From: jpbonsen@Athena.MIT.EDU
Message-Id: <9308060153.AA22467@m66-080-15>
To: rdshydur@Athena.MIT.EDU
Subject: comet to hit jupiter!
Date: Thu, 05 Aug 93 21:53:08 EDT


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Date: Thu, 5 Aug 93 18:05:51 PDT
From: GRAPS@galileo.arc.nasa.gov
Subject: Comet Shoemaker-Levy 9
X-Original-To: vince.k@applelink.apple.com, tcmay@netcom.com, extropians@gnu.ai.mit.edu
X-Extropian-Date: Remailed on August 6, 373 P.N.O. [01:06:52 UTC]
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Hello extropi-oids:

It's been quite a while since I posted to this list.. Maybe next 
time my funding runs out, I shouldn't accept every job that comes
along and, perhaps, just work at one????

Anyway...
At the Extropian Bay Area lunch today, a discussion popped up about
Comet Shoemaker-Levy 9 (aka 1993e). Some of the e-folks had just
recently heard about about this comet, while others hadn't heard about
it yet.

WHERE HAVE YOUR MINDS BEEN??!! Wrapped up in a computer monitor or
something? :)  :)

So, I, your friendly extropian astronomer, am here to educate you about
this delightful, explosive (if you like explosions or such- Keith H are
you out there?), once-in-a-lifetime celestial event.

- ---------------------------------------------------------------------

	"Jupiter is Offering a Crash Course in Comets"
	(many excerpts from ref 1)

Astronomers will be aiming virtually every telescope in the world
towards Jupiter in late-July next year to see what happens when big
chunks of a shattered comet punch into the backside of Jupiter at
almost 40 miles/sec, exploding with energy equivalent to almost a
billion megatons of TNT. For the first time since the invention of the
telescope, a comet is hitting a planet. Next year's rare, lucky glimpse
of the "before," "during," and "after" should provide an unprecendented
opportunity to test theories on, among other things, the nature of
comets, the nature of Jupiter, how planetary rings form, how craters
form, and the dynamics of high-megaton atmospheric explosions. It's as
close to a controlled laboratory experiment on a planetary scale as
we're likely to come.

The comet (perhaps 12 miles in diameter) almost hit the planet Jupiter
on its last pass a year ago, David Levy (one of the discoverers) says.
At that time, the planet's powerful gravitational forces ripped it into
a swarm of mini-comets enveloped in an enormous cloud of dust and
rubble.  By the time Earth telescopes spotted it in March, the comet
pieces had spread out like a string of pearls (also referred to as a
"nuclear train").  Scientists counted 17 major pieces then, but further
studies have revealed 21.

Uncertainty about whether a collision will occur has all but
disappeared in recent weeks, says Donald Yeomans of NASA's JPL.  "There
is almost no question that major chunks will hit Jupiter." Calculations
by Yeomans and colleague Paul Chodas (see my information from them
below) indicate a probability of more than 99 percent- a virtual dead
certainty in astronomical terms. But Yeomans hedges. "Mother Nature
sometimes thumbs her nose at us." In fact scientists emphasize that
there are so many unknowns in their calculations that they can only
make educated guesses about what's going to happen next July.

Unfortunately, we won't be able to directly see the event from Earth.
Astronomers calculate that the comet train will strike the far side of
Jupiter on July 21-23, 1994, causing repeated flashes over two or
three days. Jupiter completes one rotation every 10 hours, so the impact
zone will soon spin into view and they/we can inspect the results.

The impacts probably will, for a time, "crater" Jupiter's turbulent
atmosphere as if it were a a hard surface, causing some of its
previously unseen innards to "well up."  The shock could linger in the
form of a another semi-permanent storm similar to Jupiter's trademark,
the Great Red Spot. The impacts could create a series of fireballs
emerging from deep within Jupiter's clouds and blowing out the top, the
largest perhaps 400 miles across- a mere dot on the vast face of
Jupiter- with a flash "as bright as the surface of the sun," and a rain
of debris leaving glowing trails all around. Scientists have estimated
that the plume caused by the explosion will be 10-100 times brighter
than the rest of the surface of Jupiter. If the bulls-eye were on the
side facing Earth, Levy says, it would be visible in the daylight
without a telescope.

Astronomers will indirectly observe the event by measuring reflected
light  that bounces  off  Jupiter's nearest moons, Io and Europa. The
effects may be visible in backyard telescopes and even binoculars. The
visibility  of  the  fireworks depends in part on the size and
composition of the incoming objects.

Astronomers are hoping to directly observe the event by using several
space probes. The partially crippled Galileo will arrive at Jupiter 16
months too late to see the event up-close, but it should be able to
observe it from-a-distance. Calculations from Paul Chodas show that
Galileo will be able to see the impact on Jupiter's terminator. Gene
Shoemaker (the other discoverer) is urging NASA to power up instruments
aboard Voyager 2, now cruising beyond the planets. It could record the
flash even across such a distance.

- ---------------------------------------------------------------------

For all of you hard-core techies, who maybe want to see the numbers,
I located the following "bulletins". The astronomers that I work
with in my building are really excited about this event, and so 
everyone has graphs of impact simulations and other calculations
relating to the event posted everywhere.

- ---------------------------------------------------------------------

From IAU Circular No 5800 and 5801 (ref 2), 
"Almost 200 precise positions of this comet have now been reported
(mostly from CCD images by S. Nakano and T. Kobayashi in Japan and by
E. Meyer, E. Obermair and H. Raab in Austria).  These observations are
mainly of the the center of the nuclear train, and this point continues
to be the most relevant for orbit computations. The presumed encounter
with Jupiter that created the train occurred during the first half of
July 1992. The comet(s) has a period of roughly 2 years.

... more than half of the nuclear train could collide with Jupiter,
over an interval approaching 3 days.  Preliminary computations by A.
Carusi, Rome University, show that surviving nuclei are likely to
remain as satellites of Jupiter or be thrown closer to the sun on
short-period heliocentric orbits (depending on which side of Jupiter
they pass)."   [A mini-solar system in the making!!!!]

- ---------------------------------------------------------------------
From Don Yeomans and Paul Chodas, JPL, (ref 3) 
"The following jovicentric impact circumstances for comet 1993e
apply:

Time of entry = 1994 July 20.4995
Atmospheric entry at Jupiter latitude = -46 degrees
Atmospheric entry defined as 69,000 km at this latitude
Relative velocity at entry = 60.5 km/s
Sun-comet-Jupiter angle at entry = 66 degrees (night side)

Orbital eccentricity at entry = 0.99877
Jovicentric periapses = 33,761 km
Orbital inclination with respect to Jupiter equator = 84 degrees"

- ---------------------------------------------------------------------

ref 1)	July 19-25, 1993
	The Washington Post National Weekly Edition
	Page 38.

ref 2) 	May 22, 1993
	Central Bureau for Astronomical Telegrams, 
	Int'l Astronomical Union
	Smithsonian Astrophysical Observatory,
	Cambridge MA 02138

ref 3) 	June 1, 1993
	in NASAMail to D. Morrison, Space Sciences Division, 
	NASA-Ames.

- ---------------------------------------------------------------------

Amara Graps
NASA-Ames Research Center
MS 245-5
Moffett Field, CA  94035
graps@gal.arc.nasa.gov


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