Editor’s note: Update after confirming re-entry.
The 22-ton core stage of a Chinese rocket fell to Earth on Saturday, the third time in two years that China has allowed such a large rocket to re-enter the atmosphere unattended. There were no immediate reports of debris or damage on the ground. 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 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.
The US Space Command, which tracks objects in orbit, confirmed that the stage of the Long March 5B rocket re-entered the atmosphere around 12:45 p.m. EDT (1645 GMT). Any remaining debris from the rocket landed in the Sulu Sea at 9.1 degrees north latitude and 119 degrees east longitude, the China Manned Space Agency said in a statement.
Several social media posts, including the one below, showed what appeared to be debris from the Long March 5B rocket burning in the atmosphere. The tweet below shows a video taken in Kuching, Malaysia, on the island of Borneo.
– Nazri Suleiman (@nazriacai) 30 July 2022
There were no immediate reports of any debris falling near populated areas, but unguided re-entry has raised concerns about China’s space junk disposal practices.
“The People’s Republic of China (PRC) did not share specific trajectory information as the Long March 5B rocket fell to Earth,” NASA Administrator Bill Nelson said in a statement Saturday.
“All space-faring nations should follow established best practices, and do their part to share this type of information in advance to allow reliable predictions of potential debris impact risks, especially for heavy vehicles, such as the Long March 5B, which are very risky,” said Nelson. Doing so is critical to the responsible use of space and to ensuring the safety of people here on Earth.”
The Long March 5B rocket orbited between 41.5 degrees north and south during every hour and a half around the Earth. The land between these latitudes is home to about 88% of the world’s population.
Ted Muelhaupt, an Aerospace Corp consultant and expert on re-entry of space debris, said on a conference call with reporters before returning.
It was impossible to predict exactly when and where the rocket would re-enter the atmosphere, but the remaining debris was expected to fall over a long and narrow distance of hundreds of miles and up to a few tens of miles. Statistically, rocket debris was more likely to fall into the ocean or into uninhabited areas.
This was the third time that China had left the main Long March 5B stage in 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 lack of knowledge of the rocket’s orientation and the ever-changing density of the upper atmosphere, which is driven by solar activity causing the atmosphere to expand or touch, accordingly. Mulhaupt.
The re-entry estimate window shrinks as the event time 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 eventually slowed the rocket down enough to allow Earth’s gravity to pull back into the atmosphere, where most of the boost stage would burn up. Moulhaupt estimated that about 4 to 9 metric tons, or 20% to 40% of the missile’s dry mass, would survive 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 Mullhaupt said the primary stage on Long March 5B that fell to Earth on Saturday was the sixth largest object to enter the atmosphere again, with the exception of the space shuttle.
Aerospace Corp has estimated that the probability of killing or injuring a person is between 1 in 230 and 1 in 1,000 in part of the main Long March 5B phase, which means there is a 99.5% chance there will be no casualties. 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 has been 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 and that’s the generally accepted guideline,” Muelhaupt said a few days before the re-entry. “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 anyone returning was 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 missile 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.”
Wang Wenbin, a spokesman for China’s Foreign Ministry, said at a press conference last year that it was a “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” .
China plans to launch the next space station module on another Long March 5B rocket in October. The primary phase of that mission is expected to lead to another unsupervised comeback one to two weeks after launch.
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