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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