PLEASE NOTE:


*

CCNet 54/2001 - 6 April 2001
----------------------------


Wishing all CCNet readers a happy Passover & restful Easter break.
--Benny J Peiser, CCNet Moderator


"Circling the Earth in the orbital spaceship I marvelled at the
beauty of our planet. People of the world! Let us safeguard and enhance
this beauty - not destroy it!"
- Yuri Gagarin


(1) NEPTUNE ATTACKS: A POSSIBLE SOURCE FOR THE LATE HEAVY BOMBARDEMENT?
    New Scientist, 7 April 2001

(2) WAS JOHNNY APPLESEED A COMET?
    NASA Science News for April 5, 2001

(3) YURI'S NIGHT: THE WORLD'S SPACE PARTY, 12 APRIL 2001
    http://www.yurisnight.net/spaceparty.htm

(4) NEO NEWS
    David Morrison <dmorrison@arc.nasa.gov>

(5) BIG-ORBIT OBJECT CONFOUNDS DYNAMICISTS
    Sky & Telescope, 5 April

(6) EUROPE FEARS IT MAY SOON FALL BEHIND CHINA IN SPACE RACE
    European Space Agency, 5 April 2001

(7) ASTEROID 1998 OX4
    Jens Kieffer-Olsen <dstdba@post4.tele.dk>

(8) IMPACT CRATERS: IMPLICATIONS FOR BASEMENT HYDROCARBON PRODUCTION
    Michael Paine <mpaine@tpgi.com.au>

(9) MUST A FREE-FLOATING PLANET BE A "SUB-BROWN DWARF"?
    Stephen Ashworth <sa@astronist.demon.co.uk>

(10) TUNGUSKA GEOCHEMICAL ANOMALIES
     Andrei Ol'khovatov <olkhovatov@mtu-net.ru>

===============
(1) NEPTUNE ATTACKS: A POSSIBLE SOURCE FOR THE LATE HEAVY BOMBARDEMENT?

From New Scientist, 7 April 2001
http://www.newscientist.com/newsletter/features.jsp?id=ns22851
  
Neptune attacks! The cataclysm that made the man in the Moon began in the
far reaches of the Solar System, says Ivan Semeniuk

THIS MUCH we know: the Solar System is not a safe place. Sixty-five million
years ago, the dinosaurs were clobbered by a wayward asteroid or comet. A
hundred and eighty million years earlier, a similar event appears to have
swept the trilobites and most of their contemporaries into the dustbin of
prehistory. And a far greater assault occurred just as life was gaining a
foothold.

There is new evidence that a sudden barrage of deadly debris crashed against
the Earth and Moon 3.9 billion years ago. Thousands of giant impacts
pummelled the Earth--most as big as the event that wiped out the dinosaurs,
and some much larger. Those vast impacts left behind continent-sized craters
and liberated enough heat to vaporise oceans.

What triggered this onslaught? "Something in the structure of the Solar
System must have changed," says Harold Levison of the Southwest Research
Institute in Boulder, Colorado. Levison has his own personal Solar System, a
virtual model, which he is using to simulate those cataclysmic events. And
he is pointing the finger at two unlikely instigators: Uranus and Neptune.
If he's right, these two distant giants caused the worst assault on Earth's
surface since life began.

The evidence of an ancient barrage has been staring at us for aeons. Even a
casual glance at the full Moon reveals the image of a hollow-eyed face,
frozen in a Munch-like scream. The tortured expression of the man in the
Moon is actually an arrangement of giant circular impact basins. These
"seas" are filled with dark lava that stands out against the rougher and
brighter highlands. The basins are the most obvious sign today of giant
impacts in the Moon's distant past.

The bodies that created these seas were probably at least 50 or 100
kilometres across, travelling at tens of kilometres a second. They would
have excavated, melted and scattered vast amounts of debris across the lunar
surface, leaving a gigantic crater. Later, volcanic activity below the
surface flooded many of these low-lying basins with darker material.

The fact that there were giant impacts isn't too surprising. After all,
rocky moons and planets are all made of smaller rocks that collided and
coalesced. It is reasonable to assume that these collisions continued for
some time after the Solar System formed, gradually tapering off as available
material was absorbed or parked in stable orbits. On Earth, the combination
of oceans, atmosphere and tectonic activity wiped out all visible traces of
these formative collisions, but on the unaltered crust of the Moon the scars
accumulated.

Formative collisions

But this picture changed after Apollo. The Apollo astronauts bobbed across
the lunar terrain in search of geological souvenirs, and returned hundreds
of kilograms of rock to Earth. Geochemists then dated these rocks by
extracting argon-40. This element is produced by the radioactive decay of
potassium, building up slowly and steadily inside the lunar rocks. But if a
rock is heated to its melting point, any argon-40 in it is released as a
gas. So the amount of argon-40 tells you how much time has passed since a
rock last melted.

Lunar scientists had believed in a gradual decline of collisions, so they
expected the ages of lunar rocks to cluster towards the time of maximum
bombardment, namely right after the Moon's formation 4.5 billion years ago.
But most of the Apollo rocks proved to be about 3.9 billion years old--more
than half a billion years younger than the Moon. Only a few, from the Apollo
16 site, were closer to 4.5 billion.

As most of the Apollo rocks came from the giant impact basins, it appeared
that the basins did not accumulate gradually, but instead were created long
after the Moon formed, in a period now called the "late heavy bombardment".
It's hard to see how this could have been the tail end of the process that
formed the Moon, says Graham Ryder of the Lunar and Planetary Institute in
Houston, Texas. "That would mean you'd have to have at least that amount of
impacting--probably an increasing amount--prior to 3.9 billion years ago,"
he says. To Ryder, that's just too much bombardment; a sudden cataclysm
around 3.9 billion years ago seems more likely.

But other researchers were less sure, pointing out that the Apollo samples
are biased towards the equator of the Moon--so they might only be giving us
the history of a few impact basins, rather than the full story.

Then last December, Barbara Cohen of the University of Tennessee, Knoxville,
published her investigation of a group of lunar meteorites--rocks from the
Moon that have landed on Earth. Cohen chose meteorites that differed in
composition from the Apollo samples, so that they should have a wider range
of origins. "We went in expecting to ref-ute the cataclysmic theory," says
Cohen. But they couldn't. None of the lunar meteorites could be dated before
3.9 billion years ago.

So it seems that the Moon was subjected to an intense bombardment around 3.9
billion years ago, lasting perhaps 100 million years. Planetary scientists
are hard pressed to explain what caused it. "This is one reason why many
people are uneasy with the idea of a cataclysm," says Ryder.

As the only concrete evidence comes from lunar samples, the fateful event
might have been confined to the Earth-Moon system. Perhaps Earth started off
with a second moon, or a loose cluster of moonlets. As moon number one--the
Moon we know today--moved outwards in its orbit, it could have destabilised
the orbits of the other moons, so that they came close to Earth and got torn
into fragments by our gravity.

But there are doubts that this can deliver enough hits quickly enough to
account for the cataclysm. Moreover, says Levison, there are indications
that the late heavy bombardment was more widespread. Huge craters similar to
the Moon's impact basins can be found on Mars and Mercury. And meteorites
from Mars and the asteroid Vesta show signs of heavy impacting around 3.9
billion years ago.

What could have unleashed such widespread devastation? One theory is that
two huge asteroids collided and scattered fragments throughout the inner
Solar System. But to explain the number and size of the impacts, these
asteroids would have had to be an unlikely 10,000 times as massive as the
whole asteroid belt today. Levison thinks we must look further afield for
the culprit.

The two outermost giants in our Solar System have a reputation for unusual
goings on. Uranus spins on its side, presumably because of a collision that
knocked it over. Miranda, a satellite of Uranus, has a crater so huge that
the collision that formed it must have nearly blasted the moon to bits.
Neptune's largest moon, Triton, orbits backwards in relation to the rest of
the Solar System. "It all suggests there were large bodies roaming around
out there in the past," says Joseph Hahn of the Lunar and Planetary
Institute. Large bodies with a violent streak, what's more. But what could
all this have to do with our Moon?

In 1975, George Wetherill of the Carnegie Institution of Washington proposed
that leftovers from the formation of Uranus and Neptune caused the late
heavy bombardment. He reasoned that Uranus and Neptune might have formed
significantly later than the other planets. These planets are thought to
have grown from a swirl of icy bodies called planetesimals. Out beyond the
orbit of Saturn this raw material was thinly spread, and might have taken
hundreds of millions of years to gather into planets. "People who work with
simulations tend to fail to produce Uranus and Neptune in what might be
regarded as a reasonable amount of time," says Hahn. "People are still
arguing about how much time you really need to form these planets."

Whenever they finally formed, the gravity of these new planets would have
catapulted leftover ice and rocks in all directions. Some of this material
would have been kicked out to the limits of the Sun's gravitational
influence, joining the Oort Cloud, which occasionally sends us comets today.
Other chunks of ice would have been diverted inwards, to wreak havoc on the
inner Solar System.

When Wetherill tested the idea with a computer simulation he found he could
reproduce a kind of late bombardment, but the timing was wrong. Instead of a
narrow spike 3.9 billion years ago, the bombardment was staggered over a
much longer interval. That made the Uranus-Neptune theory a poor match for
the measured ages of the lunar samples.

A generation later, Wetherill's idea has been reborn. In 1995, Levison was
using a simulation of the early Solar System to study the formation of the
Oort Cloud. Unlike Wetherill's original model, which relied on statistical
approximations to predict where debris ends up, Levison's is a true
simulation. At any given moment, it calculates the gravitational pull on
each individual object and how that object moves, then it steps forward in
time by a short interval and does the entire calculation again.

Levison's simulation followed comets ejected from the neighbourhood of
Uranus and Neptune. To his surprise, a sudden surge of these icy bodies was
sent spiralling down into the inner Solar System, lasting about as long as
the late heavy bombardment. "I realised I could get the narrow spike," says
Levison.

Excited by this result, Levison set about creating a new series of
simulations, this time with the late heavy bombardment in mind. The key
question was no longer the duration of the bombardment, but its intensity.

The odds of any one comet hitting the Moon are low, so to plaster the Moon
with impact basins requires a vast number of comets. But planetary
scientists think the total mass of planetesimals beyond Saturn was less than
50 times the mass of the Earth, or else Neptune and Uranus would have grown
larger than they actually are.

Fortunately, Levison doesn't need too much raw material. In a paper to be
published in the June issue of Icarus, he says that his simulation would
produce a cataclysm on the Moon as long there were at least 32 Earth masses
of planetesimals available. "If I needed a thousand Earth masses, or ten
thousand, I knew no one would believe it," says Levison. "When the number 32
jumped out of my computer I said to myself, 'This is it! This has got to be
right!'"

The strange picture that Levison's model paints is of an early Solar System
that ends at Saturn. Uranus and Neptune struggle into existence more that
half a billion years late, emerging out of the icy flotsam and jetsam beyond
Saturn's orbit. And once they have grown to a critical size, after perhaps
700 million years, their gravity becomes strong enough to cast icy
planetesimals inwards. Twenty or so carve out the shadowy face on the Moon,
and many more hit the Earth.

For Levison, the best test of this model will be whether we find evidence
for cataclysmic impacts beyond the inner Solar System. "On its way in, this
stuff should have knocked the hell out of Jupiter's satellites," he says.
The Jovian moon Callisto has a surface mostly made of ice, and Levison's
version of the late heavy bombardment should have melted it. "If we find
that Callisto never melted, that could rule the model out."

Levison notes a curious side effect of his idea. All these icy comets would
have dumped enough carbon dioxide on Mars to produce a thick, insulating
atmosphere that would have allowed liquid water to exist on the Red Planet's
surface. Photographs taken in the past year by the Mars Global Surveyor
spacecraft do show apparently water-carved landforms, raising scientists'
hopes that life once existed there. So while the missiles flung by Uranus
and Neptune battered early organisms on Earth, they might have allowed a
brief flourishing of life on Mars.

Life in hell

The Moon may still bear the scars of the late heavy bombardment, but it was
our home planet that got the worst of it. With its larger size and mass,
Earth would have attracted at least ten times as many impacts as the Moon,
no matter what the cause of the bombardment. "Earth got beaten up, there's
no doubt about it," says Hal Levison.

Yet some traces of microbial life date back to the time of the bombardment,
suggesting that the very first life forms were around earlier still. So
could life have survived 200 impacts, each big enough to boil an ocean?

"These are some really catastrophic scenarios," says Barbara Cohen of the
University of Tennessee, Knoxville. But even after an impact of the most
destructive kind, she thinks, conditions on Earth could have got back to
normal within 10,000 years--roughly the average time between hits. "Maybe
life could go dormant for that long and survive, perhaps in a spore form,"
she says.

Or maybe it found places to shelter. The ocean floor may have provided the
best haven from the hellish conditions closer to the surface. If so, we may
have evolved from organisms that once thrived around deep-sea vents.

Copyright New Scientist, RBI Limited 2001

==========
(2) WAS JOHNNY APPLESEED A COMET?

From NASA Science News for April 5, 2001
http://science.nasa.gov/headlines/y2001/ast05apr_1.htm?list20392

A new experiment suggests that comet impacts could have sowed the seeds of
life on Earth billions of years ago.
 
April 5, 2001 -- Four billion years ago Earth was bombarded by a hail of
comets and asteroids. The shattering collisions rendered our planet
uninhabitable during a period scientists call the Late Heavy Bombardment
(LHB).

It surely sounds like the LHB was an awful time for the beleaguered young
planet -- but perhaps the pelting was a good thing after all, say
researchers. Kamikaze comets could have delivered important organic
molecules to Earth -- sowing the seeds for life.

Genesis by comets is a controversial idea, but it's just received an
important boost. A NASA-supported experiment reveals that complex molecules
hitchhiking aboard a comet could have survived an impact with Earth.

"Our results suggest that the notion of organic compounds coming from outer
space can't be ruled out because of the severity of the impact event," says
Jennifer Blank, a geochemist at the University of California, Berkeley.
Blank and colleagues simulated a comet collision by shooting a soda-can
sized bullet into a metal target containing a teardrop of water mixed with
amino acids - the building blocks of proteins.

Not only did a good fraction of the amino acids survive, but many
polymerized into chains of two, three and four amino acids, so-called
peptides. Peptides with longer chains are called polypeptides, while even
longer ones are called proteins.

"The neat thing is that we got every possible combination of dipeptide, many
tripeptides and some tetrapeptides," said Blank. "We saw variations in the
ratios of peptides produced depending on the conditions of temperature,
pressure and duration of the impact. This is the beginning of a new field of
science."

Freezing the target to mimic an icy comet increased the survival rate of
amino acids, she added.

Blank's ballistic test was designed to simulate the sort of impact that
would have been frequent in Earth's earliest history when rocky, icy debris
in our solar system combined to form the planets. Much of the debris would
have resembled comets - dirty snowballs thought to be mostly slushy water
surrounding a rocky core - slamming into Earth at velocities greater than 16
miles per second (25 km/sec).

The severity of the laboratory impact was akin to that of an oblique
collision between the rocky surface of Earth and a comet coming in at an
angle of less than 25 degrees from the horizon.

Benton Clark, chief scientist of Flight Systems at Lockheed Martin
Astronautics, proposed in 1988 that if comets are slowed sufficiently -- by,
e.g., drag from Earth's atmosphere, which would be greatest at low impact
angles -- some water and organic compounds might survive the collision.

"At very low angles, we think that some water ice from the comet would
remain intact as a liquid puddle concentrated with organic molecules," ideal
for the development of life, Blank said. "This impact scenario provides the
three ingredients believed necessary for life: liquid water, organic
material and energy."

Though comet hunter Eugene Shoemaker estimated that in Earth's early history
only a few percent of comets or asteroids arrived at low enough angles, the
bombardment would have been heavy enough to deliver a significant amount of
intact organic material and water, according to Blank's estimates.

One well-known model for the beginnings of life on Earth posits that
terrestrial life sprang from complex molecules such as amino acids and
sugars produced by electrical discharges in a primeval atmosphere replete
with gases such as methane, hydrogen, ammonia and water. The famous
Miller-Urey experiment in 1953, conducted by Stanley Miller and Harold Urey
of the University of Chicago, demonstrated that a lightening-like discharge
in a test tube filled with these molecules could produce amino acids.

Other scientists believe that the building blocks of life on Earth arrived
from space. Astronomers have detected many kinds of organic molecules in
space, floating in clouds of gas or bound up in dust particles. They range
from the simplest - water, ammonia, methane, hydrogen cyanide and alcohols,
including ethyl alcohol - to more complex molecules.

Interestingly, of the more than 70 amino acids found in meteorites, only
eight of them overlap with the group of 20 which occur commonly as
structural components of proteins found in humans and all other life on
Earth.

To test whether water and organic compounds could survive the high pressures
and high temperatures of a collision, Blank and her colleagues worked for
three years to design a steel capsule that would not rupture when hit with a
mile-per-second (1.6 kilometer-per-second) bullet fired from an 80-mm bore
cannon at the University of Chicago and later at Los Alamos National
Laboratory. The target she and her team developed - a two-centimeter
diameter stainless steel disk about a half-centimeter thick - was able to
withstand about 200,000 times atmospheric pressure without bursting.

They filled the small cavity with water saturated with five amino acids:
three from the list of 20 that comprise all proteins in humans
(phenylalanine, proline and lysine) and two varieties detected in the
Murchison meteorite (aminobutyric acid and nor-valine). That meteorite
plummeted to the ground in 1969 in Australia and is thought to be from the
core of a comet.

The survival of a large fraction of the amino acids and their polymerization
during the collision makes the idea of an extraterrestrial origin of organic
compounds a strong contender against Miller-Urey style theories, Blank said.

"About one comet per year arriving in a low-angle impact would bring in the
equivalent of all the organics produced in a year in an oxidizing atmosphere
by the Miller-Urey electric discharge mechanism," Blank estimated. "An
advantage is you get all of it together in a puddle of water rather than
diluted in the oceans."

The next hitchhikers she plans to subject to a shock test are bacterial
spores, which some have proposed arrived on Earth via comets to jump-start
evolution.

Ivan Semeniuk is a science writer and broadcaster based in Toronto

==========
(3) YURI'S NIGHT: THE WORLD'S SPACE PARTY, 12 APRIL 2001

http://www.yurisnight.net/spaceparty.htm

On Thursday, 12th April 2001, Earth will celebrate the fortieth anniversary
of Yuri Gagarin's historic trip into space with a series of events small and
large, quiet and raucous. The parties may be different, but the vision is
the same.

FOR MORE INFORMATION AND THE NEAREST PARTY see:
http://www.yurisnight.net/spaceparty.htm

=========
(4) NEO NEWS

From David Morrison <dmorrison@arc.nasa.gov>

NEO News (04/04/01) Miscellaneous items

Dear Friends & Students of NEOs:

This edition of NEO News contains four short news items or follow-up to
previous stories.

David Morrison

========================================
TUMBLING STONE: NEW NEO WEB-MAGAZINE

Our Italian colleagues have started publication of a new web-based magazine
called Tumbling Stone, aimed at a broad public readership. Andrea Milani,
one of the co-founders, writes that "Tumbling Stone, the online newsletter,
is a joint initiative of NEODyS (me and my coworkers at the University of
Pisa and elsewhere) and the Spaceguard Foundation (A. Carusi and his
coworkers at CNR Roma and elsewhere). As explained by Andrea Carusi in the
first edition, "It is dedicated to a better understanding, in the field of
Near-Earth-Objects, of the meaning and importance of announcements
concerning these bodies, their relationships with our planet, and the degree
of hazard they may represent for mankind. We feel that such an information
tool is needed for non-specialists."

The address for Tumbling Stone is
http://spaceguard.ias.rm.cnr.it/tumblingstone/ It will also be linked
directly from my impact hazard webpage.

=========================================
MORE ON GERVASE OF CANTERBURY

In the March 16 edition of NEO News I reported on recent work indicating
that the lunar crater Giordano Bruno could not very well have been formed in
1178, which is an interpretation often quoted for the report by Gervase of
Canterbury that five men witnessed an event on the Moon that might have been
an impact. I concluded with the comment that discounting the identification
of this event with the crater Giordano Bruno undercuts the reality of the
medieval report, or at least its relevance to NEO impacts and the Taurid
meteor stream.

Alan Harris of JPL wrote that "I am pleased to see yet another "debunking"
of this old Hartung chestnut". But he notes that a similar argument was made
8 years ago, that if a large lunar impact had occurred in 1178 there would
have been an associated meteor storm when the ejecta reached the Earth.
Harris continues: Regarding the Giordano Bruno claim, I pointed out (Harris,
JGR 98, 9145-9149, 1993) that "our final conclusion must be, even in the
extraordinarily
unlikely event that such a compact clump of ejecta were injected into
heliocentric orbit from the Moon, perturbations from the Earth would have
quickly dispersed the clump [so that] annual showers of more or less
constant intensity should be expected. . . Either by initial dispersion in
the velocity or by subsequent perturbations, an event such as Hartung
proposes [the Giordano Bruno impact] should have the result that the sky
would really light up every midsummer for a week or two." The bottom line
is, I agree completely with Withers that there should have been an
"apocalyptic" level meteor storm in the days following the claimed event,
but furthermore it should have been followed by a pretty spectacular meteor
storm on each anniversary since then.

Benny Peiser replied to my note in CCNet for March 19 as follows: "Not so
fast, my friend. While I agree that the Giordano Bruno crater appears to be
too large indeed to be convincingly associated with a hypothesised lunar
impact in 1178, neither Gervase's report nor the possibility of an observed
impact on the Moon - and not even the speculation about a link with the
Taurid meteor stream have been "debunked." As Ed Vega rightly stresses, it's
Carl Sagan's (and others') association of the eyewitness report with the
Giordano Bruno crater that has been "debunked", not the report and its
impact-interpretation itself. BJP"

In reply to Peiser, I believe that in the absence of physical evidence (such
as a young lunar crater), the report from Gervase of Canterbury has little
weight. The report is already inconsistent internally with an impact on the
Moon, since it says that "this phenomenon was repeated a dozen times or
more, the flame assuming various twisted shapes and then returning to
normal. Then, after these transformations, the Moon from horn to horn, along
its whole length, took on a blackish appearance". The above does not sound
to me like the description of an impact. The story held some credibility
only because of the claimed association with crater Giordano Bruno. Each
person makes his or her own judgement about the veracity of ancient
documents, but for me the debunking of the association of this 1178 report
with any physical evidence on the Moon effectively undercuts the entire
connection of Gervase's report with NEO impacts.

David Morrison

MODERATORS NOTE: I don't agree with David Morrison that we most discount
Gervaise's 1178 moon-flash report, just because we can now discount its
connection with the Giordano Bruno crater. If we were to follow David's
logic, we would have to discount nearly all lunar impact reports. After all,
the Leonid flashes in 1999 did not produce any observable craters either.
Admittedly, the 1178 flash report is questionable in several aspects and
would, in any case, suggest a more significant event. But to rule out the
possibility of an observed lunar impact in 1178 only because it couldn't
produce the Bruno crater is simply illogical. Sure, many people were
reluctant to accept the theory of Leonid lunar impacts. Some may never have
accepted them without the fact that the Leonid track went so close to the
moon. But the fact of the matter is that - from a scientific point of view -
there is no hard evidence to either confirm *or deny* the 1178 lunar impact
report. So let's refrain from wishful thinking. BJP

=========================================
WAS TUNGUSKA AN ASTEROID OR A COMET?

This question is still often raised. Old astronomy books almost always
assert that the Tunguska impactor was cometary. However, two independent
analyses published almost simultaneously in 1993 provided strong arguments
in favor of the asteroidal hypothesis. The publications were:

Chyba, C.F., P.J. Thomas, and K.J. Zahnle: The 1908 Tunguska explosion:
atmospheric disruption of a stony asteroid. Nature 361:40-44 (1993)

Hills, J.G. and M.P. Goda: The fragmentation of small asteroids in the
atmosphere. Astronomical J. 105:1114-1144 (1993)

In both cases these teams argued that models of the rapid deceleration and
fragmentation of the impacting body in the atmosphere showed that a comet
would have disintegrated at a much higher altitude, while only a projectile
of rocky strength and density could have penetrated to within 8 km of the
surface, the height at which the Tunguska explosion occurred.

Modeling the impact physics on the computer, of course, has its
uncertainties. Therefore it is worthwhile repeating a simple physical
argument developed at the time by Kevin Zahnle of NASA Ames, and recently
recalled by Chris Chyba of the SETI Institute. Independent of modeling
details, a dense stony object (asteroid) will penetrate deeper than a less
dense icy object (comet). Also, there are many more asteroids than comets in
Earth-crossing orbits. Thus if Tunguska were a comet, and it exploded at 8
km, then asteroids of similar size and energy must reach the surface to
produce craters. If asteroids are more common that comets (one estimate is a
factor of 10), then there would be many more asteroid impacts in the 10-20
megaton range
than comets, and we should see their associated craters. For every
Tunguska-like airburst, we might expect ten craters the size of Meteor
Crater. But these craters are not there, thus refuting the hypothesis that
Tunguska was cometary. The alternative, that Tunguska was asteroidal, and
that only the rare, very strong iron objects make it to the ground in the
10-20 megaton energy range, is of course entirely consistent with the rarity
of features like Meteor Crater (which was produced by the impact of an iron
object).

Chyba recently succinctly summarized this argument as follows: " I am
impressed with Kevin Zahnle's argument that if comets of Tunguska size
almost penetrate the atmosphere, then stony asteroids of that size certainly
will, in which case there should be far more 1 km-size craters on Earth than
in fact there are. This seems like a very difficult argument for the
"Tunguska was a small comet" point of view to counter."

==========================================
MIR AND NEO IMPACTS: A RISK COMPARISON

Recent "the sky is falling" news stories have dealt with the fall from orbit
of the Mir Space Station rather than NEO impacts. The press and public are
clearly interested in the risk of falling objects, but sometimes risks are
perceived in ways that are quite different from the hazard as calculated
numerically. The following is a very rough estimate of risks, in "order of
magnitude" terms only. It is intended to be illustrative, but certainly not
precise. By risk I mean the chance or probability that any individual will
be killed as a result of either a spacecraft atmospheric entry or the impact
of a NEO.

Let's start with the risk of death as a result of being struck by a piece of
Mir on the assumption that the fragments could land anywhere on Earth.
Suppose 1000 large metal fragments survive to hit the ground, and that if
you are within 1 meter of the impact point you will be killed. Thus 1000
square meters are at risk, out of a total surface area of the Earth of about
100 trillion (10**14) square meters (not counting the Polar Regions). This
is one part in 100 billion of the Earth's surface, and that is the risk to
each individual. Multiplying by the Earth's population of 6 billion, we get
a chance of about 1 in 20 that one person on Earth would be killed.

In fact, the Mir atmospheric entry was far from random. It was steered to an
impact point in the mid-Pacific Ocean. Unless you lived in that part of the
world, the risk to you was zero (not allowing for an uncertainty in how well
this controlled entry would be executed). Since the total population of the
Pacific is only a few million, the chance that someone would be killed was
less than 1 in 20,000. The folks who sold the Russians a $200 million
insurance policy were very unlikely to have to pay off any claims.

For comparison, consider the annual risk of dying as a result of an NEO
collision with the Earth. A number of studies (e.g., the paper that Clark
Chapman and I published in Nature in 1994) have shown that this risk is
dominated by near Earth asteroids of about 2 km diameter. There is a roughly
1 in a million chance of such an impact each year, with estimated death of
1-2 billion people. Thus the annual risk to each of us of from NEO impacts
is about 1 in a few million, or more than 10,000 times greater than the risk
from an uncontrolled Mir entry. (Note: This is a conservative estimate; many
would argue for a NEO-impact risk that is higher by an order of magnitude.)

We see from these simple calculations that the risk (per year) to each of us
from asteroid impact is thousands of times greater than from an uncontrolled
Mir entry, and millions of times greater than from a controlled Mir dive
into the Pacific. Yet no one is taking out insurance policies to protect
from cosmic impacts, and this risk receives less news coverage than the
demise of Mir. Why the disparity? For one thing, the death of Mir was a
known event that provided a good story, while we have no specific prediction
of any NEO impact. For another, Mir was a human-built object over which we
had some control (and responsibility), while an NEO impact is considered an
"act of God". But I suspect that the difference also reflects the fact that
very few reporters tried to make a quantitative comparison of these risks.
If they had, the results might have surprised them!

David Morrison

+++++++++++++++++++++++++++++++++++++++++++

NEO News is an informal compilation of news and opinion dealing with Near
Earth Objects (NEOs) and their impacts. These opinions are the
responsibility of the individual authors and do not represent the positions
of NASA, the International Astronomical Union, or any other organization. To
subscribe (or unsubscribe) contact dmorrison@arc.nasa.gov.  For additional
information, please see the website: http://impact.arc.nasa.gov. If anyone
wishes to copy or redistribute original material from these notes, fully or
in part, please include this disclaimer.

=============
(5) BIG-ORBIT OBJECT CONFOUNDS DYNAMICISTS

From Sky & Telescope, 5 April
http://www.skypub.com/news/news.shtml#bigorbit

To Marc W. Buie (Lowell Observatory), 2000 CR105 was at first just another
distant discovery among dozens found during his team's ongoing search for
trans-Neptunian objects using 4-meter telescopes in Arizona and Chile. But
its uniqueness became apparent when dynamicist Brian G. Marsden started
cranking out possible orbits for this 24th-magnitude find. "It was obviously
far away, 53 to 55 astronomical units," Marsden recalls, well beyond most
objects known to inhabit the Kuiper Belt.

In the months that followed, an international team led by Brett Gladman
(Nice Observatory) quietly tracked the dim interloper. Thanks to their
year-long pursuit, it's now clear that 2000 CR105 has a highly eccentric
orbit that stretches out to roughly 400 a.u. - more than 10 times Pluto's
mean distance from the Sun and far larger than that of any known Kuiper Belt
object. But more puzzling to dynamicists is the orbit's perihelion distance.
At 44.5 a.u. (6.7 billion km), it is well beyond the perturbing influence of
Neptune, whose gravity has flung countless other bodies out to the solar
system's most distant fringes. So how did 2000 CR105 end up stranded out
there?

Gladman and six colleagues offer several possibilities in an article
submitted to the journal Icarus and summarized here. According to coauthor
Matthew Holman (Harvard-Smithsonian Center for Astrophysics), the orbit of
2000 CR105 is dynamically chaotic and may simply be the consequence of eons
of erratic drift. But that's a statistical long shot, so the team has
explored other ideas. Perhaps the Kuiper Belt formed with numerous
planet-size bodies in its midst, which wreaked orbital havoc before
themselves being heaved out of the region. Or Neptune itself may have
ventured farther out before settling into its present orbit. And though the
notion is highly speculative, a massive perturber may yet await discovery
beyond the Kuiper Belt's known boundary. Holman notes that a body the size
of Mars 200 a.u. away could easily have escaped detection to date.

Resolving this mystery will take time, but one implication is already clear.
Objects like 2000 CR105 should be exceedingly rare, so if others are found
then the Kuiper Belt is likely much more massive than currently envisioned.

- J. Kelly Beatty

2001 Sky Publishing Corp. All rights reserved.

===========
(6) EUROPE FEARS IT MAY SOON FALL BEHIND CHINA IN SPACE RACE 

From European Space Agency, 5 April 2001
http://sci.esa.int/content/news/index.cfm?aid=1&cid=1&oid=26740

EUROPE SHOULD INVEST MORE IN SPACE SCIENCE

5-Apr-2001 Greater investment in space science would help nurture Europe's
scientific community and consequently build up the knowledge-based society
that Europe's heads of state declared they wanted at the European Union
summit in Lisbon last year. This was a common message delivered to the
General Assembly of the European Geophysical Society in Nice last week by
the outgoing and incoming directors of ESA's science programme.

Europe must take a political decision to maintain its second-ranking
position in space science, if countries such as China are not to overtake it
within a decade, said Roger-Maurice Bonnet, who retires as Director of the
ESA Science Programme at the end of April. In particular, incentives are
needed to prevent Europe's scientific community dissipating to the United
States and other greener pastures, he continued.

David Southwood, who takes over from Bonnet on 1 May, echoed this message
and suggested that ESA's future policy needed to dovetail with that recently
outlined by Philippe Busquin, EU Research Commissioner, to build up a
European research area of networked laboratories and to make Europe draw in
researchers. One way in which ESA could help to do this is by networking
with research institutes to give support for advanced payload development,
according to Southwood.

In his talk on Space science and exploration in the 21st century, Bonnet
said that the revolution in knowledge already brought about by space science
can only be maintained with additional investment in advanced technologies,
especially for astronomy. Although there is a growing interest in planetary
exploration within Europe, the issue is how to carve a niche in the face of
far greater resources invested by the United States.

Bonnet's great achievement during his directorship has been to institute the
Horizons 2000 programme, which has allowed for forward planning, said
Southwood in his talk on The future of the European space science programme.
However, a reducing budget, or one with no inflation compensation (an idea
that has been floated), would compromise this approach.

The programme of new missions approved by ESA's Science Programme Committee
last October (except for Eddington, a mission to search for Earth-like
extra-solar planets) can be achieved by 2013 if the buying power of ESA's
space science budget is kept constant, which is the minimum assumed by the
SPC. Although this programme is broad, it has gaps, no flexibility to
respond to new scientific or technical developments, and expects scientists
to wait too long for new mission opportunities. However, a budget increase
of 5% a year for five years, would enable the new missions, including
Eddington, to be completed by 2011, giving 40% more science, concluded
Southwood.

Bonnet gave his address after being awarded an EGS Fellowship Medal "for his
authoritative and wide-ranging support of the space sciences putting Europe
at the forefront of Solar System exploration". The EGS general assembly,
which was attended by several thousand geophysicists and lasted all week,
also heard about several of ESA's solar system missions. For further
details, go to the Huygens and Mars Express pages.

============
(7) ASTEROID 1998 OX4

From Jens Kieffer-Olsen <dstdba@post4.tele.dk>

Re: NEW OBSERVATIONS ELIMINATE COLLISION PROBABILITIES FOR ASTEROID 1998 OX4

From Spaceguard Central Node
http://spaceguard.ias.rm.cnr.it/SSystem/NEOCS/1998ox4.html
1998 OX4: no impact in 2012, 2014, 2017, 2022 and 2023

At the end of January 2001, following an excellent visibility
opportunity, some of the virtual impactors of 1998 OX4, currently the
biggest known NEA with remote collision probabilities, were removed.
This is the result of a dedicated observing campaign and of general data
analysis from two NEO survey programs, conducted by the Spaceguard Central
Node.

These two observing efforts for virtual impactors, respectively
denominated 2014 VI- 1998 OX4 and 2012-2014-2017-2022-2023 VIs, led to
the conclusion that 1998 OX4 is not going to impact the Earth on either
of the following years: 2012, 2014, 2017, 2022 and 2023.


Dear Benny Peiser,

How about the year 2078, when - according to the below article quoted in
CCNet 117/2000  -  the risk from 1998 OX4 is so severe that relevant
information could have been deliberately removed from the public domain?

Your World USA, November 12, 2000
http://www.yowusa.com/Archive/November2000/Asteroids/asteroids.htm

Yours sincerely
Jens Kieffer-Olsen, M.Sc.(Elec.Eng.)
Slagelse, Denmark

MODERATOR'S NOTE: As far as I am aware, there is no evidence whatever for a
risk of asteroid 1998 OX4 colliding with Earth in 2078. The uncertainty in
the probability calculation for a date that far in the future would be
enormous. Given the 10-day observed orbital arc, there is no way that there
could be a "severe" danger from this object under that circumstance. In any
case, none of the individuals and teams around the world involved in orbital
calculations have reported any problem regarding OX4 in 2078. If, as claimed
in the Nov 12 article, there have been some inconsistencies on the NEODyS
Risk Page regarding 1998 OX4, I would advise concerned CCNet readers to
contact Andrea Milani who should be able to dispell some of the concerns in
question. BJP

==========
(8) IMPACT CRATERS: IMPLICATIONS FOR BASEMENT HYDROCARBON PRODUCTION

From Michael Paine <mpaine@tpgi.com.au>

Dear Benny

The Institute for Exploration and Development Geosciences (EDGE) has just
made available in PDF format (2.9Mb) a 'classic' 1981 geology paper by
Richard Donofrio. Richard assisted me with my 1999 Space.com article
'Prospecting for Oil? Look in asteroid impact craters'

(Links at http://www4.tpg.com.au/users/tps-seti/reading.html#ez6 )

regards
Michael Paine

Basement Impact Crater Paper Available in PDF
http://www.edge.ou.edu/news/news.htm#basement
The Journal of Petroleum Geology/Scientific Press has provided EDGE with a
digital version (PDF) of a classic paper that proposed the existence of
commercial hydrocarbons in basement/crystalline rock impact structures a
decade before the Ames discovery.
Impact Craters: Implications For Basement Hydrocarbon Production by Richard
Donofrio

==============
(9) MUST A FREE-FLOATING PLANET BE A "SUB-BROWN DWARF"?

From Stephen Ashworth <sa@astronist.demon.co.uk>

Dear Dr Peiser,

In CCNet 52/2001 (4 April 2001), item (6) "FREE-FLOATING PLANETS", Robin
Lloyd of Space.com reports:

In fact, the International Astronomical Union (IAU) recently made
public its working definition of a planet. That definition excludes objects such
as Lucas and Roche have named as planets, encompassing only objects with a
certain mass orbiting around "solar-type stars." Free floaters with
that mass (less than 13 times the mass of Jupiter) are to be called
"sub-brown dwarfs," according to the working group, not planets.

Speaking from the non-professional sidelines, I would say the IAU working
group cannot be right in this.  In an article in *Astronomy* (March 2000),
Geoff Marcy and Paul Butler expressed the opinion that smaller planets may
be gravitationally ejected from a system by the larger ones: this conclusion
seems to be an inevitable corollary of the observed fact of Jupiter-mass
planets being found close to their parent star, or in highly eccentric
orbits. Therefore a planet like the Earth may exist in an earthlike orbit,
or in an eccentric one at the fringes of a planetary system, or
"free-floating" in interstellar space. Marcy and Butler speculate (p. 45):
"Our Galaxy must be filled with trillions of Earth-size rogue planets --
dark, rocky hulks wandering aimlessly through interstellar space."

If we set aside the excusable anthropomorphism "aimlessly", we are still
left with the point that it would be wrong to assume that planets, by
definition, can only be found in orbit around a star.  Surely it would be
absurd to call one and the same object a planet when it is found in one
energy state, and a "sub-brown dwarf" or "low-mass brown dwarf" (Mark
McCaughrean's term, quoted in the same CCNet item) when it is in another? --
and even more absurd to restrict the definition of a planet to those
orbiting a "solar-type star"!

Yours sincerely,

Stephen Ashworth
Oxford, UK
5 April 2001

============
(10) TUNGUSKA GEOCHEMICAL ANOMALIES

From Andrei Ol'khovatov <olkhovatov@mtu-net.ru>

Dear Dr Peiser and All,

In CCNet of April 4, 2001, there was a post TUNGUSKA AS AN EXPLODING UFO by
Matthew Genge. In there, he pays attention to articles mentioning
discoveries of geochemical anomalies in the Tunguska epicenter, and
interpreting them as remnants of a comet. But this interpretation has, at
least, several problems:

1) Even authors of the discoveries had to recognize that "small content of
Ir points to the low content of dust in the Tunguska comet that sharply
differs it from Halley's comet" [Planet. Space Sci. v.47, p.905 (1999)]

2) Discovered deiterium-to-hydrogen isotope ratio shift is strongly opposite
to those in all investigated comets.

3) The epicenter is right in the center of old "hotspot" paleovolcano, where
the same anomalies are expected. Moreover, the Tunguska region is rich in
ore (and other) deposits. For example, just 1200 km from the epicenter there
are  large REE (including Ir) ore deposits.

In other words, the cometary interpretation is rather problematic, and the
discoveries (taken alone) don't oppose the antimatter-hypothesis of the
Tunguska event.

Sincerely,
Andrei Ol'khovatov
Russia, Moscow

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