[meteorite-list] Did a Collision Cause Comet 17P/Holmes' MysteriousOutburst?

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Wed, 7 Nov 2007 20:42:29 -0600
Message-ID: <0a3a01c821b1$01a815e0$c944e146_at_ATARIENGINE>

Hi, List,

    You would think with all the new (and old) scientists
examining the collisional possibilities of Comet Holmes
passing through the Asteroid Belt, some of them might
have noticed that Comet Holmes DOES NOT PASS
"through" the Asteroid Belt!

    I'm being sarcastic about this because I made exactly the
same mistake myself, until an astronomer, List member
Larry Lebofsky, pointed out that because of its high inclination
(19.12 degrees), Comet Holmes does not pass through the
ecliptic plane in the Asteroid Belt, but way out at the inside
edge of Jupiter's orbit, at 4.86 AU.
http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=17p&orb=1

    The vast majority of the asteroids in the Main Belt have
inclinations of less than 19 degrees. Of course, it is possible
that Holmes could collide with a "less inclined" asteroid; it
depends on the orientation of the asteroidal orbit. But, but it's
really a very thin chance, with a small subgroup of an already
widely scattered population. In non-numerical terms, Comet
Holmes essentially passes over (and under) the Asteroid Belt,
rather than "through" it.

    However, Holmes does plunge through the ecliptical plane
in the position where thousands of Jupiter Trojan asteroids
co-orbit with the planet, making passes that repeat the same
orbital configuration every 81-point-something years. The
odds of a collision with something in Jupiter's Trojans is
dramatically higher than with a Main Belt asteroid.

    There are two goups of Trojans, ahead and behind Jupiter
at 60 degrees, but since they are themselves generously
distributed ahead and behind their Trojan points, along about
1/3rd of the Jupiter orbit, Holmes is exposed to such "Trojan"
encounters for about 1/3rd of its orbits.

    The two possible causes of the outburst, collision or thermal,
can be summarized as the "Bump" or "Burp" theories. I think an
endogenous cause of the outburst is more likely than a collision,
as both the great outbursts, the discovery outburst and the present
one, occured after perihelion passage with some delay. From June
16, 1892 to November 6, 1892 is 143 days. From May 4, 2007
to October 24, 2007 is 173 days. (There are some uncertainties
about dates of perihelion.) Passage through the ecliptic plane at
2.05 AU (right at the inner limit of the Asteroid Belt) occurs 4-5
months earlier than perihelion. At the times of the outbursts, the
comet was high above the ecliptic plane (the ecliptic plane being
where collisions would be most likely).



Sterling K. Webb
---------------------------------------------------------------------------
----- Original Message -----
From: "Ron Baalke" <baalke at zagami.jpl.nasa.gov>
To: "Meteorite Mailing List" <meteorite-list at meteoritecentral.com>
Sent: Wednesday, November 07, 2007 5:46 PM
Subject: [meteorite-list] Did a Collision Cause Comet 17P/Holmes'
MysteriousOutburst?



http://www.newscientist.com/blog/space/2007/11/did-collision-cause-comet-holmess.html

Did a collision cause comet's mysterious outburst?
Kimm Groshong
New Scientist space blog
November 06, 2007

Comet 17P/Holmes has certainly given sky-watchers - backyard and
professional astronomers alike - a thrilling chance to see a cometary
outburst on a grand scale. After we posted my story
<http://space.newscientist.com/article/dn12880> about on-going
speculation about what could have caused this outburst (and the one 115
years ago), many readers posted comments related to two questions: Could
this have been triggered by a collision with an object in the main
asteroid belt? And why can't we see more of a tail on this comet?

Here's what I found out: Michael Mumma at the Goddard Center for
Astrobiology says such a collision in the asteroid belt is theoretically
conceivable. He noted that comet guru Fred Whipple suggested that a
collision with a small asteroid could have provided the right amount of
energy to produce the ejecta and brightening observed in the comet's
1892 flare-up.

But Mumma himself thinks it would be "very surprising" if a collision
were the cause of the outburst. He says part of the difficulty in
weighing this possibility is that it's very hard to estimate how many
small boulders are in the asteroid belt. These tiny objects - on the
order of one-metre across - are beyond the detection limits of telescopes.

Brian Marsden, former
director of the International Astronomical Union's Minor Planet Center
<http://cfa-www.harvard.edu/iau/mpc.html>, says plainly that he doesn't
believe this to be a viable explanation for the outburst. He says it's
hard to believe that this comet, among all those that pass through the
asteroid belt, has been struck twice by objects in the belt - once in
1892 and again this year.

Many people have been discussing whether or not this comet has a tail.
Comets typically have two types of tail - a
dust tail and an ion tail. The dust tail is made of fine dust from the
comet's main body, or nucleus, that has been swept out by the Sun's
radiation. It usually points in the direction from which the comet came.
The ion tail is caused by the Sun's magnetic field sweeping ions (which
start out as neutral gas particles on the body of the comet) into a line
that always points directly away from the Sun.

Marsden says the there isn't much of either type of tail. He says it's
possible that there just isn't enough very fine dust in the material
coming off the nucleus to be pushed by sunlight into a nice dust tail.
(He says there may only be "fairly hefty dust" in the comet's coma.)

Some people have argued that we can't see the comet's ion tail because
the orientation of the Sun, Earth and comet means the tail is mostly
pointing away from Earth.

But though the tail does look fore-shortened, Mumma says the accumulated
surface brightness would be greater seen from one end than if it were
seen spread out, from the side. He likened it to looking at the contrail
of a plane. If we saw the stream from the side, we would basically see
right through it. But if we were in front of or behind the contrail,
looking into it, it would appear many times brighter.

Marsden says the comet is so far from the Sun that the solar wind is not
interacting strongly enough with the ionised gas to produce a fantastic
ion tail. But he says it doesn't bother him that it doesn't have much of
a tail. After all, the "fuzzy head" is putting on such a great show of
its own.


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Received on Wed 07 Nov 2007 09:42:29 PM PST