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The video below Space debris over Puerto Rico (2/7/2022) SAC seems likely to show one or more reentering Starlink satellites from the 40 "Starlost" lost to atmospheric heating due to a geomagnetic storm.

Wouldn't it be great if one could generate alerts to reentering Starlinks so more videos of those pesky astronomer-foiling Starlinks experiences its fiery demise can be recorded?

Question: How might a Starlink reentry alerting script work? Besides scouring TLEs and looking for low altitudes, what might a script look for either within the TLE or from other sources to get an idea that a Starlink is in imminent danger of burning up?

How might an "imminent Starlink reentry in your area" alerting script work? Besides scouring TLEs for very low altitudes and SGP4 propagating below the Karman line, what could it do and what other sources of information are available?

See also

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Well, I can't give a great definitive answer, but I can at least relate my experiences writing my own Starlink reentry analysis script.

There are several challenges that make it difficult to create a script that produces even remotely accurate results:

  1. TLEs are bad for propagation as the spacecraft dives deeper into the atmosphere. The rapid change in drag forces means that a TLE fit to "yesterday's data" will not extrapolate well into the next day. You have to take a TLE state and convert it to a full numerical integration.
  2. Lack of knowledge of Starlink de-orbit maneuvers. Obviously a Starlink's reentry path is heavily influenced by any de-orbit maneuvers to speed the descent, and SpaceX doesn't communicate them. (although that's not an issue for these particular Starlink reentries)
  3. Lack of knowledge of the Starlink attitude as it descends. Its attitude profile will obviously change the drag force it experiences, and it's also unclear at what altitude the spacecraft loses attitude control and begins tumbling. That is a function of the amount of torque the ACS can deliver, which is also unknown (at least publicly).
  4. Difficulty predicting space weather. Variability in space weather makes it difficult to predict drag forces, and therefore reentry times/locations. Often this is boxed with high/low drag predictions, or else some sort of Monte Carlo is applied to give a range of possibilities.

So, with that all being said, I did the following:

  1. I downloaded TLEs from Space-Track, used a high-fidelity force model to fit a numerically integrated state to that TLE
  2. I used that fitted state to propagate forward, under high/nominal/low drag predictions and marked the lat/lon locations of the spacecraft as it passed down from about 95 km to 80 km altitude.
  3. I also did a check if the locations were in local day or night, as an attempt to predict if the reentry would be easily visible.

For my attempts, I had issues with the fact that those Starlinks were being actively de-orbited, and therefore maneuvering, so my predictions didn't fit all that well.

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  • $\begingroup$ I suppose a custom-modified B-star extrapolating SGP4_alt might work as well, something that checks the last week's worth of TLEs and gets a feeling for how fast drag is increasing, then propagates the TLE with a custom-written time-dependent B-star might be an ugly way to address this as well, but of course a real numerical propagator is the better choice. $\endgroup$
    – uhoh
    Feb 16 at 0:21
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    $\begingroup$ @uhoh yes TLEs are a mean element set, and they don't work very well if the mean keeps changing on you. There's really no way around switching to a numerical integration. $\endgroup$ Feb 16 at 16:51
  • $\begingroup$ Yes I am sure you are right. Any thoughts on the following? "*Besides scouring TLEs and looking for low altitudes, what might a script look for either within the TLE or from other sources..." I'm kinda wondering if there are other sites where predictions of reentries are flagged by the experts, a bit like the way SOCRATES flags potential conjunctions. $\endgroup$
    – uhoh
    Feb 16 at 21:58
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    $\begingroup$ Sure, Aerospace Corporation provides their predictions for reentries here: aerospace.org/reentries That's the best source I'm aware of. There are other groups/individuals that post reentry predictions on Twitter too $\endgroup$ Feb 18 at 15:22

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