Larry wrote:
"1. As far as I know, scientists still do not know where SL9 came from
("beyond Neptune"). Probably a captured comet that happened to come too
close to Jupiter on its first pass or one of its first passes into the
inner part of the Solar System."
Hi Larry, Listees,
IMO, it was far from that dramatic sort of initial Jovian fishing
expedition in those passes, in that SL9, before it's chaotic Jupiter
capture looked like one of our favorite kinds of asteroids with an
orbit likely CONFINED inside the main asteroid belt and with
sufficiently of low inclination (though with lower probability it could
have been stuck a little further out, at most, into the zone between
Jupiter and Saturn). While all short period comets like SLP have a
pinball aspect to their orbits before getting stuck within, say, inside
Neptune's orbit, SL9 just did what any meteoroid with potential would
have done in that it got too close to Jupiter and stretched out its
orbit like pulling a rubber band. As this is thought to have happened
right at aphelion, the comet was basically at a standstill when Jupiter
bumbled by and it transferred into a Jovian orbit by basically falling
into Jupiter in an extremely eccentric orbit (as you point out), and
from there on, just got too close to Jupiter as Jupiter and the Sun
ironed out tyheir differences without JPL pushing the comet's
outgassing buttons.
Here is a sc
ienific eplanation and a graphical evolution of the capture
orbits as calculated by astrophysicists:
http://tinyurl.com/742lbr
Of course, where SL9, or anything else for that matter before being in
the e.g., asteroid belt, came from, whether 25 or 2.5 billion years
earlier, makes for good philosophy.
A minor sampling of thoughts on this event from a meteoritical
perspective...and for all good hearted Comet-fearing humans:
The collision of SL9 with Jupiter was a great event to have been alive
to have observed, but should be put in the appropriate context
regarding orbit dynamics and the inner Solar system (read: frequencies
of collision with Earth). While such a Jupiter collision may well be a
once in 6000 (as you suggested) year event, one very pleasing rigorous
analysis concluded that such event:
" In particular, we show that, for Jupiter-interacting* comets of
greater than 1 km diameter, a Jupiter impact takes place every 500 to
1000 years, and an Earth impact every 2 to 4 million years."
The sort of study great pops the ballon of theories suggesting that
comets frequently strike Earth (and shape evolution frequently in
thousands or tens of thousands of years periods). The important detail
lies within the observation that the residence time for comets in the
Terrestrial (inner) Solar system is so short and chaotic from an
orbital perspective, and the planets so small (for example, see Larry's
cross section
, but he was actually waaaaay overestimating it since we
need to also consider the inclinations of the comets), that there is
virtually nil chance = 2 - 4 million years vs. what we saw happen on
Jupiter. Furthermore, what led to that collision, as has already been
suggested is that Jupiter was able to capture the comet to start with.
That is not something the Earth is adept at doing considering the
relative size of the Sun and its gravitational potential vs. ours in
the uptown part of the Solar neighborhood. Here is the excellent
statistical vs. observational treatise by T. NAKAMURA (National
Astronomical Observatory, Japan) and H. KURAHASHI (Sano-Fuji Optics
Company, Japan), THE ASTRONOMICAL JOURNAL, 115:848-854, Feb. 1998.
http://www.iop.org/EJ/article/1538-3881/115/2/848/970144.html
*ok, so have the wild card of hyperbolic comets and highly inclined
rogue comets and the likes of the kitchen sink of things that don't fit
nicely into the Solar system intro textbook. Perhaps this provides
some SOLice for the frequent Terrestrial-cometary collision proponents.
Anyway, this is my take on it Larry, and it is based on oldies but
goodies regarding the papers cited. I can't find, and don't know that
anyone has done anything particularly revolutionary since then.
Best wishes, great health and keep looking up,
Doug
-----Original Message-----
From: [email protected]
To: [email protected]
Cc:[email protected]
Sent: Sun, 11 Jan 2009 6:54 am
Subject: Re: [meteorite-list] Comet smashes triggered ancient famine ???
Hi Graham and Rob:
Some of this is from memory and some of this I had to look up. David
Levy
was actually working part time for me at the time doing education
outreach, so I know some of the details.
1. As far as I know, scientists still do not know where SL9 came from
("beyond Neptune"). Probably a captured comet that happened to come too
close to Jupiter on its first pass or one of its first passes into the
inner part of the Solar System.
2. At some point in time (1960s to 1970s), the comet(?) soon to be
called
SL9 was captured in a 2-year orbit around Jupiter. Good for it, a new
moon
of Jupiter!
3. However in July of 1992, SL9 passed within 30,000 to 40,000 km of
Jupiter's cloud tops (Jupiter radius is about 71,500 km). This is within
the Roche limit of Jupiter (gravity-induced tides from Jupiter stronger
than the strength of the material that makes up the body; a little more
complicated than that, but good enough for this).
4. Observed first seen by Carolyn Shoemaker (observers Gene and Carolyn
Shoemaker and David Levy; interesting story). Then confirmed by Jim
Scotty
here in Arizona (the first famous image of the string of pearls).
5. Soon determined to be in orbit around Jupiter (though only "seen"
once
prior to that but not noticed
by the person who took the image). A
highly
elliptical orbit that had it going as far as 50,000,000 km from Jupiter
(but still in orbit). Repeating myself, a 2-year orbit, probably in
orbit
for 20 or 30 years.
6. Soon to be determined that its orbit was continually changing
slightly
(gravity of the Sun and mass loss of the comet which alters the orbit
slightly). The result was that in Juuly of 2004, it would be at its
closest to Jupiter again, but this time, its closest distance to Jupiter
would be 45,000 from the center of Jupiter, 26,500 BELOW the cloud tops
of
Jupiter!
7. So, the reason that all of the pieces hit Jupiter was that they were
all in the same orbit, just strung out in space (and time along an orbit
that got the pieces way too close to Jupiter).
8. I think that the best estimates (not all agree) are that the largest
pieces were at most 1-2 km in diameter with most pieces less that 1 km
in
diameter. This makes this event a once in a thousand-year event (give or
take).
9. This helps explain crater chains on two of Jupiter's satellites:
Europa
and Ganymede (16 total?). A comet gets too close to Jupiter, breaks up
and
you get a string of comets that, on their way away from Jupiter run into
one of the satellites, leaving a crater chain. Too close together and
there would not be a chain. Too far apart and only one or two would hit
the satellite and the othe
rs would miss.
To get crater chains on Earth, you would have to have a comet or
asteroid
break up before hitting the Earth, either by a close approach to Earth
(unlikely) of the Sun. However, it is unlikely that this object would
get
captured by the Earth (they are moving fast and Earth not that massive).
A breakup as the object was approaching the Earth (say in the
atmosphere)
would not give the pieces time enough to spread out and make multiple
craters (the long discussion on double craters on Earth). So the pieces,
as I said before, would have to be close enough together in order for
the
individual pieces to hit the moving Earth target and really close to
make
a chain on Earth. The Earth is moving at 30 km/s and a comet is moving
at
probably 40 or 45 km/s. So, you can easily figure out how close the
pieces
have to be.
A bunch of impacts over a few thousand years is another story that is
beyond the above discussion.
Larry
On Sun, January 11, 2009 2:49 am, [email protected] wrote:
Hi Rob,
Went to a lecture at our astronomy society about Jupiter and it
acting to
capture or perturb objects (friend or foe etc) the other night....
and I
believe it was said that SL9 only made two passes before it met
jupiter
again on jupiters next turn around the sun and was thus flung out
never
to be seen again. eg
Jupiter was on the opposite side of the sun on
SL9's
first time round with no effect and thus was very close 2nd time round
and able to change its orbit again.
I hope I remembered that right!
Graham Ensor, UK
---- Rob McCafferty <[email protected]> wrote:
Fair point, but it may well be a poor choice of words on my point.
The "Swarms"/"showers" you mention are what are suggested in the book.
Several objects arriving in quick sucession are not unusual, however.
There is evidene of it happening on most solid bodies. They all have
strings of impact craters where many objects obviously arrived in a
matter of hours producing chains of craters. My problem with this is
that
the authour is perhaps suggesting several over the last few millenia.
If
the "chain" events were that prevalant, one would expect them to
dominate
on solid bodies and they don't.
Your points are well made. I was not aware that SL9 was in orbit of
Jupiter. The implications of this are complex and I'll need to check
how
long for. I Doubt it was for long but even so, how this is related to
comets and the earth is beyond me at this time.
Rob
--- On Sat, 1/10/09, [email protected]
<[email protected]>
wrote:
From: [email protected] <[email protected]>
Subject: Re: [meteorite-list] Comet
smashes triggered ancient famine
???
To: [email protected]
Cc: "tracy latimer" <[email protected]>,
[email protected] Date: Saturday, January 10, 2009,
11:47 PM
While I have not read this book, generally, comets cannot
hit the Earth over a short interval like SL9. SL9 was in orbit around
Jupiter. It is
highly unlikely that a comet could be captured in orbit around Earth.
Continuous bombardment on Earth only happens in movies
unless there is a massive swarm of objects (like in a meteor shower).
The Earth is a moving target, so if one comet piece were to
hit the Earth, it is unlikely that a second or third one in a similar
orbit would hit, unless the cluster was VERY bunched together. The
Earth
would be long gone!
The Earth's orbital velocity is about 30 km/s and its
diameter is about 12,750 km. So the Earth moves its diameter in
about 425
seconds. If the comet pieces were farther apart than that, only one
piece would hit.
Larry
On Sat, January 10, 2009 4:06 pm, Rob McCafferty wrote:
This is not a new idea. Mike Baille's book
"Exodus to Arthur" makes
interesting reading on the idea that comets may have
triggered many human
catastrophies in the past. His book is based on
dendochronology with
support from other sources. At the time of publishing
20c.2000, there was a
gap in the Greenland Ice core during the 6th Century.
The first third of the book is compelling reading but
for me does little
to convince me that it was anything other than
volcanic eruptions. The
latter part of the book is based on written accounts,
myths and legends
to make a the suggestion that clusters of small comets
may have been
involved, small fragments arriving in short interval
like SL9 did on
Jupiter in 1994.
He's as objective as he can be but is clearly
convinced of the cometary
contribution in at least a few cases.
Rob McC
--- On Fri, 1/9/09, tracy latimer
<[email protected]> wrote:
From: tracy latimer <[email protected]>
Subject: Re: [meteorite-list] Comet smashes
triggered ancient famine ???
To: "Paul" <[email protected]>,
[email protected]
Date: Friday, January 9, 2009, 7:15 PM
From what little research I did, I had
understood that a
substantial chunk of the sun-blotting fog was
actually 'vog', which
outgassed from major eruptions in Iceland.
Iceland underwent several
periods of volcanic activity during the 'Dark
Ages', where multiple
volcanic vents burped out stifling clouds of gas.
The gas periodically
0A>>
got so thick and noxious that it poisoned
vegetation, killed animals,
and sickened almost everyone else; there was at
least one major exodus
of survivors around 770 a.c.e.
Tracy Latimer
----------------------------------------
Date: Fri, 9 Jan 2009 07:12:59 -0800
From: [email protected]
To: [email protected]
Subject: [meteorite-list] Comet smashes
triggered
ancient famine ???
Comet smashes triggered ancient famine
January 7, 2009 by Ker Than
http://www.newscientist.com/article/mg20126882.900-comet-smashes-trigge
red-ancient-famine.html?DCMP=OTC-rss&nsref=online-news
Abbott, D. H., P. Biscaye, J. Cole-Dai, and D.
Breger,
2008,
Magnetite and Silicate Spherules from the
GISP2 Core
at the 536 A.D. Horizon
American Geophysical Union, Fall Meeting 2008,
abstract #PP41B-1454
http://www.agu.org/cgi-bin/SFgate/SFgate?&listenv=table&multiple=1&rang
e=1&directget=1&application=fm08&database=%2Fdata%2Fepubs%2Fwais%2Finde
xes%2Ffm08%2Ffm08&maxhits=200&=%22PP41B-1454%22
and
http://adsabs.harvard.edu/abs/2008AGUFMPP41B1454A
Yours,
Paul
H.
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