What are the odds a free-falling rocket will kill one person somewhere in the world? There’s about a 10 percent chance over the next decade if current practices in the space industry stay the same, according to the authors of a new paper published in the journal Nature Astronomy.
While that’s not a huge risk, the threat is significantly bigger in some parts of the world than in others. In particular, many countries in the Global South are likely to deal with a larger share of space trash even though they’re not responsible for it, according to the analysis. And it could become a bigger issue as rockets launch into space more frequently to ferry up a growing number of satellites.
“It’s a statistically low risk, but it’s not negligible, and it’s increasing — and it’s totally avoidable,” says Michael Byers, lead author of the analysis and a professor in the Department of Political Science at the University of British Columbia. “So, should we take available measures to eliminate casualty risks? I think the answer should be yes,” he says.
As rockets propel themselves into space, they typically drop dead weight — shedding “stages” or rocket bodies that contain empty fuel tanks and engines that are no longer useful for launch. Some rockets drop boosters before reaching orbit and can usually aim for the ocean with some precision (it helps that oceans cover most of the Earth’s surface).
If a rocket has already made it into orbit, it’s possible to guide that equipment back down to Earth safely, again into the ocean, using engines that can reignite. SpaceX has also become famous for landing parts of its Falcon 9 rocket so that they can be reused again, and the company also performs controlled deorbits of the parts it cannot save.
Some rocket stages are still left abandoned in orbit after launch — which are the focus of this new paper.
To date, there hasn’t been a documented death from an uncontrolled rocket reentering the atmosphere. But in 2020, a 12-meter-long pipe and other debris from China’s Long March 5B rocket crash-landed into two villages in Ivory Coast.
There was another nail-biter last year when a 100-foot-tall Chinese rocket stage weighing in at around 20 metric tons plummeted back down to Earth. It finally dropped down in the Indian Ocean after scaring cities like New York and Madrid under its path. That was the spark for the research Byers led, with help from his son, an undergraduate at the University of Victoria who is another author on the paper.
Looking back at the last 30 years of rocket launches, Byers and his colleagues found that Jakarta, Indonesia, Mexico City, Mexico, and Lagos, Nigeria are at least three times as likely to see an uncontrolled rocket body reenter the atmosphere above them as Washington, DC and New York City in the US.
“The risk at an individual level is really, really small ... [but] if you’re living in a densely populated city at 30 degrees north latitude, then it should be of more concern to you,” Byers said. That’s because a lot of the debris from uncontrolled reentries comes from rockets launching payloads to geosynchronous orbit, which roughly follows the Earth’s equator and allows a satellite to match the Earth’s rotation. There’s also “significantly increased risk” about 30 degrees north of the equator because of the population density at that latitude, according to Byers.
If governments mandate changes and the space industry is willing to take on the additional costs, that risk can disappear. The newly published paper points to international agreements that could serve as an example, like the 1987 Montreal Protocol that’s phased out much of the ozone-depleting substances previously used as refrigerants in air conditioners and refrigerators. It might mean carrying smaller payloads so there’s enough fuel left to guide a discarded rocket stage back down to Earth safely. Luckily, it looks like industry is already starting to adapt.
“Current common practice is still to go, ‘Oh, well, that’s too bad. We’ll leave the rocket stage in geotransfer orbit and have it reenter uncontrolled.’ And that’s starting to change, particularly in the US,” says Jonathan McDowell, an astrophysicist at Harvard. The Space Force, for example, now requires that their launch providers deorbit rocket stages. And SpaceX is designing a next-generation rocket called Starship that is supposed to be completely reusable.
“The general practice with regards to aviation is to maximize safety. And we believe that same approach should be taken to space launches,” Byers says. So while it’s still pretty unlikely that pieces of a free-falling rocket will land on anyone’s head, Byers thinks there’s more that can be done to make spaceflight as safe as can be.