Seven years ago, when Samantha Lawler moved to rural Saskatchewan to run a farm with her partner, the glorious darkness overhead was one of the biggest perks. Undiminished by light pollution, countless stars filled the night sky. It was a pristine celestial view that city dwellers can only dream of seeing. That matters a lot to Lawler, who studies the universe as an astronomer at the nearby University of Regina when she’s not tending goats and chickens.

Yet ever since Lawler’s rural relocation, a new type of artificial star has increasingly encroached on the heavens above her homestead: satellites, constellations of them, most beaming the Internet around the globe. “Now there are many satellites all the time,” Lawler says. “I really could notice the change that was happening.”

In 2019, the same year Lawler moved to Saskatchewan, Texas-based SpaceX began launching its Starlink megaconstellation in earnest, lofting an initial 60 satellites into orbit that May. The constellation has since exploded in size. This past March, SpaceX reached a milestone of 10,000 active Starlink satellites. More than 10 million customers worldwide now use its Internet service. Other companies and countries are racing to catch up. There are plans to put nearly two million satellites in orbit. If even a fraction of those launch, it will make the current crop of about 15,000 satellites look paltry by comparison.

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The rapid proliferation of satellites brings many benefits on the ground, offering faster, more reliable Internet access in remote locations than ever before. Oceanic ships and passenger planes can benefit from broadband communications wherever they are. Emergency services can better respond to natural disasters even when local Internet infrastructure fails. Soldiers on the battlefield can remotely pilot drones and coordinate with commanders half a world away. But the surge has also raised questions about just how many satellites we can safely launch, whether there might be some kind of carrying capacity for Earth orbit and where the limitations lie.

Until now we’ve managed to operate several thousands of satellites relatively smoothly, but what does the future hold? “So far it seems manageable” through active coordination, says Giovanni Lavezzi, a research scientist who specializes in orbital capacity at the Massachusetts Institute of Technology. “The problem is how much can we push.”

Experts are wary of putting a single number on how many satellites we can fit in low-Earth orbit, the region up to 2,000 kilometers above our planet that is prized for most satellite constellations. One 2022 study attempted an answer, suggesting that as many as 12.6 million spacecraft could occupy orbits between 200 and 900 kilometers in altitude. But that simplified scenario assumes an almost perfectly ordered system, something far from reality; a more nuanced appraisal published in 2024 found that the limit would be between about 10 million to 100 million satellites—an unhelpfully broad range for any policymakers seeking guidance for regulatory action.

Either way, with so many satellites, even a tiny number of mishaps would lead to chaos. Hundreds of thousands of collisions would happen every year, each unleashing its own swarm of hazardous, high-speed orbital shrapnel. Satellites could keep operating only because some of that debris would naturally, gradually drift down into Earth’s atmosphere, but some regions of orbit would essentially become unusable. The point of the 2022 study was not to rule out the sheer physical space of low-Earth orbit as the real constraint. “There probably is a capacity,” says Miles Lifson, an orbital-capacity expert at Aerospace Corporation, headquartered in Virginia, and a co-author on both studies. “But the number of satellites is a really ill-posed way to think about it.”

We could, in principle, encircle Earth with concentric shells of satellites, each shell being an optimally spaced, carefully interwoven, machine-packed grid ever present in the sky. In practice, though, orbital capacity depends on a web of interacting factors such as hardware failure rates, collision-avoidance maneuvers and fluctuating amounts of atmospheric drag (which removes objects more quickly at lower altitudes than higher ones). Hugh Lewis, a space debris expert at the University of Birmingham in England, notes that at around 2,000 kilometers, debris can persist for 100,000 years—a timescale comparable to the 300,000 or so years humans have been on the planet.

Carrying capacity is more like the physiological concept of homeostasis, Lewis says. “It’s like how human bodies regulate different systems,” he says. “Your blood sugar levels, your temperature, and so on—they’re all regulated. But each one of those systems affects the others.” In orbit we find something similar. “There are different kinds of systems at play,” Lewis says. Put it all together, “and that’s orbital carrying capacity.”

“We are ripe for a major event to occur. And all of that debris will rain down through all the other operational satellites.” —Darren McKnight

LeoLabs

Atmospheric effects in particular can play a big role. Last March, Matthew Brown, a space weather expert at the University of Birmingham, and his colleagues found that adding more greenhouse gases to the atmosphere could shrink the carrying capacity of Earth orbit by up to half. “Carbon dioxide is released in the lower atmosphere, but in the upper atmosphere it’s a lot less dense,” he says, which causes cooling there rather than heating. “So we get cooling in the upper atmosphere, the density drops, and atmospheric drag on satellites reduces,” meaning material stays in space longer.

Yet even without those effects, Lewis argues, “we have already exceeded the critical number of objects that can be safely put into low-Earth orbit.” This alarming assertion is based on the idea of the Kessler syndrome, in which the number of satellites in orbit surpasses a critical threshold, creating a nigh endless cascade of collision-generated debris. “The environment runs away from you,” Lewis says. According to his calculations, at all altitudes above 550 kilometers, the number of satellites is already high enough that even if all launches stopped today, the amount of debris in space would continue to grow for the foreseeable future. “The spacecraft will collide at a rate that produces fragments greater than the rate at which those fragments get removed” by atmospheric drag, Lewis says.

But launches are not stopping. Quite the opposite. Last year more satellites launched than ever before. That record is on track to fall by the end of this year. As for the previous records? They were first set in 2022, then 2023, then 2024. Space is becoming busier and busier with no sign of slowdown. In 2026 alone the Chinese government has filed for 200,000 new satellites. In the U.S., Starcloud of Washington State has applied through the Federal Communications Commission for 88,000 satellites and Blue Origin, also in Washington, for 51,000. SpaceX CEO Elon Musk announced a breathtaking plan for one million orbital-data-center satellites this past January.

It’s not clear whether these plans are genuine or merely an effort to gain first-mover advantage in orbit—basically an attempt to reserve space for hypothetical satellites. Ruth Pritchard-Kelly, an expert in satellite regulation based in Washington, D.C., says SpaceX’s million-satellite application might have been just “shock and awe,” perhaps intended to bolster the nearly $2-trillion valuation the company received after its record-breaking IPO in June. “A million satellites?” she says. “Give me a break.” But launching even a fraction of this number would still add tens of thousands of satellites to orbit in the coming years. And the sky is already crowded.

Five years after moving to Saskatchewan, Lawler came across an unusual report. In a SpaceX document published online in February 2025, the company revealed that a 2.5-kilogram piece of a Starlink satellite had survived atmospheric reentry and crashed, completely by chance, on a farm in Saskatchewan—not Lawler’s—the prior summer. The piece of debris was “the only known Starlink fragment” to have made it to Earth’s surface, according to the SpaceX document. The company did not respond to a request for comment.

After reading the report, Lawler jumped on a local radio show to try to learn who had found the debris. “I got in touch with the farmer. He sent me some pictures of it, but he had already sent it back to SpaceX,” she says. Nevertheless, he had confirmed that a piece of a Starlink satellite had fallen back to Earth and just three hours down the road from her home. “It’s just such a wild story that this really happened,” Lawler says.

The story highlights an indisputable fact: as the number of satellites in orbit skyrockets, so, too, does the potential for adverse events. The risk of debris falling back to Earth and harming someone or something is slim but not zero. Even if debris completely burns up during reentry, the effect of so much metal and other aerospace materials being dumped into our planet’s fragile upper atmosphere is unclear. Upcoming studies will seek answers. One of the most notable is the European Space Agency’s Draco mission, targeted for 2027, which will fly a sensor-laden spacecraft designed to break apart during reentry so researchers can learn more about this dynamic, poorly understood process.

Orbital “traffic control” to avoid collisions between satellites is another key concern. Only one such crash has ever occurred: a U.S. Iridium satellite hit a defunct Russian satellite in 2009. The disastrous event produced more than 2,000 pieces of trackable debris larger than an AirPods case, about half of which is still in orbit today. Another collision at some point is a certainty, says Stijn Lemmens, a space debris analyst at the European Space Agency. “We have left mass in orbit that is completely dead and will be there for decades to centuries,” he says. “It’s a mathematical game. [Objects] will keep crossing each other [in orbit], and eventually they will hit each other.”

Some altitudes are more worrisome than others. Darren McKnight, a space debris expert at LeoLabs in California, says that two in particular—those at 840 kilometers and 975 kilometers—have high collision risks because they contain lots of discarded rocket stages the size of school buses. He has calculated that at the latter altitude, there’s a 29 percent chance of a collision happening this year. “We are ripe for a major event to occur,” McKnight concludes. “And all of that debris will rain down through all the other operational satellites.”

A major collision, especially at higher altitudes, could result in enough long-lasting debris to place parts of Earth orbit effectively off-limits. “It’s not that space will be unusable,” says Brian Weeden, director of civil and commercial policy at Aerospace Corporation. “It’s just a question of what costs [people] are willing to bear because either you have to invest in avoidance technologies or you have to have more satellites. There might be some orbits that become too costly for pretty much anyone to operate in.”

Collision avoidance is becoming increasingly important as satellite numbers grow. SpaceX’s Starlink constellation currently performs an average of 1,000 maneuvers a day to dodge potential collisions, a number that would have seemed absurd just a decade ago.

To prevent an in-space smash-up, a satellite must fire its thrusters to move out of the way of a projected “conjunction” with another satellite that would pose a significant collision risk. Before Starlink, a typical satellite would do this three or four times a year, says Zeno Pavanello, a collision-avoidance expert at the Polytechnic University of Milan. These maneuvers, planned days in advance, would be overseen by a human operator, who might have to negotiate by e-mail or phone with another satellite operator to decide which object would move.

That leisurely, labor-intensive scenario is often not possible. Now autonomous onboard collision-avoidance systems that can predict and dodge collisions without human input are becoming the norm. “This process that is completely manual and requires a lot of work hours is going to become unsustainable,” Pavanello says. “We’re going to end up in a situation in which satellites experience more than a conjunction a day.”

SpaceX operates the largest autonomous collision-avoidance system currently in orbit. Last year the company revealed the number of collisions it was already avoiding, reporting to the fcc that its Starlink satellites cumulatively dodged about 300,000 collisions over the course of the year, an average of 40 maneuvers per satellite. SpaceX is reluctant to divulge details on how this works, but last year a NASA experiment called Starling clarified parts of the process. The experimenters flew four satellites in close formation through Starlink’s orbital domain to gain some insight into how two satellite constellations might avoid each other.

According to information SpaceX provided to NASA, every 10 minutes each Starlink satellite recalculates its position and predicts its trajectory for the next 48 hours. The satellite then compares this path with those of objects in a database maintained by the U.S. military. If it sees the potential for a collision, the Starlink satellite will fire its thrusters until the risk of collision is less than one in 30 million. (This number is much more conservative than the industry standard of one in 10,000.)

So far this process has worked smoothly, but it is only going to get more complicated. By Lewis’s calculations, if SpaceX were to follow through on its plan to launch a million data-center satellites, the constellation would have to perform a billion collision-avoidance maneuvers every year. “It’s ridiculous,” Lewis says. “You can’t grasp that. There aren’t that many seconds in a year!”

And Lewis’s appraisal accounts only for Starlink. If you add other constellations of comparable size to the mix, such as some being planned in the U.S. and China, the picture gets much more complicated. So far there is no sizable competitor to Starlink; the next biggest, the European OneWeb constellation, has only 650 or so satellites, and they live at a higher altitude. But as other constellations come online, regulatory policy, and not physics, might become the biggest factor determining orbital carrying capacity.

The primary laws governing space today were set by the Outer Space Treaty of 1967. Although the treaty did not predict the rise of megaconstellations, it did lay out some key parameters for spacecraft. Notably, the treaty’s Article II dictates that space cannot be appropriated “by means of use or occupation.”

That dictum poses problems for constellations like Starlink, which dominates the altitude at which it operates, about 350 to 550 kilometers above Earth. As other constellations seek to come online, who will decide which ones get to use certain preferred altitudes—and how? “There’s tons of potential capacity, but there’s only so much beachfront property,” says William Parker, a space scientist at Aerospace Corporation. And what if debris-generating collisions between active satellites create orbital “no-go zones”—would operators be liable? “This is the Pandora’s box of space law right now,” says Michelle Hanlon, executive director of the Center for Air and Space Law at the University of Mississippi School of Law. “There’s a first-mover advantage built into the Outer Space Treaty, not intentionally but because of its vagueness.”

There is also the issue of the impact on the night sky, which continues to affect Lawler and other astronomers. Streaks from satellites are already tarnishing observations by telescopes both on Earth and in space. A study published last December found that adding half a million satellites to orbit would mean that almost every single telescope image taken anywhere would contain a satellite. Increase that to a million, and there would at times be more visible satellites in the sky than stars. “We wouldn’t be able to do astronomy,” Lawler says. “There’s just no way.”

No international body officially governs orbital traffic. The closest thing is the International Telecommunications Union (ITU) in Switzerland, part of the United Nations, which allocates radio spectra for satellite transmissions and helps to coordinate assigned orbits. But the ITU tends to review satellite application without considering the carrying capacity of Earth orbit, whether it’s China asking for 200,000 satellites or SpaceX asking for a million. The system is “definitely broken,” Pritchard-Kelly says, a procedure from a bygone era when building just a single satellite might take years.

Alexandre Vallet, head of the ITU’s space services department, says the organization has “struggled a bit” with the rapid increase in satellites. Attempts are now being made to bring the ITU up to speed, however. At the October 2027 ITU World Radiocommunication Conference, to be held in Shanghai, delegates will discuss options such as reserving parts of low-Earth orbit for each member of the U.N. so that everyone has a chance to launch their own constellation, preventing any single country or company from effectively monopolizing all of orbit.

Time, however, is very much of the essence. The number of satellites, many experts say, is already bordering on unsustainable. Last December, Lawler and her colleagues published a new metric called the CRASH Clock, which estimates what would happen in orbit if every single satellite suddenly became inoperable. That might sound like a far-fetched thought experiment, but it’s not a complete impossibility. There have been plenty of powerful, spacecraft-frying solar flares in our planet’s history.

The results of the study were alarming. Within five days after the satellites went dark, there would be a crash in orbit. Lawler and her co-authors have since revised that estimate to less than three days because so many satellites have gone up since their study was published. “The clock is getting shorter and shorter as we launch more satellites,” Lawler says. “That means we have less wiggle room if there’s a mistake.” She adds that it “highlights how completely reliant we are on SpaceX, mostly, continuing to perfectly execute all of its collision-avoidance maneuvers.”

What SpaceX has demonstrated so far is no doubt impressive. “If everyone behaved like Starlink, you’d get a lot more satellites up there,” McKnight says. But there is no guarantee future constellations will be so refined or that SpaceX will maintain its current standards as it seeks ever increasing growth. Ultimately we might find that the capacity of Earth orbit, rather than being in the millions of spacecraft, may not be much more than the number of satellites already in orbit today.