Rocket roulette: Everything you need to know about China’s uncontrolled rocket reentries

Last week, one of China's Long March 5B rocket core stages once again performed a potentially dangerous uncontrolled reentry into Earth's atmosphere.

The fiery reentry came after China's space administration launched the final part of its Tiangong space station to orbit using Long March 5B rocket.

Plaudits over the country's achievements in space were quickly drowned out, though, when the media turned its attention to a 21-ton piece of the rocket that would soon fall out of the sky — potentially over a populated area.

Though the rocket core stage eventually fell over the Pacific Ocean, the seriousness of the situation was highlighted by the fact that Spain had briefly closed part of its airspace an hour or so before reentry due to fears the rocket debris could down a passenger airliner.

Here's a rundown of key information about China's uncontrolled rocket reentries.

How many uncontrolled Long March 5B reentries have there been so far?

Last week's rocket reentry marked the fourth time China allowed a Long March 5B core stage to make an uncontrolled reentry into Earth's atmosphere. It came after China launched the third and final module of its Tiangong space station, the Mengtian laboratory module, into orbit.

China's first uncontrolled rocket reentry came after Long March 5B launched in November 2020, and it was the biggest near-miss to date, resulting in large pieces of debris falling onto a village in the Ivory Coast, Africa.

The second reentry occurred after Long March 5B launched in April 2021 and was fairly innocuous, with debris from the rocket's core stage dropping into the Indian Ocean near the Maldives.

The third uncontrolled reentry came in July this year, and resulted in a rocket debris reentry that occurred over the ocean, but was dangerously close to villages in Borneo.

Why does China allow its rocket core stages to perform uncontrolled reentries?

The reason China allows its 21-ton Long March 5B rocket core stage to perform an uncontrolled reentry is likely down to the cost of performing controlled deorbits and the associated risk of damaging private property and life.

The country's space administration likely sees the risk of damage as low enough not to merit the added cost of fitting extra engines onto its rocket for a controlled deorbit — the most likely outcome is always a reentry over water as the Earth's surface is roughly 70 percent ocean.

In an email interview with IE last week, Harvard astronomer and astrophysicist Jonathan McDowell explained that for China to prevent its Long March 5B core stages from performing an uncontrolled reentry, it would likely have to "add a boost motor to the payloads, and that would be expensive."

As a point of reference, in 1990, the U.S. passed a law prohibiting uncontrolled reentries of any object or spacecraft that weighs more than 10 tons due to the associated safety risks.

Why are China's rocket core stages so hard to track?

Only a short while — roughly an hour — before last week's Long March 5B reentry over the Pacific Ocean, news emerged that Spain had closed its northern airspace due to safety concerns as the rocket core stage flew overhead — approximately 300 flights were delayed for 40 minutes, the majority of them from Barcelona's busy international airport, El Prat.

The rocket ultimately fell thousands of miles away over the Pacific Ocean.

The chaos of the situation perfectly highlighted how tricky it is to track orbital rocket debris and provide an accurate estimate for the reentry point. The reason Spain closed its airspace, even though the rocket debris eventually fell over the Pacific, is that the rocket core travels at roughly 17,000 mph (27,360 km/h). This means that the estimated reentry point has to be constantly revised and changed alongside continuous tracking efforts. As an example, if the prediction is off by an hour, that means the predicted reentry point will be off by roughly 17,000 miles (27,360 km).

China's lack of transparency on the issue only makes things worse. In July, after China's last uncontrolled rocket core reentry, NASA Administrator Bill Nelson tweeted, "the People’s Republic of China did not share specific trajectory information as their Long March 5B rocket fell back to Earth. All spacefaring nations should follow established best practices and do their part to share this type of information in advance to allow reliable predictions of potential debris impact risk, especially for heavy-lift vehicles, like the Long March 5B, which carry a significant risk of loss of life and property."

Without transparency from China regarding its reentries, it falls on entities like the U.S. Space Command, part of the U.S. military, and the Aerospace Corporation to track the falling debris and compile an ever-changing "debris footprint" map.

Have other countries and organizations performed dangerous uncontrolled reentries?

Perhaps the most famous and notorious uncontrolled reentry is that of NASA's Skylab orbital station in 1979.

Parts of the space station fell over rural Australia. While some held viewing parties to watch the station fall out of the sky, safety warnings were issued throughout large parts of the world as NASA was unable to pinpoint the exact location of reentry. The orbital station weighed close to 100 tons.

A piece of SpaceX's Crew-1 Dragon capsule also fell over farmland in Australia earlier this year, having spent roughly 20 months in orbit. In an interview with IE shortly afterward, Dr. Brad Tucker, an astrophysicist from the Australian National University, said, "we are lucky" the capsule chunk landed in a rural area. It is worth noting that the capsule shard would weigh a lot less than the 21-ton Long March 5B core stage.

Today, the U.S. applies the Orbital Debris Mitigation Standard Practices (ODMSPs) to all of its launches. ODMSP requires that the risk of a casualty from a reentering rocket body is below a 1-in-10,000 threshold.

However, a study published this year in the journal Nature Astronomy highlights the fact that the U.S. Air Force waived these requirements for 37 of its 66 launches between 2011 and 2018. NASA also waived the requirements for seven launches between 2008 and 2018, including for an Atlas V launch where the casualty risk was projected to be 1 in 600.

As a point of comparison, the Aerospace Corporation estimates that there are between one in 1,000 and one in 230 risks of a casualty from each of China's Long March 5B core stage reentries.

When can we expect another uncontrolled Long March 5B reentry?

Since its rocket debris landed over a village in the Ivory Coast in 2020, China has been seemingly unmoved by public criticism and has continued to launch Long March 5B rockets to orbit, despite the risks.

China's space administration has at least one Long March 5B launch planned for 2023 to launch its Hubble-rivaling telescope, Xuntian, into orbit.

Officials in China have referred to Western coverage of its Long March 5B rocket reentries as a smear campaign against the country's space industry — due to sour grapes over the country's recent advances in spaceflight.

In his email exchange with IE last week, Harvard astronomer and astrophysicist Jonathan McDowell said China is "mistaken," adding that "these reentries are objectively worse than what other countries are doing. I praise China's space program when it does good things, as it often does. And I criticize them when they are bad. This is bad."