mars or the Moon that could not be obtained at a lower price and with lower risk on Earth. And if, by any chance, we were to run out of a given commodity on Earth, history tells us that we will devise an economically beneficial substitute. There is no such thing as unobtainium. Natural resources are worthless in themselves, it’s the services they provide us that are valuable.

Absent any economic usefulness to the main market, Earth, and given the cost and difficulty of transporting things to and fro, it’s impossible to see how a faraway advanced base on Mars could turn into a major metropolis (and note: I am not even talking about the inconvenient fact that Mars cannot retain its atmosphere and block deadly radiations due to its weak magnetic field). That is the fundamental flaw in Elon Musk’s grand plan. A Martian colony lacks the key ingredient of a vibrant and dynamic city: a central location at the confluence of major trade routes. Because of its eccentric location, Mars City would not even be a way-station. It would be situated at the end of a very long and treacherous spoke in humanity’s urban network, far, far away indeed from where the action is, an afterthought almost.

Complete isolation at the extremity of the network would translate into complete dependence on the mainland, or rather the main planet. A Mars colony could only hope to be like one of these research stations at the South Pole, tethered to the rest of the world by airfreight and government contracts.

Not that there is anything wrong with that. Scientific research on the Red Planet is a worthy enough goal and it should be pursued relentlessly. Sending a crewed mission to Mars, like in Ridley Scott’s movie (The Martian), is an incredibly inspiring challenge. What it requires is solid public funding, rather than grandiose and slightly unhinged calls for urban oases in space. Also, let us not forget that the meme of space colonization has been part of NASA’s public relations arsenal for a long time (remember those Pan-Am tickets to the Moon?). It was even used by the Soviets to good effect (see Pavel Klushantsev’s 1957 Road to the Stars). At the end of the day, Elon Musk’s SpaceX and Jeff Bezos’ Blue Origins are government contractors with a side business in awesomeness. No wonder they partake in the same retro fantasies that keep the public excited and the contracts flowing.

As for the Star Trek-like long-term plan of becoming an interstellar species, I am not even convinced that it is worth discussing at this point. Absent an impending catastrophic event that would make our plant unlivable, why are we rushing toward space? The sun will shine for a few more billion years here and we know that we can continue to achieve amazing things together in the awe-inspiring cities on our little pale blue dot.

For now at least, the spaceship is Earth.

Manu Saadia, the author of Trekonomics, hails from Paris, France. He lives in Los Angeles where he helps tech startups get off the ground. His first and only passion is the future.