(meteorobs) Correction re: Mars meteor

David Entwistle david.entwistle at dial.pipex.com
Wed Jun 8 07:57:13 EDT 2005


In message <022101c56b82$253e2f40$0300a8c0 at bcom>, Bruce McCurdy
<bmccurdy at telusplanet.net> writes
>
>    I guess it's me who doesn't understand the concept of radiant. Thanks to 
>Brian Skiff who responded to me privately rather than risking embarrassing 
>me in this forum, but my view is that the facts are much more important than 
>my misinterpretation of them. Brian wrote:
>*****
>>         In fact the 'Nature' paper indeed says the radiant was below the
>horizon by about 10 degrees.  And, no, the odds of it being merely a
>sporadic was _not_ evaluated in the paper.  So this is a fairly speculative
>result, mainly resting on the intersection the great-circle path with the
>radiant, and the event occurring within a few days of the predicted
>Martian Cepheids meteor shower (or node intersection).  Also it was moving
>fairly slowly, consistent with association with the shower. ... The paper 
>says the radiant can be as low as -20 deg and still get grazing meteors. 
>The meteor was something like 115 deg from the radiant---I've seen that on 
>Perseids when the radiant was up!
>*****

Interesting. I think I may be confused, so could anyone correct me if my
understanding of what is being discussed is wrong?

I haven't seen any radiant-over-the-horizon meteors myself. I'll check
my radio data, but that has added complications in itself.

I'm assuming that the phenomena, of meteors appearing from a radiant
below the horizon, is due to zenith-attraction. And that this happens
because a meteoroid has its path altered by the planet's gravity, as it
approaches. Consequently, the slower the meteoroid's initial approach
velocity (V_inf) the greater the zenith attraction. This allows meteors
to be seen when the radiant is approximately 15 degrees (for Earth)
below the horizon.

If this is correct, has anyone got a modified observability function,
which takes account of zenith attraction and allows for zenith angles
greater than 90 degrees?

On the other hand, seeing meteors at large angles from a radiant, when
the radiant is up, isn't unusual.

For anyone who doesn't know, Marco Langbroek has kindly provided a
spreadsheet to quantity the zenith attraction.

http://marcolangbroek.tripod.com/metsoft.html
-- 
David Entwistle


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