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Comet Freight Train to Collide with Jupiter

Significance

The impact of comet Shoemaker-Levy 9 onto Jupiter represents the first
time in human history that people have discovered a body in the sky
and been able to predict its impact on a planet more than seconds in
advance. The impact will deliver more energy to Jupiter than the
largest nuclear warheads ever built, and up to a significant fraction
of the energy delivered by the impact which is generally thought to
have caused the extinction of the dinosaurs on Earth, roughly 65
million years ago. Earth-bound observers are taking this opportunity
to observe and study the comet's collision with a planet to gain more
understanding of one of the fundamental physical processes within the
solar system, impacts. The discovery has spawned scientific thinking
about the frequency with which comets fragment and implications
related to the inventory of small bodies in the Solar System and how
they modify the surface and atmospheres of the planets.

Introduction

The fast approaching collision of segmented Periodic Comet
Shoemaker-Levy 9 with the planet Jupiter has peaked the interest of
professional and amateur astronomers worldwide. Scientists expect a
spectacular 51/2-day event from July 16-22 and anticipate some
observations. For the first time in history, scientists have advance
notice of such a collision and the technological capabilities to
observe it.

Astronomers predict the comet's 20+ segments will hit Jupiter's dark
night side, where they will be hidden from telescopes on Earth. Some
observers may be able to view the phenomenon indirectly in light
reflected from Jupiter's inner moon or off ring particles. Other
observers anticipate viewing the impacts and expected explosions
through observations from NASA's Galileo and other spacecraft or by
studying the aftereffects on Jupiter's atmosphere.

Thousands of planet-watchers are readying observatories on the ground
and in space for what they hope will be a remarkable encounter. For
comet experts and planetary specialists around the world, this may be
the most important event of their careers because of the discoveries
they may make about the nature of comets and the makeup of Jupiter's
atmosphere and magnetosphere. This knowledge may help them explain
similar high-energy events on Earth.

Background

The fragmented comet was discovered by Eugene and Carolyn Shoemaker
and David Levy on March 24, 1993. It was the ninth periodic comet
found by this team of professional and amateur astronomers. They
identified the comet through a photograph taken with the 18-inch
Schmidt telescope at Mt. Palomar Observatory near Los Angeles,
California. Subsequent imaging conducted by James Scotti at the
Spacewatch Telescope on Kitt Peak in Arizona, and by Jane Luu and
David Jewitt at the Mauna Kea Observatory in Hawaii revealed the
comet's peculiar form: it is actually a string of numerous fragments
of comet, "a string of pearls."

P/Shoemaker-Levy 9 probably split apart during July 1992, when
scientists think it traveled within 113,000 kilometers of Jupiter's
center. During this pass in its orbit around Jupiter, the planet's
tidal forces tore it apart. The comet is designated, "P," for
"periodic," because even before its capture in a death grip by
Jupiter, its original orbit around the Sun was closed and contained
within our solar system.

Its fragments vary in size, with about six relatively large pieces, a
dozen medium-sized ones, and assorted smaller debris. The average
chunk is estimated to be two kilometers in diameter, although no one
knows for certain. The size or mass of the fragments however, will
determine the nature of their impact on Jupiter's atmosphere.

Predicted Effects on Jupiter

As the comet string nears Jupiter, its associated dust coma will be
bombarded by charged particles trapped in Jupiter's magnetosphere. Gas
and dust ejected from the comet may be swept up by Jupiter's magnetic
field, possibly causing large changes in the density and composition
of Jupiter's aurora. The comet fragments will plummet into the planet
one by one, like a freight train falling off a bridge. The explosions
within Jupiter's atmosphere may inject atmospheric ingredients into
the magnetosphere, altering Jupiter's radio emissions. As the comet
fragments enter Jupiter's stratosphere, they will be heated and lose
some mass and energy by aerodynamic forces. Because they are fragile,
they may break up after penetrating about 300-400 kilometers into the
atmosphere. The largest and strongest fragments will descend another
50-200 kilometers. At this point, the fragments will release the
majority of their kinetic energy in a spectacular explosion in a
little over a second. This explosion may create a fireball, like a
nuclear burst, that could rise above Jupiter's cloud tops in a matter
of minutes.

Ordinarily, events occurring 150-200 kilometers below visible cloud
tops would be invisible beyond the planet. In this case, however, the
shock wave from the airburst may blow through the planet's atmosphere
carrying the gases far above Jupiter's clouds. This deeper gas
contains volatile materials that ultimately will condense high in the
atmosphere. The gas may form unusual clouds, which could last a long
time if the comet's ice particles are small. The impacts could create
a thermal anomaly or a tremendous storm, similar to Jupiter's Red Spot
but not as big. This new turbulence or spot might be visible through
the most powerful Earth-based telescopes.

When these comet fragments dissipate in the atmosphere, they could be
very bright - possibly as bright or brighter than Jupiter itself. The
light from the impact of the largest fragments could brighten a
well-placed inner moon of Jupiter enough to be detected by powerful
Earth telescopes. The Hubble Space Telescope is scheduled to make more
pictures of Jupiter during the days of impact. Thousands of
Earth-bound astronomers throughout the world will point their
telescopes towards Jupiter to look for some evidence of the crash.

World-Wide Effort

In the United States, NASA and The National Science Foundation have
jointly funded a coordinated program to support research efforts for
this event, using many ground-based observatories and several
spacecraft: Galileo, Hubble Space Telescope, the International
Ultraviolet Explorer, the Extreme Ultraviolet Explorer, Ulysses, and
Voyager 2. The program includes listening for radio signals, visible
and thermal imaging, modeling, theory, and data analysis. Since 1993,
an electronic bulletin board on the Internet, organized by astronomer
Michael A' Hearn at the University of Maryland, has kept the world's
planetary-scientific research community advised of the latest
information on the pending collision. In January 1994, 175 astronomers
from the United States and Europe met to identify the specific
physical phenomena they would try to observe to gain the most
knowledge from the event. They established a continuous worldwide
series of Jupiter observations during the collision's six-day time
frame. They also agreed to disseminate their scientific information
within three to six months after obtaining it. Throughout the planning
process, scientists in varied U.S. and international organizations
have displayed a spirit of cooperation in their quest for discoveries.
Astronomers hope to gain more knowledge about the composition of
comets and the makeup of Jupiter's atmosphere. Analysis of the new
data may teach us more about the role of comets, meteors, and other
space objects in the disappearance of the dinosaurs more than 65
million years ago. Additional measurements and observations may test
theories of other mass extinctions on Earth, the behavior of
high-energy shock waves and cloud formation in planetary atmospheres,
the makeup of comets, and even the origin of planets.

Comet Shoemaker-Levy 9 References

Foley, Theresa M., "Comet heads for collision with Jupiter," Aerospace
America, pp 24-29, April 1994.
Levy, David H., "Pop! Pow! Smash!," The Sciences, pp 31-35, May/June
1994.
Smith, Douglas L., "When a Body Hits a Body Comin' Through the Sky,"
Engineering & Science, pp 3-13, Fall 1993.
[1][LINK] Background Page

References

1. http://www2.jpl.nasa.gov/sl9/background.html