The 22-ton core stage of a Chinese rocket is expected to fall back to Earth some time Saturday, the third time in two years China has allowed such a large booster to re-enter the atmosphere uncontrolled. The unguided re-entry poses a low but avoidable risk to the world’s population, space debris experts said.

The Long March 5B rocket took off July 24 with the Wentian module for China’s Tiangong space station, hauling one of the heaviest payloads launched into orbit in recent years. The nearly 100-foot-long (30-meter) core stage of the Long March 5B rocket fired its two hydrogen-fueled engines for about eight minutes to inject the Wentian module into orbit.

Four strap-on boosters burned their propellant and jettisoned a few minutes after launch to fall into the South China Sea. But the design of the Long March 5B, one of the most powerful operational rockets in the world, means its core stage accelerates to orbital velocity.

Most launchers carry an upper stage to finish the job of placing a payload into orbit, leaving the booster to fall back to Earth in the ocean or to be recovered for reuse, as SpaceX does with its Falcon 9 rocket.

As of early Saturday, the Long March 5B core stage was forecast to re-enter the atmosphere in a period between 1615 GMT (12:15 p.m. EDT) and 1815 GMT (2:15 p.m. EDT), according to a prediction by the Aerospace Corp., a California-based federally-funded non-profit research institute.

The rocket’s orbit takes it between 41.5 degrees north and south latitude during each hour-and-a-half lap around Earth. The land between those latitudes is home to about 88% of the world’s population.

“It’s low risk on a global scale, but it’s unnecessary risk, and it can affect people, so that’s why we’re talking about it,” said Ted Muelhaupt, a consultant at Aerospace Corp. and an expert on the re-entry of space debris.

It’s impossible to predict exactly when and where the rocket re-enter the atmosphere, but surviving debris will likely fall in a long, narrow footprint hundreds miles long and up to a few dozen miles across. The rocket wreckage will most likely to fall into the ocean or in unpopulated areas.

This is the third time China has left a Long March 5B core stage in orbit to come back to Earth in an unguided manner. The uncontrolled re-entry of the first Long March 5B core stage in 2020 spread debris over the Ivory Coast. The Long March 5B re-entry last year occurred over the Indian Ocean, and no debris was found.

The window of uncertainty around when the rocket will re-enter the atmosphere is largely due to unknowns about the rocket’s orientation and the ever-changing density of the upper atmosphere, which is driven by solar activity that causes the atmosphere to expand or contact, according Muelhaupt.

The window shrinks as the time of re-entry gets closer. Five days before re-entry, experts estimated the window with an error of plus or minus one day. By Saturday morning, just a few hours before re-entry, the error reduced to plus or minus one hour.

Aerodynamic drag will eventually slow the rocket’s velocity enough to allow Earth’s gravity to pull back into the atmosphere, where most of the booster stage will burn up. Muelhaupt estimates about 4 to 9 metric tons, or 20% to 40% of the rocket’s dry mass, will survive the scorching heat of re-entry and reach Earth’s surface.

Abandoned rocket bodies and dead satellites regularly re-enter the atmosphere. Around 50 human-made objects weighing more than a ton re-enter the atmosphere in an uncontrolled manner each year, according to Muelhaupt.

But the Long March 5B core stage will be the sixth largest object to re-enter the atmosphere, not including the space shuttle, Muelhaupt said.

The Aerospace Corp. estimates there probability of a piece of the Long March 5B core stage killing or injuring a person to be between 1-in-230 and 1-in-1,000, meaning there is a 99.5% chance there are zero casualties from the re-entry.

But U.S. government policy guidelines call for managers of space missions to ensure the risk of a death or injury from a re-entry to be no higher than 1-in-10,000. The risk of harm from the Long March 5B re-entry is estimated to be at least 10 times the standard risk threshold for U.S. space missions.

“When it comes down, it will certainly exceed the 1-in-10,000 threshold that is the generally accepted guideline,” Muelhaupt said. “And one of the reasons we’re paying particular attention to this is that in May of 2020, the first test launch of this let debris come down in Africa.”

The risk from the re-entry for any single person is even lower — 6-in-10 trillion, according to the Aerospace Corp. assessment.

“The reality is there are a number of things that you can do about this type of thing, particularly if you’re thinking ahead with your with your mission,” said Marlon Sorge, executive director of Aerospace’s Center for Orbital and Reentry Debris Studies.

For example, designers can select materials that are more likely to burn up during re-entry, reducing the risk of any debris surviving to impact Earth’s surface.

“With the rocket bodies, they’re just so big that it doesn’t really matter what you do during during your design phase in terms of what you make it of. You’ve got huge chunks of metal where the engines are,” Sorge said.

“But there are other approaches that you can do if you think head, and one of those is controlled re-entry,” Sorge said. “Basically, once you’re done delivering your payload, you turn your rocket around, fire the engine and drive it back into the ocean somewhere, usually, someplace where there’s no population. You do that, and you have pretty much mitigated the risk right there. And that’s one of the things that is done by the U.S. government to mitigate these types of risks.”

After the most recent Long March 5B launch and re-entry last year, NASA Administrator Bill Nelson said China was “failing to meet responsible standards regarding their space debris.”

“Spacefaring nations must minimize the risks to people and property on Earth of re-entries of space objects and maximize transparency regarding those operations,” Nelson said in a statement last year.

Wang Wenbin, a spokesperson for the Chinese Foreign Ministry, said in a press conference last year that it is “common practice” for upper stages of rockets to burn up while re-entering the atmosphere. He incorrectly referred to the Long March 5B rocket body as an upper stage, and said that “most of its parts will burn up upon re-entry, making the likelihood of damage to aviation or ground facilities and activities extremely low.”

But no other launcher in the world leaves such a massive component in orbit to fall back to Earth. Dead satellites and old rocket stages regularly re-enter the atmosphere, but re-entering objects with masses of more than a few tons are rare.

“Why are we worried? Well, it did cause property damage the last time (a Long March 5B re-entered),” Muelhaupt said this week. “People are having to do preparation as a result.

“And furthermore, this is not needed,” he said. “We have the technology to not have this problem. Every time you see a Falcon 9 land, that core stage is not going to fall somewhere randomly. Bringing things down deliberately in the ocean, when they’re big enough to cause damage, that is the practice we’d like to encourage.”

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