The 22-ton core stage of a Chinese missile is expected to return to Earth sometime on Saturday, the third time in two years that China has allowed such a large missile to re-enter the atmosphere uncontrollably. Space debris experts said an unguided return poses a low but avoidable risk to the world’s population.

A Long March 5B rocket blasted off on July 24 with the Wentian module of China’s Tiangong Space Station, carrying one of the heaviest payloads ever launched into orbit in recent years. The approximately 100-foot (30 m) core stage of the Long March 5B rocket fired two hydrogen-fueled engines for about eight minutes to inject the Wentian module into orbit.

Four installed boosters packed up their fuel and were dumped a few minutes after launch to fall into the South China Sea. But the design of the Long March 5B, one of the world’s most powerful operating rockets, means its core stage is accelerated to orbital speed.

Most launchers carry an upper stage to finish the task of putting the payload into orbit, leaving the booster to return to land in the ocean or retrieve it for reuse, as SpaceX does with its Falcon 9 rocket.

As of early Saturday, the Long March 5B primary stage was expected to re-enter the atmosphere between 1615 GMT (12:15 PM EST) and 1815 GMT (2:15 PM EST) eastern United States), according to aerospace forecasts. Corp. , a federally funded nonprofit research institute headquartered in California.

The orbit of the rocket rotates between 41.5 degrees north and south latitude during every hour and a half around the Earth. The land between these latitudes is home to about 88% of the world’s population.

said Ted Muelhaupt, a consultant with Aerospace Corp and expert in re-entry of space debris.

It is impossible to predict exactly when and where the rocket will re-enter the atmosphere, but the remaining debris is likely to fall over long and narrow distances of hundreds of miles and up to a few tens of miles. Missile debris is more likely to fall into the ocean or into uninhabited areas.

This is the third time that China has left the main Long March 5B stage of orbit to return to Earth in an unguided fashion. The uncontrolled return of the first core stage of Long March 5B in 2020 has spread debris over Ivory Coast. The return of Long March 5B occurred last year over the Indian Ocean, and no wreckage was found.

The window of uncertainty about when the rocket will re-enter the atmosphere is due in large part to the unknowns about the rocket’s direction and the ever-changing density of the upper atmosphere, which is driven by the solar activity causing the atmosphere to expand or touch, accordingly. Mulhaupt.

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

The aerodynamic drag will eventually slow the rocket down enough to allow Earth’s gravity to pull back into the atmosphere, where most of the boost stage will burn up. Mullhaupt estimates that about 4 to 9 metric tons, or 20% to 40% of the missile’s dry mass, will withstand the scorching heat of re-entry and reach the Earth’s surface.

The corpses of abandoned missiles and dead satellites regularly return to the atmosphere. According to Moelhaupt, about 50 man-made objects weighing more than a ton re-enter the atmosphere in an uncontrollable way each year.

But Mulhaupt said the core stage on Long March 5B would be the sixth largest object to reenter the atmosphere, excluding the space shuttle.

Aerospace Corp. estimates that there is a probability of a portion of the Long March 5B primary killing or injuring someone between 1 in 230 and 1 in 1,000, which means there is a 99.5% chance of no injuries. Re-entry.

But US government policy guidelines call on space mission managers to ensure that the risk of death or injury from re-entry is no more than 1 in 10,000. The risk of damage from Long March 5B re-entry is estimated to be at least 10 times the standard risk threshold for US space missions.

“When it goes down, it will definitely cross the 1 in 10,000 threshold which is the generally accepted guideline,” Muelhaupt said. “And one of the reasons we pay special attention to this is that in May of 2020, the first test launch of this wreck was landing in Africa.”

The risk of re-entry for any single person is even lower – 6 in 10 trillion, according to an assessment by Aerospace Corp.

“The truth is that there are a number of things you can do about this kind of thing, especially if you think ahead about your mission,” said Marlon Sorge, executive director of the Space Center for Orbital and Debris Return Studies.

For example, designers can choose materials that are more likely to burn during re-entry, reducing the risk of any debris remaining on the Earth’s surface.

“With rocket bodies, they are so big that it doesn’t really matter what you do during the design phase in terms of what you make. You have huge pieces of metal in place of the motors,” Sorge said.

“But there are other ways you can do it if you think carefully, and one of those approaches is controlled reentry,” Sorge said. “Essentially, once you have your payload delivered, you turn your rocket, fire the engine, and put it back into the ocean somewhere, usually, somewhere where there’s no population. You do that, and you’ve greatly mitigated the risks there. And that’s one of the things that you do the US government to mitigate these types of risks.”

After the latest launch and re-entry of Long March 5B last year, NASA Administrator Bill Nelson said China “failed to meet responsible standards with respect to space debris.”

“Space-faring nations should reduce the risks to people and property on Earth from the re-entry of space objects and increase transparency regarding these operations,” Nelson said in a statement last year.

Wang Wenbin, a spokesman for China’s Foreign Ministry, said at a press conference last year that it was “common practice” for the upper stages of missiles to burn up as they re-enter the atmosphere. He incorrectly referred to the Long March 5B missile’s body as an upper stage and said that “most of its parts will burn up on re-entry, making the potential for damage to aviation or ground-based facilities and activities extremely low.”

But no other launcher in the world leaves such a massive component in orbit to return to Earth. Dead satellites and old rocket stages return to the atmosphere regularly, but objects over a few tons in mass are rare to re-enter.

“Why are we worried? Well, I did property damage last time (Long March 5B was reintroduced), Muelhaupt said this week. As a result, people have to do the preparation.

“Furthermore, it is not necessary,” he said. “We have the technology so we don’t have this problem. Every time you see a Falcon 9 Earth, that base stage won’t fall off somewhere randomly. Deliberately dropping things into the ocean, when they’re big enough to cause damage, that’s the practice we’d like to encourage” .

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