Title: Comet collision. (cover story)
Subject(s): COMETS; JUPITER (Planet)
Source: Popular Science, Jul94, Vol. 245 Issue 1, p42, 5p, 1 diagram,
2c
Author(s): Graham, David
Abstract: Reports that astronomers throughout the world are poised to
watch as a speeding comet, officially named Periodic Comet
Shoemaker-Levy 9, crashes into Jupiter on July 16, 1994, with the
force of 50 million atomic bombs. Predictions about what will happen
when collision occurs; How collision will help scientists test their
models of catastrophic impacts. INSETS: Comet crash or asteroid
attack: What killed the..., by R.L; Identifying celestial objects, by
Robert Langreth.
AN: 9407087633
ISSN: 0161-7370
Note: Tucson-Pima Public Library subscribes to this magazine.
Database: MasterFILE Elite

COMET COLLISION

A speeding comet will crash into Jupiter this month with the force of
50 million atomic bombs.

The comet hunters were started when they turned their gaze toward
Jupiter last year. Working at California's Palomar Observatory, they
discovered a comet that was not only trapped in orbit discovered the
giant planet but had shattered into a train of 21 distinct pieces
tugged apart by Jupiter's gravitational forces.

Now the "string of pearls," officially named Periodic Comet
Shoemaker-Levy 9, has an even more dramatic fate. Later this month, it
will plunge into the Jovian atmosphere with an explosive force that
could reshape the appearance of the planet. The event will mark the
first time in recorded history that anyone has seen a comet collide
with a planet.

The 21 cometary fragments will pelt the solar system's largest planet
during six days starting July 16. The first chunk is calculated to hit
at 19:28 Greenwich Time (3.28 p.m. in the eastern United States).

The impacts at 60 kilometers per second could churn Jupiter's
signature cloud bands, produce fireballs the size of Texas, and form
clouds that encircle the gaseous giant for months. It could even
generate a Jovian version of the aurora borealis in red and yellow
hues.

"This is a phenomenal event, and its amazing it's happening in our
lifetime," says Lucy-Ann McFadden, the planetary scientist
coordinating viewing plans for NASA.

When the bombardment begins, says McFadden, "virtually every
observatory in the world is going to be looking at Jupiter."
Astronomers have had months to prepare for the event and plenty of
time to book telescope reservations. Satellites and planetary probes
have been reprogrammed to watch too; chief among them is the Galileo
spacecraft. Already speeding toward Jupiter for a rendezvous next
year, it is expected to have the best view of the collision.

Because the impacts will occur on the night side of Jupiter, only
Galileo will have a dose look at them as they occur. Other
observatories will see the results within an hour or so, when the
impact sites rotate into daylight.

Eugene and Carolyn Shoemaker, a husband-and-wife team at Lowell
Observatory in Flagstaff, Ariz., and colleague David Levy are among
the many scientists eagerly awaiting the celestial fireworks. The
veteran three-some finds comets all the time, but the "squashed" comet
Carolyn Shoemaker spotted in images taken the night of March 23, 1993,
has become the most memorable of all.

Calculations now show it had been orbiting Jupiter for perhaps a
century, too distant and too faint to be seen from Earth. It's
classified as a periodic comet, one that passes through the solar
system more than once. Had it not shattered into multiple
light-reflecting fragments as it swept past Jupiter in July 1992, it
would not have been noticed in time for astronomers to observe the
impending collision.

"This probably happens only once in many lifetimes," Eugene Shoemaker
says of this month's event. "We're extremely lucky." Jupiter may
capture a comet, if only temporarily, two or three times a century,
but a collision of this sort probably occurs only once every few
thousand years.

Although the impact has been widely anticipated and discussed, no one
knows exactly what to expect. Scientists cannot accurately predict the
effects of the collision until they ascertain the size of the comet
fragments headed for Jupiter. This information is likely to be elusive
until the moment of impact.

Images taken by the Hubble Space Telescope a year ago showed 21
distinct pieces, with the largest appearing to be about four
kilometers in diameter. Since that time, three of the pieces have
disappeared, reducing the number of fragments to 18. But three others
have broken apart, returning the count to 21.

Hubble looked at the comet again in January and late March, after
corrective optics had been installed to improve the telescope's
vision. The best estimates from the Hubble Space Telescope Institute
now show the fragments may be no larger than one kilometer across and
possibly smaller. But even with updated images from Hubble,
astronomers acknowledge there is much uncertainty. Each fragment spews
highly reflective dust, which dramatically increases its visibility
but also helps obscure the nucleus where most of the mass resides.

Assuming the larger pieces are a kilometer in diameter, astronomers
calculate that these fragments will penetrate 100 kilometers beneath
Jupiter's cloud layer before exploding in the planet's thick
atmosphere, composed primarily of hydrogen and helium gases. Each
chunk will explode with a force greater than all the nuclear weapons
on Earth, and together they will pack a million-megaton punch --50
million times more powerful than the atomic bomb dropped on Hiroshima.

If a comet the size of Shoemaker-Levy bit Earth's solid surface, it
would explode instantaneously--altering the global climate and
devastating human civilization. But the effects of a comet plunging
into Jupiter's thick, gaseous atmosphere are less certain.

To predict what will happen when the collision occurs, researchers at
Sandia National Laboratories in Albuquerque, N.M., are using the
world's fastest coupled with software originally developed to model
nuclear weapon blasts. Their forecast: The fragments will plummet
through the uppermost reaches of the planet's atmosphere as rigid
objects, creating bow shocks like those produced by supersonic jet
fighters. As the fragments penetrate deeper, they will be deformed by
the ever-thickening atmosphere and will explode after a few seconds.
The trail of hot air and debris left behind each fragment will then
expand, creating an enormous fireball that will erupt above Jupiter's
cloud tops. The Sandia scientists are not sure whether these fireballs
will be visible from Earth.

Another computer model, developed by comet expert Zdenek Sekanina at
NASA's Jet Propulsion Laboratory in Pasadena, Calif., predicts even
more violent effects. While many astronomers suspect that most of the
comet's brightness is light reflected by dust, Sekanina believes the
light indicates a larger cometary core that could be some four
kilometers in diameter, with other fragments perhaps two kilometers
across.

The explosions of these chunks would roil Jupiter's well-known ammonia
cloud bands and possibly induce storm vortices that would be visible
from Earth, says Sekanina. His calculations show that the explosions
could send cometary particles and dust rippling through the entire
atmosphere, where sunlight would reflect off the spray, brightening
the planet. Europe had a similar period of "white nights" in 1908,
after a large extraterrestrial object exploded above Tunguska in
Siberia. The atmospheric blast kicked Up dust that was carried west by
prevailing winds. At night, the particles reflected distant sunlight,
brightening the sky.

If the explosions of the comet fragments are bright enough, the light
might reflect off Jupiter's atmosphere and its four moons. In effect,
they could all become mirrors of the event. Astronomers at Lowell
Observatory are deploying telescopes around the world to look for just
such reflections.

Although Jupiter can be seen with the naked eye as a bright dot in the
sky, it may be difficult for most people to notice any light
reflections caused by the comet impact, even with binoculars.
Sophisticated and expensively equipped amateurs may be able to detect
some signs of the crash, however. "If I were an amateur astronomer
with a telescope, I'd be looking," says Clark R. Chapman of the
Planetary Science Institute in Tucson, Ariz.

The clouds now visible on Jupiter consist of ammonia ice, and the rust
color comes from a "smog" that develops when sunlight changes the
clouds' chemical composition. Explosions in Jupiter's atmosphere could
liberate additional ammonia gas, flinging it high into the
stratosphere where it would freeze to create a new cloud band that
could encircle the planet for months, says Reta Beebe, a New Mexico
State University astronomer who studies Jupiter. Jupiter's winds would
spread any new clouds around the planet in a band south of the famous
Great Red Spot.

Gases splashing from deep within the Jovian atmosphere could offer
clues about the composition of the mysterious inner regions of the
planet. By studying the spectral signatures of the gases emitted,
researchers may learn whether water, carbon monoxide, and other
compounds are present.

Dust from the comet's breakup may even be captured by Jupiter's
magnetic fields to create a new ring around the planet, according to
some researchers. A new ring will probably form within a decade and
could be as bright or brighter than the current ring, according to
Mihaly Horanyi of the University of Colorado at Boulder.

Sekanina emphasizes there is a limit to the value of predictions. "We
have absolutely no precedent for this event in human experience," he
says. "Everything you hear from us is an extrapolation from what we
know about much smaller events."

The comet fragments may be so weakly bound that they will break up
further as they fall inexorably toward Jupiter. If the remaining
material is not tightly packed, it might produce little effect,
cautions McFadden. The result could be similar to "tossing a rock in
the ocean," she says.

But even if the fragments disintegrate into close groupings of
house-size objects, they should produce a "fantastic meteor shower,"
emitting brilliant flashes of blue-white light as they vaporize in the
atmosphere, says Chapman. To the Galileo spacecraft, the blue-white
flashes would look "like a fireworks show seen from a great distance,"
he says.

"Lots of energy will be deposited in Jupiter," adds Chapman, who
describes the comet impact as the most widely anticipated event in the
history of modern astronomy. "I think we simply don't know which
scenario will happen."

One effect is certain: The Shoemaker-Levy impact has turned the solar
system into a giant laboratory. Astronomers typically discover
explosive events, such as supernovas, many light-years after they have
happened. But this time, they have a chance to watch the drama as it
unfolds.

Virtually all of the major research telescopes on Earth will be
watching, as will other observatories in space. Galileo's operating
programs have been rewritten in months of marathon sessions. Even so,
the craft will be able to view only 12 of the fragment impacts, says
Chapman, who directs the optical imaging team organizing the
spacecraft's study of the collision.

Furthermore, Galileo will return less than five percent of the images
it captures, because its primary antenna for relaying information is
real functioning. Astronomers will compare the calculated impact times
with Hubble's views and retrieve the most interesting images from
Galileo, Chapman says. The craft's infrared and ultraviolet
spectrometers will also be operating.

The Ulysses spacecraft, speeding toward the sun, will listen for radio
waves from electrical storms generated by the friction of dust and gas
passing through Jupiter's atmosphere. Meanwhile, the Earth-orbiting
International Ultraviolet Explorer will look for the Jovian equivalent
of an aurora borealis.

The Kuiper Airborne Observatory, a C-141 jet, will use its infrared
telescope to search for evidence of water liberated from beneath
Jupiter's ammonia clouds. The observatory can fly at 40,000 feet,
above most of the water in Earth's atmosphere.

And in Hawaii, NASA's Infrared Telescope Facility will look for signs
that Jupiter has a solid core, perhaps made of metallic hydrogen. If
the core is solid, McFadden says, the comet's explosions should create
"seismic waves": Sound waves from the explosions would bounce off the
core, heating the atmosphere slightly. The infrared telescope should
be able to detect this change to within less than one-tenth of one
degree centigrade.

Even Voyager 2, retreating from the solar system, is participating.
Its ultraviolet spectrometer will be trained on the planet it once
visited. It has a direct view of the impacts, so it can measure
brightness.

Scientists hope the dazzling display will reveal some of the secrets
of the giant planet, which is one-hundredth the mass of the sun and an
orb that some scientists regard as a failed star. One of the most
provocative theories being discussed concerns Jupiter's role in
altering the path of comets as they swing through the solar system.

Jupiter has such a tremendous gravitational tug that it routinely
lures small comets, if only pulling them off course a bit. In 1886,
after noticing that Comet Brooks 2 had split apart, astronomers
calculated that the breakup could have occurred as the comet passed by
the planet.

Only rarely do comets collide with Jupiter. In a more typical
scenario, comets are drawn toward the planet, fractured, and flung
into space. Chapman says evidence of this may be seen on Jupiter's
moons, Ganymede and Callisto, which bear odd chains of craters seen in
images from Voyager 1. These moons are relatively small targets. For
them to have been hit suggests to Chapman that there may have been
millions of cometary breakups at Jupiter in the past billion years.
Most of the fragments would have been hurled toward the inner solar
system.

This scenario of capture, breakup, and liberation would mean that
comets are not pristine bodies from afar, but rather broken fragments
of earlier bodies, Eugene Shoemaker says. Perhaps more important, it
would mean they often arrive in concentrated storms.

The fractured Comet Shoemaker-Levy is fueling the tantalizing
hypothesis that Jupiter acts as a gravitational fulcrum, breaking up
comets and raining them on the inner solar system in "comet showers."
Some scientists believe the extinction of the dinosaurs 65 million
years ago may have resulted from a comet shower set in motion years
earlier at Jupiter, some four billion miles away (see "Comet Crash Or
Asteroid Attack: What Killed The Dinosaurs?"). Shoemaker, for example,
observes that, "You could have had a bombardment of these comets with
Earth."

Scientists studying a layer of iridium dated to 65 million years ago,
the time the dinosaurs vanished, argue that the rare metallic element
was deposited by a lone asteroid or comet impact. But Shoemaker says
the evidence shows two separate layers, laid down by successive
impacts, perhaps from a comet storm.

Regardless of whether it was a single asteroid or a comet shower that
wiped out the dinosaurs, Comet Shoe-maker-Levy's collision with
Jupiter should help scientists test their models of catastrophic
impacts. The comet will strike with roughly the same amount of energy
as the impact believed to have occurred 65 million years ago on Earth.

DIAGRAM: The distant Voyager 2 may detect a flash of ultraviolet light
at that moment of impact. The shattered comet will strike Jupiter's
night side, south of the Great Red Spot. The Hubble Spacemen Telescope
will capture the most detailed images but won't have a direct view of
the crash site. Headed for a 1995 rendezvous with Jupiter, Galileo,
will have the best view of the collision.

PHOTOS: String of pearls: Hubble Space Telescope mosaic image shows
the fragmented comet headed for Jupiter.

~~~~~~~~

By DAVID GRAHAM

Edited by Chris O'Malley
_________________________________________________________________

Inset Article

IDENTIFYING CELESTIAL OBJECTS

Quick! What's the difference between an asteroid and a comet? If you
can't answer, don't worry: Astronomers themselves often hard
difficulty telling these objects apart. To clear up the
confusion, here's a brief rundown on six often-misused terms
for solar-orbiting objects smaller than planets:

• Meteoroid: Any small rocky or metallic object that orbits the
sun.

• Asteroid: A large meteoroid. How large depends on whom you ask,
but generally anything more than a few hundred feet across
qualifies. Most asteroids orbit in a belt between Mars and
Jupiter.

• Comet: An orbiting object composed primarily of ice and frozen
gases. Many comets, such as Halley's, have long eccentric
orbits that swing them past Earth every few dozen years.

• Near-Earth Object: Any meteoroid , asteroid, or comet whose
orbit frequently brings it near Earth.

• Meteor: Any meteoroid, asteroid, or comet that actually enters
Earth's atmosphere, creating meteor showers.

• Meteorite: A meteor that makes it to Earth's surface.
Generally, an object must be at least 100 feet across to
survive atmospheric entry, but small pieces of larger objects
sometimes reach the ground.
_________________________________________________________________

Inset Article

COMET CRASH OR ASTEROID ATTACK: WHAT KILLED THE DINOSAURS?

Sometimes it takes decades for a scientific theory to gain acceptance.
This seems to be the case for the hypothesis that a massive
meteorite wiped out the dinasours and other creatures 65
million years ago. When Nobel Prize-winning physicist Luis
Alvarez first proposed the theory in 1980, many of his peers
professed skepticism. But over the years, geologists have
slowly unearthed evidence, and "the consensus is moving ever
more toward the impact theory," says Clark R. Chapman of the
Planetary Science Institute in Tucson, Ariz.

The tide of opinion began to turn in 1991, when geologists located the
probable impact site: a 120-mile-wide submerged crater on
Mexico's Yucatan Peninsula. After more precisely measuring the
slight gravity variations produced by the crater, researchers
now calculate that its diameter is about 185 miles. If this is
confirmed, the new size estimate would appear to eliminate
objections that the Yucatan meteorite was not large enough to
cause worldwide catastrophe.

Paleontologists have long remained the prime objectors to the Alvarez
theory, arguing that the fossil record for many species
indicates a gradual extinction. But they too are changing their
tune. In a recent blind test of the Alvarez theory, University
of Miami sedimentologist Robert Ginsburg sent soil samples
containing fossils of marine microorganisms to several
paleontologists for identification. To eliminate unintentional
bias, Ginsburg kept the ages of the fossil samples to himself.
When he finally pooled the results, sure enough, he learned
that almost all the species from the dinosaur era remained
alive right up to the time before the impact.

So was it an asteroid or a comet that hit the Yucatan Peninsula? "No
one really knows," says planetary scientist David Morrison of
the NASA Ames Research Center in Moffett Field, Calif. The
traceable effects of the impact, he explains, could been caused
by either type of celestial object.
_________________

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Source: Popular Science, Jul94, Vol. 245 Issue 1, p42, 5p, 1 diagram,
2c.
Item Number: 9407087633