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 advances the progress of a second outpost in orbit where humanity is able to conduct scientific research in a microgravity environment.
China plans to operate the new Tiangong station for at least a decade, inviting other nations to take part. Tiangong is smaller than the aging International Space Station, which is to be retired in 2030 under NASA’s current plans, although Russia has given conflicting signs of how long it will continue to participate.
But as with two earlier space missions by China, Sunday’s launch resulted in a 23-ton booster stage from the Long March 5B rocket orbiting the planet. The booster, part of China’s most powerful rocket, is expected to fall back to Earth during the coming day, and no one knows exactly where it will land.
China’s lack of any way to guide the booster down leaves the uncomfortable possibility that debris could descend in a populated area, causing property damage, injury and even death on the ground.
When will the rocket come down?
As of Friday afternoon, the Aerospace Corporation, a nonprofit organization that performs research and analysis including the tracking of space debris, is predicting re-entry on Saturday at 1:24 p.m. Eastern time near Mexico’s west coast.
But the uncertainty is still significant — give or take one hour — and because the booster takes only 1.5 hours to circle all the way around the world, the re-entry point could still occur over much of the planet.
While China’s space agencies are providing public data on the orbital path of the rocket body, they are not predicting where or when it will re-enter. They did not respond to requests for comment before Saturday.
How much risk does the rocket pose to anyone on the ground?
If you’re in Chicago or anywhere else above 41.5 degrees north latitude or in Antarctica or the southern tip of South America below 41.5 degrees south latitude, you are perfectly safe.
The trajectories on Saturday during the period when the booster is predicted to re-enter also do not pass over Europe or much of northern Africa.
Even if you live somewhere where the rocket will pass over, you have a better chance at winning the Mega Millions lottery than getting hit by a falling piece of rocket debris.
But the cumulative risk of someone being injured is higher than experts would like. (Someone will win the Mega Millions; it almost certainly won’t be you.)
“This is a real concern,” said Ted Muelhaupt, a space debris expert at the Aerospace Corporation. “The Chinese shouldn’t do this.”
But he added, “It isn’t a cause for panic. Nobody ought to be walking around with football helmets on just in case of falling space debris.”
Exactly how much risk the booster poses is difficult to estimate because the details of the rocket’s design influence how much debris survives re-entry and reaches the ground.
Space agencies in China have not provided those details or released their estimates of the risk. But they might have decided this was an acceptable risk, betting that the danger for a small number of launches is not high enough to justify the costs of changing how the rocket operates.
So far, there have been two other Long March 5B launches. The first booster fell on villages in Ivory Coast in western Africa, causing some property damage but no injuries. The second booster splashed in the Indian Ocean.
When NASA’s Upper Atmosphere Research Satellite, which was the size of a city bus, made an uncontrolled re-entry in 2011, NASA calculated a 1-in-3,200 chance that someone could be injured. It ended up falling in the Pacific Ocean.
Typically 20 percent to 40 percent of a rocket or satellite survive re-entry, Mr. Muelhaupt said, which would suggest that 10,000 to 20,000 pounds of the Chinese booster would reach Earth’s surface.
For the most part, organizations launching large rockets and satellites these days take precautions to make sure that their space debris does not fall over populated areas. Sometimes, it still occurs, as in 2021 when a malfunction on the second stage of a SpaceX Falcon 9 rocket prevented its engines from directing it to a safe re-entry. Debris fell on a farm in central Washington. There were no injuries from that incident; the four-ton Falcon 9 second stage is considerably smaller than the 23-ton Long March 5B booster.
In 2003, when the space shuttle Columbia disintegrated as it re-entered the atmosphere, debris scattered across eastern Texas and southern Louisiana. Nearly 85,000 pounds of debris from Columbia were recovered; none of the pieces caused any injuries.
The Long March 5B is unique for modern rocketry in that China has not put any effort to controlling the re-entry of something so big.
So why is China operating such a big rocket this way?
Most big rockets have two or more stages. The first stage, the biggest piece of the rocket, typically drops off a few minutes after launch without ever reaching orbit. That way, there is no surprise where it is going to come down. (One reason the Kennedy Space Center is in Florida is the location near the Atlantic Ocean, where the first stages of rockets fall.)
The Long March 5B, which was designed to lift the Tiangong modules, is different. Chinese officials have referred to the booster as the second stage, trying to draw parallels to the Falcon 9 second stage that fell over Washington State. But the Long March 5B has no second stage. The big central booster that ignites at liftoff accompanies the payload all the way to orbit, and the Chinese did not design any way to bring the booster back down from orbit. (Four strap-on boosters do drop off harmlessly during the launch.)
The booster’s engines are not designed to restart, so those cannot be used to guide the booster back into the atmosphere. The rocket’s designers could have incorporated thrusters for that task, but they would have added weight and complexity.
On Wednesday, Zhao Lijian, spokesman of China’s Ministry of Foreign Affairs, said that the Long March 5B rocket is designed with special technology, although he did not specify what kind. The overwhelming majority of its components would burn up during the re-entry into the atmosphere, he added.
“The probability of this process causing harm to aviation activities or to the ground is extremely low,” he said.
Is this going to happen again?
There are two more Long March 5B launches planned: one to launch Mengtian, a second laboratory module, to Tiangong in October and one next year to launch a space telescope, Xuntian, that will orbit close to the space station.