Construction of the Chinese space station Tiangong continued smoothly this week with the launch and docking of Wentian, a laboratory module. The lab facility advances the progress of a second orbiting outpost 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 participate. Tiangong is smaller than the former International Space Station, which is due to be retired in 2030 under NASA’s current plans, though Russia has given conflicting signals on how long it will continue to participate.
But as with two previous space missions from China, Sunday’s launch resulted in a 23-tonne booster stage of the Long March 5B rocket orbiting the planet. The rocket, part of China’s most powerful rocket, is due to fall back to Earth during the next day, and no one knows exactly where it will land.
China’s lack of any way to guide the thruster down leaves the uncomfortable possibility that debris could descend into a populated area, causing property damage, injury and even death on the ground.
When will the rocket fall?
As of Friday afternoon, Aerospace Corporation, a non-profit organization that conducts research and analysis, including tracking space debris, predicts re-entry on Saturday at 1:24 pm eastern time off the west coast of Mexico.
But the uncertainty is still significant – an hour or so – and as the thruster takes just 1.5 hours to circle the globe, 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 someone 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’re perfectly safe.
The trajectories on Saturday during the booster re-entry period also do not pass through Europe or much of North Africa.
Even if you live somewhere where the rocket is going to go, you have a better chance of winning the Mega Millions lottery than getting hit by a piece of rocket debris.
But the cumulative risk of someone being injured is greater than experts would like. (Someone will win Mega Millions; it will almost certainly not be you.)
“This is a real concern,” said Ted Muelhaupt, a space debris specialist at Aerospace Corporation. “The Chinese shouldn’t do that.”
But he added: “There is no reason to panic. No one should be walking around in football helmets just in case space debris falls.”
Exactly how much risk the booster poses is difficult to estimate because rocket design details influence how much debris survives reentry and reaches the ground.
Space agencies in China have not provided these details or released their estimates of the risk. But they may have decided that was an acceptable risk, betting that the danger for a small number of launches isn’t high enough to justify the costs of changing the rocket’s operation.
So far, there have been two other Long March 5B releases. The first reinforcements landed on villages in Ivory Coast, West Africa, causing some material damage but no injuries. The second booster dropped into 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 into the Pacific Ocean.
Typically, 20% to 40% of a rocket or satellite survives re-entry, Muelhaupt said, which suggests that 10,000 to 20,000 pounds of the Chinese rocket would reach Earth’s surface.
For the most part, organizations that launch large rockets and satellites these days take precautions to ensure that their space debris does not fall into populated areas. It still sometimes occurs, as in 2021, when a malfunction in the second stage of a SpaceX Falcon 9 rocket prevented its engines from directing it towards safe re-entry. The wreckage landed on a farm in downtown Washington. There were no injuries in that incident; the second stage of the four-ton Falcon 9 is considerably smaller than the 23-ton Long March 5B booster.
In 2003, when the space shuttle Columbia disintegrated upon re-entering the atmosphere, debris scattered across eastern Texas and southern Louisiana. Nearly 85,000 pounds of debris from Columbia were recovered; none of the parts caused any injuries.
The Long March 5B is unique for modern rockets in that China has made no effort to control the re-entry of something this large.
So why is China operating such a big rocket in this way?
Most large rockets have two or more stages. The first stage, the biggest chunk of the rocket, usually falls a few minutes after launch without ever reaching orbit. That way, there’s no surprise where it will land. (One reason the Kennedy Space Center is in Florida is its location near the Atlantic Ocean, where the first stages of rockets fall.)
The Long March 5B, which is designed to lift Tiangong modules, is different. Chinese officials have referred to the booster as the second stage, trying to draw parallels with the second stage of the Falcon 9 that crashed over Washington state. But the Long March 5B has no second stage. The large central thruster that lights up on takeoff tracks the payload into orbit, and the Chinese have not designed any way to bring the thruster back from orbit. (Four tether boosters drop harmlessly during launch.)
The booster’s engines are not designed to restart, so they cannot be used to guide the booster back into the atmosphere. The rocket’s designers could have incorporated thrusters for this task, but they would have added weight and complexity.
On Wednesday, Zhao Lijian, a spokesman for China’s Ministry of Foreign Affairs, said the Long March 5B rocket was designed with special technology, although he did not specify what type. The overwhelming majority of its components would burn during re-entry into the atmosphere, he added.
“The probability of this process causing damage to aviation activities or the ground is extremely low,” he said.
Will this happen again?
There are two more launches of Long March 5B planned: one to launch Mengtian, a second lab module, to Tiangong in October and another next year to launch a space telescope, Xuntian, which will orbit close to the space station.
Read You contributed research.