Planned satellite megaconstellations could generate runaway space debris

Sixteen thousand satellites currently whiz above our heads on a 3D space superhighway. To the consternation of astronomers and dark-sky lovers, that number may increase 10-fold within the next decade as operators launch internet and telecoms satellites en masse. If SpaceX CEO Elon Musk gets his way, the number could breach 1 million not long after. A new analysis posted to the preprint server arXiv last week, however, suggests those plans face a physical limit, stemming not from the number of satellites, but from the space junk generated as they collide. The study finds that the swarms of satellites, known as constellations, would reach a threshold known as Kessler syndrome—a runaway proliferation of debris that would make the swarms themselves unsustainable. And those predictions come from evaluations of individual constellations, without accounting for existing satellites or the hundreds of thousands of other planned satellites. “In the absence of having perfect information about what these satellite operators are planning, this is as good as it gets,” says University of Regina astronomer Samantha Lawler, who was not involved in the new analysis. “It just so nicely shows how detached from reality these proposals are.” Kessler syndrome, proposed in 1978 by NASA astrophysicist Donald Kessler, describes a scenario in which the collisions between spacecraft produce space junk faster than what falls naturally out of the atmosphere because of drag. Over time, that leads to an exponentially increasing amount of debris, posing threats to other satellites, limiting safe launch windows for future spacecraft and astronauts, and jeopardizing human infrastructure when the debris falls back to Earth. When Kessler defined his eponymous syndrome, megaconstellations were far-future fantasies—not serious proposals being touted by publicly traded companies and weighed by government regulators. So Hugh Lewis, a space debris scientist at the University of Birmingham and a past collaborator of Kessler’s, wanted to see how today’s commercial satellite proposals fared against this dystopian threshold. Lewis compiled data from the Federal Communications Commission, the European Space Agency (ESA), and longtime satellite-tracking astronomer Jonathan McDowell on 16 planned or deployed satellite constellations. He fed the known properties of each constellation’s satellites—such as their numbers, orbits, sizes, and lifetimes—into a simple model that tracked how often a satellite or any resulting space junk passed through a single point in the atmosphere, to determine how quickly those objects’ modeled orbits decayed with atmospheric drag. He also considered two factors that could slow the generation of space junk: how well the planned satellites could maneuver away from potential collisions and whether they were designed to fall back safely to Earth when their lifetimes ended. Of the 16 planned constellations, comprising nearly 1.7 million satellites, six passed the threshold into runaway debris, among them China’s Guowang, Cowboy Space Corp.’s Stampede, and Blue Origin’s Project Sunrise. Perhaps the most notable generator of debris would be Starmind, SpaceX’s plan for 1 million satellites to act as orbital data centers. Three more proposals reached an unstable state, where debris would increase but not endlessly. Lewis found problems even with less ambitious constellations. One, proposed by Eutelsat, became unstable with a mere 528 satellites, in part because of those satellites’ proposed orbital layout; another, from E-Space, induced Kessler syndrome with “nanosatellites” of just 10 kilograms apiece, based on the assumption that such small satellites lack the propulsion capabilities necessary for collision avoidance. To Lewis, the proposals collectively are “not meeting what you’d argue to be a requirement to maintain or achieve sustainability and safety in space.” The true risk may be even less “rosy” than the study suggests, Lawler says, because it considered every constellation independently. The study “makes all of these assumptions that are as good as possible for the megaconstellation operators, and you still end up in the runaway collisional regime,” she says. “That’s just really quite horrifying.” Clayton Swope, deputy director for the Aerospace Security Project at the Center for Strategic and International Studies, a policy research organization, is skeptical of Lewis’s simplistic model, preferring the more complex ones NASA and ESA use to account for the behavior of individual satellites. But Aaron Boley, a space sustainability researcher at the University of British Columbia who was not involved in the study, says the analysis highlights a crucial point: just how few rules have been set for operating in space, including who must maneuver in the case of a collision course. Any one company may set its own policies—on average, SpaceX satellites conduct one collision avoidance maneuver every 75 seconds—but “it’s really hard to see, if we go to the situation in which we have hundreds of thousands of satellites, that we’re not going to see some serious catastrophes,” Boley says. That reasoning alone should compel satellite companies to re-evaluate their proposals, the researchers say, even if only to avoid losing spacecraft and money. In fact, Lewis’s study suggests small tweaks—such as increasing a satellite’s area so it experiences more drag and drops out of orbit before colliding, or lengthening a satellite’s operational lifetime so fewer replacement launches are needed—could bring a proposal back from the brink. The goal is a world in which crucial communications, research, and navigation functions are preserved, all while the sky remains safe for launches and clear for astronomers. With the models, “there is uncertainty in what would happen in the real world,” Lewis acknowledges. “But at some point, we have to make decisions within that uncertainty.”