Chris Peterson wrote:
In order for a much smaller object to persist to low height would require a very unusual combination of conditions.
This was 20 years ago, this particular meteorite; Maryville,TN 1983, I observed coming in around 4 am in the morning so it had a high relative speed; a combination of Earth's velocity around the sun (15kps?)plus what ever approach speed the meteoroid had. The recoverd stone was very smooth and egg/bun shaped, with small area of secondary fussion crust confirming the sliver of surface ejected in the bolide terminus. Dr Larry Johnson at the University of Tennessee did the analysis and we had a good chat about our different prespectives. I thought this extra high velocity might account for how the stone got so low before going "bolide". I immediately pulled over the car and got out standing in awe. When I did the quick calculations I remembered that I had my personal M2 compass with me which allowed a very accurate measurement of vertical angles--I unfortunately forgot that I had a camera somewhere buried in the car and failed to get a photograph of the cloud (sigh). I remember standing outside the car and looking up at a column of smoke going up what I know now to be 100-130-? miles high.
I forget my data but I think I remember the results being that the bolide itself was 400-800 meters wide, 3-4 times wider than the smoke trail, about 3.6 to 4 miles(approx. 5.5-6 km) altitude (I believe this was corrected to sea level but the ground level was between 800-1100 ft and insignificant). I was between 5-7 miles away , I'd have to consult a map again. My estimate was pretty good as I recall, some 10 years afterwards when I found the exact location of the recovery I checked it. The meteorite itself was about 2 kg. It had to lose alot of mass coming down given the smoke trail it left.
Chris Peterson also asked:
Can you describe the artillery observer methods that you used to estimate the low height for the fireball you saw?
This is sort of dry technical but I'll try to lay out the procedure. This outlines a field expedient method of locating an event which has both visible and aural components --aka flash and bang. At that time I was a tactics instructor at US Army Infantry School and an Expert Infantryman Badge holder-- there are some things so impressed by training they are second nature such as starting to count automatically. This calculation relies on the "mil relation formula". A circle can be divided into 6400 mils . There are 17.78 mils per degree. A distance between two points (diameter, height above ground, etc ) can be measured/estimated if one knows a distance and uses mil relation formula. The mil relation formula states that a 1 mil arc will subtend 1 meter at 1000 meters or in other words at 1 kilometer, 1 mil is 1 meter wide-- at 2 kilometers it is 2 meters wide , at 10,000 meters or about 6.2 miles, 1 mil is 100 meters wide and so forth.
To estimate a distance or "range", when one sees a flash -- of lightening or in this case a bolide, immediately start counting seconds and/ or glancing at a watch to get the most accurate transit time --called flash-to-bang time. Sound moves approx 350 meters per second. (330 meters/sec is also used by other than Military units) Multiply the elasped seconds by 350 / 330 or 340 as a compromise, as you prefer, to get the distance in meters.
Mils are most accurately measured with an M2 compass or Aiming Circle (thedolite), however without one, there are techniques to estimate the mil angle (width). At arms length the index finger is 30 mils wide, the first two fingers closed 70, three 100, four 125 five closed 180, five spread 300 thumb and index in an "L" Index finger is 200 high thumb is 125 wide. A lot of practice will let you know what your specific widths if you are extra skinny or like me have large hands (ahem). So without getting a compass out one can make a quick estimate of size, height, etc. Fifteen mils is about the closest accuracy one can achieve with fingers but you can use a pen/pencil, coin edge etc to get smaller measurements. This is a general and accepted standard with its inherent inaccuracies. You know the old saying " close only counts in horseshoes , hand grenades, and artillery"
If one wants to be prepared for fireball observations they can also sit in the driver's seat and sight the elevation and azimuth angles of top, bottom, sides of their windshield in halves, quarters, thirds etc --as they say "It is another technique".
Elton ______________________________________________ Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
