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Comet Rushes Toward Collision with Jupiter   (4/94)

by Karen Pendlebury

This document is taken from the Spring 1994 issue of Educational Horizons.

Shoemaker-Levy 9 is a 125,000 km train of cometary fragments roaring through
Space at 40 miles per second.  In July 1994, part of the train may crash into
the far side of Jupiter, giving scientists their first chance since the
invention of the telescope to study a planetary collision between a comet and a
planet.


Discovered by astronomers Carolyn and Eugene Shoemaker and David Levy on March
24, 1993, the comet has already had several near misses with the planet.  The
last time it brushed by Jupiter, Shoemaker-Levy was only 50,000 km above the
Jovian cloud, so close that the planet's gravity tore it into a string of at
least 21 pieces.  The largest fragments are thought to be about 2 to 4
kilometers across.  When the comet crashes into Jupiter, it will explode with
energy equivalent to more than 1 million megatons of TNT. To put this into
perspective, the largest nuclear explosion that ever took place on Earth
equalled 60 megatons.


This comet, also known as 1993e, is unique because it is not orbiting the Sun
as other comets do.  Shoemaker Levy appears to have had several close calls
with Jupiter over the past 20 years, was captured in the planet's strong
gravitational field, and is now following an elliptical path around Jupiter.


The possibility that Shoemaker-Levy will have a second near-miss with the
planet is slim, but scientists warn that it is difficult to calculate the
orbits of each individual piece of the comet.  Comets are thought to be
composed of dust and ice.  When they are broken into pieces, most of the nuclei
break down and disappear completely.  For this reason, Daniel Green of the
Smithsonian Observatory says the current orbit is meaningless.  As the moment
of impact looms closer, scientists will be better able to judge the actual
orbits of each fragment.


If Shoemaker-Levy collides with Jupiter, there will not be a single impact.
Several explosions will occur over a period of two to three days as the pieces
collide with the giant planet.  A few may pass through the atmosphere only to
be perturbed into completely new orbits.  Some might travel toward the Sun.
Some might stay near Jupiter.


There is only one factor that may dampen scientists' excitement about this
historical event.  Astronomers on the Earth will probably not be able to see
the event first-hand because the comet is expected to hit the far side of
Jupiter between July 16 and July 22.  It will probably plunge into the edge of
the dark side, coming into view less than two hours later.


Most scientists agree that this event will not be visible with the naked eye or
even binoculars or small telescopes.  However, larger telescopes around the
world are eagerly awaiting their chance to record the collision.


No one really knows what will be learned from this event, and there is a great
deal of controversy surrounding its possibilities.  The spacecraft Galileo is
hurrying toward Jupiter but will arrive 16 months after the impact.
Nevertheless, it will be in an excellent position to view at least some of the
impacts in real time and will be able to study the after-effects of these
collisions as it nears Jupiter.


Shoemaker-Levy is expected to plunge at least 200 km below Jupiter's cloud deck
and then spew material up into Space much like a falling drop of water causes
the surface of a pond to rebound.  This will be helpful to scientists who know
little about the material hidden below Jupiter's clouds.


Astronomers are also eager to study the effects in the upper atmosphere of the
planet.  According to Green, scientists believe another semi-permanent
disturbance such as the Great Red Spot may appear.  Some think a ring similar
to those around Saturn may be formed around Jupiter if the comet doesn't
directly collide with the planet but breaks up into many small fragments.


Christopher Chyba of NASA's Goddard Space Flight Center says one of the primary
areas of study will focus on the atmospheric effects of a massive explosion.
This will be the first true opportunity scientists will have to apply their
theories.


Whatever may happen next summer, everyone agrees there is much to be learned
from this event, if it is as spectacular as we believe it will be.  A collision
of this magnitude, an impact equal to that which wiped out the dinosaurs on
Earth, has never been observed by human eyes.  It is estimated that such large
comet collisions occur only every 100 million years.


"This is a very exciting thing because we've never seen anything like this
happen before," says Green, "It's uncharted territory for us, and it will be
great just to sit back and watch."


In commemoration of the 25th anniversary of the Apollo lunar landing, The
Planetary Society (818/793-5100) will host educational events tied to the
expected July 21 impact of Comet Shoemaker-Levy 9 with Jupiter.


In addition, the Jet Propulsion Laboratory (JPL) will be hosting a "Comets,
Asteroids & Meteors" educator conference August 12-14, on the Caltech campus in
Pasadena. The conference will cover early science results from the Comet
Shoemaker-Levy 9 impact of Jupiter, recent asteroid observations including the
Galileo spacecraft encounters of Gaspra and Ida, and meteor showers like the
August 11 Perseids. For more information, contact the JPL Public Education
Office, Mail Stop CS-530, 4800 Oak Grove Drive., Pasadena, CA 91109;
818/354-6916.