Should you be worried about debris from China’s big rocket booster?

Construction of China’s Tiangong space station continued smoothly this week with the launch and docking of Wentian, a laboratory module. The installation of the lab furthers the progress of a second orbiting outpost where humanity will be able to conduct scientific research in a microgravity environment.

China plans to operate the new Tiangong station for at least a decade and invite other nations to participate. Tiangong is smaller than the aging International Space Station, which NASA’s current plans are to decommission in 2030, though Russia has given conflicting signs of how long it will be participating.

But as with two of China’s previous space missions, Sunday’s launch resulted in a 23-ton booster stage of the Long March 5B rocket orbiting the planet. The booster, part of China’s most powerful rocket, is scheduled to fall back to earth over the coming day, and no one knows exactly where it will end up.

China’s inability to guide the booster downward leaves the uncomfortable possibility that debris could fall into a populated area, causing property damage, injury and even death on the ground.

On Friday afternoon, Aerospace Corporation, a non-profit organization that conducts research and analysis including space debris tracking, predicts reentry Saturday at 2:16 p.m. Eastern time over the Indian Ocean.

But the uncertainty is still considerable – more or less five hours – and given that it only takes the booster 1.5 hours to circumnavigate the world, the re-entry point could still happen over much of the planet.

While China’s space agencies provide public data on the rocket body’s orbit, they don’t predict where or when it will reenter. They did not respond to requests for comment before Saturday.

If you are in Chicago or anywhere else above latitude 41.5 degrees north, or below latitude 41.5 degrees south in Antarctica or at the southern tip of South America, you are completely safe.

Saturday’s trajectories during the period when the booster is expected to re-enter also do not pass over Europe or much of North Africa.

Even if you live somewhere where the rocket will fly over, you have a better chance of winning the Mega Millions lottery than getting hit by falling rocket debris.

But the cumulative risk of someone getting hurt is higher than experts would like. (Someone is going to win the Mega Millions; it almost certainly won’t be you.)

“It’s a real problem,” said Ted Muelhaupt, a space debris expert at Aerospace Corporation. “The Chinese shouldn’t do that.”

But he added: “There’s no need to panic. Nobody should be walking around in football helmets just in case space junk falls.”

The exact risk the booster poses is difficult to estimate, as the details of the rocket design affect how much debris will survive re-entry and reach the ground.

Space agencies in China have not provided these details or published their risk estimates. But they might have decided that was an acceptable risk and bet that for a small number of launches the hazard isn’t high enough to justify the cost of changing the way the rocket works.

So far there have been two more launches of the Long March 5B. The first booster fell on villages in Ivory Coast, West Africa, causing property damage but no injuries. The second booster splashed in the Indian Ocean.

When NASA’s city bus-sized upper-atmosphere research satellite made an uncontrolled re-entry in 2011, NASA calculated a 1 in 3,200 chance that someone could be injured. It eventually ended up in the Pacific Ocean.

Typically, 20 to 40 percent of a rocket or satellite survives re-entry, Muelhaupt said, which would indicate 10,000 to 20,000 pounds of the Chinese booster would reach Earth’s surface.

Most organizations launching large rockets and satellites these days take precautions to ensure their space debris does not fall over populated areas. Sometimes it still happens, like in 2021 when a second stage malfunction on a SpaceX Falcon 9 rocket prevented its thrusters from guiding it to a safe re-entry. Debris fell on a farm in central Washington. There were no injuries in this incident; The Falcon 9’s four-ton second stage is significantly smaller than the 23-ton Long March 5B launch vehicle.

When the space shuttle Columbia disintegrated while re-entering the atmosphere in 2003, debris strewn across east Texas and southern Louisiana. Nearly 85,000 pounds of Columbia debris was recovered; None of the pieces caused injuries.

The Long March 5B is unique to modern rocketry as China has made no effort to control the re-entry of something this large.

Most large rockets have two or more stages. The first stage, the largest piece of the rocket, usually falls off a few minutes after launch without ever reaching orbit. That way there’s no surprise where it’s going to come down. (One reason the Kennedy Space Center is in Florida is its location near the Atlantic Ocean, where the first rocket stages are launched.)

Unlike the Long March 5B, which was designed to lift the Tiangong modules. Chinese officials have dubbed the booster a second stage, trying to draw parallels with the Falcon 9 second stage that fell over Washington state. But the Long March 5B doesn’t have a second stage. The large central booster that ignites at launch accompanies the payload into orbit, and the Chinese have not devised a way to bring the booster back out of orbit. (Four strap-on boosters fall harmlessly during launch.)

The booster’s engines are not designed to be restarted, so these cannot be used to propel the booster back into atmosphere. The rocket’s designers could have built in engines to do the job, but they would have added weight and complexity.

On Wednesday, Zhao Lijian, spokesman for China’s Foreign Ministry, said the Long March 5B missile was constructed using special technology, although he did not specify what type it was. The vast majority of its components would burn up on re-entry into the atmosphere, he added.

“The likelihood of this process causing any harm to aviation or the ground is extremely small,” he said.

Li You contributed to the research.

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