HELSINKI — The large first stage of the Long March 5B rocket which sent the Mengtian space station module into orbit earlier this week is set for an uncontrolled reentry Friday.

China launched the third and final module for its Tiangong space station Oct. 31. The Mengtian module successfully docked with the station 13 hours later.

As with China’s previous space station module launches, the roughly 21-metric-ton dry mass Long March 5B first stage also acts as the upper stage for the mission and entered orbit, which is exceptional in international spaceflight. 

The Long March 5B first stage is also unable to restart its engines to effect a controlled reentry into the atmosphere. 

Analysis by the Aerospace Corporation’s Center for Orbital and Reentry Debris Studies (CORDS) gives a latest predicted reentry time of 1:56 p.m. Eastern (17:56 UTC) Nov. 4, with a window of uncertainty of six hours either side. 

The center of the window would see a reentry over the Gulf of Mexico, but the rocket stage is traveling at nearly eight kilometers per second, meaning a deviation of even a few minutes means a reentry more than a thousand kilometers away.

The latest prediction from the U.S. Space Force’s 18th Space Defense Squadron predicts a reentry at 8:16 a.m. (12:16 UTC) plus or minus five hours.

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The large window for the reentry is due to the challenges of modeling, including variables such as atmospheric fluctuations which affect how quickly the orbit of an object decays. Prediction windows will narrow closer to the reentry event.

The three earlier launches of this rocket ended with uncontrolled reentries. The previous launch sent the Wentian module into orbit and saw the first stage reenter the atmosphere over Southeast Asia less than a week later.

Ted Muelhaupt, a consultant with The Aerospace Corporation’s Corporate Chief Engineer’s Office, opened a briefing on the situation Nov. 2 with the words “Here we go again.”

Muelhaupt noted that the risk to any individual is vanishingly small, and no-one needs to alter their lives due to the event. There is a “99.5% chance that there’s zero casualties, but it’s high enough that the world has to watch and prepare and take precautionary steps and that has a cost which is unnecessary,” Muelhaupt said.

Most of the rocket stage will burn up during the high-speed reentry into the atmosphere, but 10 to 40 percent of the mass of a large object will reach the ground, depending on the design of the object.

The most likely scenario is that the stage reenters over the oceans. There is however a non-zero probability of the surviving debris landing in a populated area, with over 88 percent of the world’s population living under the reentry’s potential debris footprint, according to a presentation by Muelhaupt.

The stage’s orbit sees it reach a latitude a little farther north than New York, Madrid, and Beijing, and as far south as southern Chile and Wellington, New Zealand. Ground tracks for individual orbits are shown in an Aerospace Corporation illustration.

“Two out of three of these [rocket reentries] have dumped big chunks of metal where people are. So far, no casualties. Property damage, maybe, which I can’t confirm. But in one case, they took precautionary evacuations. So there’s a real cost to this uncertainty and it’s avoidable.”

First stages for orbital launches typically do not reach orbital velocity and fall within a calculated area downrange. Some upper stages, including China’s Long March 2D, restart their engines after delivering a payload into orbit and deorbit themselves to reduce space debris and the risk of collisions in orbit. 

However, many upper stages make uncontrolled reentries. Chinese foreign ministry spokesperson Wang Wenbin said last year ahead of the reentry for the booster that launched Tianhe that it was “common practice across the world for upper stages of rockets to burn up while reentering the atmosphere.” 

The Long March 5B reentry is a large and prominent symptom of a wider problem. A recent Nature Astronomy paper published earlier assesses that current practices mean there is a 10 percent chance of uncontrolled reentries causing one or more casualties over a decade.

Around 50 objects with a mass of more than one ton reenter the atmosphere randomly each year, according to the Aerospace Corporation.

The previous Long March 5B reentries have accounted for three of the five largest objects making uncontrolled reentries. The reentries of the U.S.’s Skylab in 1979 and the Soviet Union’s Salyut 7 in 1991, at roughly 77 tons and 40 tons respectively, are the only higher mass events.

Article updates at 9.54 a.m. Eastern, Nov. 3 to update Aerospace Corporation prediction.

Andrew Jones covers China's space industry for SpaceNews. Andrew has previously lived in China and reported from major space conferences there. Based in Helsinki, Finland, he has written for National Geographic, New Scientist, Smithsonian Magazine, Sky...