Coloma, CA 22 Apr 2012 1452 UTC

Two words every meteorite hunter or hobbyist loves to hear – “daytime fireball”! This one was powerful. Bill Cooke at NASA MSFC says it produced energy equivalent to 3.8 kT of TNT, and it threw sonic booms over a very wide area.

It also appears on three out of four nearby radars! Even better.

This event appears and disappears quickly. The three different radars catch it at different altitudes, from about 20,000′ AGL down to about 10,000′ AGL, but only traces are seen in other sweeps if at all. Contrast that against the West, TX (“Ash Creek”) fall that persisted in radar data for about 10 minutes. What that tells me is that this event produced a number (dozen? dozens?) of larger (kg-mass to 100′s of grams) meteorites but not a lot of smaller material. I estimate the mass given the ~3 minute interval between sonic boom reports and appearance of the meteorites on radar. It takes about 2 minutes for a multi-kg meteorite to fall from 25 km altitude.

The locations of these meteorites should be directly under the radar signatures.

Only two out of the three radar signatures exported properly to Google Earth. I’ll try again with the third one later, but here’s the first two. DOWNLOAD

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UPDATE: 26 Apr 2012

I’ve got two items in this update – a strewn field estimate, and an estimate of total recoverable meteorite mass. The mass estimate is something I’ve been working on for a while, but first here’s the strewn field.

Strewn Field
At the time of the fall, winds were as high as 23 m/s (~50 mph) at 11 km above ground level (AGL). Luckily, the best radar data occurs when the meteorites are only 4.8 km AGL where winds were calm. Dark flight model results show negligible lateral drift from 4.8 km altitude. This makes calculating the strewn field pretty easy – the meteorites landed directly under where they appear on radar.

FIGURE: Winds over the fall site. This data is from a weather balloon launched from Reno, NV about three hours before the meteorite fall, and so is a good estimate of the conditions at the fall. We see that winds peaked at about 23 m/s (orange line) coming out of an azimuth about 240 degrees, or WSW. Falling meteorites would have been pushed towards the east as they fell, after the fireball burned out and the surviving rocks plummeted to the ground.

Now, a bit of an apology. I’ve been pretty busy this week (too busy to drive out there and search myself, regrettably!) and hurried through the mass estimation in my first pass. You can see that in how short my original post was! I originally said that the fall consists largely of meteorites in the 100s of grams to kgs. After spending some serious time with the radar data, I need to revise that number downward. It appears that I confused an earlier-appearing radar signature with the large one from the KDAX radar. That is where Robert Ward and Petrus Jenniskens found the first pieces, and they are down in the 10s of grams mass range – that appears to be the correct value for that area of the strewn field. With that said, however, larger masses should appear from this fall. Here’s the breakdown of radar data, to include a new one:

FIGURE: KDAX First Appearance: Appears at 1452.10 UTC. Assuming the fireball occurred very close to 1452 UTC, this one registers the passage of a meteorite or meteorites in the 10s of kgs range, and approximately 0.25m in size. It may not look like much – its the single green stripe in there – but this radar signature appears at a whopping 14.1 km above the ground and appears so quickly after the fireball that it must be a large object or objects. I have included a first-pass, very rough estimate of the potential fall area for this meteorite (or meteorites). This estimate could use some refinement by people with a better sense of the meteorites’ direction and velocity at this point (…Rob Matson? You readin’ this?). I also lack a good measurement of the altitude of the fireball at terminus.

KRGX 1450 UTC 1.5 degree sweep: Appears approximately 220 s after the fireball and should represent meteorites in the 100s of grams mass range. This one is shown in the image at the very top of this page.

KDAX 1450 3.5 degree sweep: Appears approximately 420 s after the fireball (~7 minutes) and should represent meteorites in the 10s of grams range. This is where meteorites have been found so far. This is the second picture from the top on this page.

FIGURE: My first draft at the strewn field location, as drawn from close analysis of the radar data. The green outline shows the highest probability of finding meteorites, with meteorite size increasing towards the southwest. The yellow outline is my projection of the extended strewn field – where I believe meteorites should have fallen, but there aren’t direct radar measurements showing them in these locations. The white line is my current estimate of the fireball direction, which is a very important factor in calculating the potential fall location of the very large meteorite(s) seen in the “first appearance” (orange rectangle). That orange rectangle will certainly move around with an improved picture of the direction, altitude, and declination of the fireball.
Winds were out of the west, and so they pushed the meteorites towards the east as they fell. Since smaller masses take longer to fall, winds blew those meteorites further to the east. As a result the strewn field tends to curve towards the east at the northeast corner of the strewn field where the smaller meteorites fell. …and why are the smaller meteorites up in that corner? Because more massive meteorites tend to be carried further downrange (towards the SW in this instance) by their greater momentum.

What it means: The “big end” (where the bigger meteorites are found) of this strewn field is towards the southwest from the first find locations.

Mass Estimation
I’m not ready to divulge the details yet, but I’ve been working on a means of estimating the mass of a meteorite fall from its radar reflectivity. It turns out this is not straightforward, but I’m getting the hang of it. I’ve devised an estimate using data from other meteorite falls observed with weather radars. Unfortunately, making this estimate forces me to rely on the total recovered masses of the other falls. As any meteorite hunter knows, there are dozens of factors in recovering meteorites that can hinder meteorite recovery. So what we end up with is a very coarse estimate.
For the Coloma event, radar reflectivity is very strong as compared to other meteorite falls. I could go on about that for quite a bit, but lets skip to the chase – the calculation for this meteorite falls yields an estimated 53.5 kg of recoverable meteorites on the ground. This is a high number! To be honest, however, I don’t trust it to any precision. Don’t believe the number as gospel, but the fact that 53.5 kg popped out of the calculation is evidence that this was a sizable fall and a lot of material remains to be recovered.

None of the above should be considered the last word on this fall, of course. Our collective understanding of this meteorite fall will evolve as people get involved and talk about it, as it should be for any scientific endeavor. It would be helpful to get input from some of the other folks who have been modeling this event. The fall site for the large, first-appearance meteorite would benefit from more attention as well. But for now, to the people out there hunting – I’d suggest moving southwest from the original find site as larger meteorites should lie in that direction.