[meteorite-list] BREAK! For the love of meteorites, STOP -- COMET 73/P

From: Pete Pete <rsvp321_at_meteoritecentral.com>
Date: Sun May 14 17:27:56 2006
Message-ID: <BAY104-F14259D131E648A3B0D6B5DF8A20_at_phx.gbl>

Hi, Sterling and Doug,

Thanks for your valued input.

Regarding the 2022 shower, I was wondering how different that spectacle will
be considering it won't be the normal dust-to-pea-sized coma debris, but
more likely some considerable chunks included, due to the current and nicely
timed disintegration.

Armegaddon!? What side of the planet should we be on then? (-Rhetorical ;])

At this rate of break-up, is it possible that there won't be a comet left
for a return trip from around the sun?

Cheers,
Pete


From: "Sterling K. Webb" <sterling_k_webb_at_sbcglobal.net>
To: "Meteorite List"
<meteorite-list_at_meteoritecentral.com>,<joseph_town@att.net>,"Pete Pete"
<rsvp321_at_hotmail.com>
Subject: Re: [meteorite-list] BREAK! For the love of meteorites, STOP --
COMET 73/P
Date: Sun, 14 May 2006 03:08:50 -0500

Hi, Pete,

    Your message came just in time. I was typing a snide remark
about the Hematitic Lump From Mars. (Somebody forward to
this guy Göran Axelsson's picture of the identical "tone rock"
in Sweden at a church, and explain to the guy that he got God's
message all mixed up -- he's supposed to use his rock as the
bell for his church, not sell it on eBay!)

    What bothers me about Comet 73P is this: It can't be a
"new" comet (even though we discovered it in 1930). The orbit
is too stable for the comet to have recently been thrown in
there. It's been around for centuries, probably millennia, in
this same orbit. Yet, it has unraveled so quickly and easily.

    Once it started to come apart, sometime between 1990
and 1995, it has split, re-split, fractured. If you go back and
read the earliest studies this pass, the authors clearly expected
that whatever splits had occured at the time they wrote to
be the extent of splitting when they passed the Earth.

    Three fragments would be visible, they said. Whoops, make
that six fragments. Uh-oh, make that 9, 12, 30. I'm not making fun
of the researchers, but our experience of split comets is that
this disintegration takes a while. 73P has just gone to hell
overnight. It must be very, very weak, they say.

    OK, BUT... If it's that weak, what has been holding it
together for the last 75 years (and for centuries before
that)? Thermal stress is pegged as the likely culprit for the
breakup, but it's been exposed to the Sun for a long time.
How could it have survived so long if it was this fragile?

    My guess answer is that the fragile material was probably
adhered to something that wasn't fragile, like a small rocky core.
This small dark object would have been completely shielded
from the Sun by the weak porous fluffy ices that surrounded
it and made up the outer body of the comet.

    But once a good chunk of those ices cracks off from a tiny
impact or from thermal stress, it exposes a portion of the dark
rock core to sunlight; the rock warms and more fragments of
icy fluff soon come loose. They're too small to survive and rapidly
break up in a cascade of fragments, as we've seen. A bare dark
rock object is left behind in an orbit similar to the other fragments,
but it's too distant to be detected... yet.

    I'm looking forward to the discovery of a small Earth-crossing
asteroid in 2011, 2016, 2022 with an orbit very like Comet 73P!
It would not be a big one. The pre-breakup 73P nucleus was only
1000-1200 feet in diameter; a core is unlikely to be more than
a few hundred feet across (30 to 80 meters), I hope, instead of
400 meters.

    Despite the fact that meteor showers are so showy, no fall
has ever been associated with them. Only one fall was ever
witnessed during a meteor shower and recovered, and it
was an iron, a complete coincidence. The biggest fragments
in a meteor shower are smaller than a pea, moving very fast,
and in for a short bright ride, then Pffft! Small junk never
makes it through the atmosphere.

    Predicting future meteor shower orbits is the most thankless
job in number crunching. Some people like it for that very
reason. Every little piece of cometary material is capable of
puffing little jets of gas; every little jet is a thrust; every thrust
alters the precise orbit somewhat; the thrusts go on for months
with progressive orbital changes, like ion engines. Some jets
are on rotating bodies, so the thrusts are like pinwheel jets,
pushing this way then that way. To quote Charlton Heston
in Planet of the Apes, "It's a madhouse! A madhouse!"

    How spectacular a close comet approach is depends
not only on how close but on how big. The close approach
of a really big fresh long-period comet is probably the most
spectacular thing that is visible in the sky, apart from a
Type II supernova 700 light years away. We haven't had
one really spectacular one for over a century and a half,
but the century before that was blessed with some giant
"apparitions," as they are called, in 1729 and 1744,
and the 19th century had flashier big meteor showers
than the 20th. Maybe we're due for one.

    As for people who worry about close approaches,
here's a table of the 20 closest approaches of comets
that were discovered after 1700 (although some historical
close approaches are included in the list), courtesy of
Harvard http://cfa-www.harvard.edu/iau/lists/ClosestComets.html
    One of the brightest recent ones is Hyakutake in
1996 and it was a really fine sight. Comet 73P is on
the list at Number Ten (in 1930 when it was discovered).

    I added the distance to the Moon for comparison, and
in all this time, nothing has gotten closer than six times the
Moon's distance.

    I say, let's keep it that way.

Distance Date (TT) Permanent designation
  (AU)
0.0026 Distance to the Moon
0.0151 1770 July 1.7 D/1770 L1 (Lexell)
0.0229 1366 Oct. 26.4 55P/1366 U1 (Tempel-Tuttle)
0.0312 1983 May 11.5 C/1983 H1 (IRAS-Araki-Alcock)
0.0334 837 Apr. 10.5 1P/837 F1 (Halley)
0.0366 1805 Dec. 9.9 3D/1805 V1 (Biela)
0.0390 1743 Feb. 8.9 C/1743 C1
0.0394 1927 June 26.8 7P/Pons-Winnecke
0.0437 1702 Apr. 20.2 C/1702 H1
0.0617 1930 May 31.7 73P/1930 J1 (Schwassmann-Wachmann 3)
0.0628 1983 June 12.8 C/1983 J1 (Sugano-Saigusa-Fujikawa)
0.0682 1760 Jan. 8.2 C/1760 A1 (Great comet)
0.0839 1853 Apr. 29.1 C/1853 G1 (Schweizer)
0.0879 1797 Aug. 16.5 C/1797 P1 (Bouvard-Herschel)
0.0884 374 Apr. 1.9 1P/374 E1 (Halley)
0.0898 607 Apr. 19.2 1P/607 H1 (Halley)
0.0934 1763 Sept.23.7 C/1763 S1 (Messier)
0.0964 1864 Aug. 8.4 C/1864 N1 (Tempel)
0.0982 1862 July 4.6 C/1862 N1 (Schmidt)
0.1018 1996 Mar. 25.3 C/1996 B2 (Hyakutake)
0.1019 1961 Nov. 15.2 C/1961 T1 (Seki)
Sterling K. Webb
-------------------------------------------------------------------------
----- Original Message ----- From: "Pete Pete" <rsvp321_at_hotmail.com>
To: <joseph_town_at_att.net>
Cc: <meteorite-list_at_meteoritecentral.com>
Sent: Saturday, May 13, 2006 11:06 PM
Subject: RE: [meteorite-list] BREAK! For the love of meteorites, STOP


>
>Hello, List,
>
>There has been over thirty posts in this thread with barely an
>interruption. Nothing else to talk about?
>

>I'm hoping Sterling K. Webb will give us his analytical dissection of the
>disintegration of Comet 73P, in his usual interestingly descriptive way!
>>From the top, Sterling!
>And some thoughts about its predicted meteor shower in 2022, if you
>will.(any speculation as to survivors to the surface then?)
>http://www.space.com/scienceastronomy/060510_comet_spitzer.html
>http://www.space.com/scienceastronomy/060510_comet_spitzer.html
>
>Cheers,
>Pete
>
Received on Sun 14 May 2006 05:27:48 PM PDT