Time to Watch for a Falling Chinese Rocket Stage Again

This Monday, The China Manned Space Agency (CSMA) launched what's being reported as the third and final module for their Tiangong space station.  Like other modules, this one was launched atop their Long March 5 rocket.  As with the previous launches, the upper stage of the rocket is now in decaying orbit and is going to crash back to earth uncontrolled, with no attempt to steer the rocket into a ocean splashdown in TMFN - The Middle of Nowhere (you can figure out what the F stands for yourself). 

You may well remember the previous incidents. 

In July, between 5.5 tons to 9.9 tons (5 to 9 metric tons) of another Long March 5B crashed into the Indian Ocean after surviving the fall through Earth's atmosphere. Another Long March 5B fell into the Indian Ocean in April 2021 after China's space agency did not perform a controlled deorbit. And in 2020, after the rocket's debut launch, pieces of the vehicle's core stage reportedly hit the ground in Ivory Coast. 

In my opinion Eric Berger at Ars Technica gives a better explanation of the story than that first link to Space.com.

Typically during a launch, a rocket's large first stage will provide the majority of thrust during the first minutes of launch and then drop away before reaching an orbital velocity, falling back into an ocean harmlessly. A smaller second stage then takes over and pushes the rocket's payload into orbit. However, the modified version of the Long March 5B has no upper stage. Rather, it consists of a core stage with four strap-on boosters.

The boosters power the rocket off the pad, but then the core stage, with its two YF-77 main engines, pushes the space station modules to low-Earth orbit. At that point, the core stage lacks the capability to restart its main engines and make a controlled entry into Earth's atmosphere. Typically, rocket upper stages and other used space hardware is disposed of by aiming for the remote Point Nemo, in the Pacific Ocean, but that will not happen in this case.

Not all space hardware needs to be disposed of in such a manner. Vehicles such as the Russian Progress spacecraft are small enough to burn up in the atmosphere. But that's not the case with the core stage of the Long March 5B rocket, which has a mass of more than 20 metric tons. Large pieces of metal will make it to the surface of the Earth.

Even Eric, though, kind of let an important piece of info go unemphasized.  Where he says, "the core stage lacks the capability to restart its main engines..." that's the reason they can't aim for Point Nemo. Why don't they design a system that can restart?  China apparently just doesn't care.  They have, after all, dropped boosters in populated areas of their own country.  

Still, the odds of anyone in particular being affected by a falling rocket upper stage are staggeringly small.  Ted Muelhaupt, a consultant with The Aerospace Corporation's Corporate Chief Engineer’s Office, was quick to point out that "nobody has to alter their lives because of this," but also pointed out that "88% of the world's population is at risk, and so 7 billion people are at risk" from the Chinese space debris falling on them.  The surface of the Earth is mostly water, of which most is ocean water and most of the 7 billion people live in concentrated cities.  Muelhaupt added, "the risk to an individual is six per 10 trillion. That's a really small number."  Small number?  That's about a million times less likely than winning the Lotto.  

Where he gets the "88% of world's population" number is from the amount of the Earth's surface under the Long March 5B's orbit, shown here in blue and yellow.

Aerospace Corporation is saying they'll provide data and refine predictions as newer data becomes available.  Earlier this evening, they tweeted:

Our latest prediction for #CZ5B rocket body reentry is: 04 Nov 2022 23:17 UTC ± 10 hours Reentry will be along one of the ground tracks shown here. It is still too early to determine a meaningful debris footprint.

That's 7:17 PM on Friday - plus or minus 10 hours.  If you don't care to use Twitter, it will be on Aerospace's website.  Expect that time to change before Friday.