PLEASE NOTE:


*

CCNet DIGEST 4 June 1998
------------------------

(1) HOW WE'RE GETTING USED TO COSMIC CATASTROPHES IN OUR SOLAR SYSTEM:
    TWO COMETS IMPACTED SUN EARLY THIS WEEK
    NASA NEWS <NASANews@hq.nasa.gov>

(2) ERIK ASPHAUG'S ARTICLE IN NATURE
    Phil Burns <pib@nwu.edu>

(3) ERIK ASPHAUG'S IMPACT SIMULATION ON THE WEB
    Erik Aspaugh
    http://WWW.es.ucsc.edu/%7Easphaug/castalia_sim.html

(4) NUKES 'MAY NOT BE ENOUGH' TO SAVE EARTH FROM ASTEROIDS
    Press Agency News <http://www.pa.press.net>

(5) TUNGUSKA YIELD
    Mark Boslough <mbboslo@valinor.sandia.gov>

(6) THE TEMPO OF THE PERMIAN MASS EXTINCTION
    S.A. Bowring et al., MIT

(7) OBSERVING KUIPER BELT OBJECTS
    K.L. Maslennikov et al., RUSSIAN ACADEMY OF SCIENCE

(8) THERMAL PHYSICS OF ASTEROIDS
    J.S.V. Lagerros, ASTRONOMICAL OBSERVATORY UPPSALA

(9) MUINONEN'S GAUSSIAN SHAPE HYPOTHESIS FOR ASTEROIDS AND COMETS
    K. Muinonen, UNIVERSITY OF HELSINKI

================
(1) HOW WE'RE GETTING USED TO COSMIC CATASTROPHES IN OUR SOLAR SYSTEM:
    TWO COMETS IMPACTED SUN EARLY THIS WEEK

From NASA NEWS <NASANews@hq.nasa.gov>

Douglas Isbell
Headquarters, Washington, DC             June 3, 1998
(Phone:  202/358-1547)

Bill Steigerwald
Goddard Space Flight Center, Greenbelt, MD
(Phone:  301/286-5017)

NOTE TO EDITORS:  N98-38

SOHO SPACECRAFT SEES TWO COMETS PLUNGE INTO SUN

In a rare celestial spectacle, two comets have been observed plunging
into the Sun's atmosphere in close succession, on June 1 and 2. This
unusual event on Earth's own star was followed on June 2 by a likely
unrelated but also dramatic ejection of solar gas and magnetic fields
on the southwest (or lower right) limb of the Sun. 

The observations of the comets and the large erupting prominence were
made by the LASCO coronagraph on the Solar and Heliospheric Observatory
(SOHO) spacecraft. Science instruments on SOHO have discovered more
than 50 comets, including many so-called sun grazers, but none in such
close succession. The eruption of solar gas was directed away from
Earth and does not pose a hazard to our planet or orbiting astronauts. 

Video footage of these intriguing events will be uplinked on the NASA
TV Video File at 11 a.m. and 3:55 p.m. EDT on Thursday, June 4.

Selected images and an image sequence of the new observations can be
found on the World Wide Web at the NASA SOHO Website:

     http://umbra.nascom.nasa.gov/comets/SOHO_sungrazers.html

SOHO is a joint undertaking of NASA and the European Space Agency. 
Development of the LASCO instrument was coordinated by the U.S. Naval
Research Laboratory, Washington, DC. Dr. Donald Michels of the LASCO
science team led the team that observed this rare phenomenon.

NASA Television is carried on GE-2, transponder 9C, 85 degrees West
longitude, vertical polarization, frequency 3880 MHz, audio 6.8
Megahertz.

========================
(2) ERIK ASPHAUG'S ARTICLE IN NATURE

From Phil Burns <pib@nwu.edu>

Several articles discuss research by Erik Asphaug and colleagues into
asteroids as rubble piles which appeared in the June 4th issue of
NATURE:

  http://cnn.com/TECH/space/9806/03/asteroid.impact/index.html
  http://www.abcnews.com/sections/science/DailyNews/asteroid980603.html
  http://www.cbs2chicago.com/prd1/now/template.display?p_story=52336&p_who=wbbm
  http://wire.ap.org/APnews/center_package.html?FRONTID=SCIENCE&PACKAGEID=meteor&STORYID=APIS6LQPSFO0

One important point is that diverting threatening rubble pile asteroids
may require different approaches then diverting solid asteroids.

-- Phil "Pib" Burns
   Northwestern University, Evanston, IL.  USA
   pib@nwu.edu
   http://pibweb.it.nwu.edu/~pib/

=================
(3) ERIK ASPHAUG'S IMPACT SIMULATION CAN BE ACCESSED ON THE WEB

From Erik Asphaug's homepage
http://WWW.es.ucsc.edu/%7Easphaug/castalia_sim.html

IMPACTS INTO CASTALIA

This work was conducted by E. Asphaug together with colleagues from
Iowa State University (D.J. Scheeres), Washington State University
(R.S. Hudson), the University of Bern, Switzerland (W. Benz), and from
NASA JPL (S.J. Ostro, E.M. De Jong and S. Suzuki). Most simulations
were computed on a fast SGI workstation; recent high-resolution
simulations were performed on the Cray T3E supercomputer at NASA
Goddard Space Flight Center.

Why Castalia?

4769 Castalia is a near Earth asteroid (NEA) whose shape, however
unusual to us, may in fact be representative of many small planetary
bodies. This impact simulation begins with an 8m (50 foot) radius
projectile (the white dot in the first frame) impacting at 5 km/s
(11,000 miles per hour), a typical encounter speed for meteoroids in
the asteroid belt. The impactor weighs 6000 tons compared with over 1
billion tons for Castalia. This impact speed, squared, times the mass
of the impactor equals the impact energy: 17 kilotons, equivalent to
the Hiroshima explosion.

The second through fourth frames show impact fragmentation expressed on
the surface, where the colors blue through red represent increasing
levels of damage. Click here for a cross-section view, and for figures
with captions from our letter to Nature.

The last three frames show the surface waves induced by the impact
shock progressing across the target, color coded by particle speed.
Click here for an mpeg movie of a higher-resolution simulation, run on
the Cray T3E at NASA GSFC and rendered by DIAL-JPL. Click here for a
high-resolution snapshot from the movie, showing a cut-away view of
fragmentation in the asteroid's interior.

===================
(4) NUKES 'MAY NOT BE ENOUGH' TO SAVE EARTH FROM ASTEROIDS

From Press Agency News
     http://www.pa.press.net

NUKES 'MAY NOT BE ENOUGH' TO SAVE EARTH FROM ASTEROIDS

By John von Radowitz, Science Correspondent, PA NEWS

It would be far from easy to save the Earth from an incoming asteroid
if the crisis depicted in the action film Deep Impact ever became
reality, a study showed today.

It had previously been thought that nuclear blasts could be used to
deflect or break up an asteroid on collision course with Earth. But
scientists have found that some types of asteroid could soak up a
powerful nuclear explosion with little or no effect.

Astronomer Erik Asphaug, a researcher associate at the University of
California, Santa Cruz, used computer simulations to study the effects
of powerful impacts on asteroids with different internal structures.
And he and his colleagues found that the outcome of blasting an
asteroid depends entirely on its structure.

Many asteroids are not single rocks, but are aggregates of debris left
over from previous collisions. They might consist of a few large
fragments held together by gravity or "rubble piles" consisting of
numerous smaller pieces.

The new study, published today in the journal Nature, shows that the
porous nature of such asteroids would dampen down shock waves from a
nuclear explosion, limiting its effects to a localised area.

"It's lot more difficult to nudge these asteroids around than we had
thought," said Asphaug. "More work needs to be done before we can
decide whether nuclear warheads provide a viable deterrent."

The researchers created a computer model of an asteroid one mile across
based on radar images ofa near-Earth asteroid called Castalina. They
gave the peanut-shaped body three different internal structures: solid
rock, a pair of solid rocks in close contact, and a rubble pile with
pore space taking up 50% of its volume.

For each of these, an impact by a house-sized rock travelling at five
kilometres per second was simulated. This is equivalent to the energy
produced by the atom bomb dropped on Hiroshima.

In the rubble pile model, the impact shock wave died out quickly
leaving a large crater in one spot, without disturbances to the rest of
the asteroid. At the opposite extreme, a solid rock asteroid may
shatter into many smaller pieces when blasted. The pieces form a family
of smaller asteroids, or come together to form a rubble pile.

To predict the effect of a nuclear explosion on any particular
asteroid, scientists would need to understand the object's internal
structure, said Asphaug.

There are hundreds of thousands of asteroids in near-Earth space which
if they struck the planet would have as big an impact as the largest
thermonuclear device ever exploded.

Although the likelyhood of one of these asteroids hitting the Earth was
small, the consequences would be disasterous. Asphaug said: "Asteroids
are not an imminent threat, and I am far more concerened about what
human beings are doing to the planet. But in case we ever identified 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."

Copyright 1998, Press Agency

=================
(5) TUNGUSKA YIELD

From Mark Boslough <mbboslo@valinor.sandia.gov>

Duncan,

What you say is true, but it is really a wash because it would mean
that a "Tunguska event" has been downgraded. If we are correct about
the yield, it doesn't change the frequency of 3.5 - 5 Mt events, it
just means that we are calling these smaller events Tunguska class, so
it would just be the terms that change, not the risk.

Tunguska was a remarkable event, but I think its effects have been
exaggerated. Much of the standing forest there consisted of dead and
rotting timber, according to the old Russian literature. That means
the blast wave was not as strong as has been claimed.

Regards,

Mark

====================
(6) THE TEMPO OF THE PERMIAN MASS EXTINCTION

S.A. Bowring*), D.H. Erwin, Y.G. Jin, M.W. Martin, K. Davidek, W. Wang:
U/Pb zircon geochronology and tempo of the end-Permian mass extinction.
SCIENCE, 1998, Vol.280, No.5366, pp.1039-1045

*) MIT, DEPARTMENT OF EARTH ATMOSPHERIC & PLANETARY SCIENCE, CAMBRIDGE,
   MA, 02319

The mass extinction at the end of the Permian was the most profound in
the history of life. Fundamental to understanding its cause is
determining the tempo and duration of the extinction. Uranium/lead
zircon data from Late Permian and Early Triassic rocks from south China
place the Permian-Triassic boundary at 251.4 +/- 0.3 million years ago.
Biostratigraphic controls from strata intercalated with ash beds below
the boundary indicate that the Changhsingian pulse of the end-Permian
extinction, corresponding to the disappearance of about 85 percent of
marine species, lasted less than 1 million years. At Meishan, a
negative excursion in delta(13)C at the boundary had a duration of
165,000 years or less, suggesting a catastrophic addition of light
carbon. Copyright 1998, Institute for Scientific
Information Inc.

=================
(7) OBSERVING KUIPER BELT OBJECTS

K.L. Maslennikov*), O.P. Bykov, Y.N. Gnedin, S.V. Zharikov, I.N. Lvov,
N.A. Tikhonov: BTA observations of objects from the Kuiper belt.
ASTRONOMY LETTERS-A JOURNAL OF ASTRONOMY AND SPACE ASTROPHYSICS, 1998,
Vol.24, No.2, pp.180-184

*) RUSSIAN ACADEMY OF SCIENCE, PULKOVO ASTRONOMICAL OBSERVATORY, ST
   PETERSBURG 196140,RUSSIA

The BVRI photometry and position measurements obtained for three
objects of the transneptunian asteroid belt (the Kuiper belt) with the
BTA telescope are presented. During the observations, an unknown
asteroid of the Main belt was detected, and its color properties were
compared with those of the Kuiper-belt objects. Copyright 1998,
Institute for Scientific Information Inc.

====================
(8) THERMAL PHYSICS OF ASTEROIDS

J.S.V. Lagerros: Thermal physics of asteroids - IV. Thermal infrared
beaming. ASTRONOMY AND ASTROPHYSICS, 1998, Vol.332, No.3, pp.1123-1132

ASTRONOMICAL OBSERVATORY, BOX 515,S-75120 UPPSALA, SWEDEN

The so called beaming is an important factor when studying the thermal
emission from the atmosphereless bodies of the solar system. The
emission is non-Lambertian and has a tendency to be 'beamed' in the
solar direction. The small scale surface roughness is probably the main
source of the effect. In this paper, the problem is studied
theoretically by adding roughness to Lambertian surfaces, and
investigate the change in the emission. The radiative transfer problem
in the visual and IR is considered and solved simultaneously with the
heat conduction problem. This gives the temperature variations over the
surface, as caused by shadows and varying slopes on the surface. The
beaming is derived by comparing the emission from the rough and smooth
surface. Two types of roughness approaches are considered. Analytical
solutions to the temperature variations are given for surfaces covered
with hemispherical segment craters. Numerical methods are used for
stochastic surfaces. The latter approach to the roughness is probably
closer to the real surfaces, but the former is far easier to apply in
practise. The qualitative results of the two surface types are the
same. The beaming is enhanced by the increasing the root mean square
(r.m.s.) slope, or by increasing the albedo. The beaming produced by
the two approaches for a given r.m.s. slope is rather close for a 60 %
crater coverage in the spherical crater case. The combined effects of
heat conduction and the surface roughness is studied. In many cases it
is, however, possible to treat them separately which is of major
practical importance. As an example, IRAS data (Tedesco et al., 1992)
of the asteroid 3 June is compared to model thermal light curves
produced, with good agreement. Copyright 1998, Institute for Scientific
Information Inc.

==========================
(9) MUINONEN'S GAUSSIAN SHAPE HYPOTHESIS FOR ASTEROIDS AND COMETS

K. Muinonen: Introducing the Gaussian shape hypothesis for asteroids
and comets. ASTRONOMY AND ASTROPHYSICS, 1998, Vol.332, No.3, pp.
1087-1098

UNIVERSITY OF HELSINKI, OBSERVATORY, POB 14, FIN-00014 HELSINKI,
FINLAND

A hypothesis is presented that the irregular shapes of asteroids and
cometary nuclei can be modeled by using lognormal statistics (Gaussian
random sphere). The Gaussian sphere is fully described by the mean and
covariance function of the radius. A suitable covariance function is
devised here for the generation of sample Gaussian spheres that closely
resemble the shapes observed for asteroids. To collect more evidence
for the Gaussian hypothesis, assuming simple Lommel-Seeliger and
Lambert scattering laws, lightcurves are computed for rotating Gaussian
spheres. The results show striking similarities to asteroid
lightcurves. For example, the observed increase of lightcurve amplitude
with increasing solar phase angle appears to be at least partly
explained by the numerical simulations. Making further use of the
Gaussian random sphere, a statistical model is developed for albedo
variegations on asteroids, and for characterizing active regions on
cometary nuclei. Copyright 1998, Institute for Scientific Information
Inc.

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