I picture an ordinary evening. In a city on Mars, the shops are closing. Children are coming home from school. On a terrace, two people drink coffee and talk about their work. None of them checks the schedule of the cargo flights. The city is no longer waiting for anything that has to come from Earth.

This picture is further away than the plans of the space industry suggest. For years, Elon Musk has spoken of a million people on Mars by 2050. Around a thousand Starships over two decades of launch campaigns are meant to carry it. In early 2026, SpaceX set Mars aside in favour of the Moon. The first flight there has moved into the early to mid 2030s.

So the date of the first landing is not what interests me. What interests me is the question behind it: what has to be true for this evening to be possible? Anyone who takes that question seriously does not arrive at thirty years. They arrive at about two hundred. This number is not a guess. It follows once the physics, the population and the economy of such a city are worked through together.

Under Rock, Not Under Glass

Most images of a Martian city show glass domes over gardens. Physics argues against them. The air pressure on Mars is less than one percent of Earth's. A dome filled with breathable air therefore pushes outward with enormous force. At half of Earth's pressure, the force on each square metre equals the weight of more than ten tonnes on Mars. A roof over an entire city would not need to be held up. It would need to be held down.

Then there is radiation. Mars has no global magnetic field, and its atmosphere is thin. Measurements from the Curiosity rover show that a person on the surface absorbs as much radiation in a few days as on Earth in an entire year. Glass stops little of it.

A city meant to last two hundred years therefore moves under rock. Mars offers a template. Volcanic activity has left lava tubes beneath its surface. Because gravity is lower, they could be considerably wider than their counterparts on Earth. A few metres of rock overhead shield against radiation. The same rock holds the pressure. Daylight enters through sealed openings, and the green grows under lamps.

Mars thus remains a place that humans can only inhabit inside a closed shell. But that shell can be a city. The question is then no longer whether people can live there. It is how many people it takes for that city to sustain itself.

How Many People a City Needs

When I first read the well-known figures, they sounded encouraging. In 2020, the French researcher Jean-Marc Salotti arrived at 110 people for a self-sustaining Martian settlement. A group at George Mason University arrived at just 22 in simulations. Both figures, however, rest on an assumption that is easy to miss. The simulated settlements still receive supplies from Earth every two years. They survive. They are not independent.

A city that no longer needs anything from Earth must make everything itself. Food and water are the easier part. Chips, medicines, machine tools and spare parts for the plants that produce the air are much harder. The physicist Casey Handmer has calculated how many people such a complete industry requires. On Earth, it takes between ten and a hundred million. Only a handful of economic regions today can truly make everything. For Mars, Handmer considers a million people a realistic order of magnitude, provided manufacturing becomes far more efficient by then.

A comparison from Earth shows how wide the gap between survival and independence is. Iceland has around 350,000 inhabitants. Without imports, Handmer estimates, the country would fall back to an eighteenth-century standard of living within a few years. Iceland sits on a planet with air. A city on Mars would have no such margin.

Twenty-Six Months

A million people do not arrive at once. They arrive in waves. Celestial mechanics sets the spacing of those waves. Earth and Mars line up for a flight at reasonable cost only once every 26 months. Missing a window means waiting more than two years for the next one.

Handmer has run these numbers too. If the population doubles with every window, it reaches a million in about fifty years. No place in history has ever grown like that. Each wave would have to bring housing, air, food and work for as many people as already live there. A doubling over two or three windows is more realistic. The million then arrives closer to a hundred years after the first landing.

Even then, the city is not yet independent. An industry that makes everything grows more slowly than the number of people. It needs mines out in the open, smelters, chemical plants and factories for precision parts. Each of these steps has to be learned anew on Mars. On top of that comes a generation born there that has grown to adulthood there. Taken together, the evening from the beginning lies between 150 and 250 years away. I put it at two hundred years, because that is the honest middle.

What the City Lives On

Even a city that makes almost everything itself needs income. Some things will long remain unprofitable or out of reach on Mars. They have to be paid for. Robert Zubrin, who shaped Musk's Mars plans early on, puts it plainly. A Martian civilization must either be fully self-sufficient or export something that pays for its imports.

Shipping raw materials to Earth hardly pays for that. Transport eats up every profit. Zubrin therefore bets on deuterium. In Martian water, it is about five times more common than on Earth. As fuel for future fusion reactors, it could be in demand. He expects even more from ideas. People living with a constant shortage of labour are forced to invent. Their patents could be licensed on Earth.

More important than either seems to me to be location. Whoever one day mines the asteroid belt will need food, fuel and spare parts from nearby. Mars is better placed for this trade than Earth. The city could become the port of an economy that does not yet exist.

Critics object that no colony in history has financed itself through ideas rather than goods. The objection is fair. It also shows what matters. A city whose income depends entirely on Earth remains a colony, whatever its constitution says. Independence follows the balance sheet.

The First Child

A city can have factories and income. It becomes a home only through people who were born there. This point concerns me more than any other. It is where our knowledge ends. Gravity on Mars is 38 percent of Earth's. No one yet knows whether a human pregnancy can proceed healthily under these conditions. Animal experiments in weightlessness have been inconclusive, some of them troubling. For the partial gravity of Mars, there is practically no data.

Then there is radiation. For an unborn child, it is far more dangerous than for adults. Under rock, it can be limited. Whether that is enough, only the attempt will show.

Even if everything succeeds, something new emerges. A person who grows up at a third of Earth's gravity develops bones, muscles and circulation for that world. Whether they could ever set foot on Earth without strain is an open question. For them, Mars would not be the destination of a journey. It would be their origin. With this child, the city changes. People who arrived become people who come from there.

When Earth No Longer Has a Say

At this point a question arises that no technology can answer. The first settlements on Mars will be run by space agencies or companies. There will be a mission command. There will be contracts. The facilities will have owners. Even the air will at first belong to whoever built the machine that makes it.

As long as the settlement depends on supplies, this order works. Whoever delivers has a say. Yet everything that makes the city independent removes the ground this order stands on. Those who make things themselves need no deliveries. Those who earn their own income need no backers. Those born there never signed a contract. At some point, Earth is governing a city whose inhabitants it never asked.

The history of colonies on Earth knows this moment. It was rarely negotiated. Mostly it was fought for. Mars adds something particular. Depending on the positions of the planets, a message takes between three and twenty-two minutes. A city that waits for an answer from Earth before every decision cannot function. Its self-government therefore begins in everyday life, long before anyone proclaims it.

I do not believe this moment can be prevented. The only open question is whether we prepare for it or run after it. Whoever plans the first settlements today decides who owns the facilities. They also decide how that ownership will one day pass to those who live there. Both will shape two hundred years.

What Begins Today

The city on Mars is not a project for one generation. Yet it begins now. Centrifuges in orbit can clarify how mammals develop under low gravity. What is learned there will decide the first child. Closed loops for water, air and food can be tested on Earth before they face their first real trial on the Moon. Manufacturing from local rock will be needed on the Moon anyway. It is the school for the industry that Mars will one day have to carry. The rules of the first settlements are being written in the same years, whether they are called a constitution or operating procedures.

I gladly give up the pace of the promises for this. We gain a city that lasts. In it, a child will be born whose great-grandparents still lived on Earth. For this child, Mars is not humanity's second world. It is simply home.

If this city one day declares that it no longer needs Earth: may it then decide about itself alone, or does the Earth that built it keep a say?

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