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SpaceX enjoys press.

So the unexpected events popping up in the news is good for them.

March 26, 2021:

October 2, 2021:

Questions:

  1. Do reentering capsules always make a(n Earth-shattering ka-)boom...
  2. ...and do they always look like a fireball if reentering at night?
  3. Can the time and epicenter be predicted with publicly available information?

For #3, based on ISS TLEs, various ocean and air traffic warnings and other published data, could a reasonable prediction of these events be made so that folks could look and/or listen for it?

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    $\begingroup$ Is question 2 a serious question? $\endgroup$
    – Organic Marble
    Oct 2, 2021 at 13:15
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    $\begingroup$ @OrganicMarble yes it is a serious question. They are certainly bright locally (e.g. looking out the capsule window) but I don't know if they always appear so bright that they would appear "fireball-like" to an observer on the ground. I simply don't know enough about reentries and heat shields and light production to be certain that all corners of the envelope are "fireball-bright" when seen from Earth. I'm coming up on 2.5 kiloquestions here in Space SE, is there any evidence that I've asked a non-serious question? $\endgroup$
    – uhoh
    Oct 2, 2021 at 14:41
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    $\begingroup$ Well, there is now. $\endgroup$
    – Organic Marble
    Oct 2, 2021 at 14:53
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    $\begingroup$ @uhoh - I think there is a very real question in point two but not sure on the current wording, I'd sorta of expect heat shield design to play a role here. Ablative heatshields/complete burnups would produce a different trail of hot solid particles than a space shuttle style re-entry that in theory is just trailing hot atmosphere. So a spaceX return may be more visible than space shuttle was. Unsure how to word that for a stackX question though since both types would make a 'fireball' and look similar if people use different zooms looking for pleasing composition. $\endgroup$
    – GremlinWranger
    Oct 2, 2021 at 23:40
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    $\begingroup$ @uhoh - some image and video searches seems to suggest that both space shuttle and crew dragon look similar - steadily moving stable bright dot leaving a long trail covering most of the sky, only the second stage re-entries produce what I'd call a 'fireball' with multiple bright spots, flickering and things clearly 'not right'. So possible answer here is that 'neither crew dragon nor space shuttle (when operating correctly) make a fireball'. Very much sematics, though splitting second stage burnup from re-entry may be useful. $\endgroup$
    – GremlinWranger
    Oct 3, 2021 at 0:19

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Partial answer addressing Q3 for SpaceX Dragon2 missions. Extensive leverage of my answer to Dragon re-entry flight profile? & my answer to What is the heat shield refurbishment procedure for a crew Dragon capsule?.

There are 7 normal splashdown sites all located near northern Florida for crewed flights:

splashdown sights

They are not appreciably far enough apart to warrant distinction when considering the relatively low accuracy of my "armchair" modeling.

Given the 51.6° inclination of the ISS there are 2 possible tracks that a returning Crew Dragon could take, north(L) and south(R):

north track south track

(Personal work)

The track of choice for a given splashdown time can be inferred by using ISS TLEs to generate a "full orbit" ground track plot at the splashdown epoch (no SGP4 needed, mean elements suffice!). Then simply see which way the ground track crosses northern Florida:

CRS-23 ISS "ground track" (Personal work, CRS-23 example)

I know the Demo-2 mission used the southern track, and some crude TLE work (like above) shows both Crew-1 & Crew-2 used the southern track, while CRS-23 (the October 2nd example in the question) used the northern track as shown above. Note how even 5 days out it is easy to see which track is going to be used.

I think it's safe to assume that the higher the instantaneous reentry heating, the more luminous the capsule will appear in the sky (at the very least it's a good approximation). Here is the entry trajectory where the heating rate is expressed as a colour using MATLAB's "hot" colormap (lighter = hotter, linear scale):

north hot South Hot

(Personal work, heating estimated from here)

This looks promising considering the news reports from Alabama & Georgia given in the question and the peak brightness occurring over those states. Here is a plot of heating VS altitude, the higher the capsule is the farther away it should be visible from:

heat vs alt (Personal work)

The time from entry interface until parachute deploy is about 12 minutes and estimated splashdown times are usually announced.

SpaceX also does west coast splashdowns of some CRS missions near Los Angeles (iirc):

west north west south

(Personal work)

People fortunately located near these tracks should be able to reliably predict and see SpaceX capsule reentries.

I have created KML files (Google Earth Pro, it's free!) for the 4 trajectories, found on Github. This is powerful for observation planning. For example, here is the view from the US Space and Rocket Center in Huntsville, AL for a Florida north track trajectory (moves right to left):

USSRC view (Personal work)

This is also near peak heating so it would make for a very cool photograph/video (sadly no longer in HSV or else I would try this!)

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    $\begingroup$ Future work: add heating color representation to KML files $\endgroup$
    – BrendanLuke15
    Feb 17, 2022 at 1:54
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    $\begingroup$ This is of course wonderful and I understand it draws from other work, where does the actual trajectory (altitude and speed vs time) come from in your simulation? Is it the result of tracking, or some optimization, or something else? $\endgroup$
    – uhoh
    Feb 17, 2022 at 21:40
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    $\begingroup$ @uhoh I've simulated (low fidelity, 3D) the entry trajectory using available/inferred specs of the capsule. I guess determination of the final orbit (thus initial conditions for entry) could be called "tracking" though I wouldn't be that bold (I just watched and listened to the Demo-2 splashdown livestream for altitude call outs b4 entry). There is some "optimization" of the argument of periapsis and RAAN of final orbit to make it land near the right spot on the Earth's surface, but nothing too serious. $\endgroup$
    – BrendanLuke15
    Feb 17, 2022 at 23:54
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    $\begingroup$ Reentry trajectories have a lot more flexibility when it's in a computer and can't burn up. I'm simply curious if you tried to match a real reentry trajectory which would be optimized for minimum burnupedness, or just dove into the atmosphere in order to hit the ground. For example, I took great delight in roasting these guys but in this scenario there wasn't an other possible outcome. $\endgroup$
    – uhoh
    Feb 17, 2022 at 23:59
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    $\begingroup$ @uhoh Yes, not fitted to minimum heat, but certain parameters fitted to known values. The only "free variables" in my sim are the nose radius and $L/D$. See here ($L/D$ -> maxG) & here ($R_n$ -> max temp). $\endgroup$
    – BrendanLuke15
    Feb 18, 2022 at 0:52

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