Title: Target: Earth! (cover story)   
Subject(s): ASTEROIDS -- Environmental aspects
Source: Astronomy, Oct95, Vol. 23 Issue 10, p34, 8p, 1 graph, 5c, 2bw
Author(s): Morrison, David
Abstract: Examines the possible effects of asteroid impacts on the
Earth's environment. Implications of the crash of Comet Shoemaker-Levy
9 into Jupiter; Average accumulation of extraterrestrial material by
the Earth; Formation of tsunamis; Combination of fire storms and ice;
Estimation of the number of near-Earth-objects (NEOs); Efforts to
detect NEOs. INSETS: They stand on guard for thee; Cold War weaponeers
gaze skyward.
AN: 9509276740
ISSN: 0091-6358
Note: Tucson-Pima Public Library subscribes to this magazine.
Database: MasterFILE Elite

TARGET: EARTH!

Searing fireballs, massive tidal waves, decade-long winters . . . such
a fate could befall Earth at any moment. Is there an asteroid
out there with our planet's name on it?

The crash of Comet Shoemaker-Levy 9 into Jupiter in July 1994 was the
most widely witnessed event in astronomical history. For a few hectic
days professional astronomers turned every telescope in the world (and
above it) toward Jupiter. They were joined by legions of backyard
observers who could see Jupiter's Earth-sized impact scars in scopes
with apertures as small as 2 1/2 inches. For years to come, such
sights will remain etched in the minds of those who saw it first hand.

This indelible legacy of the Great Comet Crash of 1994 raised
humanity's awareness of catastrophic impacts in the solar system.
Watching Jupiter suffer hit after hit left scientists and laymen
wondering aloud if the same thing could happen to Earth.

Truth is, planetary scientists have known for a long time that Earth
is subject to bombardment from space. You have only to look at the
Moon's battered landscape to realize that we live in a bad
neighborhood. Cosmically speaking, Earth and Moon are very close
neighbors, and whatever rain of debris hit our satellite must have had
a similar effect on Earth. Were it not for the constant reworking of
Earth's surface by erosion and plate tectonics, our planet would be as
densely cratered as the Moon.

The real scientific question is not whether Earth has been battered by
impacts over its long geological history. We know that for a fact.
What's revolutionary in today's thinking is that our fragile
environment can be disrupted by relatively small impacts. And it's not
just the immediate effects of an impact we should be concerned with;
S-L 9 has helped us get a feel for consequences that can linger long
after the initial strike.

All this is a far cry from barely 20 years ago, when few scientists
even considered the possibility that asteroid hits could shuffle
Earth's biological deck.

Survival of the Toughest 

Our thinking about how asteroid impacts influence Earth's environment
began to change with a serendipitous discovery in 1978. Lius and
Walter Alvarez of the University of California at Berkeley were
searching geological rock layers for a largely extraterrestrial
element called iridium. Iridium, they figured, drizzled down on Earth
at a nice, steady rate in the form of meteor dust. Measuring the
amount of iridium in these layers could help scientists figure out how
fast sediments collected on the floors of ancient oceans.

What they found instead was a major infusion of the element in a rock
boundary between the Cretaceous and Tertiary eras, which marks when
the age of dinosaurs ended some 65 million years ago. There was enough
iridium in the K/T boundary, they calculated, to equal a
10-kilometer-diameter asteroid.

Based on this evidence, the team announced that a 10-km asteroid, a
Great Extinctor, caused an environmental catastrophe that ended the
Cretaceous era. They concluded that the dinosaurs suffered a global
cool-down caused by dust thrown up into the stratosphere by the
impact. Today we now know that this dust cloud is just one of several
environmental catastrophes associated with an impact of this size.

As if this astounding discovery weren't enough, the Alvarezes went on
to propose a sweeping generalization. If a hit of this magnitude
killed the dinosaurs, perhaps similar impacts might have happened at
other times in Earth's history and played a major -- perhaps dominant-
role in biological evolution. With asteroid impacts in the picture,
suddenly one of a species' most important traits is its ability to
survive random cosmic assaults. The winner of an evolutionary race is
not a species that is stronger or faster or even smarter, but one that
survives an occasional intruding asteroid or two. Natural selection
still works, but the context has changed. Asteroids enter the
evolutionary game as wild cards.

Over the past 15 years, evidence for a dinosaur-snuffing catastrophe
has continued to accumulate, culminating with the 1991 discovery of a
telltale crater (named Chicxulub) under Mexico's Yucatan peninsula.
Scientists now estimate from the crater's size that the progenitor
asteroid hit with an astounding energy equivalent to more than five
billion Hiroshima atomic bombs (100 million megatons).

With that sobering statistic in mind, today geologists sift the
geological record for hidden evidence that Earth might have suffered
other impact-induced mass extinctions. Although that evidence is slow
in coming, the events that killed the dinosaurs 65 million years ago
are no longer a matter of serious dispute.

Skies of Fire, Tracts of Ice 

Aside from the immediate effects of heat and concussion, two major
post-impact events did the dinosaurs in: a firestorm followed by
excessive cold. What we saw on Jupiter during Shoemaker-Levy's 1994
impact was a window to this lethal combination of fire and ice.

After fragments of Comet S-L 9 exploded in Jupiter's atmosphere they
produced huge fireballs of hot gas and dust that rose high above the
planet's clouds.

Because Jupiter's atmosphere was literally blown away from above the
impact site, hot gas and dust were funneled up to altitudes of 3,500
km. But in Jupiter's strong gravity, whatever went up had to come
down. So for about 20 minutes after each impact, the dust-laden plumes
fell back into the atmosphere, reentering with a horrendous release of
energy. The heat from this reentry was so intense it was easily
detected from Earth.

The same thing happened on Earth 65-million years earlier with the
Chicxulub impact. But in our case, the plume included large quantities
of rock and dust blown out from the crater. When this ejecta rained
down about 30 minutes after the impact, it produced a meteor shower of
almost unbelievable proportions. Meteors turned the sky red-hot and
ignited terrestrial forests and grasslands. Telltale soot from this
firestorm is found in sediments from the K/T boundary at sites all
over the world. Most land animals probably perished by fire.

Then came the cold.

On Jupiter, large, black dust clouds remained clearly visible at each
impact site for weeks, suspended in the stratosphere. Even more than a
year after the impacts, not all the dust has settled back into the
deep atmosphere. These bruises have substantially reduced the amount
of sunlight that reaches Jupiter's lower cloudtops.

In Earth's case, the Great Extinctor hit in shallow water and
excavated several times its mass in finely fractured target rock.
Indeed, the total amount of material measured in the worldwide K/T
boundary layer is about a hundred times greater than the mass of the
10-km asteroid that actually hit. Most of the layer has large grains
that were blown horizontally by the impact and plume particles that
were heated during atmospheric reentry. But a sizable portion of the
layer is in the form of fine dust that remained suspended in the
atmosphere for months, blocking photosynthesis and plummeting
temperatures on the dark surface beneath the clouds.

Suppression of photosynthesis lead to a breakdown in the ocean food
chain that killed most marine creatures. Survivors had to hunker down
to a global drop in temperature not unlike the "nuclear winter"
scenario postulated during the later days of the Cold War.

Still other environmental catastrophes drove the K/T mass extinction
65 million years ago, including global acid rain and destruction of
the ozone layer. Species that survived one stress may have succumbed
to another. But it was the thermal pulse from re-entering ejecta and
the subsequent darkness caused by suspended stratospheric dust that
did most of the killing.

Death by the Megaton 

It's hard to imagine that all this carnage came from a relatively
small asteroid no more than 10 km wide. But what the Great Extinctor
lacked in size, it made up for in kinetic energy, measured in units
the same way we measure the yield of thermonuclear weapons.

The energy of the Chicxulub impact was about 100 million megatons
--that is, about five billion times the size of the atom bomb dropped
.on Hiroshima 50 years ago. Fortunately, events like Chicxulub are
quite rare, occurring at intervals of 100 million years or more. But
what about smaller impacts? Could these produce similar environmental
effects?

Brian Toon and Kevin Zahnle at the NASA Ames Research Center have
calculated the effects of smaller impacts. According to their work, a
3-km impactor, only 2% the mass of a Great Extinctor, would kick up
enough dust to block photosynthesis. And given what we know about the
number of small, near-Earth objects, such small hits are statistically
possible during our lifetimes.

The Earth accumulates about 100 tons of extraterrestrial material
every day under a constant rain of interplanetary debris. Most
meteoroids enter the atmosphere and burn up unnoticed. Some survive
the fiery heat of entry and are slowed down by air friction to a speed
of about 320 km per hour. What's left hits the ground as a meteorite.

Big Ones Dig Deep 

It's quite a different story when Earth's atmosphere can't slow down
large meteoroids. Striking at their space velocity of 15 to 20 km/s,
they explode on impact with an energy 100 times their mass in TNT.
Since such an event would be much more destructive than a simple
meteorite fall, it's natural to wonder what size meteoroid could
devastate the Earth's surface.

Calculations by Christopher Chyba and Kevin Zahnle show that large
objects entering Earth's atmosphere undergo tremendous stress due to
air resistance. As the object fragments and covers more area along its
line of travel, it creates more atmospheric resistance. Soon this
process leads to a catastrophic break-up that causes the asteroid to
decelerate explosively.

If such an explosion takes place at altitudes above 15 km, there is
little harm done near Earth's surface. The outcome isn't so rosy if a
larger object digs deeper into our atmosphere.

Ordinary stony asteroids, which have a rather crumbly composition,
must be larger than 50 meters across (half the size of a football
field) to do any damage at the ground. Such a projectile packs about.
10 megatons of energy, comparable to the largest nuclear bomb. Thus,
Earth's atmosphere protects us from smaller impacts, but not from
those with 10 megatons or more energy. We can expect a 10-megaton
impact about once per century on average.

To find one of the largest atmospheric explosions in recorded history,
we need not look to hieroglyphs or deduce hidden imagery in ancient
mythology. Earth reeled under a tremendous assault as recently as 87
years ago with the great Tunguska bolide of 1908.

On June 30 of that year, a 15-megaton explosion took place in the
atmosphere above a desolate region of central Siberia, devastating
more than 2,000 square km of forest. The Tunguska region is so remote,
it wasn't until 17 years after the explosion that a scientific
expedition visited the site. When the team arrived, they found no
craters or evidence of the meteoritic fragments.

Many astronomy textbooks will tell you that the Tunguska impactor was
probably a comet. If the Tunguska impactor had been cometary, however,
it would have exploded at a higher altitude and done no damage. Had it
been composed of denser iron, it would have reached the ground and
made a crater. Meteor Crater in Arizona was formed by just such a
metallic meteorite, with the same 15-megaton energy as Tunguska.

We now recognize the culprit as a rocky asteroid about the size of a
city office building that decelerated and exploded at an altitude of 8
km. Last year, scientists apparently even found tiny fragments of this
rocky object imbedded in tree resin at the impact site.

Playing Chicken With Oblivion 

If "ground zero" for Tunguska (or Meteor Crater for that matter) had
been a city, the destruction would have been terrible -- equivalent to
that of a large nuclear bomb. Such impacts happen about once every 300
years anywhere on Earth's surface, or about once per millennium on the
land. Even if 10% of Earth's land were densely populated, this amounts
to a hit on a city only once in 10,000 years.

There are other ways cities can suffer the effects of an impact. Were
a Tunguska-class object to land in the ocean, it would produce
tsunamis with tremendous potential for destruction. Tsunamis travel
much farther from the impact site than the direct blast wave,
producing widespread destruction in coastal areas thousands of
kilometers distant.

For every person killed directly by the impact, about ten more would
be killed by an impact-induced tsunami. Yet even the tsunami risk is
less than that of succumbing to an ecological disaster brought about
by still larger impacts, which could trigger crop loss and starvation
on a global scale. Your risk of dying from such a global event is
about ten times greater than that from a tsunami, or 100 times greater
than that of a direct hit.

But a simple comparison of risk between impacts and their secondary
effects doesn't tell the whole story. No other natural disaster we
know of has global dimensions. Even the worst earthquake or flood
affects only a few percent of Earth's population. An impact-induced
ecological catastrophe, however, could kill a billion people and
destabilize civilization. It would end the world as we know it.

In other words, larger impacts, ones caused by asteroids as small as
100 meters right on through the 6-km or-larger Great Extinctors, pose
the greatest risk to humanity. Even though such massive hits are
extremely rare, their global consequences bode worse for humanity than
the cumulative effects of smaller and more frequent hits.

Since the chance of a major impact in our lifetime is so low, impacts
are a danger most people ignore. Therein lies the problem with
statistical arguments. Being human, the weight we assign to threats in
large part determines how "real" they become in our collective mind.
Either there is an asteroid up there in an orbit that will lead to a
collision with Earth within the next century, or there is not. If not,
then we have nothing to worry about. But if there is a big rock up
there with our name on it, we had better find it and take action to
protect ourselves.

There are probably 1,000 near-Earth objects (NEOs) -- whose orbits
cross that of Earth's -- large enough to cause a global environmental
collapse if they hit. But of these 1,000 objects, we have actually
discovered fewer than 100, less than 10 percent. We know from orbital
calculations that none of these 100 objects threaten Earth. But that
says nothing about the 900 we haven't discovered. One of them could
hit at any time, perhaps without warning.

Earth's Guard is Down 

The sad reality is that we humans are not doing a very good job of
policing the inner solar system, where threatening asteroids lurk
undetected. Virtually all the discoveries of near-Earth objects are
made by just four teams of astronomers, one of which is terminating
its search (see "They Stand On Guard For Thee"). This leaves only a
handful of people -smaller than the staff of one McDonald's restaurant
-- to carry out the beginnings of what could be an effort to save
humanity from the next mass extinction.

While the search effort continues, albeit haltingly, another facet of
the humans-versus-asteroids story is gathering momentum. Former Cold
War adversaries have teamed up to explore ways of deflecting or even
destroying threatening asteroids (see "Cold War Weaponeers Gaze
Skyward").

But the first step in any asteroid defense program is to carry out a
census of potentially threatening objects, beginning with the most
dangerous -- those larger than 1 km in diameter. In 1990, Congress
requested that NASA study ways to increase the discovery rate of these
near-Earth objects. An international team of astronomers proposed a
program called the Space-guard Survey, designed to obtain a complete
census of these larger asteroids.

The results were reported to the Congress in 1992, and NASA made
available about $1 million in additional funds to upgrade existing
search programs. At the same time, the International Astronomical
Union appointed a working group on near-Earth objects to promote
international cooperation in this search. Since everyone on Earth is
at risk from cosmic impacts, it seemed only reasonable to share the
cost of the survey'.

And even before the dust of Comet Shoemaker-Levy 9's impact completely
settles on Jupiter, Congress is asking NASA to act, this time with a
greater sense of urgency. They want the agency to work with the US Air
Force, which already maintains a network of telescopes used to track
faint Earth satellites, to find a way to carry out an NEO survey
within the next decade. A team led by Gene Shoemaker is currently
preparing a report on ways to carry out this mandate.

So in a way, Earth's fragile lifeforms are finally coming to grips
with a persistent problem. For the first time in our planet's history,
through the intellect of its human species, life can avoid a fate that
up to now has been inevitable. After observing the geological evidence
for mass extinctions and bearing witness to Jupiter's horrific
pounding, we the people of Earth are getting positioned for action.

Earth, the planet of life, need never be a target again.

Some risky asteroi sizes 

Small Extinctor 

* 100 gigatons (100,00 megatons)
* 1 Km diameter
* Affects a hemisphere

Regional Bludgeon 

* 100 MT
* 0.1 Km diameter
* Destroys a continent

Tunguska 

* 10 MT
* 50 meter diameter
* Wipes out a city

Great Extinctor 

* 1000 TT (100,000,000 MT)
* 10 Km diameter
* Global disaster

PHOTO (COLOR): Illustrates the crash of Comet Shoemaker-Levy 9 into
planet Jupiter

PHOTO (COLOR): 1994 Jupiter weathers a barrage of impacts

PHOTO (COLOR): 1972 Earth narrowly misses a 10-megaton impact

~~~~~~~~

By David Morrison

David Morrison heads the Space Science Division at NASA's Ames
Research Center in California, and recently received the agency's
Outstanding Leadership Medal for his work on impact hazards.
_________________________________________________________________

Inset Article

COLD WAR WEAPONEERS GAZE SKYWARD

It was called the International Technical Meeting on Active Defense of
the Terrestrial Biosphere from Impact by Large Asteroids and
Comets. What it dealt with, in fewer words, was how to defend
the Earth against cosmic impacts.

Last May's workshop at the Department of Energy's Livermore National
Laboratory in California was the third in a series of
international meetings on planetary defense sponsored by
nuclear weapons labs in the United States and Russia. Experts
in nuclear weapons and missile defense systems, as well as a
sprinkling of astronomers and government policy makers,
pondered what to do if a comet or asteroid were on a collision
course with Earth. It turns out there are two options: Change
the threatening object's orbit or smash the offending meteoroid
into bits that would burn up in Earth's atmosphere.

Sessions focused primarily on dealing with objects greater than 1 km
in diameter, since they pose the greatest risk to life on
Earth. Nuclear explosives would be better at deflecting
asteroids of this size, assuming an asteroid search could
provide a lead warning of years or decades.

Some astronomers advocate quick development of such a survey,
contending that a nuclear defense system could be developed
after such a threat is found. However, many weapons scientists
feel we should build anti-asteroid defenses now and test them
against non-threatening asteroids to hone our
deflect-or-destroy abilities.

In addition to the nuclear options, Star Wars-based technology might
physically "slice up" asteroids with diameters of tens of
meters. Indeed, some scientists think this approach could
eventually deal with larger objects as well.

One of the strongest proponents of testing is Edward Teller, the
father of America's hydrogen bomb. He gave a banquet talk on
the "need for experiments on comets and asteroids." Teller
focused on the measurable likelihood that a small asteroid,
perhaps the size of a football field, could hit in the 21st
century. Such an impact would create tsunamis large enough to
devastate continental coastlines. Monster waves, he said, would
"cause billions of dollars in damage and kill thousands --
perhaps many thousands --of people." In his opinion, protection
against such a catastrophe justifies a defense program in the
range of "perhaps $100 million per year."

Such a program could ride the coat-tails of Clementine, the Pentagon's
successful mission to the Moon. Defense scientists remain
interested in similar scientific missions to asteroids, leading
toward active experiments, both nuclear and non-nuclear.
Missions might use decommissioned 300 Russian SS-18 missiles.
The reconfigured rockets could launch small payloads to
asteroids.

But such work must happen between now and the year 2002; all SS-18s
have to be destroyed, as mandated by the START nuclear weapons
reduction treaty. After years of contributing to the nuclear
stalemate, perhaps these Cold War rockets might hasten the day
when Earth can protect itself against rogue comets and
asteroids.

GRAPH: Risk is a matter of perception

PHOTO (BLACK & WHITE): Dr. Edward Teller, Father of America's Hydrogen
Bomb

ILLUSTRATION: A Star Wars Answer to Small Asteroids
_________________________________________________________________

Inset Article

THEY STAND ON GUARD FOR THEE

Sadly, more people work a typical shift at one fast-food restaurant
than scan the skies for near-Earth asteroids. But what the
searchers lack in numbers, they make up for in determination.
Here's a brief listing of some searches underway or being
planned:

• The California Institute of Technology's Palomar
Planet-Crossing Asteroid Survey (PCAS) has been photographing
the sky with an 18 1/2-inch Schmidt telescope from Palomar
Mountain in California since 1972. Films are carefully
scrutinized under a stereo microscope for any objects that have
moved between exposures. Program track record: 45 near-Earth
objects, as well as 14 comets and several thousand main-belt
asteroids.

• Another project associated with Cal Tech, the Palomar Asteroid
and Comet Survey (PACS), runs in a similar fashion to the PCAS,
but involves the resources of Arizona's Lowell observatory for
follow-up observations. Since 1983, PACS has used Palomar's 18
1/2-inch Schmidt telescope to discover 43 NEOs, plus now-famous
comet Shoe-maker-Levy 9, which collided with Jupiter in 1994.
At press time, this search program is being phased out.

• The University of Arizona's Space-watch survey is an all-CCD
camera operation, using a rather sizable chip of 5 cm by 5 cm
that covers a 38' field of view through a 36 1/2-inch telescope
on Kitt Peak. Astronomers turn off the telescope's clock drive
and allow the sky to trail through a 2 12-minute exposure. The
final image shows stars as parallel streaks. NEOs appear as
sideways streaks about 26 pixels long. The program's track
record since 1981: An average of three NEOs each month -the
smallest they've found is only 6 meters across.

• The Anglo-Australian Near-Earth Asteroid Survey (AANEAS) uses
photographic plates taken through the 1.2-m U.K. Schmidt
telescope operated at the Anglo-Australian Observatory in
Siding Spring, Australia. Exposures are usually done for
reasons other than an asteroid search; AANEAS merely tags
along, looking for tiny but telltale streaks that betray the
presence of an asteroid. The longer the streak, the closer the
object is to Earth and the likelihood of it being an NEO. Up to
250 NEOs may lurk on plates yet to be scanned.

• The Lowell Observatory's Near-Earth Object Survey (LONEOS) is
poised to come on-line near Flagstaff, Arizona. It uses a
four-chip CCD camera mounted to a 16-inch Schmidt camera at
Lowell.

• A larger survey effort called Space-guard might use LONEOS's
prototype CCD technology. First proposed in 1992. Spaceguard
will station six 2.5-m survey scopes at strategic locations on
Earth to cover the whole sky.

PHOTO (BLACK & WHITE): 1991 JX passes within 3 million miles of Earth
_________________

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Source: Astronomy, Oct95, Vol. 23 Issue 10, p34, 8p, 1 graph, 5c, 2bw.
Item Number: 9509276740
_________________________________________________________________