Possible Kenyan Meteorite Fall

[David C]

There is a pic on the metlist today of one of the stones. It's awesome!!!!

[Kieron]

Hi Folks,

I notice that one of the Kenyan stones shows distinct signs of orientation/flow lines. This leads me to pose the following question. Can anyone tell me at what height a meteorite would have to fragment in order that individual pieces would have time to become oriented like this?


Regards, Kieron

[David Entwistle]

Can anyone tell me at what height a meteorite would have to fragment in order that individual pieces would have time to become oriented like this?

Hi Kieron,

It's an amazing looking thing. I've added a link to Dirk Ross's blog article for anyone who hasn't seen it.

http://lunarmeteoritehunters.blogspot.c ... orite.html

I did play around with modelling the equations provided by Ernst J. Öpik in Chapter 4 of Physics of Meteor flight in the Atmosphere. I'm not too sure how far I got and I've never had the opportunity to check the results. There are a lot of assumptions to simplify the equations and there are plenty of input parameters which include the following (with assumptions used):

start mass
density 3000 kg/m^3 for dense rock
shapeParameter (assume a compact sphere)
contourParameter (sphere)
coefficientOfDrag = 1.0;
heatTransferCoefficient = 0.01
latentHeatOfVapourization = 1.0e6; // J/kg
altitude (degrees from horizontal)

Based on my implementation of that model and a modest sized stony object, deceleration peaks at 60 - 70 km and ablation is greatest between 60 and 40 km and have tailed off by 35 km height above ground. That suggests, for the oriented individual, fragmentation occurred at more than 50km and probably above 60km.

I'll try and run through my calculations again and add some charts.

[Kieron]

Good info, David. Thanks very much.


Regards, Kieron

[David Entwistle]

Hi Kieron,

I got the height of maximum deceleration wrong in my previous post.

Im not sure this is a correct representation of Öpik's equations, but it makes some sense. The following is based on a 20Kg stone (about 20 cm across at the start) entering the Earth's atmosphere at 30 degrees from horizontal and with velocities of 12, 16 and 20 km/s.

Remaining as a single body, only a fraction of the slower body survives to the ground - about 0.5 kg. The bodies with greater initial velocities are fully ablated, but see below.

Mass remaining (kg)

The quicker the entry velocity the less time there is to loose mass early in the meteoric flight.

Velocity remaining (km/s)

As a consequence, the deceleration is more dramatic for the quicker entries and almost certainly the rock would not survive as one piece. There'd be fragmentation and each fragment would loose part of its cosmic velocity. Fragmentation would start at 55 km, for a relatively robust object entering at 20km/s, and higher for quicker and less robust objects.

Deceleration (g)

Looking at my calculations, I see that I haven't applied the "jet correction factor" (see Opik equation 4-68) to the deceleration calculation. As I say, I'm not sure the rest is correct, but if anyone fancies working on this, I'd be happy to pass on what I've done. It's written in a handful of Java objects. I have a copy of G. W. Wetherill and D. O. ReVelle's Which fireballs are meteorites? A study of the Prairie Network photographic meteor data, which would be a good place to start looking to validate the model.

[Matt Smith]

Impressive work David, introduced me to a side of meteoritics I'm largely ignorant of. Time for me to do some more reading I think