Friday, 31 January 2020

California 30 January 12:30 UT: the "space debris" reentry that wasn't

On 30 January 2020 near 12:30 UT (10:30 pm PST), a bright, slow, spectacularly fragmenting fireball swooped over southern California. It was seen and reported by many in the San Diego-Los Angeles area. The video above was obtained by a dedicated fireball all-sky camera operated by Bob Lunsford. The fireball duration approached 20 seconds.

In the hours after the fireball, the American Meteor Society (AMS) initially suggested that this was a Space Debris reentry, i.e. the reentry of something artificial from earth orbit.

But it wasn't.

Immediately upon seeing the video, I had my doubts. Upon a further look at the video, those doubt grew. To me, the evidence pointed to a meteoritic fireball, a slow fragmenting fireball caused by a small chunk of asteroid entering our atmosphere.

A discussion ensued on Twitter, until NASA's Bill Cooke settled the issue with multistation camera triangulation data, which showed that this was an object from an Apollo/Jupiter Family comet type heliocentric orbit with a speed of 15.5 km/s. In other words: my doubts were legitimite. This was not a space debris reentry but indeed a chunk of asteroid or comet.

I've already set out my argumentation about my doubts on Twitter yesterday, but will reitterate them again below for the benefit of the readers of this blog.

My doubts started because while watching the video I felt that the fireball, while slow and of exceptionally long duration, was still a tad too fast in angular velocity in the sky, and too short in duration, for this to be space debris. In the video, it can be seen to move over a considerable part of the sky in just seconds time.

The image below shows two stills from the video 6 seconds apart in time. The fireball passes two stars, alpha Ceti and beta Orionis, that are 35 degrees apart in the sky, and it takes the fireball a time span of about 6 seconds to do this, yielding an apparent angular velocity in the sky of about 5-6 degrees per second. That is an angular velocity that is a factor two too fast for reentering space debris at this sky elevation, as I will show below.

stills from the fireball video, 6 seconds apart, with two stars indicated

Orbital speed of a satellite is determined by orbital altitude. Reentering space debris, at less than 100 km altitude, has a very well defined entry speed of 7.9 km/s. This gives a maximum angular speed in the sky of about 5 degrees/second would it pass right above you in the zenith (and only then): but gives a (much) slower speed (2-3 degrees/second) when the reentry is visible lower in the sky, such as in the fireball video.

To gain some insight in the angular velocity a reentering piece of space debris would have at the elevation of the California fireball, I created an artificial 70 x 110 km reentry orbit over southern California that would pass the same two stars as seen from San Diego.

The map below shows that simulated track, with the object (marked by the green rectangular box) at 70 km altitude and positioned 6 seconds after passing alpha Ceti (marked by the green circle):

Simulated reentry track. click to enlarge
The angular velocity in the sky for a reentering object at this sky elevation suggested by this simulation is barely half that of the fireball. During the 6 seconds it took the fireball to move over 35 degrees of sky passing alpha Ceti and beta Orionis, the simulated reentering object would have moved over only 15 degrees, i.e with an angular velocity of 2.5 degrees/second rather than the 5-6 degrees/second of the fireball.

So this suggested that the fireball was moving at a speed a factor two too high for space debris. This therefore pointed to a meteoritic fireball, not a space debris reentry.

There were other reasons to doubt a reentry too. There were no matching TIP messages on Space-Track, the web-portal of CSpOC, the US military satellite tracking network. A reentering object as bright as the fireball in the video would have to be a large piece of space debris: this bright is clearly not the "nuts and bolts" category but suggests a large object like a satellite or rocket stage. It is unlikely that CSpOC would have missed a reentry of this size.

To be certain I ran a decay prediction on the full CSpOC catalogue with SatEvo myself: no object popped up that was expected to reenter near this date either, based on fresh orbital elements.

The fragmentation in itself, one of the arguments in the AMS' initial but mistaken conclusion of a "space debris reentry", is not unique to space debris reentries. It is also a common occurence with slow, meteorite dropping asteroidal fireballs, especially when they enter on a grazing trajectory. Take the Peekskill meteorite fall from October 1992 for example:

Likewise, while a 20-second meteor is not everyday, it is not a duration that is impossible for a meteor. Such durations (and even longer ones) have been observed before. Such long durations are especially the case with meteors that enter in a grazing way, under a shallow angle.

At the same time, a 20 seconds duration would be unusually short for a satellite or rocket stage reentry. Such reentries are usually visible for minutes, not a few seconds or a few tens of seconds.

So, to summarize:

1) the angular velocity in the sky appeared to be too large for space debris;
2) the fireball duration would be unusually brief for space debris;
3) and there were no obvious reentry candidates.

On the other hand:

a) the angular velocity would match those of slow ~15 km/s meteors;
b) the 20 second duration, while long, is certainly not impossible for a meteor;
c) the fragmentation observed occurs with slow asteroidal origin meteors as well.

Combining all these arguments,  my conclusion was that this was not a space debris reentry, but an asteroidal origin, slow meteoritic fireball. This was vindicated shortly later by the multistation camera results of Bill Cooke and his group, which yielded an unambiguous speed of 15.5 km/s and as a result a heliocentric orbit, showing that this was not space debris but a slow chunk of asteroid or Jupiter Family comet.

In defense of the American Meteor Society (who do great work on fireballs): it is not easy to characterize objects this slow, certainly not from single camera images and visual eyewitness reports. Given the slow character and profuse fragmentation, it is not that strange that the AMS initially (but incorrectly) thought it concerned a space debris reentry. It does go to show that you have to be extremely careful in drawing conclusions about slow moving fireballs: not every very long duration fragmenting fireball is space debris.

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