Jay Lake (jaylake) wrote,
Jay Lake

[writing|science] Geeking out over asteroid strikes

One of the things I love about writing is research. One of the things I love about research is interviewing experts. An acquaintance (and fan) is an asteroid geologist and runs a meteorite laboratory. I asked her about the kinetic strike on Seattle that occurs in my METAtropolis: Green Space novella, "Rock of Ages". Here is part of what she sent me, redacted somewhat for clarity and confidentiality:

400 tons seems low. I've had to search a bit for reputable estimates of the Chelyabinsk meteor, but numbers around 17-18 meters in diameter seem good (slightly under 60 feet across). People are assuming 3.6 g/cm^3 for density, which is the density of the individual stones, but might be too high for the rock overall (asteroidal densities are much lower, as they appear in many cases to be rubble piles).

But if you take the volume of a sphere (4/3 Pi R^3) and plug in 9 meters for the radius, and then multiply by the density, you get a mass of approximately (I did rounding) 12,000 short tons. The reputable sources that gave me the diameter, calculated 11,000 tons, so I'm in the ballpark. I ran Jay Melosh's simulator for Chelyabinsk, assuming it stayed as one piece on the way down (which it didn't). I assumed a 20 degree angle and 11 km/sec (it should be 11.2, which is escape velocity if being dropped from orbit). It makes a 466 meter (about 1500 feet across hole, with ejecta spreading at least that far on all sides, releasing 0.15 megatons of energy. And stones are likely to break into pieces on the way down. I suspect 400 tons is a bit small. If you give me an idea of the damage you want (I haven't read past this point, remember), I can try to create a somewhat realistic mass.

H. Jay Melosh and Ross A. Beyer

Results for computing crater size from projectile diameter
Your Inputs:

Projectile Descriptors
Projectile Diameter 18 meters
Projectile Density 3600 kg/m3

Impact Conditions
Impact Velocity 11 km/sec
Impact Angle 20 degrees

Target Descriptors
Target Density 3000 kg/m3
Acceleration of Gravity 9.8 m/sec2
Target Type competent rock or saturated soil

The three scaling laws yield the following transient crater diameters
(note that diameters are measured at the pre-impact surface.
Rim-to-rim diameters are about 1.25 times larger!)
Yield Scaling 2.34 x 102 meters
Pi Scaling (Preferred method!) 2.99 x 102 meters
Gault Scaling 1.43 x 102 meters
Crater Formation Time 3.56 seconds

Using the Pi-scaled transient crater, the final crater is a Simple
crater with a rim-to-rim diameter of 4.66 x 102 meters.

This impactor would strike the target with an energy of 6.65 x 1014
Joules (1.59 x 10-1 MegaTons).

Which she then followed up with:
I can rework numbers making assumptions, such as what you've extracted is mostly metallic material (higher density), and since it has been artificially formed into an airfoil, it isn't spherical, and see where I get to.

There are two other things I want to check - I'm not sure the payload would still be on fire two miles up--my memory is that fireballs go out higher up--I can also check Apollo capsule and shuttle information. I also want to look at the shuttle insertion angle--presumably that is the optimal one for not having a problem. I can look that up as well (I think it is around 35-40 degrees, but not sure of my memory).

Meteorites come in at escape velocity or higher (11.2 km/sec--not meters/sec, but km/sec). Again, I'll have to look up the data on the shuttle. I have the numbers for the Columbia disaster (see below). Their velocity is considerably slower than that of a meteorite. I just need to see if the values are typical. Also, the velocity will depend on how high up in orbit you start. Higher up = faster. Are you in geosychronous? Low Earth Orbit?

Pre De-orbit burn data

Orbit Inclination: 39°

Location (latitude/longitude in degrees): -35.00000 S. / 85.0000 E.

Time: 13:15:00 GMT

Altitude: 929,016 Ft. (175.95 statute miles)

Velocity: 17,496 mph (25,661 Ft./Sec.)

I practically had a nerdgasm over these emails.

Tags: cool, friends, science, writing

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