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China has launched another Long March 5B rocket that seems liable to fall anywhere

Although the overall risk of harm to people is low—there is only a 0.5 percent chance of injury or death to a human, based on one model—these risks are nonetheless higher than accepted by most spacefaring nations, said Ted Muelhaupt, a reentry and debris expert at The Aerospace Corporation.

In an industry that can tell you things with relatively high precision, this seems odd. Why don't we know where this will fall? Is it related to how the Chinese rockets work?

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    $\begingroup$ My impression is that we don't know exactly where most first stages will end up, it's just that when you launch out of Cape Canaveral or Baikonur, "somewhere off the coast" or "somewhere in the tundra" is considered acceptable precision for the purpose. $\endgroup$
    – Cadence
    Nov 3, 2022 at 19:01
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    $\begingroup$ @Cadence Most of those stages do not go into orbit. This one did, and has been in orbit for several days. $\endgroup$ Nov 3, 2022 at 20:34
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    $\begingroup$ @DavidHammen I see, I missed that this discussion was about an orbital stage. $\endgroup$
    – Cadence
    Nov 3, 2022 at 21:00
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    $\begingroup$ Great answers, but I feel they miss an important part of the question (in the last sentence): is this specific to the way the rocket was launched or its design (and they could have done something different to better control when and thus approximately where it falls), or is it a generic issue with all launches (or a large number of them)? In other words, can we blame the Chinese as some imply, or is this a more general issue? $\endgroup$
    – jcaron
    Nov 4, 2022 at 14:50
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    $\begingroup$ @jcaron I should note that the Chinese state-run media states (incorrectly) that these rockets burn up entirely on re-entry. They're literally a "fire and forget" space program, in this sense. My question about it being related had more to do with any intentional design decisions that made it unpredictable. Apparently that's not the case. The rocket is just floating around up there, waiting on its inevitable fiery demise. The unpredictability is due to the sun, apparently. $\endgroup$
    – Machavity
    Nov 4, 2022 at 15:50

2 Answers 2

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If the orbital period is about 90 minutes, that means ±45 minutes error at predicting the moment of landing means randomizing that point all around the globe. At the moment the prediction error is ± 10 hours.

The rocket is skirting the upper atmosphere experiencing very low drag - but exactly how low is extremely variable, depending a lot on solar weather, Earth weather below its trajectory, how it orients itself in space (quite randomly) and as such, it randomizes the time until entry quite thoroughly. One can estimate the time using models that assume the atmosphere and drag change in a deterministic way, but they won't. Weather is not deterministic. Solar activity is unpredictable. And small changes in these - shifting the prediction by, like, 15 minutes - shift the impact point by half of Earth away.

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    $\begingroup$ So the Chinese are almost literally playing roulette with people's lives? $\endgroup$ Nov 6, 2022 at 1:45
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    $\begingroup$ Yes, they are. Thankfully, this concern applies only to the Long March 5B, which only launches very infrequently. The only manifested payloads for it are the space station modules (which is now complete, at least for the initial configuration) and the space telescope (launch NET December 2023). There is an option to extend the station from 3 to 6 modules, but is not clear whether this will happen, and if it happens whether the Long March 5B might have gained control capability. Other rockets in the family do have either relight capability or at least grid fins for control during reentry. $\endgroup$ Nov 6, 2022 at 5:28
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Why don't we know where this will fall?

Because it went into orbit and it did not have any propellant remaining to perform a targeted reentry.

The launch vehicle core stage has been in orbit for several days as of now (launched 31 October, expected reentry 4 November). That's more than enough to add a good deal of uncertainty regarding where it will reenter. All it takes is one little poof from the Sun (the Sun is no longer inactive, BTW) to make the upper atmosphere expand unpredictably. The current uncertainty is ±10 hours, which means anywhere along the orbital track, but with a peak probability in the western Indian Ocean.

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    $\begingroup$ Small nitpick, the rocket can't quite fall anywhere, it's limited to the latitudes covered by its orbital inclination. Debris decaying from an equatorial orbit can only land on the equator, while debris decaying from a polar orbit could indeed land just about anywhere. This particular rocket has an inclination of around 40 degrees, so we know that it will not land in Greenland, for example. $\endgroup$ Nov 4, 2022 at 13:46
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    $\begingroup$ @NuclearHoagie You are right; it cannot land in Greenland. I changed "anywhere" to "anywhere along the orbital track". $\endgroup$ Nov 4, 2022 at 13:48
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    $\begingroup$ Not quite "no propellent" Source (arstechnica.com/science/2022/11/…) reads: "Because this core stage lacks the capability to relight its engines". Too bad it doesn't have the capacity that we built into Saturns long ago; all it has to do is point and vent the leftover propellant through the engine bell; this provides quite a bit of Delta V on an otherwise empty stage. $\endgroup$
    – Joshua
    Nov 5, 2022 at 4:50
  • $\begingroup$ Saying "along its orbital track" sounds more specific than it actually is, since changes in drag at different latitudes may affect the rocket's surface-relative path differently. $\endgroup$
    – supercat
    Nov 6, 2022 at 5:09

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