Is It Rational to Live in Space?

On a summer night, looking up at the sky, I sometimes find myself wondering whether humanity really needs to live in space. The humid air moves slowly, and distant stars flicker faintly. In that quietness, the grand story of space settlement suddenly comes back down to the scale of reality. The romance is beautiful. But structurally speaking, does it make sense? That kind of idle question rises gently in the summer dark.

Building cities in orbit, living on Mars, creating colonies on the Moon—these futures are spoken of almost casually in films and news stories. Yet when I think about them structurally, their rationality is hard to find. If anything, the romance carried by the phrase “space settlement” may be covering up the severity of the actual conditions.

What comes to mind first is the old story of “land shortage” that circulated during Japan’s period of rapid growth. People seriously believed that the population would keep growing until there would no longer be enough places to live. But now the population is declining, vacant homes are increasing, and cities have begun to shrink. The “land shortage” of that era was a social narrative that helped absorb anxiety about the future. Space settlement has a very similar structure. “Earth will become too small,” “resources will run out,” “we will need a new frontier”—these phrases are born from a human tendency to answer future anxiety through expansion.

The actual environment of space, however, is far harsher than any difficult place on Earth. Mars has only about 0.6% of Earth’s atmospheric pressure, an average temperature near −60°C, radiation exposure hundreds of times higher than Earth’s surface, toxic soil, and no breathable oxygen. Its gravity is only about one-third of Earth’s, so a long stay would rapidly reduce bone density and even alter the shape of the heart. The Moon is harsher still: daytime temperatures around 120°C, nighttime temperatures around −170°C, no atmosphere at all, and dust as sharp as glass fragments—potentially fatal if it enters the lungs.

If humanity truly had the technology to live in such places, then Arctic cities, Antarctic cities, underwater cities, and underground cities should already be common. Earth’s polar regions are cold, but they have oxygen. The deep sea is dark, but radiation is low. Underground spaces can feel closed in, but pressure is stable. Compared with Mars or the Moon, all of these are almost paradise. And yet humanity does not really live there.

In other words, the assumption that space settlement is feasible does not fit very well with the reality we already see on Earth.

There is also the question of population decline. In an era when populations peak and may begin to fall—not only in Japan and other developed countries, but eventually at the global level—the rationality of seeking a “new land” fades away. When population decreases, pressures around resources, land, and survival all weaken. Cities shrink, space becomes available, and redistribution within Earth becomes overwhelmingly more rational. Structurally speaking, the need to look for a new world in space may simply disappear.

So why is space settlement discussed so often? Because its value is less rational than narrative. Nations can display technological power. Private companies can offer investors a dream. Media can stage the romance of the future. Science-fiction-like hope captures the human heart. Space settlement functions not as structural rationality, but as a social necessity—a story people want to tell.

The realistic uses of space will probably remain closer to research bases and resource extraction. A lunar base may be useful as an observation site, and helium-3 mining might one day become an energy-related possibility. Mars may host robotic laboratories. Orbital space stations have value as places for scientific experiments. But as an expansion of humanity’s everyday living space, the rationality is almost nonexistent. The return is too small for the cost.

After looking up at the summer night sky and thinking about this, I arrive at a simple conclusion. Space settlement is beautiful as a romance, but it has almost no structural rationality. The future humanity should turn toward may not be somewhere beyond the stars, but the redesign and redistribution of life within this planet called Earth.