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(meteorobs) Austrailia fireball

In response to: Bruce McCurdy's message re sonic boom over Western Australia
9 Oct 2003.

Bruce:
    The conception of a meteoroid breaking the sound barrier is a false one
as per your posting to meteorobs.  The popular term "sound barrier" is not
an entirely correct one.  A sonic boom is not the result of the meteoroid
going "through" the so called sound barrier.  When a meteorite enters the
upper atmosphere the velocity can be anywhere from 11.2 km/sec to around 71
km/sec, this range being much more than the speed of sound.  Below the speed
of sound air molecules pass around the object.  At or above the speed of
sound the air molecules can no longer get out of the way but form a shock
front.  Actually there are two shock waves.  One at the leading edge of the
object and one at the trailing edge.  In the case of a small meteoroid only
one sonic boom is heard.  With the space shuttle two distinct booms can be
heard because of the long length of the shuttle.  The boom is heard when the
shock wave reaches the observer.  This happens at the speed of sound and all
speeds above this, NOT because the object has passed through the so called
sound barrier.  What is referred to as a bolide, a meteoroid the explodes
high in the atmosphere, also can generate a shock wave that can sometimes be
heard.  In this case the shock wave is a spherical one unlike the conical
one for the entering meteoroid.  If such a meteoroid is broken into several
pieces multiple sonic booms may be heard as each entering fragment develops
its own shock wave.  Sound phenomenon is complex.  One observer may hear the
boom but a short distance away nothing may be heard.  This has to do with
refraction and the way the sound is propagated.  If the shock wave reaches
you, you will of course hear the boom.

Ed Majden - MIAC Associate
Sandia Bolide Detection Station - Courtenay B.C.
CANADA

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