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VANCOUVER — At any given moment, thousands of pieces of space debris orbit Earth, and some are heading back toward us. According to new research from the University of British Columbia, there’s a 26% annual probability that one of these massive objects will reenter Earth’s atmosphere over a region with high air traffic density, potentially disrupting hundreds of flights and thousands of travelers.

This risk was dramatically illustrated in November 2022 when Spanish and French authorities closed parts of their airspace due to a falling 20-tonne Chinese rocket body, affecting 645 flights and delaying passengers by an average of 29 minutes. Some aircraft already in flight had to turn around or divert their paths, creating cascading disruptions throughout European airspace.

“The recent explosion of a SpaceX Starship shortly after launch demonstrated the challenges of having to suddenly close airspace,” explains Ewan Wright, the study’s lead author and an interdisciplinary studies doctoral student at UBC, in a statement. “The authorities set up a ‘keep out’ zone for aircraft, many of which had to turn around or divert their flight path. And this was a situation where we had good information about where the rocket debris was likely to come down, which is not the case for uncontrolled debris re-entering the atmosphere from orbit.”

Why is rocket debris such a big problem?

The problem stems from how modern spaceflight operates. When rockets launch satellites into orbit, large portions of these rockets are often abandoned in space. If these leftover rocket stages have a low enough orbit, they eventually fall back to Earth in an uncontrolled manner. While most materials burn up during reentry, significant pieces can survive the descent.

The study, published in Scientific Reports, reveals concerning statistics about high-traffic airspace. Using Denver, Colorado, as a reference point—which had the highest air traffic density in the dataset at approximately one aircraft every 18 square kilometers—researchers calculated the probability of rocket debris reentering the atmosphere over different traffic density thresholds.

For regions with air traffic similar to that found in major transit corridors like the northeastern United States, northern Europe, or parts of the Asia-Pacific, the probability of a rocket body reentering busy airspace is 26% per year. However, this probability does not mean that reentry will occur over the same location every four years—only that such an event is likely somewhere within airspace that meets these traffic conditions.

Rocket launches are increasing, as is the risk

The situation appears even more pressing when considering current trends. In 2023, there were 212 successful rocket launches, with 128 uncontrolled rocket body reentries. The number of rocket bodies still in orbit exceeds 2,300, each one destined to eventually fall back to Earth. Meanwhile, air passenger numbers are projected to increase by almost 7% in 2025, according to the International Air Transport Association.

“The space industry is effectively exporting its risk to airlines and passengers,” says Dr. Aaron Boley, associate professor in the Department of Physics and Astronomy at UBC and co-author of the study.

This risk transfer occurs because when space debris threatens busy airspace, aviation authorities must either take preventive action—by diverting flights or closing airspace—or gamble by allowing flights to continue.

The annual probability of space rocket debris actually colliding with an aircraft stands at 1 in 430,000—a relatively low number, but one that still carries catastrophic potential. Even small pieces of debris pose significant risks due to their high velocity and the speed at which aircraft cruise.

Solutions and future outlook

The researchers emphasize that these risks are not inevitable. “Uncontrolled rocket body reentries are a design choice, not a necessity,” explains Dr. Boley. The industry could instead use rockets designed to reenter the atmosphere in a controlled way after use, directing them to crash harmlessly into remote ocean areas.

However, implementing this solution requires international cooperation, according to co-author Dr. Michael Byers, a UBC political science professor. “Countries and companies that launch satellites won’t spend the money to improve their rockets designs unless all of them are required to do so,” he explains. “So, we need governments to come together and adopt some new standards here.”

Until such standards are implemented, aviation authorities worldwide will continue facing difficult decisions about managing airspace safety while minimizing disruptions to air travel, all while the number of both flights and space debris continues to grow.