[meteorite-list] Fireballs with Sonic Booms 101

From: Robert Verish <bolidechaser_at_meteoritecentral.com>
Date: Thu Apr 22 10:28:30 2004
Message-ID: <20031016070919.7905.qmail_at_web60305.mail.yahoo.com>

In my reply to a post regarding a "Fireball with Sonic
Boom in Australia", I commented that the sonic boom
precluded that there were meteorites produced by this
event. My statement was considered to be inaccurate
by listees Jim Gamble and Ed Majden. In their efforts
to educate me on the details of sonic booms they
produced a synopsis on this phenomenon that I thought
would be of interest to others. I would like to share
this information with you now:
Bob V.

----------------- Attached Test ---------------

Basically, a sonic boom is not created by the
transition from subsonic to supersonic,
i.e. the popular term "sound barrier".
There is no barrier in the true sense! During sub
sonic flight the air molecules pass around the object.
When the velocity of sound is achieved the air
molecules can no longer pass around the object but
form a shock wave. This shockwave is present at the
speed of sound and higher velocities. At supersonic
speed a conical shock front is developed. If this
shock front reaches the observer a sonic boom is
heard. This boom is delayed by distance from the
object producing the shock wave. Actually there are
two shock waves. One from the leading edge and one
from the trailing edge. In the case of a meteor, this
is heard as a single bang as the size of the meteoroid
is generally rather small.

The space shuttle at supersonic speed has two distinct
bangs because of the length of the shuttle. Jim Gamble
pointed out that multiple sonic booms were reported
from the October, 1997 El Paso superbolide with no
recoverable debris. In this case the meteoroid broke
apart at high altitude and each surviving piece
created its own shockwave and sonic boom. Stone
meteorites often break apart because of the high
stresses encountered. This could certainly be the
case with the Aussie fireball as well. An iron has a
better chance surviving in tact. Just because one
hears a sonic boom does not necessarily mean that a
meteorite has dropped. It does however point to the
*possibility* of one surviving to reach the ground.

A bolide can explode at altitude often leaving nothing
to survive except perhaps dust. This explosion
generates a spherical shock front which may or may not
be heard. Propogation of sound is complex and can be
refracted away from the observer so he hears nothing.
Sound shockwaves are generally heard from meteors
below 50 km or so but are more likely at around 10 km.
 This has nothing to do with the "dark flight"
portion of the meteors flight path where ablation is
no longer taking place. Indeed, the meteor may not be
visible to the observer as the sound is delayed by
minutes and the meteor is long gone. An observer
farther down the flight path may still see the meteor.
 Sound travels in mysterious ways. One person may
hear the sonic boom from his vantage point but a short
distance down range it may not be heard.
Dr. Jeremy Tatum from MIAC pointed out that refraction
is the consideration here. Some time ago I asked
Dr. Jiri Borovicka about the ablation process of a
meteoroid entering the Earth's atmosphere. He said
that the average velocity where ablation stops is
around 3 km/second. Then the meteoroid enters the so
called "dark flight" portion of its penitration. This
can be as high as ~5 km/sec (which is still
supersonic) and this can occur from about 30 km down
to zero altitude depending on the size of the
meteoroid. The lowest value for a photographically
recorded fireball was 13 km.

The velocity of a meteorite at impact is proportional
to mass. For small meteorites (the usual case) this
is around 100 meters/second at impact. Giant
meteorites don't loose much of their cosmic velocity
and generally vaporize at impact forming a large
impact crater. A prime example is the Arizona crater.

Ed Majden - MIAC Associate - EMO Sandia Bolide
Detection Station, Courtenay, B.C. Canada.

Jim Gamble - El Paso, Tx Station-Sandia Meteor
Detection Network


References:

J. B. Tatum. Fireballs: Interpretation of Airblast
Data
       Meteoritics and Planetary Science 34,
571,(1999)

J.B. Tatum, L.C. Parker and L.L. Stumpf. Sound from a
Fireball - Distinguishing between the Hypersonic Shock
Front and Terminal Burst.
         Planetary & Space Science, 48, 921, (2000)






__________________________________
Do you Yahoo!?
Yahoo! SiteBuilder - Free, easy-to-use web site design software
http://sitebuilder.yahoo.com
Received on Thu 16 Oct 2003 03:09:19 AM PDT