Why It's So Hard to Predict Where China’s Out-of-Control Rocket Will Crash

George Dvorsky
·6 min read
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The Philippine Coast Guard inspecting debris from a Long March 5B rocket launched on July 24, 2022.
The Philippine Coast Guard inspecting debris from a Long March 5B rocket launched on July 24, 2022.

An out-of-control booster from a recently launched Long March 5B rocket is expected to reenter Earth’s atmosphere on Friday. The 22-metric-ton object poses a threat to human lives and property, but determining exactly when and where it will crash is proving to be fiendishly difficult, and here’s why.

The rocket, carrying the third and final module for China’s Tiangong space station, took off from Wenchang Space Launch Center in Hainan on October 31. As expected, the core stage didn’t perform a reentry burn and instead entered into a very low orbit. This is now par for the course, as China’s Long March 5B rocket pulled the exact same stunt during its three previous launches. It’s how China’s space agency chose to design the rocket, forcing us to dread each launch and wonder where debris from the 108-foot-long (33-meter) booster will crash.

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The Aerospace Corporation’s Center for Orbital Reentry and Debris Studies (CORDS) is currently tracking the object and providing regular updates, which it does using data from the U.S. Space Force’s Space Surveillance Network. As of this posting, the core stage is expected to perform an uncontrolled reentry on Friday, November 4 at 8:24 a.m. ET (12:24 p.m. UTC), give or take about 4 hours. A CORDS map shows the possible places where the debris could fall, but it’s still too early to know the exact location or timing.

That’s frustrating, and even a bit scary. The Aerospace Corporation says somewhere between 10% and 40% of the booster will survive reentry, representing some 2 to 9 metric tons of falling space junk. Ideally, we would warn the threatened regions in advance, but the unpredictability of these reentries makes this unfeasible. In 2020, debris from the first test launch of the Long March 5B rocket fell onto the Ivory Coast. The core stage from the rocket’s second flight fell into the Indian Ocean, but the reentry from the third flight in July 2022 was visible from Malaysia, with debris falling onto parts of Indonesia and Philippines. None of this was predicted in advance, at least not to any meaningful precision. Thankfully, no one has been injured or killed by incoming Long March 5B debris, that doesn’t mean it can’t happen.

The Space Surveillance Network knows the speed and location of the booster, which is critically important data that allows CORDS to map the possible paths along which the debris might fall and the areas that will be spared. The potential places where the booster might crash can be determined in advance, but the exact timing of the reentry cannot be known, making it difficult to nail an exact location. As Ted Muelhaupt, a consultant with Aerospace’s Corporate Chief Engineer’s Office, told reporters yesterday during a press conference, it’s “easier to say where it won’t fall.”

We’re accustomed to pin-point accuracy when it comes to predicting the motions of bodies operating in the space environment, but it’s a different story for objects coming in from space, particularly if those objects are tumbling and out of control.

A critical factor that makes predictions so tough is the intense speed at which these boosters are moving when they hit the atmosphere. Objects spinning around Earth move at speeds reaching 17,500 miles per hour (28,160 kilometers per hour), so for every incorrect hour in the predictions, the debris footprint will be off by that exact distance: 17,500 miles. For reference, Earth’s circumference is 29,900 miles (40,075 km). This all makes sense, of course, as objects in low Earth orbit complete one full revolution of the planet every 90 minutes or so.

Ted Muelhaupt said the core stage from July arrived 20 minutes later than the company’s predictions. That may sound like a fairly close guess, but it amounts to a reentry some 5,800 miles (9,334 km) farther away than anticipated. That CORDS hadn’t received new data from the Space Space Surveillance Network in seven hours didn’t help matters, but that’s yet another factor that makes predictions so difficult.

It’s also important to note that the speed and shallow one-degree angle at which the core stages reenter the atmosphere results in a rather larger debris footprint. These objects don’t disintegrate in a single moment, and instead gradually fall apart. Muelhaupt said debris footprints can be very long, at over 44 miles (70 km) wide and over 1,240 miles (2,000 km) long. The heaviest objects land last, Muelhaupt added.

Another reason for the unpredictability has to do with the atmosphere itself. Ultimately, it’s atmospheric drag that causes an object to perform a reentry, but the atmosphere’s size and density is in a state of constant flux owing to solar effects. The Sun is currently going through a period of intense solar activity, causing the atmosphere to bloat and expand. For objects in orbit, including expelled core stages, this results in increased atmospheric drag. Measuring this and then calculating the resulting atmospheric effects on an inbound object is currently beyond our capacities.

Adding to this complexity is the motion and composition of the object itself. Should the booster be tumbling, which is very possible, it will interact with the atmosphere in dynamic ways. To make predictions about when a tumbling object will reenter, experts would need to know the tumbling motion and the angle at which it’s spinning, according to Muelhaupt. To further understand these interactions, they’d also need to know the materials used to construct the rocket booster, in addition to the exact shape and internal configuration of the object. “We would need to know what’s in front of what,” he said, as certain components could serve as veritable heat shields during reentry, allowing some parts to survive longer than others. You need “a lot of data” to really predict what’s going to survive a reentry, Muelhaupt said.

“The atmospheric density, the drag coefficient, the mass, and the cross-sectional area all combine into a single term for drag in force models, and uncertainty in any makes the quantity uncertain,” he said. “Density is the biggest, but we need details of the physical characteristics for the others.”

China could provide some of these details to improve predictions, but its secretive space agency isn’t in the habit of disclosing sensitive information about its rockets. Ultimately, China needs to get its act together and ensure that future rocket designs incorporate controlled reentries during launch, in which an engine would steer the booster away from populated areas and toward the ocean. For now, people living in threatened regions can only hope to avoid the incoming debris, which has to fall somewhere.

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