PLEASE NOTE:


*

CCNet DIGEST 5 June 1998
------------------------

(1) IMPACT FREQUENCY OF NEAR EARTH OBJECTS
    Jonathan Shanklin <jdsh@mail.nerc-bas.ac.uk>

(2) TUNGUSKA-EVENTS EVERY 25 YEARS?
    Jens Kieffer-Olsen <Comp2002@jk-o.demon.co.uk>

(3) ASTEROID IMPACT STUDY FINDS EFFECTS OF COLLISIONS OR EXPLOSIONS ON
    SMALL ASTEROIDS MAY BE HARD TO PREDICT
    Andrew Yee <ayee@nova.astro.utoronto.ca>

(4) TEMPE RESIDENT REMEMBERS METEOR'S PLUNGE 86 YEARS AGO
    Rolf Sinclair/NSF Physics Division <rsinclai@nsf.gov>

(5) MILLENNIUM BUG: TIME IS RUNNING OUT FOR LIVERPOOL'S ASTRONOMICAL
    'COMPUTER' MADE IN 1600
    Liverpool Echo, 4 June 1998

(6) HYPERVELOCITY IMPACT CRATERING OF CO2 ICE
    J. Leliwa Kopystynski et al., UNIVERSITY OF WARSAW

(7) HYPERVELOCITY IMPACT ON SODA LIME GLASS
    E.A. Taylor et al., UNIVERSITY OF KENT

(8) EJECTA FROM HYPERVELOCITY IMPACTS ON CO2
    W. Brooke Thomas et al., UNIVERSITY OF KENT

(9) HYPERVELOCITY IMPACT ON BRITTLE MATERIALS
    E.A. Taylor et al., UNIVERSITY OF KENT

(10) HYPERVELOCITY IMPACT EXPERIMENTS ON THETHER MATERIALS
     D. Sabath & K.G. Paul, TECHNISCHE UNIVERSITAET MUNICH

(11) HYPERVELOCITY IMPACT EXPERIMENTS ON SOLID CO2
     M.J. Burchell et al., UNIVERSITY OF KENT

(12) MORPHOLOGY OF IMPACT CRATERS
     V.V. Silvestrov et al., RUSSIAN ACADEMY OF SCIENCE

===============
(1) IMPACT FREQUENCY OF NEAR EARTH OBJECTS

From Jonathan Shanklin <jdsh@mail.nerc-bas.ac.uk>

I have extracted the two messages from sci.space.science and have
pasted them below.

Regards,
Jon Shanklin
j.shanklin@bas.ac.uk
British Antarctic Survey, Cambridge, England
http://www.nbs.ac.uk/public/icd/jds

==============================
(2) TUNGUSKA-EVENTS EVERY 25 YEARS?

From: Jens Kieffer-Olsen <Comp2002@jk-o.demon.co.uk>

I paid a visit to-day to http://cfa-www.harvard.edu/iau/lists/ where
Near Earth Objects are tabulated in various contexts.

From the site's tables I compiled the below top-10 list of close
encounters with objects of a diameter greater than 1.5 km (i.e. 1 mile)
and known to pass within 5 mio. km of Earth some time over the next 50
years.

Mo/Year     Size (km)    Distance (mio. km)    Discovered   Name
Aug 2003       1.5              4                  1994      PM
Sep 2004       4                1.5                ?         Toutatis
Feb 2017       2                5                  1992      FE
Jan 2022       1.5              2                  1994      PC1
May 2022       1.5              4                  1989      JA
Jun 2027       6                4.5                1990      MU
Oct 2028       1..5              1                  1997      XF11
Sep 2033       2                4.5                1996      EN
Aug 2042       2                3.5                1994      CN2
Aug 2046       1.5              4                  ?         Castalia 


Assuming the sphere of the Earth to be hit proportionally the same
number of times, 10 hits in 50 years within a radius of 5 mio. km would
match a probability of one such destruction of Earth every 2.5 mio.
years.

Objects sized down by a factor of up to 100 ( 15m, Tunguska-type)
should occur about 100,000 times ( 100**2.5 ) more often than those
tabulated, that is once every 25 years.         

But seeing that virtually all these asteroids have been discovered less
than 10 years ago, isn't it reasonable to expect at least twice as many
impacts?

---------------------

From Frank Crary <fcrary@rintintin.Colorado.EDU>

It's actually a bit worse than that. The Earth's gravity tends to focus
impactors, and increase the probability of a hit. That process depends
on the relative velocity of the impactors, but I believe it's typically
a factor of a few for near Earth objects. By the way, your table isn't
quite right when it comes to the names of the objects: For example, you
list 1994PM as discovered in 1994 and named PM. Strictly speaking, that
object doesn't have a name and the 'PM' is part of the date of
discovery. The system used by the minor planets center identifies
objects by date of discovery pending an official name and number. The
letters (if memory serves) identify the two week period and day of
discovery within that period. E.g. 1994PM was discovered in the 13th
day of the 16th two-week period of 1994 (M is the 13th letter of the
alphabet and P is the 16th) or day of year 237 of 1994 (the 25th of
August.)

>  Objects sized down by a factor of up to 100 ( 15m, Tunguska-type )
>  should occur about 100,000 times ( 100**2.5 ) more often than those
>  tabulated, that is once every 25 years.         
>  But seeing that virtually all these asteroids have been discovered
>  less than 10 years ago, isn't it reasonable to expect at least twice
>  as many impacts?

That isn't obvious to me. The fact that all these objects are recent
discoveries suggests that we haven't found all the 1.5 km or larger
objects out there. But I don't understand why you say we are under counting
by at least a factor of two. Objects that size should have an absolute
magnitude of around 16, which is bright enough for the current detection
programs to easily find. So I suspect we've probably found the majority
of them (with low albedo ones, like C-type asteroids, being an exception.)
For smaller objects, the under counting is probably more severe, but
you really need to know the details of the observing programs that
have looked for these objects, before you can attach a number to that.
Finally, your scaling is probably conservative. A 2.5 power law
may understate the number of smaller bodies. Cratering records suggest
power laws in the 2 to 4 range, and I think the large-size end of
the asteroid population is similar (the small asteroids can't be
used for this, since there are biases due to under counting.) But
those estimates are uncertain: They are for a different size range,
population of impactors, and (in the case of craters) an average
over a long period rather than the current population. Unfortunately,
we don't really have good statistics on the current size distribution
of small, near Earth impactors.

                            Frank Crary
                            CU Boulder

=====================
(3) ASTEROID IMPACT STUDY FINDS EFFECTS OF COLLISIONS OR EXPLOSIONS ON
    SMALL ASTEROIDS MAY BE HARD TO PREDICT

From Andrew Yee <ayee@nova.astro.utoronto.ca>

Contact:
Tim Stephens, science writer
University of California
Public Information Office
1156 High Street
Santa Cruz, CA  95064
Phone: (408) 459-4352   Fax: (408) 459-5795
stephens@cats.ucsc.edu

ASTEROID IMPACT STUDY FINDS EFFECTS OF COLLISIONS OR EXPLOSIONS ON
SMALL ASTEROIDS MAY BE HARD TO PREDICT

SANTA CRUZ, CA -- An analysis of collisions between asteroids may help
explain the structure and evolution of these small planetary bodies and
also raises concerns about the feasibility of disrupting or deflecting
an asteroid in the event that one is discovered hurtling through space
towards Earth.

Astronomer Erik Asphaug, a research associate at the University of
California, Santa Cruz, used computer simulations to study the effects
of powerful impacts on asteroids with different internal structures. He
and his colleagues found that the outcome of such impacts depends on
the degree to which the asteroid has been fractured and made porous by
earlier collisions.

Asphaug said he is primarily interested in understanding the geophysics
of asteroids and the evolution of small bodies in the solar system. But
the implications of his findings for deterrence of an asteroid
collision with Earth are compelling. Nuclear explosions have been
proposed as one way to break up or alter the course of an asteroid
headed towards Earth. But Asphaug found that some types of asteroids
could absorb a powerful explosion with little or no effect.

"It's a lot more difficult to nudge these asteroids around than we had
thought," said Asphaug, who completed the study while working at NASA
Ames Research Center and the SETI Institute in Mountain View, CA. "More
work needs to be done before we can decide whether nuclear warheads
provide a viable deterrent," he added.

Previous studies by Asphaug and others suggested that many of the
asteroids in our solar system are aggregates of debris left over from
previous collisions -- either a few large fragments held together by
self-gravity or "rubble piles" consisting of numerous smaller pieces.
The new study shows that the porous nature of such asteroids damps the
propagation of shock waves, thereby limiting the effects of an impact or
explosion to a localized area. Asphaug and his collaborators at several
other institutions published their results in the June 4 issue of the
journal Nature.

For their simulations, the researchers started with a computer model of
an asteroid 1.6 kilometers (1 mile) across, based on radar images of a
near-Earth asteroid named Castalia. They gave this peanut-shaped target
asteroid three different internal structures: solid rock, a pair of
solid rocks in close contact, and a rubble pile with pore space
accounting for 50 percent of its volume. The researchers subjected each
of these to impact by a house-sized rock traveling 5 kilometers per
second, a typical speed for collisions in the asteroid belt. This is
equivalent in energy to the 17-kiloton Hiroshima bomb, although impacts
are more devastating than explosions of equal energy, Asphaug said.

The results may explain some of the bizarre shapes and structures
scientists have observed in recent years as they have begun to get
detailed images of near-Earth asteroids. For example, in June 1997 the
Near Earth Asteroid Rendezvous (NEAR) spacecraft sent back remarkable
images of the asteroid Mathilde, showing five huge craters, some larger
in diameter than the radius of the asteroid itself. The impacts that
created these enormous craters did not break the asteroid apart, did
not erase or disturb preexisting craters, and left no sign of fractures
on Mathilde's surface.

"A rubble-pile model provides a good explanation for a low-density body
like Mathilde, because an impact can blast the heck out of a local area
and have little effect on the rest of the asteroid; the shock wave dies
out quickly, so a large crater can be excavated without the rest of the
asteroid noticing what happened," Asphaug said.

At the opposite extreme, he noted, an asteroid consisting of solid rock
throughout may shatter into many smaller pieces when hit by another
object. Depending on the energy of the impact, those pieces might
disperse to form a family of smaller asteroids or remain aggregated,
forming a rubble pile. A solid asteroid might also break into several
large pieces plus debris.

Many asteroids (including Castalia) have a twin-lobed structure that
suggests they consist of two separate pieces held together by gravity.
Such "contact binaries" proved somewhat resistant to impacts in
Asphaug's simulations, because the shock wave reflects off the boundary
between the two components. As a result, one side of a binary asteroid
could be blown apart by an impact, while the other side remains
relatively unaffected, Asphaug said.

These results suggest that to a large extent the fracture pattern
resulting from one impact determines the outcomes of future impacts.
"Once an asteroid has been broken, it becomes more resistant to
subsequent events because the impact-generated shock waves can't
propagate across the fractures," Asphaug said.

The same would hold true of explosions. Therefore, to predict the
effect of a nuclear explosion on any particular asteroid, scientists
would need to understand its internal structure, Asphaug said. The
internal structures of asteroids, however, are probably just as diverse
as their external shapes, he added.

Asphaug's ongoing research may shed light on the evolution of the solar
system from a disk of gas and dust to the familiar set of planets
circling the Sun. The asteroids are presumably representative of the
small bodies that eventually grew into planets by accumulating
additional matter. This accretion process must have involved numerous
collisions between smaller bodies, and Asphaug is trying to find out
what factors in a collision favor the accumulation of mass (allowing
for the growth of planets) rather than disruption and loss of mass.

"Asteroids may be like a snapshot of the first stages in planetary
evolution," Asphaug said. "We're in the midst of an epoch of discovery
in which we are just beginning to see what asteroids look like and to
understand how they got to be the way they are."

Additional studies may explain many of the key processes involved in
the evolution of the planets. Of course, one process involving
asteroids will always be of special interest to the inhabitants of
planet Earth, and that one is currently playing in theatres nationwide.
Movies such as Deep Impact dramatize the scenario in which an asteroid
collision threatens life on Earth.

According to Asphaug, there are hundreds of thousands of asteroids in
near-Earth space whose impact on Earth would be equivalent to the
largest thermonuclear device ever exploded.  Thousands, like Castalia,
are larger still. While the probability of such an impact occurring in
the near future is extremely small, the consequences would be
disastrous.

"Asteroids are not an imminent threat, and I am far more concerned
about what humans are doing to the planet," Asphaug said. "But in case
we ever identify an asteroid or comet on a collision course, it would
be best to know our enemy so that we can get it before it gets us."

#####

IMAGES AND VIDEO AVAILABLE -- Contact Tim Stephens at (408) 459-2495 or

stephens@cats.ucsc.edu.

Editor's note: You may reach Erik Asphaug at (408) 459-2260 or
asphaug@earthsci.ucsc.edu, or through his web page at
http://www.es.ucsc.edu/~asphaug/.

===================
(4) TEMPE RESIDENT REMEMBERS METEOR'S PLUNGE 86 YEARS AGO

From Rolf Sinclair/NSF Physics Division <rsinclai@nsf.gov>

Hi Benny -- The following is from the Southwest Archaeology e-mail
distribution.

Rolf

---------------
TEMPE RESIDENT REMEMBERS METEOR'S PLUNGE 86 YEARS AGO

30 May 1998

MESA, Ariz. (AP) _ Pauline McCleve of Tempe doesn't need to go to the
movies to see scary scenes of meteors streaking toward frightened
people. She can just rerun one of the memories in her head. Now 103,
McCleve remembers the explosion in the sky when a rock from outer space
fell near Holbrook in northern Arizona on July 19, 1912. ``That was the
loudest sound I ever heard in my life,'' she recalled recently.
``There was no sound from us except a gasp of terror.''

She was 17, standing outside her family home in Holbrook with her
parents and some of her 10 brothers and sisters. The meteor dominated
the early evening sky. ``It was coming right toward us. We thought we
were going to die. ``The closer it came, the more frightened we were.
We just stood there paralyzed.'' The boom was heard as far away as 100
miles north and south of the city, according to newspaper accounts from
that week. ``People ran into the streets and stared at the sky,'' the
Holbrook News reported. Witnesses in Winslow, 30 miles farther west,
saw a smoky trail streaking eastward toward Holbrook. McCleve
remembered it as a glowing fireball with a bright tail. The boom came
from a chunk of asteroid shattering into thousands of pieces.

It probably was about the size of an office desk when it  first entered
the atmosphere, according to Carleton Moore, director of the Arizona
State University Center of Meteorite Studies. ``Holbrook is still the
only observed fall in Arizona,'' Moore said. ``All the other meteorites
in Arizona have just been found sitting on the ground.''

Observed falls, in which a meteorite is seen in the air and then
recovered on the ground, occur only about once every two or three years
anywhere in the world. Several pieces of the dense black stone now sit
in one of the center's public display cases on campus, including the
biggest chunk that hit the ground, weighing 14 pounds, and tiny bits
the size of peas.

McCleve remembered, ``It exploded like shrapnel.'' The pieces landed in
a 3-mile-long ellipse centered about six miles east of Holbrook. One
baseball-sized chunk knocked the limb off a tree. ``Papa said, `Oh, it
missed us, but that landed very close. I'll go out in the morning and
look for it.''' Other folks had the same idea, and many of them went
out to collect pieces of the dense black stones. More than 14,000
pieces were collected that summer, mostly from the surface of the
ground, but some of the largest were embedded up to 6 inches deep. Many
were purchased by a Philadelphia collector, Warren Foote, who wrote the
first scientific paper about the Holbrook meteorite four months later.

McCleve's father, Richard Decatur Greer, and her younger brother, Pratt
Greer, earned nearly $2,000 gathering and selling pieces of the
Holbrook meteorite, she said. The man she married the following year,
James Cyrus McCleve, made $400.

``It was hard times, and everybody was glad to get what they could,''
she said. In 1912, $2,000 was enough to buy a modest home. About 2,000
additional pieces of the Holbrook meteorite have been found since 1912,
some as recently as 1991.

Moore gave a talk about meteorites to the Kiwanis Club at the
Friendship Village retirement center in Tempe last month. Afterward, he
received a note that McCleve, a resident of the center, would like to
talk with him. Some of the pieces of the Holbrook meteorite at ASU were
part of Foote's collection, so some may have originally been picked up
by McCleve's father, Moore said. McCleve has remembered the meteor many
times in the past 86 years. ``That was the most terrifying time in all
my years,'' she said, ``Those few seconds of the meteor coming toward
us.''

http://www.swanet.org/ telnet://aztec2.asu.edu
Southwestern Archaeology (SWA) - History, Archaeology,
and Anthropology of the American Southwest!

==============
(5) MILLENNIUM BUG: TIME IS RUNNING OUT FOR LIVERPOOL'S ASTRONOMICAL
    'COMPUTER' MADE IN 1600

From The Liverpool Echo, 4 June 1998

By Richard Price

Stargazers at Liverpool Museum have discovered the oldest machine in
the world to suffer from the Millennium Bug. The devise, which is used
to pridict the position of planets, was made by an unknown astronomer
more than 400 years ago. Made out of brass and measuring about 12
inches, the price-less instrument was once a state-of-the art
astronomer's computer.

But after the year 1999 it will become useless. Museum astronomy expert
Martin Suggett said: "We don't know who made this machine but it is an
incredibly intricate piece of work called an equatorium. The only
problem is that the numbers only go up to 1999.

"When it was made - in about the year 1600 - four hundred years
peobably seemed like an eternity. But at the end of the next year that
time will have passed, and it simply won't work anymore."

One side of the divise has five moving circles to represent the planets
which were known at the time, while the other side represents the sun
and the moon. It was first donated to the museum from the collection of
Joseph Mayer, a goldsmith from Bromborough. He bought it in 1869.

Mr Suggett said: "Nobody really looked at it for about a century and we
still don't know where it comes from. A very famous astronomer called
Jeremiah Horrocks** lived in Liverpool in the early 1600s, but we don't
think it had anything to do with him. All we know is that it was made
either in England or France."

Astronomy boffins at the Museum are planning to use the equatorium for
the very last time by setting it for the last eclispe of the moon this
century. That will fall on August 11, 1999, after which it will go into
retirement.

"It just goes to show that it's not only modern computers which can't
cope with the new millennium," said Mr Suggett.

(C) 1998 Liverpool Echo

------------

** Jeremiah Horrocks (1619-41) predicted that a transit of Venus would
be observable on November 24, 1639. His research and observations were
published posthumously in his work "Venus in Sole Visa" in 1662. The
work became a cornerstone of the 'New Astronomy' (from ASTRONOMY ON
MERSEYSIDE. A BRIEF HISTORY, Telescope Technologies Ltd.)

================
HYPERVELOCITY IMPACTS IN SPACE: PHYSICS, CHEMISTRY & MECHANICS
SPECIAL ISSUE OF "ADVANCES IN SPACE RESEARCH", 1997, Vol.20, No.8,

--------------------

(6) HYPERVELOCITY IMPACT CRATERING OF CO2 ICE

J. Leliwa Kopystynski*), W. Brooke Thomas, M.J. Burchell, J.C.
Zarnecki: Hypervelocity impact cratering of CO2 ice and implications
for planetary sciences. ADVANCES IN SPACE RESEARCH, 1997, Vol.20, No.8,
pp.1577-1580

*) UNIVERSITY OF WARSAW, INSTITUTE OF GEOPHYSICS, PASTEURA 7, PL-02093
WARSAW, POLAND

We present our experimental data for hypervelocity impacts at 5km s(-1)
on compact CO2 ice. We have examinated crater morphology (depth,
diameter, shape and volume) for impacts by stainless steel projectiles
of diameter from 0.4 mm to 2 mm. Relationships between crater and
projectile parameters have been derived and extrapolation to other
projectile sizes is considered By means of comparison of these results
to impacts on H2O ice as presented in the literature, we discuss the
influence of icy target material on impact crater records in the Solar
System. (C) 1997 COSPAR. Published by Elsevier Science Ltd.

===================
(7) HYPERVELOCITY IMPACT ON SODA LIME GLASS

E.A. Taylor & J.A.M. McDonnell: Hypervelocity impact on soda lime
glass: Damage equations for impactors in the 400-2000m range. ADVANCES
IN SPACE RESEARCH, 1997, Vol.20, No.8, pp.1457-1460

*) UNIVERSITY OF KENT, PHYSICS LAB, UNIT SPACE SCIENCE & ASTROPHYSICS,
CANTERBURY CT2 7NR, KENT, ENGLAND

The results of a Light Gas Gun hypervelocity impact program on 25 mm
thick soda-lime (float) glass targets are compared to the values
predicted by an empirically determined power law spallation equation
(Paul and Berthoud, 199$). Impact velocities were in the vicinity of 5
km s(-1) projectile densities were between 1 and 8 g cm(-1) and the
projectile diameters, d(p) were 0.8 to 2 mm respectively, producing
spallation diameters between 10 mm and 50 mm. Previously published data
in the range d(p) = 7 - 1000 mu m are also used to assess the validity
of the equation. We conclude that D-spall is predicted by the
empirically determined power law spallation diameters for d(p) < 400 mu
m but only to within +/- 50%. An alternative equation to ddescribe the
response of target material for d(p) > 400 mu m is presented. (C) 1997
COSPAR. Published by Elsevier Science Ltd.

==============
(8) EJECTA FROM HYPERVELOCITY IMPACTS ON CO2

W. Brooke Thomas*), M.J. Burchell, J. Leliwa Kopystynski, J.C.
Zarnecki: Studies of ejecta from hypervelocity impacts on CO2. ADVANCES
IN SPACE RESEARCH, 1997, Vol.20, No.8, pp.1447-1450

*) UNIVERSITY OF KENT, PHYSICS LAB, UNIT SPACE SCIENCE & ASTROPHYSICS,
CANTERBURY CT2 7NR, KENT, ENGLAND

We have studied the ejecta produced by the normal incidence impact at 5
km s(-1) of I mm diameter stainless steel spheres on CO2 ice. The
ejecta size and angle of ejection were measured by use of thin witness
foils. We reconstruct the size distribution (for angles between 20
degrees and 80 degrees to the ice surface) and find it is dependent on
angle of ejection. Between angles of ejection of 20 degrees and 80
degrees; we find that the measured ejected mass flow is maximum at
(62.5 +/- 2.5)degrees, and represents approximately 1.4 % of the crater
mass. (C) 1997 COSPAR. Published by Elsevier Science Ltd.

=================
(9) HYPERVELOCITY IMPACT ON BRITTLE MATERIALS

E.A. Taylor*), L. Kay, N.R.G. Shrine: Hypervelocity impact on brittle
materials of semi-infinite thickness: Fracture morphology related to
projectile diameter. ADVANCES IN SPACE RESEARCH, 1997, Vol.20, No.8,
pp.1437-1440

*) UNIVERSITY OF KENT, PHYSICS LAB, UNIT SPACE SCIENCE & ASTROPHYSICS,
CANTERBURY CT2 7NR, KENT, ENGLAND

Hypervelocity impact on brittle materials produces features not
observed on ductile targets. Low fracture toughness and high yield
strength produce a range of fracture morphologies including cracking,
spallation and shatter. For sub-mm diameter projectiles, impact
features are characterised by petaloid spallation separated by radial
cracks. The conchoidal or spallation diameter is a parameter in current
cratering equations. An alternative method for interpreting
hypervelocity impacts on glass targets of semi-infinite thickness is
tested against impact data produced using the Light Gas Gun (LGG)
facility at the University of Rent at Canterbury (UKC), U.K. Spherical
projectiles of glass and other materials with diameters 30-300 mu m
were fired at similar to 5 km s(-1) at a glass target of semi-infinite
thickness. The data is used to test a power law relationship between
projectile diameter and crack length. The results of this work are
compared with published cratering/spallation equations for brittle
materials. (C) 1997 COSPAR. Published by Elsevier Science Ltd.

===================
(10) HYPERVELOCITY IMPACT EXPERIMENTS ON THETHER MATERIALS

D. Sabath & K.G. Paul: Hypervelocity impact experiments on tether
materials. ADVANCES IN SPACE RESEARCH, 1997, Vol.20, No.8, pp.1433-1436

TECHNISCHE UNIVERSITAET MUNICH, FACHGEBIET RAUMFAHRTTECHNIK, RICHARD
WAGNER STR 18, D-80333 MUNICH, GERMANY

Tethered systems are new and exciting means for various applications,
such as the re-entry of small payloads from the space station. Due, to
payload mass constraints of the launch vehicle, the mass of the
tethered system should be minimised. Therefore, fibres are the choice
for tether materials. The probability of a severe impact into the
tether is very high due its large surface area despite its small
diameter. Hence, the risk of an impact of a micrometeoroid or a space
debris particle cutting the tether should be investigated prior to
flight. This work reports first observations of hypervelocity impact
experiments on three different braided materials used for tether
applications. The tether samples - Dyneema, Kevlar and Spectra were
tested using the plasma drag accelerator (PDA) facility of the
Fachgebiet Raumfahrttechnik (LRT), Technische Universitat Munchen
(TUM). An overview of the morphology of such impacts is presented. The
extent of damage is compared to other materials commonly found on
spacecraft. A risk assessment of an impact cutting the tether with
current meteoroid and debris models and data from LDEF, Eureca and HST
solar arrays, is also given. (C) 1997 COSPAR. Published by Elsevier
Science Ltd.

=================
(11) HYPERVELOCITY IMPACT EXPERIMENTS ON SOLID CO2

M.J. Burchell*), W. Brooke Thomas, J. Leliwa Kopystynski, J.C.
Zarnecki: Hypervelocity impact experiments on solid CO2 targets.
ICARUS, 1998, Vol.131, No.1, pp.210-222

*) UNIVERSITY OF KENT, PHYSICS LAB, UNIT SPACE SCIENCE & ASTROPHYSICS,
CANTERBURY CT2 7NR, KENT, ENGLAND

The results of impact experiments on solid CO2 are presented. The
resulting crater morphology has been measured (crater depth, diameter
and volume). The data are for normal incidence impacts, at typically 5
km s(-1), of solid steel spheres of diameters 0.4 to 2.0 mm onto solid
CO2. The size range of the projectiles permits a determination of the
dependence of the crater parameters with impact energy. The craters are
shallow, with material removed mainly by spallation (crater
depth/diameter approximate to 0.2), Compared to similar impacts in
water ice, the craters in CO2 are smaller (by a factor of 2.2 in
diameter and 8.9 in volume). The size distribution of the ejecta is
measured and is found to vary strongly with the angle of ejection. (C)
1998 Academic Press.

=================
(12) MORPHOLOGY OF IMPACT CRATERS

V.V. Silvestrov*), A.V. Plastinin, I.V. Yakovlev, V.V. Pai: Morphology
of craters at hypervelocity impact in isotropic composites with
inclusions. COMBUSTION EXPLOSION AND SHOCK WAVES, 1997, Vol.33, No.3,

*) RUSSIAN ACADEMY OF SCIENCE, MA LAVRENTEV HYDRODYNAM INST, SIBERIAN
DIVISION, NOVOSIBIRSK 630090, RUSSIA

Results of a study of hypervelocity impact in model disperse-reinforced
composites with an epoxy or aluminum matrix with metallic (Al and Pb)
or ceramic (SiO2) inclusions are reported. The goal of the present
study is to find materials that possess a higher resistance to
penetration of a high-velocity projectile compared with materials of
separate components. This resistance is characterized by the ratio of
the depth of a crater in a sufficiently thick target to the diameter of
a spherical projectile. For two composites studied, we show that in
impact of a. steel particle With a velocity ranging from 3 to 11
km/sec, the crater depth is approximately one projectile diameter
smaller than that for lead or aluminum targets. (C) 1998 Published by
Elsevier Science Ltd.

=================

M. Katayama, S. Toda, S. Kibe: Numerical simulation of space debris
impacts on the Whipple shield. ACTA ASTRONAUTICA, 1997, Vol.40, No.12,
pp.859-869

CRC RESEARCH INSTITIUTE INC, ENGINEERING SOLUTUTIONS DEPARTMENT, KOTO
KU, 2-7-5 MINAMISUNA, TOKYO 136,JAPAN

The authors carried out three series of experimental tests of the first
bumper perforation and main wall cratering processes directly caused by
three types of projectiles with about 2, 4 and 7 km s(-1) impact
velocities but comparable initial kinetic energies, by using three
different accelerators (one-stage powder gun, two-stage light-gas gun
and rail gun), for the purpose of investigating space debris
hypervelocity impacts onto single-walled Whipple bumper shields [1]. In
the present study, after reviewing the numerical simulation method of
hydrocode for both Eulerian and Lagrangian descriptions, a number of
parametric numerical simulation analyses using multiple material
Eulerian methods were performed in order to optimize the material
properties of bumper and main wall materials through comparison with
experimental results of single target impacts by the projectiles. In
particular, the material data on the dynamic fracture phenomena are
discussed in detail in the first part. Then a couple of numerical
calculations using the interactive Lagrangian rezoning method to
simulate the overall impact process against the single walled Whipple
shield were performed and compared with the corresponding experimental
results. Both results indicated fairly good agreement with each other.
Moreover, it was demonstrated that the present method is helpful and
efficient in understanding the impact phenomena and fracture mechanism
in the space debris hypervelocity impact problem. Finally the multiple
material Eulerian method was applied to the same problems modeled by
the interactive Lagrangian rezoning method used previously, because the
former is much easier to use for almost all users, although it is more
diffusive and unclear of material boundaries than the latter. Those two
kinds of numerical results also indicated fairly good agreements with
each other. (C) 1998 Published by Elsevier Science Ltd.

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