Space, as Douglas Adams so aptly put it, is big. Compared with the confines of our own planet, the solar system’s expanse is absurdly huge. The idea that we could deplete its vast reserves seems almost laughable.

Amazon CEO Jeff Bezos touched on such sentiments on May 9, during a speech unveiling Blue Moon, his spaceflight company’s lunar lander. “If we move out into the solar system, for all practical purposes, we have unlimited resources,” the billionaire said.

Yet a recent study suggests that at current growth rates, humanity could practically exhaust the most accessible riches in our stellar neighborhood within less than five centuries. To prevent an interplanetary economic crisis, the study’s co-authors suggest that our civilization must somehow build sustainable practices into our expansion out to space, potentially setting aside more than seven eighths of our stellar system’s off-world “real estate” as protected wilderness. The findings appear in the journal Acta Astronautica.

Study co-author Martin Elvis, a researcher at the Center for Astrophysics at Harvard University and the Smithsonian Institution, began wondering a few years ago just how much metallic resources could be extracted from the asteroid belt between Jupiter and Mars. Although hard to reach, in most other respects, asteroids seem to be ideal sources for iron and other heavy metals, concentrating tremendous amounts of ore in small, easily mined space-rock packages. Elvis estimated that the belt contains roughly a million times the available iron reserves of Earth.

The question then became how long it might take to use up this bounty. “I had hoped the answer would be something like 10,000 years,” Elvis says. “It came out to 400.”

The problem is exponential growth. Like a population of prolific rabbits, in recent history, humanity’s consumption rate has swiftly expanded many times over short spans. Assuming that a space-based economy would sustain an annual 3.5 percent growth rate—similar to the increase in the use of iron since the industrial revolution—an eighth of the asteroid belt’s resources would be drained after four centuries.

An eighth might not sound significant, but Elvis and his co-author Tony Milligan, an ethicist at King’s College London, consider it a tipping point. Beyond that threshold, humanity’s potentially insatiable demands for metals would double every 20 years, meaning first a quarter, then half and eventually all of the asteroid belt’s reserves would be used, all within just six additional decades.

Of course, the concept of unchecked expansion has been used and abused to prophesy many nonsensical futures since at least the time of Thomas Robert Malthus. In 1894 a prediction based on the number of horses that would be needed to move London’s population around five decades hence suggested that every street in the city would eventually be buried under nine feet of manure. The advent of automobiles, however, rendered that dire forecast moot.

Elvis and Milligan are not interested in being modern-day Malthusian catastrophists. But even if forecasts based on an exponential turn out to be naive, their broader argument is that resources in the final frontier, as anywhere, have limits. Milligan points out that to people in the 19th century, the vast terrain and riches of the North American continent also seemed inexhaustibly huge.

“We’ve seen that film, and we don’t like the ending,” he says. “Things look big at first, but after a while, they’re not as big as you might think.”

Should the one eighth boundary ever be reached, trying to reorient an economy millions of times bigger than Earth’s current one to reduce consumption in just more than half a century could be extremely difficult, the researchers say—especially if corporations and other vested interests are raking in colossal profits by then, Elvis adds. A more prudent approach might be to avoid ever reaching that point, perhaps by declaring the large majority of the solar system off-limits to exploitation from the get-go.

Recycling materials could extend the time it takes to exhaust the solar system’s metallic wealth but perhaps not indefinitely. In Bezos’s speech, he outlined trillions of humans living and working in gargantuan spinning space stations called O’Neill habitats. Once metal has been committed to such constructions, the crews of those outposts might protest any effort to disassemble their homes for scrap.

Turning the researchers’ proposed principle into practical measures remains a work in progress. First and foremost, Elvis says, we should assess exactly how many usable resources the solar system contains. Are the total surface areas of Mars and the moon in play? What about the enormous gas stores (or metal-rich cores) of worlds like Jupiter and Neptune? Perhaps places of great natural splendor or scientific interest, such as the Red Planet’s gigantic Valles Marineris canyon, would always remain off-limits. Similarly, it might be a shame to consider the reserves of water ice in the beautiful rings of Saturn open to exploitation.

Elvis and Milligan intend for their ideas to be flexible and involve trade-offs. Should SpaceX CEO Elon Musk’s plans to bring settlers to Mars pan out, perhaps the first arrivals might agree to leave large parts of that world untouched. But if a considerable population is one day established, residents might need to push into more than an eighth of the planet’s surface. Maybe then a greater share of the asteroid belt—or some other, more desirable and valuable solar system real estate—could be given protected status.

“This is a conversation shaper, an invitation to dialogue,” Milligan says. “I would like to see whether our principle looks better than the principle next door.”

Economists and policy makers might be the first to quibble with the proposition’s assumptions. Experts in the cloistered spaceflight community are also grappling with the researchers’ conclusions. “As a general posture, it’s fantastic,” says Mark Lupisella, an engineer at NASA’s Goddard Space Flight Center, who has worked on efforts to develop environmental protections for celestial bodies. “But when you make numerical claims and advocate for a policy that’s quantified, I think you might be going too far too quickly.”

Lupisella adds that he thinks it’s good to be concrete and give people something to react to. “I just don’t think we’re in a very good position to make very specific resource preservation claims that are so big and long-term.”

The principles of one generation are not always taken up by those in the next, and even Milligan says he would be surprised if people a century or two hence live according to rationales that seem to make sense to someone today. At the same time, he adds, many great projects in human history have been multigenerational.

Now might be a good time to begin imagining how to ensure a just outcome for those who are to exist after us, he says. “I think it behooves us to have that bigger picture as part of the conversation.”

In Bezos’s optimistic vision, he intended the promise of the solar system’s nearly bottomless riches to provide a sunnier future than the stagnation that would come from remaining bound to our limited Earth. “Do we want stasis and rationing,” he said, “or do we want dynamism and growth?” Perhaps what Elvis and Milligan are offering is a third option: the ability to expand into space while recognizing that even in the infinite beyond, we will need to practice self-control.