For most of human history, expanding into the Solar System would have sounded less like a plan and more like something discussed after too much wine.
Today, it is still enormously difficult, dangerous and expensive, but it is no longer ridiculous.
We have walked on the Moon. We have landed machines on Mars. We have sent probes past every major planet, photographed worlds billions of miles away and launched spacecraft that are now travelling through interstellar space. We are still standing on the shore, but at least we have discovered that there is an ocean.
The temptation now is to talk about Mars, Jupiter, Saturn and eventually other stars as though humanity should simply pack a suitcase and get moving.
I think we should slow down.
Not because we should stay home.
Quite the opposite.
If humanity is serious about becoming a civilisation that exists beyond Earth, I believe we should approach it the same way we approached most difficult journeys in our history: one step at a time. Exploration has always involved risk, and it probably always will. There will be accidents. There will be failures. Tragically, there will almost certainly be people who do not return.
That does not mean we should manufacture additional danger through impatience.
Before Mars, I would look much closer to home.
The Moon.
Not necessarily because the Moon is the most exciting destination in the Solar System. Compared with some of the worlds waiting farther out, it is hardly showing off. No vast rings, no thick atmosphere, no mysterious ocean hidden beneath kilometres of ice. It is grey, dusty and has spent several billion years being hit by rocks.
But location matters.
The Moon could become humanity's first true stepping stone into space.
Imagine a permanent settlement gradually expanding into an industrial complex: laboratories, mines, manufacturing facilities, launch systems, living areas and eventually enormous construction yards. Instead of treating the Moon merely as somewhere astronauts visit, plant a flag and leave, we could turn it into infrastructure.
More importantly, we could build spacecraft there.
One of the fundamental problems with constructing enormous interplanetary vessels on Earth is rather obvious: once you have built the thing, you then have to get it off Earth.
Earth's gravity is wonderful when you drop your phone and need to be reminded how much a new screen costs. It is considerably less helpful when you are trying to launch thousands of tonnes of spacecraft.
A future interplanetary vessel would not necessarily need to resemble the rockets we launch today. If it never had to land on Earth, it could be designed purely for travelling through space. It might be vast — containing rotating habitats, radiation protection, gardens, medical facilities, workshops and enough redundancy to keep hundreds or thousands of people alive during a long journey.
Building something like that on Earth's surface and launching it whole would be absurdly difficult.
Building it beyond Earth's deepest gravitational well is another matter.
That is why I can imagine the Moon eventually becoming something resembling a cosmic Heathrow — except perhaps a hundred times larger, with fewer announcements telling you that your gate has changed just as you reach the original one.
People and supplies could travel from Earth to orbital stations or lunar facilities aboard smaller transport vehicles. Passengers would then transfer to ships designed specifically for deep-space journeys. Cargo could follow a similar route. The enormous vessels themselves might never come anywhere near Earth's surface.
At that point, Mars becomes considerably more interesting.
I have written before about humanity establishing itself there, so I will not repeat the entire argument. But I do think Mars represents something fundamentally different from visiting another world.
Eventually, I would want us to try to transform it.
Terraforming Mars would probably be one of the largest engineering projects our species had ever attempted. It might take centuries. Perhaps much longer. We would need to understand the planet extraordinarily well before interfering with systems we barely comprehend today.
But if it proves possible, I would not see the final objective as a slightly more comfortable research station.
I would aim for a second Earth.
Not a copy — nature does not tend to produce those — but a living world where future generations could eventually walk outside without treating the atmosphere as an immediate personal enemy.
That transformation would also teach us something far more valuable than how to live on Mars.
It would teach us how to build worlds.
And once humanity possesses that knowledge, the Solar System begins to look very different.
We would not have to stop exploring while Mars slowly changes. Robotic missions could continue farther outward. In fact, machines should probably arrive before us whenever possible. A robot does not die from radiation exposure in quite the same way a human does. It does not have children waiting at home. If it falls into a crevasse on Europa, we have lost an expensive machine rather than a person.
Still annoying.
Much less tragic.
Artificial intelligence and increasingly autonomous robotics could survey environments, construct shelters, extract resources and prepare basic infrastructure long before the first settlers arrive. I have discussed elsewhere how humans themselves may also have to change if we are serious about living permanently away from Earth. Human 1.0 was shaped for one particular planet. The Solar System did not sign a contract promising to accommodate the original specification.
The next great expansion could therefore happen gradually.
Mars first. Then farther.
The asteroid belt offers an extraordinary amount of material without requiring us to tear increasingly large holes in our home planet. Metals, minerals and other resources that are scarce or environmentally expensive to obtain on Earth may be abundant elsewhere. In a mature space economy, mining an asteroid might eventually make more sense than digging another enormous pit into the ground here.
Earth could begin to breathe a little easier.
Then come the moons of Jupiter.
Europa, Ganymede and Callisto are radically different worlds, each presenting its own opportunities and problems. Europa in particular is fascinating because beneath its frozen surface may lie a global ocean. If we ever confirm that life exists there, our priorities should change immediately.
We should not arrive with bulldozers.
A living world deserves caution.
But if a world is genuinely sterile, the question becomes more interesting. Could we eventually establish habitats there? Could we modify certain environments? Could techniques first developed while transforming Mars be adapted to entirely different conditions?
Then Saturn waits farther out, accompanied by another collection of remarkable moons.
Titan alone is almost an argument for curiosity becoming a government department.
It has a thick atmosphere, weather, rivers, lakes and seas — except many of those familiar-looking features involve liquid methane and ethane rather than water. It is simultaneously strangely Earth-like and completely alien.
Beyond Saturn lie Uranus, Neptune, their moons, dwarf planets and eventually the distant frontier where the Sun becomes little more than an unusually bright star behind us.
I do not think humanity should jump there immediately.
I think we should expand outward in layers.
Build experience. Build infrastructure. Learn from mistakes. Establish routes between settlements. Improve propulsion. Develop energy systems. Create manufacturing centres that do not depend on every important component being shipped from Earth.
A civilisation spread across the Solar System would also need energy on a scale difficult to imagine today.
Fortunately, we live next to a power station that has been operating for roughly 4.6 billion years.
The Sun.
We currently capture only an almost comically tiny fraction of its output. A civilisation expanding through space could deploy enormous solar installations far beyond anything practical on Earth. Perhaps one day there will be vast orbital energy networks or swarms of collectors surrounding the Sun — not necessarily the complete science-fiction shell sometimes imagined, but progressively larger systems harvesting more of the energy already pouring into space.
This is where the Kardashev scale becomes particularly interesting.
Humanity today is not yet a Type I civilisation. A truly advanced Solar System civilisation would eventually move beyond simply using the resources of one planet and begin exploiting the energy available from its star on an enormous scale.
But technological advancement alone would not make us mature.
There is another problem we would have to solve.
Us.
Humans are extraordinarily good at cooperation. We have built cities, scientific institutions, international projects and machines so complex that no single person could possibly understand every component.
We are also remarkably talented at dividing ourselves into groups and deciding that our group deserves something because, for reasons that become increasingly questionable under close inspection, we say so.
That part of our chimpanzee inheritance may become rather inconvenient in space.
Imagine establishing civilisation around Jupiter only to spend the next century arguing about which country owns which section of Ganymede.
We would have travelled hundreds of millions of miles only to recreate a property dispute with a better view.
I hope that by the time we reach that stage, something in us will have changed.
Future humans living on Mars may not be genetically identical to people living on Earth. Settlers around Jupiter or Saturn might eventually adapt themselves to environments that demand different solutions. After enough generations, the word human could describe a family of related forms rather than one biological template.
I do not find that frightening.
What matters is whether we still recognise one another as part of the same larger organism.
Different bodies. Different worlds. Different cultures.
One civilisation.
That may prove harder than building the spacecraft.
If we can achieve it, however, colonising the Solar System would become more than a survival strategy. It would represent the transformation of humanity from a planetary species into something larger.
And eventually we would reach the edge.
Not literally a wall with a sign saying THANK YOU FOR VISITING THE SOLAR SYSTEM. Space is inconsiderate enough not to provide convenient borders.
But there would come a point when our permanent presence extended throughout the system. By then, perhaps centuries or millennia from now, the technologies available to our descendants might make our current ideas about interstellar travel look primitive.
We have already speculated about possibilities such as shortcuts through spacetime. Whether anything resembling them can ever become practical is another question entirely. Perhaps the answer will come from physics we have not discovered yet. Perhaps future propulsion will simply become extraordinarily efficient. Perhaps the solution is something nobody alive today has imagined.
A person living before the invention of the steam engine would have struggled to design a spacecraft.
We should probably have enough humility to assume that we are equally ignorant about some technologies of the distant future.
And I do not think we need to wait until the Solar System is perfect before leaving it.
Perfection is a dangerous requirement because it conveniently ensures that nothing ever begins.
If humanity reaches a stage where Mars has been successfully transformed, permanent settlements exist across numerous worlds, our energy and resource networks span much of the system and interstellar travel becomes realistic, then we should go.
The experience gained here would travel with us.
Terraforming Mars would not merely give humanity another home. It could become the first great lesson in turning a dead world into a living one. If similar experiments elsewhere succeed, we would accumulate something extraordinary: knowledge about how to create environments where complex life can flourish.
That knowledge could eventually be carried to other star systems.
There is an important distinction here.
If we discover a planet already containing life, I believe our first instinct should be restraint. Even microbial life elsewhere would represent an independent biological history potentially billions of years old. We would have no right to erase it because we preferred the neighbourhood with trees.
But the galaxy may contain countless sterile worlds.
Rock.
Ice.
Atmosphere.
Oceans.
Chemistry.
Everything except life.
And perhaps that is where humanity's greatest purpose in space could eventually emerge.
Survival matters. Curiosity matters. Knowledge matters. Access to resources and energy matters. Spreading civilisation beyond one vulnerable planet is sensible for reasons ranging from asteroid impacts to disasters we might unfortunately invent ourselves.
But I think there is something even more compelling.
Life spreading life.
Imagine a barren planet orbiting another star. Nothing has ever grown there. Nothing has moved through its oceans. No creature has ever seen its sunrise because there has never been anything there capable of seeing.
Then, perhaps thousands of years from now, our descendants arrive.
Not to conquer it.
To wake it.
They apply knowledge that began with primitive experiments on Mars. Atmospheres change. Water cycles develop. Microorganisms come first. Ecosystems follow over centuries or millennia. Forests may eventually grow beneath the light of another sun.
And one day something takes its first breath on a world that had been silent for billions of years.
Perhaps that is where all of this eventually leads.
The Moon would not simply be a base.
Mars would not simply be a colony.
Jupiter and Saturn would not simply be destinations.
They would be lessons.
Step by step, world by world, humanity could learn how to survive beyond the place that created us — and eventually how to carry life's flame farther than nature managed to carry it on its own.
We began as one species on one small planet orbiting one ordinary star.
There is no guarantee that we will become anything more.
But if we do, I hope our greatest achievement will not be how much territory we claimed, how many worlds we owned or how far our machines travelled.
I hope it will be how much of the darkness we left alive.