PLEASE NOTE:


*

CCNet DIGEST, 2 July 1999
--------------------------

(1) WHY IMPACT PREDICTIONS CAN CHANGE DAY BY DAY
    Andrea Milani and Steve Chesley <chesley@dm.unipi.it>

(2) METEOR CRATER ASTEROID WAS SHOCK MELTED
    Andrew Yee <ayee@nova.astro.utoronto.ca>

(3) GROWING SPACE DEBRIS IMPERILS SATELLITES & ASTRONAUTS

(4) NEW RESEARCH ON DIABOLO IMPACTOR
    Ron Baalke <baalke@ssd.jpl.nasa.gov>

(5) NET USERS TAKE OVER NEWS
    BBC Online Network, 2 July 1999

===================
(1) WHY IMPACT PREDICTIONS CAN CHANGE DAY BY DAY

From Andrea Milani and Steve Chesley <chesley@dm.unipi.it>
 
Dear Benny,

In reference to yesterdays discussion of impact probabilities (CCNet
Digest July 1, 1999), we update on NEODyS
<http://newton.dm.unipi.it/neodys> the orbits of all near-Earth 
asteroids as soon as new observations are published by the Minor Planet
Center; with the new orbit, we automatically update the list of close
approaches to the Earth from 1975 to 2075. As an example, you can see
that for 1999 AN10 the new observations by Gladman and Nicholson (from
Mt. Palomar) have been already included in the fit, and the new nominal
(best fit) solution has now a close approach in 2027 at about 250,000
km. However, the nominal solution has no special significance.

When a new orbit becomes available we also recompute the impact
possibilities until 2050 and estimate the probabilities of such events;
this requires many hours of computer time, and is not yet automated,
thus the results normally come in one or more days later. As you know,
1999 AN10 is the "most wanted" asteroid in our impact risk list, being
the only km-size object for which we know an impact is possible,
although unlikely. With yesterday's new data, the least unlikely impact
could occur in August 2044; our order of magnitude estimate of the
probability of such an event is 7 parts in a million.

We need to stress once more that there is no such thing as a unique
impact probability, but the exact value depends upon the statistical
model used to describe the observation errors. In simple terms, if you
know which observation errors are more likely than the others, you also
know which orbits are more likely than the others. Our simple
computations, published on the NEODyS impact risk page at
<http://newton.dm.unipi.it/neodys/risk.html>, ignore this effect, thus
somewhat different values could be obtained by using, e.g., a gaussian
model, but the orders of magnitude would normally not change.

We also would like to stress that the existence of an impact solution,
and the impact probability estimate, will generally not change
dramatically as a result of a single observation, because they result
from the processing of all observations at once. A dramatic change is
only possible if the observation is very far from the others (e.g., at
a different apparition) or the impact is already close to being
excluded. For example, with the addition of the La Palma and Klet
observations our (uniform density) impact probability estimate for
August 2044 changed from 3 to 5 parts in a million. The further
addition of the Palomar observations increases the estimate to 7 parts
in a million.

We realize there are two points of this discussion which may not be
clear to many of your readers, and which are the source of some
discomfort; namely why the impact probabilities can change day by day,
and why the nominal solution is not important for this kind of
computations. This we will try to explain, for the benefit of those who
do not have the know-how to do these computations by themselves, but
wish to understand the process better.

One intuitive way of expressing the uncertainty of the orbit of an
asteroid is to use a finite probabilistic model. You should think to a
large number of orbits, maybe a million, all compatible with the
observations; each of these virtual asteroids has a small uncertainty,
and one of them is the real one, but we do not know which one. In a
simple model, you can think that all of them have the same probability
of being the real one; with this model, if five of the virtual
asteroids have an impact, then the probability is 5 in a million. In a
more refined model, some of the virtual asteroids are more likely than
others, and the exact value of the probability is somewhat different.
The nominal solution is just one of these one million possible
solutions, and it is, according to some statistical models, more likely
than the others, but only slightly; as an example, in the gaussian
model, the nominal solution is just 2.5 times more likely than the
average, but it is not significantly more likely than the neighboring
ones. As new observations come in, the nominal solution can change very
fast, just by jumping from one to some other of the virtual asteroids.

Now, new observations come in. The probability is just a way to
describe our ignorance, thus it is a basic fact that when new
information becomes available the probability changes. (This can
be expressed by speaking of conditional probability of an orbit
given the observations; this is the Bayesian interpretation preferred
by some, including Karri Muinonen.) In our simple model, the new
observations are such that some of the virtual asteroids are
incompatible with them, thus a number of them, say 300,000 out
of a million, are proven not to be the real one, so only 700,000
remain. If, unfortunately, the five impacting solutions are among
those still compatible with the observations, now the impact
probability is 5/700,000, significantly increased from the previous
value. With all virtual asteroids equally likely to be real, the
probability of an impact cannot decrease: either it goes to zero,
because the virtual impactors are now excluded by observation, or it
increases, roughly by the same amount by which the knowledge of the
orbit has been improved. If a more complex model, e.g., gaussian, is
used, then the changes can go both ways, but still it is the case that
both a significant increase and disappearance from the list can take
place. Since the nominal solution is the one in the middle, it is the
least likely to go away with only a few new observations, but it can
indeed go away after many new observations are reported.

As an example, the 2044 virtual impactor of 1999 AN10 is now at 1.44
sigmas, which means it is not any more very close to the nominal, thus
it is possible that it would go away as more observations are obtained
during the favorable observing window of this summer. But, this is by
no means sure, and it is indeed possible that the probabilities will
keep increasing and we shall have to wait until the 2004 radar
observation window to be sure.

We conclude by saying that we do this kind of computation on a regular
basis, as our scientific work but also with some spirit of service. We
do cross check our results with Paul Chodas and Karri Muinonen, and we
can get to an agreement with them after some discussion (sometimes
comparison of the results is not trivial). If other groups were to
acquire the capability to perform this kind of computation (of impact
possibilities and probabilities), we would be very happy to share with
more people the responsibility of such very critical job; however, we
do find it difficult to accept technical criticism from those who do
not do these computations.

Yours,

Andrea Milani and Steve Chesley

========================
(2) METEOR CRATER ASTEROID WAS SHOCK MELTED

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

News Services
University of Arizona

Contact(s):
Elisabetta Pierazzo
520-626-5065
betty@lpl.arizona.edu

Lori Stiles, News Services
Tel: 520-621-1877   FAX: 520-626-4121
lstiles@u.arizona.edu


Scientists discover that most of the asteroid that formed Meteor Crater
was shock melted

TUCSON, ARIZ. -- Most of the asteroid that blasted Meteor Crater out of
the Colorado Plateau melted, according to new evidence released today
by an international team of scientists. This new finding contradicts a
previously held theory that the Canyon Diablo meteor vaporized and
gives a glimpse of what happens when similar-sized meteors slam into
Earth every 6,000 years or so.

Meteor Crater, near Winslow, Ariz., the best-preserved impact crater in
the world, was formed 50,000 years ago -- just yesterday on the
geological time scale. Although modest by geological standards -- the
equivalent of a 20-to-40 megaton bomb -- it grabs our attention because
of its close proximity to our own time and for the story it tells about
what could happen again.

The bowl-shaped depression measures 1.2 kilometers (four-fifths of a
mile) wide and 180 meters (570 feet) deep and scientists say events
like this occur every 1,600 years, with a Canyon-Diablo-sized meteor
slamming into a land mass every 6,000 years.

In research published today (July 2) in Science, scientists conclude
that more than four-fifths of the Earth-crossing asteroid completely
melted and spread over the Four Corners Region where Colorado, Arizona,
New Mexico and Utah meet. Most of the iron asteroid, which was 30
meters (100 feet) or more in diameter, spread as an enormous expansion
plume produced by gases released from Colorado Plateau limestone. A
fraction of the melted material survived to form sand-grain-sized
particles called "spheroids."

By using complex measurements of radioactive nickel 59 and computer
modeling, the researchers determined the probable depth within the
asteroid at which these spheroids were formed. Their experimental
measurements and modeling results indicate that Canyon Diablo was
travelling faster on impact that previously believed.

The scientists include faculty members from Rutgers University, The
University of Arizona in Tucson, Australian National University, 
University of Rhode Island and University of California-Berkeley.

Keith Fifield of the Australian National University, led the team in
systematically measuring long-lived radioisotope nickel 59 in Canyon
Diablo meteorites and spheroids. Nickel 59 is a "cosmogenic nuclide"
produced in space when cosmic rays penetrate objects containing nickel
58. Nickel 58 changes to nickel 59 by absorbing an extra neutron from
cosmic radiation. Fifield used accelerator mass spectrometry to make
the measurements.

Canyon Diablo meteorites contain seven times more nickel 59 than do
recovered spheroids, meaning they had come from the surface or outer
shell of the asteroid, where exposure to cosmic radiation is greatest,
said Greg Herzog of Rutgers University.

Scientists find nickel 59 to be a far more useful cosmogenic nuclide
for such analysis than some more commonly used ones. That's because of
the mechanism by which it forms, its long half-life (76,000 years), its
low volatility and its resistance to weathering, team members add.

Elisabetta Pierazzo, a post-doctoral researcher at the UA Lunar and
Planetary Laboratory, used numerical models to simulate the impact. The
simulation, based on models developed at Sandia National Laboratories,
factored in the size and composition of Canyon Diablo and its target.
Pierazzo determined which parts of the Earth-smashing asteroid remained
solid and which melted and became spheroids. This was done by using
experimentally measured shock pressure values for melting iron/nickel
alloys. The composition of these alloys is close to that of meteorites.

The team concludes that the precursor material of the spheroids probably
came from depths of 1.3 to 1.6 meters (four to five feet) beneath the
surface of the meteor before it entered Earth's atmosphere.

Pierazzo says that only about 15 percent of the rear, outer part of the
asteroid remained solid after impact and that the other 85 percent of
the projectile melted. She bases this conclusion on combined
observational, experimental and theoretical evidence.

Impact velocities by Earth-crossing asteroids average around 15 to 20
kilometers per second. The 20 km/s velocity -- or 45,000 mph -- would
produce a melting profile that agrees with the experimental
measurements, she said. At lower velocities, a much larger fraction of
the projectile would have remained solid, leaving behind far more
meteorites.

"The model really makes sense when you match it with the hard
evidence," Pierazzo said. "The modeling confirms the experimental
results that say the Canyon Diablo meteorites came from the outer part
of the projectile, and the spheroids from a depth of 1.5 to 2 meters
below the surface.

"I feel confident that this impact was at higher velocity than many
people have believed it to be," she added. "This work gives no evidence
for vaporization. From what we know about shock pressure, melting and
vaporization of iron, the model indicates little or no vaporization of the
impact."

RELATED LINKS
http://www.lpl.arizona.edu

======================
(3) GROWING SPACE DEBRIS IMPERILS SATELLITES & ASTRONAUTS

[http://www.nando.com/noframes/story/0,2107,65248-103435-730391-0,00.html]

Monday, June 28, 1999

Growing space debris imperils satellites, astronauts
By PETER N. SPOTTS

In the market for some used rocket boosters? Skip the trip to Watto's
scrap heap on Tatooine. Near-Earth space may have just what you're
looking for.

And that troubles a growing number of people who see the path to living
and working in orbit strewn with space-age litter.

Roughly 10,000 objects large enough to track from the ground -- from
old satellites and spent upper stages to fuel tanks, lens caps,
tie-down straps, and bits of explosive bolts -- encircle the planet.
And that doesn't include the millions of smaller bits of orbital debris
ringing Earth.

Only four collisions between a spacecraft and debris have occurred
since spaceflight began. But concern is growing that as the number of
satellites multiplies -- along with Earthlings' reliance on them for
everything from cell phones to digital directions to grandmother's
house -- so are the chances that orbiting flotsam could knock out key
satellites and threaten human presence in space.

"Is this something people should worry about? Yes it is," says Scott
Pace, a science-policy analyst with the RAND Corp. in Washington.

Earlier this month, for example, a derelict Russian rocket booster
hurtled toward an uncomfortably close encounter with the unoccupied
International Space Station. And in 1997, a close encounter between the
Russian space station Mir and a U.S. satellite forced Mir's crew to
strap themselves into their escape capsule until the satellite had
passed.

Scientists are also concerned about the potential for chain-reaction
collisions if a piece of space junk slams into a satellite that is part
of a larger constellation of spacecraft.

In the current issue of the journal Nature, Alessandro Rossi of the
Italian National Research Council and two colleagues from the
University of Trieste looked at the collision risk to the Iridium
project, a global phone and paging network designed around a fleet of
66 satellites (plus six spares).

Rossi's team estimates that the risk of a catastrophic collision is
about 10 percent over a 10-year period. If that happens, debris from
the first collision has a 10 percent chance of taking out another
Iridium satellite over the next five years.

Such events, the team concludes, could set up chain-reaction collisions
that would generate debris for a century -- too slow to affect phone
service, perhaps, but three to five times faster than previous
estimates.

The space-junk problem has become sufficiently acute that orbital
debris has for the first time found a place on the agenda for the
United Nation's Conference on the Exploration and Peaceful Uses of
Outer Space, which will be held in Vienna in July.

Space junk's appearance results largely from a change in attitude on
the part of the United States, which had kept the issue off the table
at previous meetings, according to John Logsdon, director of the Space
Policy Institute at George Washington University in Washington, D.C.

When the Defense Department dominated the U.S. launch scene, "it didn't
want to expose its issues and craft to international debate," Logsdon
explains. The U.S. argued that space junk was a science and technology
issue, not a legal problem.

These days, however, private companies launch more hardware than the
federal government, bolstering economic arguments for controlling space
debris. The U.S., Logsdon says, bowed to the inevitable -- with hopes of
steering the discussion.

For now, the best hope for controlling the cosmic clutter seems to lie
in spacecraft and mission design, researchers say, rather than in
cleanup technologies.

"Basically, we've got to live with it," says Kyle Alfriend, who heads
the aerospace engineering department at Texas A&M University. "Let's
not increase the amount of stuff we leave up there, and let's let the
atmosphere help clean it out."

A few years back, NASA conducted a study on using ground-based lasers
to hit small pieces of debris. "But that idea raised significant
political problems," Alfriend says, noting that one nation's laser
"broom" is another's antisatellite weapon. Even if lasers were used
benignly, he adds, it's still possible to zap another country's
satellite by mistake.

Faced with little prospect of clearing space with an orbiting Hoover,
researchers are focusing efforts on improving computer programs that
calculate debris orbits. Such improvements, researchers say, can lead to
more accurate and more timely warnings of potential collisions.

"We do not have a good set of knowledge about what's up there," says
William Kainard, senior research scientist at the NASA Langley Research
Center in Hampton Roads, Va. "Before you can do too much, you need to
understand the population of millimeter- to centimeter-size objects,
get their sources, and orbital lifetimes."

Copyright 1999 Christian Science Monitor Service

======================
(4) NEW RESEARCH ON DIABOLO IMPACTOR

From Ron Baalke <baalke@ssd.jpl.nasa.gov>

Rutgers, The State University Of New Jersey (http://www.rutgers.edu)
Contact: Joseph Blumberg , Manager Of Science Communications
Phone: (732) 932-7084, ext. 652; Email: blumberg@ur.rutgers.edu
June 30, 1999

Rutgers Researchers Team With International Group To Investigate One Of
The Most Famous Meteorites In The World

NEW BRUNSWICK/PISCATAWAY, N.J. -- Researchers studying remains of the
Canyon Diablo impactor have been able to describe the changing
character of the meteoroid as it traversed Earth's atmosphere and hit
its surface, ascertain how the remaining fragments were formed, and
determine where within the body of the meteoroid the fragments
originated.

Meteoroid refers to a natural object that moves through interplanetary
space, as opposed to the term meteorite, which refers to such an object
after it has fallen to Earth.

The team, which included Rutgers chemists Dr. Christoph Schnabel and
Dr. Gregory Herzog together with colleagues in Arizona, California and
Australia, used ultrasensitive measurements and computer modeling to gain
insight into meteoroid dynamics.

The Canyon Diablo impactor was the object responsible for excavating
Meteor Crater, the famous Arizona landmark. It struck the desert near
Winslow, Ariz., some 50,000 years ago, producing a crater about 4,000
feet wide and 570 feet deep. This was the first crater on Earth to be
identified as having been created by a meteoroid.

"The original meteoroid was thought to have been about 100 feet in
diameter weighing approximately 60,000 tons, but little of it remains
intact today," said Schnabel, a postdoctoral associate in the
department of chemistry at Rutgers.

"Two types of material survive from the Canyon Diablo impactor - iron
meteorites, which did not melt during the impact, and spheroids, which
did," said Herzog, professor of chemistry with the Faculty of Arts and
Sciences-New Brunswick. "Our challenge has been to determine the processes
involved in the impact and the formation of the resulting products,
specifically the spheroids -- millimeter-size fragments found in the soils
around the crater."

In the July 2 issue of Science, the authors describe how they were able
to deduce the original depth within the body of the meteoroid of the
material that melted to form the spheroids. At the Australian National
University, co-author Dr. L. Keith Fifield and his group employed
accelerator mass spectrometry to analyze a rarely measured radioisotope
of nickel (59Ni). Known as a cosmogenic nuclide, the 59Ni was produced
by cosmic ray bombardment in the outer shell of the meteoroid while in
space, and the relative concentration of this nuclide serves as a good
indicator of depth of origin of the spheroid fragments.

The resulting depth figures were then compared with predictions from
computer-modeled simulations of the impact that were carried out at the
University of Arizona by another co-author, Dr. Elisabetta Pierazzo.
Conclusions based on this comparison yielded new information about the
dynamics of meteoroid strikes on Earth or other solid objects in the
solar system, information that may be applied in general to medium-size
meteoroids when they impact.

For example, the researchers were able to conclude that the trailing
hemisphere of the meteoroid was the likely location for the molten
material that gave rise to the spheroids. They further assert that
material in the leading hemisphere of the meteoroid would more readily
have mixed with and been lost in a large volume of rock at the impact
site.

Four batches of spheroids had been analyzed with average masses in each
group ranging from 1 to 10 mg. On average, the spheroids contained six
to seven times less 59Ni than the meteorites. The 59Ni measurements
yield evidence that the liquid material that formed the spheroids came
from depths of 1.3 to 1.6 meters beneath the surface of the meteoroid.

The researchers also concluded that most of the spheroids did not form
when atmospheric resistance to the incoming meteoroid melted surface
material and blew molten droplets away, as had been previously held.
Rather, computer numerical modeling of the meteoroid and its impact
suggests explosive or shock melting of most of the object and dispersal
of the spheroid fragments upon impact. They contend that little, if
any, of the meteor vaporized. Moreover, the impact modeling suggests
that the impact velocity of Canyon Diablo was higher than the velocity
normally assumed for such an impact.

Full copies of the Science article, "Shock Melting of the Canyon Diablo
Impactor: Constraints From Nickel-59 Contents and Numerical Modeling,"
can be obtained by contacting the American Association for the
Advancement of Science at (202) 326-6440; fax (202) 789-0455; or e-mail
scipak@asss.org.

NOTE TO REPORTERS: Dr. Gregory Herzog can be reached at (732) 445-3955 for
interviews Wednesday, June 30, through Friday, July 2, afternoons only.

=====================
(5) NET USERS TAKE OVER NEWS

From the BBC Online Network, 2 July 1999
http://news.bbc.co.uk/hi/english/sci/tech/newsid_383000/383587.stm

By Internet Correspondent Chris Nuttall

Online journalists have been warned that Net users are taking over 
their role, forging a new kind of people's journalism.

Steve Yelvington, Executive Editor of Cox Interactive Media in the US, 
told the annual Netmedia conference in London that the rules were
changing in the new medium of the Internet.

"We are not gatekeepers anymore, the city walls are down, we don't own
customers, we don't control information," he said.

Slashdot threatens extinction

"But they still need us as guides. They need to know what's important,
what's true and what's useful. Our new role is as a trusted guide."

Mr Yelvington said in his keynote speech that, in the new journalism,
people were telling their own stories on sites such as Geocities,
Tripod and TalkCity and he praised the News for Nerds discussion group
Slashdot:

"If Slashdot were a mammal, most of our news sites would be the
dinosaurs. Many journalists don't understand this and don't think it's
journalism."

He added that news sites should note that they are a niche product:
MSNBC rated best in a usage survey in March but was only at Number 23
behind the likes of Xoom, Excite, Ebay, AOL, Yahoo and Blue Mountain
Arts online greetings cards.

Second era of Net news

The editor-in-chief of MSNBC, Merrill Brown, declared a second era of
Internet news was beginning, in another keynote speech.

He said the first era of Internet news, in a very short space of time,
had brought "an entirely new global news infrastructure and never
before seen - or imagined - storytelling techniques that change the
entire news experience. And we've just started".

"What will this new news ultimately look like? Well, we believe that
people will, in large numbers, use video and Internet text and
applications together in a truly converged environment.

"They'll watch the evening news - when they choose to of course - and
review maps and data at the same time they're listening to the anchor
and reporter packages. This isn't decades away. It is right around the
corner.

"Similarly with software, there is certain to be a new level of product
integration as well. We're planning to provide people headlines in any
number of ways.

"How about integrating headlines and information into your daily
desktop calendar tool so that if your planner says you're headed for
London, you're automatically given headlines and weather for London and
background material on who you're going to meet.

"This critical integration of the news into the fundamental tools of
home and office computing is another part of the next generation of
Internet news."

The Netmedia conference is staged over two days each year at London's
City University. This year's features an online media roundtable of
European journalists, a Cyberlaw lecture, a Digital TV roundtable, and
the presentation of the UK's first online journalism awards.

Copyright 1999, BBC

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