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It was commented that there are currently 22 SpaceX upper stages in orbit that will eventually reenter in an uncontrolled fashion. PcMan pointed that out when I asked Why the Long March 5B wasn't deorbited intentionally, and it's a good point. If you add up all those stages, that's more mass than the 5B, I'd guess. I didn't check for the other GTO launch stages also in decaying orbits.

I'm leaving out discussion of how the difficulty of deorbiting many upper stages operating at their limits in launches to geostationary transfer orbit, compares to deorbiting 1 core stage that was pushed to the limit to put one thing in a low orbit. I'll likely ask about the difficulty of deorbiting later.

I'm just looking to understand how the amount of debris compares. The Falcon 9 upper stages will come in with more speed and are far smaller objects. How does the amount of debris that will result from the 5B reentering compare to the debris of one of those stages?

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It's very difficult to predict what may survive reentry, but I found this on Mir (a controlled deorbit):

Mir's large mass, over 143 tons (including the docked Progress M1-5 spacecraft) [...] A sizable[sic] portion of Mir, perhaps 20 tons or more, is expected to survive reentry (emphasis mine).

from The Aerospace Corporation, it was linked in the Deorbit of Mir Wikipedia page, reference 13.

One seventh or ~14% (~3000 kg for the ~20,000 kg booster) might be as good a guess as there is, though it is probably higher for rocket stages than space stations because of engines with higher heat tolerances than space station modules.

I think GTO stages don't necessarily enter with higher speeds because the drag is strongest at periapsis and thus lowers the apoapsis over repeated orbits until basically comparable to a low earth orbit reentry as seen in the LM-5 core stage:

And more dramatically in this other debris in a higher orbit:

High Earth Orbit decay

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  • $\begingroup$ Surface to volume means that larger objects are much more likely to survive, so assuming a constant ratio of surviving debris to reentering mass is wrong. $\endgroup$
    – user21103
    Commented May 5, 2021 at 15:38
  • $\begingroup$ On the contrary. It provides an upper bound. $\endgroup$
    – SE - stop firing the good guys
    Commented May 5, 2021 at 15:49
  • $\begingroup$ @tfb What about material effects? I would think engine plumbing hardware (steel, other high temp. alloys) is more likely to survive than a space station module (aluminum). $\endgroup$
    – BrendanLuke15
    Commented May 5, 2021 at 16:18
  • $\begingroup$ @SE-stopfiringthegoodguys: only, perhaps, for some fixed configuration which you scale. The Apollo CSM survived reentry with something over 80% of its mass and it was a lot less massive than Mir. So there's an upper bound which is even more useless... $\endgroup$
    – user21103
    Commented May 6, 2021 at 10:04
  • $\begingroup$ @BrendanLuke15 Yes, exactly. I think that you can neither scale things nor generalise reliably (which you say in the answer!) $\endgroup$
    – user21103
    Commented May 6, 2021 at 10:06
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In like for like orbit conditions materials definitely play a big role. Composite Overwrapped pressurant tanks are understood to be more likely to survive.

Looking at the LM 5B vs Falcon 9 upper stage and using this line of reasoning obviously depends on what such composite over-wraps are used for, e.g. if the main fuel/ox tanks are self-pressurising then there is less reason to assume that the 5B would have bigger/more tanks.

If the composites aren't the main issue I presume the next big/dense items would be the engine turbines.

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