[meteorite-list] MORE COMET HOLMES

From: mexicodoug <mexicodoug_at_meteoritecentral.com>
Date: Fri, 26 Oct 2007 01:40:22 -0500
Message-ID: <007801c8179b$16ac4ee0$4001a8c0_at_MICASA>

"you'll see that any tail (which by default points away
from the Sun) would point away from the Earth at
a very similar angle. The tail would (will) have to be
fairly long before we got our first glimpse of it and...
the coma is in the way, too."

Sterling, this is not true for "any" tail. Due to the greater distance
represented by each arcminute with such a relatively remote object to
observe, the dust tail would be expected to develop more "slowly" from our
perspective. (The Ion tail is another matter.)

As covered in a previous post on the subject to Steve when we discussed
Comet McNaught there are two tails on every comet, whether you see them
easily or not. The Ion Tail, and the Dust Tail, and the dust tail usually
forms fairly along the lines our senses would expect.

Comet Holmes, is is quite far away, well into the cold main asteroid belt
where the Solar wind is somewhat attenuated due to the distance (vs. your
typical Sungrazer or comets making it closer to the Sun) and ion tail
formation less dramatic (though certainly detectable if you were in Rosetta
along for the ride). At just 1 proton per cm^3, it is around one sixth as
dense as it is near Earth's orbit, providing this much less flux, somewhat
like a black light (UV) light, that provides most of the energy to "light
up" the ion tail. Given that the Solar Wind at that distance is a snails
crawl only about 300 Km/s :-) and importantly only one sixth that even of a
temporate Earth density, things are a little bit less chaotic in that
neighborhood which basically 'starts' somewhere around Mars (1.5 AU), and
certainly somewhat calmer by 2.5 AU of the comet.

In this case, however, it seems plausible that a major disruption could
provide a dust tail, which we have a nice perspective to view forming. Dust
tails though will form much more slowly and this could start out just being
the so-called "coma". Certainly, an Ion Tail is possible, but that is not a
reason to rule out a conventional tail composed of larger neutral "dust".

All of the interesting magnitude comments by Larry and myself become far
more astounding and outrageous when we realize that this Comet is shining so
bright * out there *. Larry has estimated what the magnitude would be if we
were closer to the comet (on Mars). He hasn't covered what it would be if
the comet were closer to us. I mentioned if the comet were where Mars is,
it would be 25% as bright as the whole planet (this was yesterday). I need
to clarify that. That would be correct if it received the same amount of
Sunlight. But at a Mars distance, it would receive a Mars share of
Sunlight. Nearly three times as much. In that hypothetical comparison the
comet would actually be 75% the brightness of Mars and today surpass the
brightness of the planet. If the comet were a typical comet we saw at say
at a moderate 0.5 AU distance from Earth as well as 0.5 AU from the Sun, the
magnitude would be over 10 times brigher due to the proximity to us, * and *
over 25 times brighter due to the increased brightness of the Sun in its
neighborhood. I'm sloppy with phase angles, but it is good to get an idea
that it would be over 250 times even brighter. That is six more magnitude
points: A diffuse object as bright as Venus!!! Then there is one more
factor I didn't include. If it were that close to the Sun there is no
telling how great the production of both tails would be, but clearly it
would be a bright daylight object even at 0.5 AU.

This hypothetical exercise may not be the situation, but thinking about it
surely helped me appreciate how fortunate we are to be observing this event
in our lifetimes. Good luck to you Walter and friends in your observations
throught the clouds. Visually, at 10:20PM CDT (2007.10.26.14 UT), I
estimate the magnitude and did not seen the super brightness increase
reported and can only estimate m1 to be 2.6 magnitude - still a health
improvement over my 2.8 magnitude yesterday.

Listmember Bob King deserves a real round of congratulations, having viewed
the comet the night before last as the situation developed from the 7 th
magnitude the evening of the event....

Best wishes and back to the sky,
Doug







----- Original Message -----
From: "Sterling K. Webb" <sterling_k_webb at sbcglobal.net>
To: "Meteorite List" <meteorite-list at meteoritecentral.com>
Sent: Thursday, October 25, 2007 11:17 PM
Subject: [meteorite-list] MORE COMET HOLMES


> Hi, All
>
> List member Chris Peterson is too busy observing
> Holmes to post it here (rightfully), but his website has
> excellent pictures of the comet and a lot of up-to-date
> information:
> http://www.cloudbait.com/gallery/comet/holmes.html
>
> Everyone mention that a tail "has not yet formed,"
> but if you look at the NASA-JPL orbit simulation:
> http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=17P;orb=1
> you'll see that any tail (which by default points away
> from the Sun) would point away from the Earth at
> a very similar angle. The tail would (will) have to be
> fairly long before we got our first glimpse of it and...
> the coma is in the way, too.
>
>
> Sterling K. Webb
> ----------------------------------------------------------------
>
> ______________________________________________
> Meteorite-list mailing list
> Meteorite-list at meteoritecentral.com
> http://six.pairlist.net/mailman/listinfo/meteorite-list
>
Received on Fri 26 Oct 2007 02:40:22 AM PDT