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 laboratory 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 join. Tiangong is smaller than the aging International Space Station, which is due for retirement in 2030 under current NASA plans, although Russia has given conflicting signs as to how long it will be involved.
But as with China’s two previous space missions, Sunday’s launch brought a 23-ton booster stage of the Long March 5B rocket into orbit around the planet. The booster, part of China’s most powerful rocket, is expected to fall back to Earth within the next day, and no one knows exactly where it will land.
The lack of any means for China to guide the thruster downward 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?
On Friday afternoon, the Aerospace Corporation, a nonprofit organization that conducts research and analysis, including space debris tracking, predicted a re-entry Saturday at 1:08 p.m. Pacific Ocean.
But the uncertainty is still significant – plus or minus an hour – and since the booster only takes an hour and a half to circumnavigate the globe, the point of re-entry could still occur over much of the planet.
While Chinese space agencies provide public data on the rocket body’s orbital path, they don’t predict where or when it will return. They did not respond to requests for comment until Saturday.
What risk does the rocket pose to anyone on the ground?
If you are 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.
Saturday’s trajectories during the period the booster is supposed to return also do not pass over Europe or much of North Africa.
Even if you live somewhere the rocket will pass, you have a better chance of winning the Mega Millions lottery than being hit by a piece of falling rocket debris.
But the cumulative risk of someone being injured is higher than experts would like. (Someone will win the Mega Millions, it definitely won’t be you.)
“It’s a real concern,” said Ted Muelhaupt, a space debris expert at the Aerospace Corporation. “The Chinese shouldn’t do this.”
But he added: “It is not a cause for panic. No one should walk around with football helmets just in case space junk falls.
It is difficult to estimate exactly the risk posed by the booster because the details of the rocket’s design influence the amount of debris that survives reentry and reaches the ground.
Chinese space agencies have not provided these details or released their risk estimates. But they could 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.
There have been two more Long March 5B launches so far. The first recall fell on villages in Côte d’Ivoire in West Africa, causing property damage but no injuries. The second booster splashed down 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 one in 3,200 chance that someone would be injured. It ended up falling into the Pacific Ocean.
Typically, 20 to 40 percent of a rocket or satellite survives re-entry, Muelhaupt said, suggesting that 10,000 to 20,000 pounds of Chinese propellant would reach Earth’s surface.
For the most part, organizations launching large rockets and satellites these days take precautions to ensure that their space junk does not fall on populated areas. Sometimes it still happens, like in 2021 when a malfunction on the second stage of a SpaceX Falcon 9 rocket prevented its engines from directing it to safe re-entry. Debris fell on a farm in central Washington. There were no injuries in this 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 upon re-entering the atmosphere, debris scattered across eastern Texas and southern Louisiana. Nearly 85,000 pounds of Columbia wreckage was recovered; none of the pieces caused injuries.
The Long March 5B is unique for modern rockets in that China made no effort to control the re-entry of something so large.
So why is China operating such a big rocket like this?
Most large rockets have two or more stages. The first stage, the biggest piece of the rocket, usually falls a few minutes after launch without ever reaching orbit. That way, there’s no surprise where it’s going to go down. (One of the reasons Kennedy Space Center is in Florida is the location near the Atlantic Ocean, where rocket first stages fall.)
The Long March 5B, which was designed to lift the Tiangong modules, is different. Chinese officials have called 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 does not have a second floor. The large central thruster that ignites on liftoff accompanies the payload into orbit, and the Chinese have not devised any means of bringing the thruster back down from orbit. (Four strap-on boosters drop harmlessly during launch.)
The booster motors are not designed to restart, so they cannot be used to bring the booster back into the atmosphere. The rocket designers could have incorporated thrusters for this task, but they would have added weight and complexity.
On Wednesday, Zhao Lijian, spokesman for China’s Foreign Ministry, said the Long March 5B rocket is designed with special technology, although he did not specify what type. The overwhelming majority of its components would burn up on re-entry, he added.
“The likelihood of this process causing damage to aerial or ground operations is extremely low,” he said.
Will this happen again?
Yes.
Two more Long March 5B launches are planned: one to launch Mengtian, a second lab module, at Tiangong in October and one next year to launch a space telescope, Xuntian, which will orbit near the space station.
Li You contributed to the research.
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