CCNet DIGEST, 15 December 1998

    Ron Baalke <>

    David Morrison <>

    Michael Paine <>


From Ron Baalke <>

Applied Physics Laboratory
Johns Hopkins University
Laurel, Maryland

Media contacts:
JHU Applied Physics Laboratory:         NASA Headquarters:
Helen Worth                             Doug Isbell
Laurel, MD 20723                        Washington, DC
Phone: 240-228-5113                     Phone: 202-358-1753
E-mail:          E-mail:


NEAR Spacecraft Closing in on Eros

The NEAR (Near Earth Asteroid Rendezvous) spacecraft is about to make
interplanetary history. On Jan. 10, 1999, after traveling more than a
billion and a half miles it will reach asteroid 433 Eros and embark on
the first close-up and comprehensive study of an asteroid. The NEAR
mission, the first launch in NASA's Discovery program, is being managed
by The Johns Hopkins University Applied Physics Laboratory (APL), in
Laurel, Md., which also built the spacecraft.

"What we now know about asteroids is very limited," says NEAR Mission
Manager Dr. Robert W. Farquhar of APL. "It's come from ground-based
observations and quick flybys. But now, for the first time, we're going
to go into orbit around an asteroid and study it intensely for a year.
We expect to get astounding information."

During its yearlong mission to unlock the secrets of asteroid Eros,
NEAR will confront the challenge of orbiting a tumbling,
irregularly-shaped body from extremely close distances. Never before
has any small body been orbited by a spacecraft, but the additional
task of maneuvering a spacecraft within 9 miles (15 kilometers) of the
asteroid's surface makes the engineering challenge even more complex.

A cluster of six instruments will take millions of measurements and
images over the entire surface of Eros from various altitudes. From
these data scientists will determine the asteroid's physical and
geological properties and its elemental and mineralogical composition.

NEAR's rendezvous with Eros requires that the spacecraft be sped up
with a series of engine burns so that it can catch up with the
faster-moving asteroid. At 5 p.m. (EST) on Dec. 20, 1998, when NEAR is
nearly 150,000 miles (242,000 kilometers) from Eros, a bi-propellant
rocket engine firing (or "burn") will increase the spacecraft's speed
by 1,500 mph (650 meters per second).

On Dec. 28, a second burn will increase NEAR's speed by 680 mph (294
meters per second) while at a distance of 13,000 miles (21,000
kilometers) from Eros, reducing the spacecraft's speed relative to Eros
to less than 70 mph (30 meters per second). On Jan. 3, 1999, a third
burn will reduce the relative speed a further 50 mph (22 meters per
second) at a distance of 3,000 miles (5,000 kilometers). At 10 a.m., on
Jan. 10, 1999, NEAR is scheduled to lock into orbit around Eros with a
final burn reducing relative speed to 19 mph (8 meters per second) at a
distance of about 630 miles (1,000 kilometers).

For the next year NASA's Deep Space Network will transmit data from the
spacecraft to NEAR's Science Data Center, at the Applied Physics
Laboratory, and commands from the Laboratory's Mission Operations
Center back to the spacecraft. Regular tweaking of the spacecraft's
orbit will be needed to ensure that spacecraft instruments are used to
their full advantage.

Dr. Joseph Veverka of Cornell University, Science Team Leader for the
mission, says the challenges that face the NEAR mission are significant.
"This will be the first characterization in detail, not only of the
surface of an asteroid, but of the interior of the asteroid, and the
history that this asteroid has gone through based on its surface
characteristics and materials composition."

The NEAR spacecraft was launched Feb. 17, 1996. Its flyby of asteroid
Mathilde on June 27, 1997, provided the program's first science return.
By mission's end, Feb. 6, 2000, scientists expect to know much more
about Eros and thus near-Earth asteroids in general. From this, they
hope to gain insight into the Earth's origin and the formation of the
solar system.

To follow the NEAR mission as it unfolds, visit the mission's Web site: Updates of mission activities and science
returns will be posted on the Web site and provided to media through
press conferences and briefings. The following conferences and
briefings are currently scheduled:

           Dec. 16, 1998      Jan. 10, 1999     Jan. 14, 1999

           1 p.m.             noon              1 p.m.

           NASA Headquarters  JHU/APL           JHU/APL

           Washington, D.C.   Laurel, Maryland  Laurel, Maryland

           Live over NASA TV  Live over NASA TV Live over NASA TV

For directions to the APL campus and information on hotel
accommodations, visit Web site: .

APL is located on Johns Hopkins Rd., 0.5 mile west of the intersection
of Johns Hopkins Rd. and U.S. Rte. 29, just south of Columbia, Md.


From David Morrison <>

It is well known that at the 1-km diameter level, we have discovered
only about 10% of the NEAs. At large sizes, however, our catalogs are
more complete, and there is no chance of an undiscovered NEA as big,
for example, as Eros (about 30 km in the long dimension).  But to what
NEA size are we complete; or, phrasing the question differently, what
is the largest undiscovered NEA? Some astronomers have speculated that
by now our survey is probably complete to about 5 km diameter. This is
roughly equivalent to saying that there are no unknown asteroids out
there large enough to cause a mass extinction. (Comets are, of course,
a different issue).

On December 1, 1998, a team of astronomers at the Beijing Observatory
Xinglong Station led by M. Teng discovered 1998XB, with a preliminary
orbit period of 1.01 years and an absolute magnitude of 14.2.  The
orbit is very similar to the orbit of the Earth itself, and it is still
not entirely clear whether this is an Apollo object or an Aten (period
less than 1.00 years). The surprising thing is its bright absolute
magnitude and inferred diameter of 4-8 km (6 km nominal for that
magnitude). In a humorous note written shortly after the discovery, Al
Harris (JPL) wrote: "Every time you find one of these guys it jacks up
my estimate of the number of NEAs with H<18.0 [>1 km diameter] by about
200 or so. And that in turn reduces my estimate of the time until the
"end of the world" by about 50,000 years. You should be more careful
when the end of the world is at stake."

Ted Bowell estimates that 1998XB is even brighter, at H=13.8, making it
correspondingly larger (possibly as large as 10 km diameter, if it is
very dark). Just a few months ago the LINEAR survey team found Apollo
asteroid 1998QS52 with H=14.1, about the same size as 1998XB. To see
any comparable-size discoveries, we must go back to two Amor asteroids
(not currently Earth-crossing), 7358 at H=14.4 found in 1995, and 5836
at H=13.9 found in 1993.

Discussing the orbit of 1998XB, David Rabinowitz (JPL) writes: "This
object, with semimajor axis so close to 1 AU, has an interesting
ephemeris. For the next 7 years, the object's peak solar elongation
will get larger and larger, until Dec 2004 when it will be within 10
degrees of opposition and the magnitude reaches V=13. By Dec of 2009,
the peak elongation will be back down to 90 degrees, about where it is
now. So every December, for the next 10 years, there will be good
observing circumstances. After 2009, however, it is another 20 years
before the object strays more than 90 degrees from the sun."

So how did we miss something this big? The problem is its orbit, which
is so nearly commensurate with the Earth's. It can spend a couple of
decades on the opposite side of the Sun from the Earth, invisible to
our telescopes, followed by a decade in which it is easily seen every
year. Clearly, there could be others out there with similar periods,
athough the odds are against many others quite this big.

These NEAs with unusual orbits will be, statistically, among the last
to be discovered in a Spaceguard Survey - among that last 10% that
simply take longer than a decade to find. Some people have suggested
that we need a space-based search telescope for Aten asteroids (as this
one may be) with orbits inside that of the Earth. But it is not the
small orbit that matters, rather it is the fact that the asteroid's
period is so close to 1 year. It is hard to find precisely because it
comes so close to the Earth. The proper strategy to find these NEAs is
not to build an expensive space-based system, but to start the
ground-based search as quickly as possible. We will find these unusual
objects not by going to space, but by surveying from the ground when
their orbital phasing bring them into view.

David Morrison (with thanks to Al Harris, Ted Bowell, and David


From Michael Paine <>

Dear Benny,

Re: Andrew Glikson's comments on "inter-planetary bacterial seeding"

In August this year I received the following email from Jay Melosh,
University of Arizona:

Thank you for your interest in "Swapping Rocks" (he was referring to my
web page A more
up-to-date version of the idea was just published in a journal called
"The Sciences", a publication of the New York Academy of Sciences. The
citation is:

Melosh, H. J. (1998) Blast Off, The Sciences 38, no. 4, 40-46.

I am also participating in a gigantic collaboration to do a careful
analysis of the transfer of viable microorganisms from one planet to
another that will be published soon in Icarus:

Mileikowsky, C., Cucinotta, F., Wilson, J. W., Horneck, G., Lindgrin,
L., Melosh, H. J., Rickman, H. and Valtonen, M. (1998) Natural transfer
of viable microbes in space, Part 1: From Mars to Earth and Earth to
Mars, submitted to Icarus.

This idea seems to have reached the mainstream and there is currently a
lot of interest in it. I am told by Paul Davies (U. of Adelaide) that
he has just submitted a book on the origin of life in which this idea
figures. The book is supposed to appear in the UK sometime this fall and
in the USA next year.

Sincerely,  Jay Melosh

Some of Andrew Glikson's questions are liklely to be covered in these


Michael Paine

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From Julian A. Hiscox <>



Geological Society, Burlington House, Piccadilly, London, W1.
11TH DECEMBER, 1998.

Organised by:

Richard L. S. Taylor
0181 764 2774

Julian A. Hiscox
01635 200103

David Hughes
0114 222 4288


Dr. Matthew Genge (Natural History Museum): The implications of
meteorites and micrometeorites for the nature of sub-critical

The material properties of impactors and how might this affect hazard
limitation efforts and impact behaviour are discussed.  The question of
what are the most hazardous comets or asteroids a problem which
generates much disagreement between US and British astronomers. From the
view point of a meteoritist the question is slightly different because
it is related to what is the difference between these two classes of
object. The evidence from micrometeorites is leading us to believe that
comets and asteroids may not be discrete classes of objects but
represent a continuous series from ice-rich comets to silicate and
metal-rich anhydrous asteroids.

As far as impact effects go the material properties of the impactor are
less important than those of the target and the kinetic energy of the
impactor, however material properties play an important role in whether
a small object penetrates to ground zero or forms an air-burst such as
in the Tunguska event.  This may be an important factor in the impact
damage since for smaller events air-bursts may cause more damage over a
wider area.

Jonathan Tate: (Spaceguard UK): The frequency of SCI impactors with
diameters in the 0.1 to 1km range

A brief definition of Sub-Critical Impactors is offered, and used as
the basis for the remainder of the paper.  The populations of sub
critical impactors, including the attrition and replenishment rates,
have been calculated and/or estimated by a number of individuals and
research teams in the past decade using a variety of methods.  This
work has led to estimates of the impact flux on the Earth from such
objects. In this paper the findings of the various researchers are
investigated, and conclusions are drawn from their results.  Although
there are intrinsic uncertainties in all of this work, there is good
agreement between the findings of the various researchers who have
contributed. Brief consideration is also given to the question of
whether the SCI flux has relevance to the widely recognised impact

Dr. David Hughes: (University of Sheffield): The cratering rate of
planet Earth.

The rate at which the surface of Earth is being cratered can be
measured by analysing the sizes and ages of the craters that are found
on certain stable areas of the Earth's land-mass.  The effects of
erosion can be easily accounted for. On the stable areas it is found
that, for example, craters larger than 2.5 km in diameter take longer
than 125 My before they can no longer be measured accurately.  The
number N(D) of craters larger than diameter D follows a relationship of
the form N(D) [sign of proportionality] D ^ a. The power a does not, as
often previously thought, have a constant value of - 2.0. This power a
is found to vary continuously as a function of crater diameter, being
typically: -0.43 around D=4km, -0.81 around D=10km, -1.31 around D=20km
and -1.70 around D=40km.  The Earth's cratering rate is such that time
interval between impacts producing craters larger than say 3 , 15 and
100 km is 180,000, 420,000 and 15,000,000 years respectively. It is
found that the extraterrestrial bodies responsible for producing craters
of diameters about 15 km are about 250 times more likely to be asteroids
than comets.

Professor Ian Smalley & Mr. Ian Jefferson (Nottingham Trent
University): Sedimentological consequences of sub-critical impacts in
sandy deserts or loess regions.

Small particle impacts on loess soil landscapes have been studied for
many years in the context of wind erosion.  At the WERU 50 wind erosion
meeting in Kansas in 1997 the concept of metastable energy in the soil
system aiding erosive forces was introduced, and can be developed in
this paper.  Very large body impacts in loess regions can be interpreted
in terms of the erosion equations developed for WERU 50 but it is
likely(inevitable) that new variables will have to be introduced.
Large impact in a sandy desert has certain similarities with impact in
a loess region.  But in addition particle breakage needs to be
considered - leading to the production of large amounts of silt-sized
material as 'Moss defects' in the quartz sand particles are activated. 
Widespread distribution of clastic material would occur but of all
terrestrial material a thick loess deposit(say ~300m thick) is probably
the best equipped continental deposit to absorb a small impact(about
the size of the Red Wing event).  A large amount of sand material(say
300-500 um) will be injected into the lower atmosphere by a desert
impact.  This will fall out rapidly; the silt sized material can go
into low level suspension and be moved tens of km.  Of the basic
primary mineral particles defined by Jefferson & Smalley the C
particles(~3-5um) travel farthest. Experimental studies using high
speed disc mills suggest that large amounts of C particles can be
formed if critical energy levels in the sand sized particles are
exceeded- and we expect impact energies to be above these critical
levels. At some distance from the impact various geomorphological
phenomena might be observed. One of the most interesting is soil
collapse. Even an attenuated stress wave might cause metastable soil or
sediment structures to collapse at large distances from impact. The
geomorphological zone needs to be distinguished from the geological zone
and the geochemical zone.

Professor Nicholas Fedoroff  (Institut National Agronomique, France):
Registration of abrupt events in loess: around 67000 yrs BP (before
present) transition reveals unusual attributes.

Abrupt events are presently well documented in ice-cores (Dansgaard-
Oeschger cycles, Younger Dryas) and deep sea-cores (Heinrich events).
Recently Heimich events have been correlated with coarse loess layers
in China, Paleoctimatologists believe that these events are caused by
earth oscillations (Milankovitch theory) and ocean upheaval.  The aim
of this presentation is to show existence of anomalous constituents and
features in the coarse loess layer dated of -67000 yr.  BP (Heinrich
event 6) in China and in Europe and to discuss their origin.  This
layer is characterised by, (1) abundant, fresh, large biotite flakes
and to a lesser extent of muscovite, besides an increase in size of
quartz grains; (2) an abrupt appearance of calcitic fragments; (3) a
few isolated and compound fibro-radiated spherules; (4) phantoms of
mediumly soluble salts, e.g. gypsum; (5) increase in heavy minerals,
e.g., green hornblende, zircon, fragments of ultra-basic rocks', (6)
increase in burnt plant fragments; (7) rounded soil matrix aggregates
(pseudo-sands); (8) fragmented microstructure (only in China); (9)
brownish black clay and fine silt coatings. (1), (2) and (7) should be
interpreted as a strong increase of the intensity of dust storms
associated with soil denudation, soil erosion and saltation of pseudo-
sands while the fragmented microstructure (8) could result from an air
blast.  Biotites were probably deflated from river beds, e.g., Yellow
river, Burnt plant fragments and the high organic content of coatings
(9) indicate occurrence of  extended wild fires.  The origin of
mediumly soluble salts is questionable as well as the increase in heavy
minerals. Fibro-radiated spherules do not form in normal soil forming
processes. Brownish clay coatings result from heavy rains on a burnt
bare soil. Evidence of extended @d fires, presence of heavy minerals
and -anomalous constituents such as fibro-radiated spherules should
imply as a working hypotheses for this sequence of events a cosmic
origin. Why such constituents and features in terrestrial sediments and
soils have not been detected earlier? Varied causes have to be put
forward in order to explain this lack of knowledge, such as: (1)
diagnostic constituents are diluted within a mass of common minerals,
e.g. quartz and moreover spatially unevenly distributed, (2) presently
used analytical methods are not adapted to detect these diagnostic
constituents and features, (3) their significance is not clearly
established, (4) radiometric techniques do not exist for measuring and
even estimating duration of these abrupt events, (5) modem analogue do
not exist, (6) existing concepts in geology do not permit
reconstruction of such abrupt events, (7) investigations on abrupt
events are closely segmented between highly specialised discipline,
Moreover the sequence of events which characterise such abrupt events
is difficult to reconstruct. Further investigations are undoubtedly
needed.  A multidisciplinary approach is required.  We need to fill the
gap between impact specialists and other specialists.  Some kind of a
task force must be gathered. Pedologists are the best suited to analyse
pre- and post abrupt pedogenic, features and fabrics as well the
features and fabrics occurring during the abrupt event.  A priority
should be given to a multiscalar, comprehensive, in-situ analysis (soil
micromorphology). Sedimentologists and geomorphologists should also be
included. Such a task force should first re-investigated the soils of
the Tougounska area, Then the Chinese toes-., more precisely the sandy
loess layers, should be approached, a special attention being devoted to
the 0.8 Ma time span of time during which Australasian tektites were
deposited, However investigations should be ex-tended to all continents
in order to confirm the global character of these abrupt events.

Professor Claudio Vita-Finzi (University College, London): Seasonal
enhancement and seismic triggering by impacts

Permanent ice covers ~ 18 % of the Earth; permafrost underlies a further
~20%. Their response to shallow impacts will vary according to season
from brittle fracture to flow accompanied by vaporization.  Seasonal
climatic contrasts will thus be enhanced by sub-critical high latitude
cratering, as particulate ejecta from icy targets will contribute in the
winter months to the 'impact winter' effect associated with large
events, and water vapour derived from thawed permafrost or melted sea-
ice to the atmospheric greenhouse effect in the summer months.  In
seismically active areas, an impact may trigger imminent events
by changing the static stress field or by ground shaking. Assessment of
the environmental consequences of a sub-critical impact thus needs to
consider its timing within the local seasonal or earthquake cycle as
well as location, size and trajectory.

Professor Chandra Wickramasinghe (M.K. Wallis and D.H. Wallis):
(University of Wales, College of Cardiff) Climatic Switches Induced by
Stratospheric Dust Loading.

Highly stable periods of glaciation in the Earth's history could be
terminated abruptly by impact of comets of masses in the range 1015 to
1016.  Such an impact releases enough water into the atmosphere to
increase the low values of water column density 0.1cm appropriate for
ice ages to 1 cm.  This leads to an immediate resumption of a
greenhouse, increase in water precipitation, decrease in albedo, rise in
average surface temperature above 280K, and a snapping out of the Earth
from an ice age.

Dr. Norman MacLeod (Natural History Museum, London): Identifying
instances of past environmental change and their causal mechanisms.

Although paleontologists commonly refer to the "big five" mass
extinction events (Tatarian, Ashgillian, Norian, Maastrichtian,
Frasnian) stage-level extinction intensities assessed for the fossil
record of marine invertebrates-the most complete fossil record
available-exhibit a quasi-continuous distribution. This continuity
undermines attempts to achieve an objective definition of "mass
extinction" (or, by implication "sub-critical" extinction), and implies
a continuity of physical processes responsible for stage-level
extinction events. Over the past 20 years there has been a tendency in
mass extinction studies to posit an ultimate physical mechanism (e.g.,
bolide impact, volcanism, sea-level change) and then test the operation
of that mechanism by determining whether disappearances of fossil taxa
are associated with evidence for mechanism's effects (e.g., Ir
anomalies, isotopic excursions). This is an inherently limited research
strategy in that it fails to consider the dynamic, multi-causal nature
of environmental perturbations and fails to capitalise on the ability of
biotic data themselves to identify changes in the physical environment.
Only by integrating temporal, geographical, and ecological data from
different microscopic and macroscopic organismal groups can a more
detailed picture of the nature, timing, distribution, and intensity of
the proximate environmental perturbations associated with mass
extinction events be constructed. These data can then be used to test
physical models of mass extinction causality. Synoptic data from marine
micro- and macro-invertebrates suggests that flood basalt volcanism
exhibits the most consistent association with stage-level extinction
intensity during the last 250 million years, though there are many
lines of evidence that are consistent with the three largest extinction
events in this interval all being multi-causal. In some instances
available biotic data are sufficient to rule out particular proximate
causal models (e.g., Strangelove Ocean) and constrain/support
particular hypotheses by documenting patterns of geographical variation
in environmental stress factors. In others the same proximate causes
supported by biotic data are associated with different ultimate causes.
Future mass extinction studies must take into consideration the ability
of biotic data to test physical models of extinction causality.
However, Raup's (1986) dictum, coupled with existing uncertainties in
our knowledge of marine invertebrate biology/ecology may circumscribe
the ability of biotic data to distinguish between some proximate
limiting mechanisms.

Dr. Julian Hiscox (IAH Compton Laboratory): Possible
biochemical/bioevolutionary consequences of SCI events.

The impacts of asteroids and comets on the Earth have played a
fundamental role on the origin and evolution of life on Earth. The vast
majority of water in the oceans are believed to have been derived from
the influx of comets.  Not only that, but many the building blocks of
life such as amino acids are present in cometary nuclei. The
possibility also arises that the impact delivery of these materials may,
depending on the oxidation state of the atmosphere, have played a role
in the origin of life on the early Earth. Also, if the early Earth was
frozen, the energy generated by the impact of comets or asteroids, may
have melted the ice layer and then life may have been given another
kick start.  Certainly the impact of an asteroid played a fundamental
role in the emergence of mammals, by probably contributing to the
demise of the dinosaurs. In fact, it is highly likely that many species
have become extinct, by going through population bottlenecks, due to
the environmental changes wrought by the impacts of comets and

Dr. Marie-Agnes Courty (National Centre for Scientific Research,
France): Recognition of instantaneous soil collapse at 3950 BP (before
present) throughout the Middle East in response to a blast wave, wild
fires and heavy rains caused by an extra-terrestrial event.

Further investigations lead us to re-examine the nature, age, causes
and effects of the third millennium catastrophe that at first we
identified as an abrupt climate change synchronous to a volcanic event,
and inferred to have initiated the collapse of the Akkad Empire at 2200
BC. More recently we refuted the occurrence at this period of a tephra
fall-out from a volcanic explosion and suggested the dust deposition -
synchronous with unusual heavy rains and large forest fires - to be the
eventual fingerprints of an extra-terrestrial impact, here we report
the preliminary conclusions of a more comprehensive exploration across
the Middle East that promotes the alternative of an impact event
although it remains obscure on many aspects, We suggest that the
ambiguity lies partly in the weak knowledge of the geoscience community
of minor collisions with Earth because of the lack analogies and, also,
inability of conventional approach to discriminate in the geological
record instantaneous events and their spatial patterns of Variations
from local to large regional scales.
The methodology we propose is based on the combined study of high
resolution sequences from archaeological sites and natural soils, The
former offer the possibility to obtain a high resolution signal with a
precision range from years to seconds.  The main constraint consists in
the diversity of human actions that often Confuses the identification
of unknown natural effects.  Thig difficulty can be raised by checking
in natural deposits from the landscapes around human settlements the
occurrence at the same time of similar manifestations, Inter-regional
comparison of archaeological sites is essential to further confirm the
natural origin of an unusual phenomena, and, when fully assumed, to
examine with the help of archaeological data how the event affected
human populations. Application of this methodology to a random sample
of regions now allows us to confirm the deposition of an unusual dust
layer that was synchronous with a violent explosion and exceptional
wild-fires across Syria, Jordan and Lebanon. Resembling manifestations
are detected in western Europe, west Asia and south America but of weak
intensity. Radiometric dating provides an age at ca. 4000 B.P. ( i.e.
4500-4100 cal B.P. at 2 s), consistent with the chronological
estimation based on archaeological data from reliable contexts: the
transition between late Early Dynastic III and Early Akkad (i.e.
2350-2300 cal BC). C14 radiometric ages partly obscure the abruptness
of the event however well documented by its micro-stratigraphic record.
The solution of a minor collision with Earth is proposed on the basis
of coherent petrographic and geochemical evidence that suggest the dust
layer to result from mixing of an impact ejecta and pulverised local
soils by the surface propagation of a violent air blast. Evidence for
exceptional heavy rains just following the explosion attest for a rapid
environmental perturbation probably caused by the massive injection in
the earth's atmosphere of dust and smoke, The subsequent wide erosion
and soil collapse, that contributed to fossilise, erase or confuse the
record of the impact event, is suggested to. express a significant
increase of rain acidity as the result of extensive biomass burning and
carbon dioxide production.  The impact is believed to be located
somewhere in the Middle East where it caused severe devastation
and to have global consequences.

Dr. Benny Peiser (Liverpool John Moores University): Current research
on Holocene impact events and its implications for impact rate

The geological, climatological and archaeological search for evidence
of hypervelocity impacts since the onset of the Holocene period
(>10,000 yrs BP) is still in its infancy. This research is not only of
geological, historical and anthropological interest. The results of
these investigations will have important ramifications for our
understanding of the cosmic environment of planet Earth. They will also
help to produce a sound methodology on which to base impact rate
calculations. Current estimates suggest that up to 100 Tunguska-type
impacts may have punctuated the Earth in the last 10,000 years. Such
punctuations (> 50 MT) have only local or regional effects with little
detrimental consequences for human and societal evolution.. In contrast,
episodes of multiple multi-megaton impacts or significant cosmic dust
loading (Super-Tunguskas) can have catastrophic effects on a hemispheric
or even global scale. Such ‘Super-Tunguska’ events may subsequently lead
to critical climatic downturns and the consequent collapse of
agricultural civilisations. Some of the most significant questions
impact research currently faces are i) whether cosmic impacts with
hemispheric or global knock-on effects did occur during the Holocene,
ii) whether such impact events in fact triggered the collapse of ancient
civilisations, and iii) how many such ‘Super-Tunguskas’ may have
occurred during the last 10,000 years. In this paper, I attempt to 
address these questions.  I will review the present research on
Holocene impact events and civilisation collapses and will assess the
implications these studies may have for impact rate estimates.

Dr. Victor Clube (Oxford University and Armagh Observatory):
Sub-cometary and sub-asteroidal impacts: historical considerations.

Not so long ago, comets were a source of extreme terror to Mankind,
giving rise to considerable social upheaval. The rational basis for
this terror was eventually suppressed however and is now virtually
unknown.  Nevertheless it has recently been established through a
variety of historical enquiries (largely outside the astrophysical
domain) that fixed stars, planets and comets accompanied by numerous
subordinate bodies in similar orbits were "the grand key to the
theology of the ancients", essentially the (heliocentric) Platonist
cosmology, whose origin can be traced to 3000 BC.  This cosmology
remained paramount in the West until ca 1300 AD after which it
continued (as a component of Protestant Christian doctrine) in
contention with Aristotelians until ca 1700 AD.  Anglo-Saxon scholars
then unexpectedly agreed that protracted fireball swarms regarded as
"revelations" essentially bore no relationship to subordinate cometary
bodies with the result that the terrestrial environment came to be
regarded as historically "uniformitarian".  The discovery of giant
comets during the late 20th century, the Tunguska event (1908) and
cometary trails (1983) inter alia have now materially altered our
perception of the terrestrial environment on both historical (kyr) and
geological (myr) time scales, essentially reinstating Platonist
cosmology. The incidence of extreme terror in the future can only be
avoided therefore by appropriate technological developments.

Professor Neville Price (formerly University College, London)
Evidence for impact as a significant and periodic geological process.

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CCCMENU CCC for 1998