[meteorite-list] 1 in 3200 odds of human impact (help)

Sun Sep 25 02:38:53 EDT 2011

Dear Doug, List,

The effective cross section of a human varies
widely with the path of the debris, its angle
relative to the Earth's surface.. It's only 2.5
sq. ft. if the debris is falling vertically. If it's
moving at a shallow angle to the horizon, the
human target is 5-6 feet high and 2 ft. wide
or about 10 sq. ft. at very shallow angles,
7 sq. ft. at 45 degrees, etc.

As far as stacking people up, you can only
stack them up on land, not by using the entire
surface of the planet. (Sea stacking sucks,
er, sinks.) The total land area of the Earth is
1,603,176,817,500,000 square feet. Give
everybody four square feet, and you only
have room for 400 trillion friendly folks.
Or with a 2.5 sq. ft. cross section, 642 trillion
friendly folks. (With so little space apiece,
they have to be friendly...)

Using the 4 sq. ft. allowance and a mere 6
billion people, only one in 66,667 "people
patches" is occupied, so the odds of a person
patch being the occupied one is 66,666-to-1.
Using an 8 sq. ft. patch, gives a result close to
32,000-to-1, ten times the NASA estimate and
1/10th your estimate.

Actual statistics is something else. What are
the sizes of the debris? Only if the debris is
much smaller than the people-patch-size are
the odds so calculated valid. If the debris is much
larger than one or two people patches, you
need to know the likelihood of people clustering
together. What if you set out one person every
66,666 patches and they all walk in to the
center of the general area to chat with each
other?

Humans cluster strongly -- cities are the result.
There was, it happens, a lot more Siberia for the
Tunguska Object to hit than there were Londons.
Things falling in Mexico City (the most populous
on Earth) have a greater chance of hitting people
than things falling in say, Montana, but there's
a lot more of Montana. We need to include a
"clustering coefficient" in the calculation.

Fortunately, I'm not up to it...

Sterling K. Webb
--------------------------------------------------------------------------------
----- Original Message ----- 
From: "MexicoDoug" <mexicodoug at aim.com>
To: <Meteorite-list at meteoritecentral.com>
Sent: Saturday, September 24, 2011 11:31 PM
Subject: [meteorite-list] 1 in 3200 odds of human impact (help)

> Hi listers
>
> I'm very suspicious of this widely quoted 1 in 3200 that is being 
> passed off as a scientific number by NASA.
>
> Not 1:3000, nor between 1:1000 to 1:10,000: but 1:3200.
>
> This foolishly precise assertation, which if you've read "The Little 
> Prince" you immediately suspect it is overstated due to the author's 
> calculations 70 years ago there...where a similar calculation is 
> actually done ...
>
> Average cross sectional area of a person? (Depends if it is in the 
> morning when everyone is praying, I guess, or in the afternoon when 
> everyone is running out of work)...let's say:
>
> Cross section per person:18 inches by 18 inches (1.5 x 1.5 sq. feet)
> World population: 6.964 X 10^9 living souls
> World Area: 196,939,900 sq miles
>
> Calculations:
>
> * Cross section per person = 2.5 sq. feet
>
> * current world population occupies 624.3 square miles
> (a wee bit bigger than Guam, and smaller than Singapore)
>
> * people that could fit on Earth's surface: 2,196,000,000,000,000 (2.2 
> million X 10^9)
>
> * Fraction of Earth's surface that's "people"  = 6.96 / (2,196,000)  = 
> 0.00000317
> = People occupy *ONLY* 3.2 parts per million (3.2 ppm) of the earth's 
> surface
>
> So, saving rounding till the end, each piece of UARS actually has a 
> 1/315,457 chance of falling on people (1/0.00000317).
> In rounded numbers, that's about 1:320,000 per fragment ==> 26 
> fragments approximately 1:12,000 chance.
>
> I guess if you are American you need more space than if you are 
> Indonesian, and changing it to a 18 inches X 17 inches would change 
> the result by 6% ie, if 3200 were right for 18X18 it would now be 
> about 1:3000, and that is one of so many assumptions making the 3200 
> number a total joke of fake scientific confidence.  If you gave 
> everyone a square yard ((91.4 cm)^2) instead, it would be in the 3000 
> range.
>
> But here are the defficiencies I think of looking at it this way:
>
> * this looks at the whole world vs. the limited satellite trace.  A 
> true measurement would do a little calculus along the path considering 
> the population density and the probability of earlier or later entry 
> which could change probabilities by an order of magnitude easily.
>
> * I think what I did would work for 26 darts, but not hunks of 
> significant size compared to a person's area unit.
>
> * Finally there is the Sylacauga effect for bouncing material that 
> will affect things another factor of 2, 3, 4 who knows...
>
> There must be a half dozen other complicating factors to do this 
> right. Does anyone know what has been considered to arrive at the 
> bogusly precise 3200-1 odds being fed to us?
>
> Love to hear any improvements on the above model (if you can call it a 
> model) which I got the 1:12,000 as a streaming (unverified) starting 
> point ...
>
> Kindest wishes
> Doug
>
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