Title: The threat from space.
Subject(s): EARTH; METEORS; COMETS
Source: Earth, Aug96, Vol. 5 Issue 4, p24, 8p, 1 chart, 12c
Author(s): Desonie, Dana
Abstract: Looks at the potential danger from the cosmic collision
involving the planet earth and meteors/comets on the human race. Mass
extinction in the earth as the result of a bombardment; Objects which
pose a threat to the earth; Odds of a lethal cosmic collision;
Possibility of defense against this phenomenon. INSET: Craters,
craters everywhere, by Alexandra Witze.
AN: 9607032823
ISSN: 1056-148X
Note: This title is not owned by Tucson-Pima Public Library.
Database: MasterFILE Elite

THE THREAT FROM SPACE

Asteroids and comets have slammed in to Earth before. They will do so
again--but when? And what can we do about it?

It could happen to you.

Just as a Triceratops may have done some 65 million years ago, you
look up and see something strangely bright in the sky. An hour later
it appears even more brilliant. Is it a supernova? An alien ship? A
comet? Unlike that dinosaur, you might have been warned: A killer
meteorite is headed your way.

The object grows larger and brighter until, three hour later, it
enters Earth's atmosphere. It ignites into a blinding fireball that
strikes the surface ten seconds later.

By chance, ground zero is shallow water; the impact generates enormous
tsunamis that flood coastal regions thousands of miles away. Dust and
gas fly heavenward and then coalesce into fireballs that fall back
toward Earth. The energy released heats the atmosphere to the
temperature of a kitchen oven on broil. Land animals roast. Forests
burst into flames. dust particles and smoke block the Sun out and day
into night. Earth plunges into a deep freeze. Without sunlight,
photosynthesis ends; plants perish and animals starve.

Sulfuric gases released from the shattered limestone rock mix with
water in the atmosphere to form acid rain. Over the next several
months, the rain falls to Earth, dissolving the shells of tiny marine
plants with its acidity. The food web of the ocean collapses. Carbon
released from the limestone combines to form the greenhouse gas carbon
dioxide. Within months the deep freeze ends, and years of intense
warming begin.

Sound farfetched? This list of horrors was compiled from the work of
many scientists to describe the environmental devastation that
resulted 65 million years ago when a meteorite six miles across struck
Mexico's Yucatan Peninsula with ten billion times the energy of the
atomic bomb that destroyed Hiroshima. Although scientists still debate
exactly what happened then, many are convinced that the collision
triggered a global mass extinction during which roughly two-thirds of
Earth's species, including the dinosaurs, were wiped out.

If such a cosmic collision were to occur today, life on Earth would
once again be utterly changed, with countless species -- including
perhaps our own -- being wiped out forever. Any humans who did weather
the blast would experience global crop failures and sweeping famine
because of the blocking of the Sun's light. "Human civilization would
be completely rebooted," says William K. Hartmann of the Planetary
Science institute in Tucson, Arizona.

Hartmann and many other scientists believe that impacts have played a
crucial role in the evolution of life on our planet by wiping out
dominant species and giving others a chance to flourish. For example,
many paleontologists think that dinosaurs were agile and cunning, that
they were well-adapted to their environments and dominated the
smaller, less highly specialized mammals. Yet in a geologic instant
the dinosaurs' 140-million-year reign ended, and mammals took over the
planet. Other extinctions have also been tentatively linked to
impacts: Scientists have speculated that the mass extinctions of the
late Devonian Period, roughly 365 million years ago, were linked to at
least one impact at the same time.

Another doomsday may be inevitable. But do we have no more control
over our destiny than the dinosaurs had over theirs? Or can we locate
our attacker before it strikes and destroy it before it destroys us?

Yes, scientists say, there are things we can do to cheat fate. We can
search the skies for an object on course to strike our planet; most
scientists agree that such a search is wise, but they disagree on
whether more should be done. Some say we should construct a defense
system now, to be ready to send missiles or even nuclear weapons into
space so that we can destroy the object or deflect it from its
collision course. Others argue that the risks are so long-term that
construction of a defense system can wait until technological advances
yield solutions that society can afford.

One thing is perfectly clear, however. Another asteroid or comet will
hit Earth again. It's only a matter of time.

Scientists know Earth has suffered repeated bombardment since it
formed 4.5 billion years ago. But this consensus was a long time in
coming. In fact, it wasn't until 1957 that scientists even agreed that
Earth had a genuine meteorite crater. After decades of debate, a
geology graduate student named Eugene Shoemaker argued convincingly
that the iron fragments, layers of broken rock and shock-melted glass
droplets found at Arizona's Meteor Crater, also known as Barringer
Crater, could only have formed by meteorite impact. An asteroid about
200 feet in diameter had broken up in Earth's atmosphere, and some of
its fragments and its shock wave slammed into northern Arizona 50,000
years ago, creating one of the best-preserved impact structures on
Earth. Since then, scientists have catalogued at least 150 impact
structures on Earth, up to 150 miles in diameter, according to impact
expert Richard Grieve of the Geological Survey of Canada in Ottawa.

The idea that cosmic collisions could shape the evolution of life on
Earth gained enormous credibility in 1980 when the father-son team of
Luis and Walter Alvarez and two colleagues suggested that a meteorite
impact annihilated the dinosaurs and the many other forms of life that
became extinct 65 million years ago. The team based their conclusion
on rich concentrations of iridium found in layers of rock in many
areas of the world, all dating to 65 million years ago. (This element
is virtually absent from Earth's surface but is abundant in
extraterrestrial rocks.) Since then, other scientists have found the
purported crater, buried offshore near its namesake village of
Chicxulub in the Yucatan.

The Alvarez theory prompted scientists to ask new questions. If a
random event could so profoundly alter the course of life on Earth,
could it happen again? And if so, when might that happen? Should we be
worried about the hazards of extraterrestrial impacts?

Such questions became even more pressing after July 1994, when we
first witnessed a collision between a large object and a planet. The
planet was Jupiter; the object was the shattered comet known as
Shoemaker-Levy 9. Over a period of several days, more than 20 comet
fragments pummeled Jupiter. Plumes of gas and dust shot thousands of
miles into the stratosphere. Astronomers on Earth detected the heat
given off as the material fell back.

If Earth had been Shoemaker-Levy 9's target, the impact would have
been "a globally devastating event," says astronomer Tom Gehrels of
the University of Arizona in Tucson. And yet such events are not
unlikely.

Our solar system is teeming with objects of all sizes, from particles
of dust to planets. Most pose no threat to Earth because they are too
small or too distant. The two main hazards to Earth, though, are
 comets and asteroids. Both types of objects can sometimes come   perilously close to our planet. On May 19 of this year, for example,
an asteroid about one-quarter of a mile across whizzed past only
280,000 miles away --approximately the distance to the Moon. This was
the largest object ever to get so close to Earth. Most unsettling,
astronomers didn't spot the object until just a few days before it
zipped by.

Asteroids are rocky or metallic objects ranging in size from particles
to tiny planets nearly 600 miles across. Most asteroids orbit the Sun
between Mars and Jupiter in what's called the main asteroid belt.
They're thought to be debris that didn't coalesce into a planet
because of Jupiter's tremendous gravitational pull.

Occasionally two main-belt asteroids collide, ejecting fragments that
veer into the inner solar system. The fragments thus become near-Earth
objects that could intersect our planet's orbit. The gravitational tug
of Jupiter can also nudge a main-belt asteroid into a near-Earth path.

Comets are composed of rock and dust bound together by ice. (The ice
changes directly to vapor when the comet nears the Sun, producing a
magnificent tail.) And they're big: The nuclei of comets can measure
as large as 150 miles across, though most of them are only about 10
miles across.

Comets come from different places. Some orbit the Sun in the Kuiper
Belt just beyond the planet Neptune. Others reside in the Oort Cloud,
which reaches a fifth of the distance to the next nearest star. Each
year, several previously unrecorded comets, such as the recent Comet
Hyakutake, blaze through the inner solar system, perhaps dislodged
from the Oort Cloud by gravitational interactions with other comets.

Between comets and asteroids, Earth faces a pretty good chance of
getting walloped. At present, more than 200 noteworthy objects with
orbits that cross Earth's orbit have been discovered, though none are
predicted to hit anytime soon. However, there is at least one
prediction, made recently by Paolo Farinella of the University of Pisa
in Italy and colleagues, for a collision between Earth and a large
asteroid. In eight computer simulations of possible orbits for a
14-mile-wide asteroid named Eros, one forecast a catastrophic impact
with Earth 1.14 million years into the future.

According to Gehrels, at the present rate of discovery --
approximately 50 a year and climbing -- it will take about a century
to catalog essentially all of the objects that we know could pose a
threat.

The odds that a lethal cosmic collision will occur in your lifetime
are a lot higher than you probably think. "The chances that you'll die
from an asteroid impact are the same as [the chances of your] dying in
a plane crash if you take one round-trip a year," says David Morrison
of NASA's Ames Research Center in California.

Large meteorite impacts are extreme cases of low-frequency,
high-consequence risks -- events that occur very infrequently but
cause enormous numbers of deaths. Many other natural disasters, such
as earthquakes, also represent low- frequency, high-consequence risks.
Yet most natural disasters are more limited in magnitude than a large
collision. For example, geologists believe that Earth's crust can
build up only so much stress before that stress must be released in an
earthquake; this restricts the size of earthquakes. An Earth-bound
asteroid, however, could measure hundreds of miles across. The 1976
Tangshan earthquake in China is thought to have killed as many as
three-quarters of a million people, but a collision with an object
larger than one mile in diameter could wipe out billions, scientists
believe.

Fortunately, the more lethal the object is, the less likely it is to
strike Earth's surface. Asteroids less than about 160 feet across burn
up in the atmosphere. A somewhat larger meteor might explode above
ground and damage the surface with its shock wave. Objects measuring
between 300 and 3,300 feet are estimated to strike every 1,000 to
200,000 years. If one of these hit the sea, it would send out a
16-foot-high tsunami that would travel thousands of miles and kill
millions of people. On land, the same impact would create a
three-mile-wide crater and destroy almost 4,000 square miles of
property. A hit in New York City's Central Park could kill 25 million.

Scientists estimate that it would take a meteorite about one mile
across to trigger a catastrophe in which more than one-quarter of the
human population dies. An asteroid of that size is thought to strike
Earth about once every 300,000 years.

Scientists disagree on which objects pose the greatest threat. Many
think enormous objects are the most dangerous though they are
relatively rare. "The largest -- bigger than a mile across -- are most
bothersome because they have the potential for global and catastrophic
effects," says Gregory Canavan of Los Alamos National Laboratory in
New Mexico. Other scientists think more frequent, smaller collisions
are the biggest danger. "What's more of a threat, in terms of global
economy, are the smaller objects, most of which we haven't discovered
at all," Hartmann says.

The first steps in addressing such hazards are to locate
Earth-threatening objects and then to calculate their orbits. Today,
few scientists are engaged in the effort to find near-Earth objects,
though the number is growing. Morrison is fond of saying "The total
staff searching worldwide is less than the total staff of one
McDonald's restaurant."

Gehrels and his colleagues at the Space-watch telescope at Kitt Peak
National Observatory near Tucson have discovered about two-thirds of
the known near-Earth objects. Other new or updated search programs are
based at the Lowell Observatory in Flagstaff, Arizona, and in
Haleakala, Hawaii. The Haleakala program alone discovered four new
Earth-crossing asteroids and a fast-moving comet last March, the first
"good weather" month for observing since the program began in December
1995. "These discoveries certainly suggest that we could face a
surprise encounter with a large, unseen object," says lead scientist
Eleanor Helin.

Still, many scientists think these efforts are not enough. "At
present, we know only one to three percent of the objects that could
do global damage or even extinguish the human race," says astronomer
Brian Marsden of the Harvard-Smithsonian Center for Astrophysics in
Cambridge, Massachusetts.

If scientists locate an asteroid or comet threatening Earth, it would
probably be far enough away that we'd have decades, perhaps centuries,
to prepare for its impact, Marsden says. What kind of strategy rye
would mount against the hazard depends not only on the length of time
available but on the size and makeup of the object.

If a meteor were identified that would hit in only a few months, "all
we could do would be to plot where the impact would occur and attempt
evacuations and civil defense-type measures," says Alan Harris of the
Jet Propulsion Laboratory in Pasadena, California. With more advanced
warning, it might be possible to deploy specially designed nuclear
warheads to blow up the object.

But there are drawbacks to such a strategy. The explosives must be
enormously powerful or else must bury themselves deep enough in the
asteroid to shatter it into fragments less than 160 feet across --
small enough to ensure that they will burn up in Earth's atmosphere.
Any large remaining fragments also on a collision course with Earth
could be just as serious a threat as the original object.

Deflecting the invader from its path appears to be a better strategy.
A small nudge, averaging less than half an inch per second over ten
years, can push an object six-tenths of a mile in diameter out of a
collision course with Earth. The thrust could be provided by a series
of one megaton nuclear explosives detonated on the asteroid's side. An
asteroid can also be deflected if its mass is decreased enough to
change its velocity. A bomb detonated near the body could cause the
outer layer to spall off and alter its mass.

Some scientists think a nuclear solution is the only sensible way to
avoid a collision. They say the technology is understood and could be
modified quickly for use. "Nuclear weapons are the highest-energy
weapons we know of and the odds-on favorite to intercede and blow the
thing up into chunks or slowly deflect it," Harris says.

Others support the use of nuclear weapons to deter collisions in only
the most extreme cases. They maintain that nuclear weapons may not be
as effective as some think because the physical properties of
asteroids are poorly understood. In addition, there are inherent risks
in the very existence of such a powerful nuclear arsenal, and, by
international agreement, it is illegal to detonate nuclear weapons in
space. A few scientists, such as Carl Sagan of Cornell University,
believe that a nuclear defense system capable of deflecting an
approaching object could be used by a terrorist organization to knock
a harmless object into a collision course with Earth.

Non-nuclear technologies to deflect an asteroid, including Earth-or
spacecraft-based lasers, rely on altering the asteroid's mass by
incinerating its outer layer. Planetary scientist Jay Melosh of the
University of Arizona has proposed vaporizing the object's surface by
focusing sunlight from an enormous solar collector. If necessary, a
solar collector defense might be realized in a decade.

Most scientists involved in these studies agree that humans probably
won't be wiped out by an impactor in the next generation. So, they
say, it is too dangerous and expensive to develop a full-scale defense
system in the absence of an identified threat. "If an asteroid is
found to be on a collision course, people will spend anything to
defend against it," Melosh says. "But I don't think large sums will be
spent to protect against anything purely hypothetical."

A very few scientists think the seriousness of the hazard justifies
beginning work on a defense system now. Edward Teller, often called
the father of the hydrogen bomb, has in recent papers and talks
advocated experiments on asteroids and comets, including testing
nuclear explosions on asteroids in space. In his opinion, expenditures
of roughly $100 million per year are warranted.

Meanwhile, Melosh and many others agree that research on defense
systems should be maintained at a low level with an emphasis on
non-nuclear defenses. They advocate locating and determining orbits
for all Earth-crossing asteroids and increasing efforts to search for
hazardous comets.

"There's a good reason to conduct surveys and to try to determine how
much hazard we have and if something is coming our way," Harris says.
"I personally don't think it's worth another Cold War build-up just in
case."

Still, questions about the future of humanity linger. If the impact
theory is correct, mammals didn't get ahead because they were better
adapted or smarter than reptiles; their rise to power was brought on
by a roll of the cosmic dice. Dinosaurs didn't survive the impact,
whereas a precious few stocks of mammals did. "It is not some imagined
superiority of mammals versus reptiles but much more a matter of sheer
chance that finally shifted the balance of control of terrestrial
ecosystems from reptiles to mammals," writes paleontologist Niles
Eldredge. When the dice are thrown again, our kind may be the next to
go.

We can allow evolution to take its natural course or we can do what
the dinosaurs did not have the capability to do: We can search and, if
necessary, we can defend. Marsden suggests that we have a
responsibility to do more than just survey for threats to Earth. He
points out that it takes only one object hitting Earth to wreak havoc
on the planet.

"Chances are it will be 100 million years in the future, but it could
be tomorrow," he says. "Considering all the tomorrows there are in the
next couple of centuries, we owe it to our descendants to do something
about it."

Meteorites striking Earth are events that don't happen very often, but
when they do they can be deadly on a global scale. Your os of dying in
a meteorite impact may be higher than you think.

Size of impactor        Frequency of impact        Odds of dying

Less than 50 meters     Burns up in atmosphere              None

50-100 meters           Every few centuries or    1 in 3 million
100,000 years in cities.

100 meters-1 kilometer  1,000-2,000 years            1 in 25,000

Greater than 1 km       Every 100,000 years         1 in 200,000
or greater

PHOTO (COLOR): An asteroid slams into Washington, D.C. in this
painting by space artist Michael Carroll. As farfetched as it seems,
this scene is entirely plausible: fragments of asteroids and comets
have battered Earth repeatedly over its history and will undoubtedly
do so again.

PHOTO (COLOR): Though buried in Mexico's Yucatan Peninsula, the
Chicxulub Crater is visible in this image of the gravity exerted by
the crust here. This crater was created when an asteroid slammed into
Earth 65 million years ago. The impact may have indirectly killed
two-thirds of Earth's species, including the dinosaurs.

PHOTO (COLOR): The South Pole-Aitken basin on the Moon, shown here as
a purple and blue splotch, is the largest impact crater known in our
solar system. An asteroid or comet slammed into the Moon with such
force that it penetrated the mantle and left behind this scar, more
than 1,300 miles across.

PHOTO (COLOR): In this mosaic based on images from the Clementine
spacecraft, Earth rises above the pockmarked surface of the Moon. Were
it not for the continuous recycling of Earth's crust by plate
tectonics, Earth's surface would probably bear many marks from impacts
as well.

PHOTO (COLOR): Some impact craters on Earth have not yet been recycled
by tectonic processes. This is Meteor Crater, also known as Barringer
Crater, in northern Arizona -- one of the best-preserved. It was also
the first to be recognized as the scar of an extraterrestrial object
slamming into our planet.

PHOTO (COLOR): The crater known as Manicouagan in eastern Quebec,
Canada, measures an impressive 62 miles across.

PHOTO (COLOR): This view of Jupiter in 1994 using ultraviolet
wavelengths shows the "bruises" left by pieces of the shattered Comet
Shoemaker-Levy 9 that punched into the giant gas planet. (The bruises
are in the southern hemisphere.)

PHOTO (COLOR): This map depicts most of the roughly 150 impact craters
that have been discovered on Earth. The apparent concentration of
craters in North America, Europe, Australia and Russia is partly due
to thorough exploration of these areas by geologists.

DIAGRAMS: The two main threats to Earth come from wayward comets or
asteroids that are nudged out of their usual paths into Earth-crossing
orbits. Asteroids are bits of rock that mainly orbit the sun between
Mars and Jupiter (top diagram). They're thought to be debris that
never coalesced into a planet because of Jupiter's wrenching
gravitational pull. Comets, on the other hand, are bits of rock held
together by ice. They reside in two main areas outside our solar
system known as the Kuiper Belt and the Oort Cloud. The Kuiper Belt
(middle diagram), which includes the planet Pluto, is a disk of rock
debris beyond the orbit of Neptune. The Oort Cloud (bottom diagram)
stretches as far as one-fifth the distance to our next nearest star,
Alpha Centauri. In this diagram, scale is rendered in astronomical
units, or a.u. One a.u. is the Earth-Sun distance.

GRAPH: The graph at left shows how often meteorites of a certain size
smash into Earth today (pink dots) and also how the frequency of
impacts has changed over the course of Earth's history. Small rocks
hit more frequently and do less damage than large ones. (Anything
smaller than about 50 meters across will burn up in the atmosphere and
leave no crater on the surface.) In its infancy, Earth was bombarded
by meteorites far more often than today. In the first 60 million years
of its existence, Earth was struck more often by larger objects
(yellow dots). Then, 4 billion years ago, bombardment began to slow
down (purple dots), until it reached the present rate.

PHOTO (BLACK & WHITE): In 1908, a mysterious object exploded above the
Tunguska Valley in Siberia, knocking residents and their reindeer
flat. After years of speculating on the cause of the blast, scientists
now think that an asteroid about 200 feet in diameter entered the
atmosphere and exploded three to six miles above the ground. The force
of the blast downed thousands of trees, as shown in this 1929 photo.

~~~~~~~~

by Dana Desonie

Dana Desonie teaches geology and oceanography at Arizona State
University in Tempe.
_________________________________________________________________

Inset Article

CRATERS, CRATERS EVERYWHERE

There's something to be said for a bird's-eye view, especially when it
comes to discovering new impact craters.

Scientists have recently been using satellite imagery to identify
features that look like possible impact craters from space. On
the ground, geologists can then positively identify the crater
by finding minerals that are characteristically formed in a
high-energy impact.

The image at left shows what may be a previously unrecognized
meteorite crater in western Wisconsin. Sanjay Limaye of the
Space Science and Engineering Center in Madison, Wisconsin,
says he has noticed this circular feature, which measures more
than 60 miles across, on satellite images of the area for 15
years. This image was taken by NOAA instruments in January
1996.

The image below was taken by the SIR-C radar system carried aboard the
space shuttle. The ring structure left of center is the
previously identified Aorounga crater in Chad. But this new
image reveals more details: a possible second and even third
impact crater, to the right of and above the confirmed one.
NASA scientists speculate that this chain of craters may have
formed when a comet or asteroid broke into several pieces
before slamming into Earth, just as Comet Shoemaker-Levy 9 did
at Jupiter.

PHOTOS (COLOR): Different craters on earth cause by slamming comets

~~~~~~~~

By Alexandra Witze
_________________

Copyright of Earth is the property of Kalmbach Publishing Co. and its
content may not be copied or emailed to multiple sites or posted to a
listserv without the copyright holder's express written permission.
However, users may print, download, or email articles for individual
use.
Source: Earth, Aug96, Vol. 5 Issue 4, p24, 8p, 1 chart, 12c.
Item Number: 9607032823