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(meteorobs) Re: flaming meteor (1972 grazer)





Quoting Tom Fleming <enders_gt1@prodigydot net>:

> 
>     Here is a clarification regarding the comments on the grazer over =
> the Tetons.
> 
> The best estimate =
> summary for the object's mass and size were 4000 English tons and a =
> diameter of about 13m (the length of a tractor trailer but 'spherical'). 

Hi Tom,

According to Ceplecha (Astron. Astroph. 283 (1994), 287-288)the oft quoted 
masses for the 1972 grazer are grossly overestimated. From his dynamical 
studies on the fireball he finds an initial mass in the order of 40 metric tons 
(much lower than the earlier estimates), of which less than one third had 
survived at the 53 km perigee. A 40 metric tons mass equals a body about 3 to 5 
meters large, depending on which density you take.
According to the study by Hills and Goda (Astron. J. 105 (1993), 1114-1144), 
such a mass with the entry velocity of 15 km/s actually is in the regime where 
(assuming it is stony) it would severaly fragment due to dynamical stress when 
descending lower in the atmosphere. Depending a bit on wether it was a soft 
stone (carbonaceous, as Ceplecha finds most likely from the dynamic behaviour), 
or hard stone (ordinary chondrite/achondrite), the largest surviving fragments 
that reach earth surface would be only 0.002-0.003 kg (soft stone) to 0.5 kg 
(hard stone), so actually very modest compared to the typical meteorite from 
smaller original body sizes! At these sizes, they will quickly decelerate and 
hit earth surface with the typical subsonic speeds for a normal meteorite, 
hence no craters but relatively small impact pits at best. Yet, there would be 
many of these, so it would result in a spectacular shower and strewnfield 
formation of relatively small meteorite fragments - but litterally thousands of 
them. Something like the famous Pultusk, Holbrook or Homestead meteorite 
showers.

Hope that answers your question. Lew: sorry for the off topic character 
(meteorites again) but since the question was asked and some answers given 
already on the list, thought I might better answer it on the list too. The 1972 
grazer is (rightfully) one of those legends in meteor history about which 
however many misunderstandings seem to exist. Also goes to show that relatively 
large masses need not drop large meteorites.

- Marco Langbroek
  Dutch Meteor Society (DMS) - meteorite section
  http://home.wanadoodot nl/marco.langbroek/dutchmet.html



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