Satellite megaconstellations continue to grow. Could their debris fall on us?

streaks of light crisscross a dark blue sky above a blurry earth below — From the International Space Station, SpaceX Starlink satellites can be seen streaking above the Earth along with lightning strikes and city lights in this photo from NASA astronaut Don Petit. (Image credit: Don Petit/NASA)

As more and more satellite megaconstellations continue to be launched into Earth orbit, some researchers are beginning to calculate the chances that people on the surface could be struck by the incoming remnants of these spacecraft reentering the atmosphere.

A new study by a team of Canadian researchers looks into eleven different megaconstellations and what would happen during their fiery reentry into Earth's atmosphere. What are the charred, declarative results? They find that there's a 40% collective risk of on-ground casualties if satellites do not burn up entirely.

The collective casualty risks modeled by Ewan Wright from the University of British Columbia and colleagues took into account well-funded constellations that, in total, represent some 73,369 satellites. As it turns out, researchers still can't quite predict what would happen if and when those fall back to Earth. "Scientific understanding of how satellites burn up in the atmosphere is not perfect and satellites may not burn up in the atmosphere entirely," Wright told Space.com. "If even a small piece of debris from each satellite survives reentry, the chance of someone getting hit could be considerable if thousands of satellites are launched in megaconstellations," he said.

"Minimum lethal amount"

In the study, the research team asked: "what happens if the minimum lethal amount of debris from each satellite does not burn up and reaches the ground intact?"

It turns out that materials used in satellites with lower melting points, such as aluminum, are more likely to demise entirely. But other spacecraft-making materials such as stainless steel, beryllium, titanium, tungsten, and silicon carbide are less likely to do so. These are commonly employed in fuel tanks and reaction wheels.

When satellites nose-dive into Earth's atmosphere, the Canadian team observes, aerodynamic forces cause them to break up. The intense heat of re-entry ablates their materials into fine particles. "However, many satellites, particularly large ones, do not burn up entirely," they explain. "Risky uncontrolled reentries of space objects should be the exception, rather than the norm."

Surviving debris

While spacecraft builders may strive for their satellites to demise entirely, Wright and fellow researchers add that there is considerable uncertainty in the degree to which total ablation can be achieved. And if a satellite does not burn up entirely, it creates a casualty risk.

"Many existing standards and guidelines only consider individual satellites, and don't consider the cumulative effect of launching and reentering thousands at once," said Wright.

Unless satellite operators purposely direct their satellites to reenter over a specific location on Earth in a "controlled re-entry" manner, that fall from space will be uncontrolled and surviving debris will be spread out over an area centered on a random location along the satellite's orbit.

Those satellite leftovers create a casualty risk to people on the ground and in aircraft in flight, as well as other problems, be they infrastructure damage and airspace closures.

Key question and recommendations

Wright and colleagues ask a key question in their analysis: Do we need so many satellites?

"It is possible to design constellations made up of fewer, higher capacity, higher quality satellites with longer operational lives. This, in turn, would reduce the risk to people on the ground and any damage to the atmosphere," they report.

As for what-next suggestions, the Canadian team offers a set of considerations.

Due to the large collective risks created by single megaconstellations, and the even larger cumulative risks from all constellations, they recommend:

"states and their national regulators should require independent verification of claims of full 'demisability'
evaluate collective casualty risks from entire constellations
pursue a smooth transition to a fair, equitable, and globally applicable controlled reentry regime"

The study "Satellite megaconstellations and collective casualty risks" was published Feb. 6 in the journal Space Policy.

Leonard David is an award-winning space journalist who has been reporting on space activities for more than 50 years. Currently writing as Space.com's Space Insider Columnist among his other projects, Leonard has authored numerous books on space exploration, Mars missions and more, with his latest being "Moon Rush: The New Space Race" published in 2019 by National Geographic. He also wrote "Mars: Our Future on the Red Planet" released in 2016 by National Geographic. Leonard has served as a correspondent for SpaceNews, Scientific American and Aerospace America for the AIAA. He has received many awards, including the first Ordway Award for Sustained Excellence in Spaceflight History in 2015 at the AAS Wernher von Braun Memorial Symposium. You can find out Leonard's latest project at his website and on Twitter.

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