PLEASE NOTE:
*
CCNet DEBATES, 28 April 1998
----------------------------
ASTEROID 1997 XF11: QUESTIONS REMAIN
Six weeks after 1997 XF11 made the world's news headlines, the
NEO-search community has come to a general conclusion: There is
no
threat whatsoever from this PHA [that is, excluding any 'effects
that
would be highly unusual']. Yet some questions remain. One of the
most
controversial is still unresolved: at what stage of observation
and
orbit calculation could this certainty have been achieved?
Early last week, Alan W Harris (JPL/NASA) notified me about IAUC
6879.
He interpreted Brian Marsden's circular as an admission that as
early
as 21 December 1997, two weeks after Jim Scotti's discovery of
XF11,
'a collision could have been ruled out.' Since I came to a
different
reading of IAUC 6879, I asked Alan to clarify a number of
questions
which had been raised by the ongoing debate.
I would like to thank Alan for trying to explain some of the
underlying
problems with asteroid 1997 XF11. I would show-off if I were to
claim
that I now understand all (or even most) of the problems
involved. But
at least I have the feeling that I am getting closer to
comprehend what
the actual mathematical complexities (and uncertanties) are.
As far as I am concerned, the whole episode has been, more than
anything else, a tremendous learning experience, not just for me
but, I
guess, for most list members of the CCNet. We shouldn't
underestimate
this positive side-effect, in spite of all the controversy. As a
result, perhaps twice or three times as many people may be able
to
participate in, assess or simply understand the accuracy of
complex
orbit calculations next time around - at least that's what I hope
will
be the case in the future.
Benny J Peiser
======================
A QUESTIONS & ANSWERS SESSION ON ASTEROID 1997 XF11
On Tue, 21 April Benny J Peiser [BJP] wrote to Alan W Harris:
Dear Alan
I am relieved to see the first attempts of reconciliation within
the
NEO-search community. I very much hope that this positive
direction and
a cooled atmosphere will make it easier to critically assess what
the
REAL problems with XF11 have been and what we can learn from this
experience. I also hope that despite all the controversy, no
bridges
have been burnt so that we can continue to cooperate in the full
knowledge that disagreement may remain on some issues.
Since our controversy has taken place under the eyes of a
critical
public (which is, I believe, crucial for upholding public
confidence in
NEO research), it would certainly be helpful to convey to the
interested public the preliminary results of this re-assessment
within
the NEO community. I will ask Brian whether the MPC would have
any
objections to make those parts of the IAUC 6879 which deal with
1997
XF11 available for the CCNet [see annotated IAUC 6879, CC DIGEST
24/04/98].
Yet some questions remain which I find difficult to understand.
1.) On Mon, 20 April you wrote [about IAUC 6879]:
> Although it is a bit concealed (MPEC 1997-Y11 contains only
> observations from the date of discovery, Dec. 6, to Dec. 21,
a 15-day
> arc!), Marsden has accepted the fact that already by then a
collision
> could be ruled out.
What you are saying is that after as little as 15 days of
observation
of 1997 XF11 "a collision could [have been] ruled out".
I understand
that as late as 10 days ago, one of your colleagues has
calculated - on
the basis of the 1997/98 data - a hypothetical collision of XF11
with
earth. Tell me if I got this wrong, but how - on this basis - can
one
rule out any impact hazard?
---------------------
On Sun, 26 April, Alan W Harris [AWH] wrote:
I believe you have got it a bit wrong. One way to test the
robustness
of a conclusion (especially a negative one) is to force a
positive
solution and see what contradictions arise. In the 1997
XF11 case, one
can insert a false "observation" into the data set,
essentially an
"observation" of XF11 in October 2028 that corresponds
to an impact on
the Earth, and then compute the orbit, to see what happens to the
fit
to the real observations. This is a quite common and
straightforward
way to demonstrate in easy to understand terms whether a
conclusion is
firm or not.
The statement that the Earth lies 30-sigmas outside of the error
envelope, and hence the formal collision probability is something
like
10^(-300), loses any possible understanding. So, we can,
figuratively
speaking, grab hold of the orbit, and "pull it over" to
pass through
the Earth in 2028, and see what is the very least displacement
from the
observations in 1997-8 that can allow that to be true.
There are some practical difficulties in carrying out this
computation,
for example some programs may have difficulty when the computed
orbit
passes through a perturbing body, and fitting an
"observation" where
the solution distance tends to zero (the false observation on the
Earth's surface) can be difficult. Anyway, several colleagues
have
pursued this method and have produced orbits to evaluate the
plausibility of a collision trajectory based on various subsets
of the
1997 XF11 observations.
But here's the rub: this "solution" is a
mathematical solution to a
hypothetical situation posed; it is not necessarily a physically
allowed "solution" to a real situation, indeed it is
demonstrably NOT
an allowed solution, which is exactly the reason for examining
it. Let
me describe in general terms what has been found from these
orbits.
The solution through all of the data, including 1990, left
"observed
minus computed" residuals to the actual observations as
large as 10
arc-minutes, or 1/3 the diameter of the moon. It is
inconceivable that
any modern competent observer could make such horrendous errors
in
observation. Even Tycho Brahe did far better without a telescope,
and
with only the star catalogs of his day.
Furthermore, the fit residuals to this forced solution are not
random
-- they fall along a very narrow curve when plotted with time,
indicating that each of the many individual observers would have
to be
making the same gross errors as all the others, of many
arc-MINUTES, to
within a fraction of an arc-SECOND. Clearly this is
inconceivable, and
demonstrates that the collision trajectory is impossible, in much
more
graphic and humanly understandable terms than stating a
probability of
"10^(-300)", or the blunt, "that's zero,
folks."
This is not quite the end of the story. Everyone agrees that,
with the
inclusion of the 1990 observations, an impact is totally out of
the
question. Since the largest residuals occur for the 1990
observations,
the above solution tends to overwhelm what might be done to
better fit
the 1997-8 observations. Ted Bowell, at some sacrifice to his
normal
time last night for a beer, made an attempt at the problem using
his
orbit determination program, inserting a couple of synthetic
"observations" a couple hours before the
"impact" in 2028. He was able
to force a new solution through these points and preserve the
impacting
trajectory, but avoid the problems that earlier caused his
program to
blow up. He thus employed this technique to recompute an
orbit,
excluding the 1990 observations, which would impact the Earth in
2028,
and computed the residuals to the 1997-8 observations. The
pattern is
the same as for the orbit including the 1990 observations,
although the
amplitudes of the systematic deviations are much smaller.
In
Declination, the residuals range from +10 arcseconds at the time
of
discovery to about -6 arcseconds on March 3-4. In right
ascension, the
residuals run from -4 arcseconds at the time of discovery to +22
arcseconds on March 3-4. As before, the plot of residuals vs.
time
fall along systematic trends to within an arcsecond or two.
Thus we
are left with the same conclusion: it is really inconceivable
that
these observations, coming from diverse observatories with
different
instruments could be that wrong, and yet so systematically
concordant
with each other but not with the forced
"solution." To be sure, the
last word is not in, and work is still in progress fine-tuning
this
result. However, the basic conclusion that an impacting
trajectory
could be ruled out appears robust, and I don't think it is a fair
assessment to imply that it has taken more than a month to
certify that
conclusion as "correct."
Finally, you question whether this situation is true for such a
short
arc of data as 15 days. Here I am merely quoting IAUC 6879. Brian
said
it, I didn't. Paul Chodas has run calculations in terms of his
"10^(-large number)" collision probabilities that would
tend to support
that conclusion. In this case, the statement in IAUC 6879
actually went
further than I expected to see.
------------------
BJP:
There are other questions which have been raised by the events of
the
last month:
Why did various NEO researchers - even after 88 days of
observation -
come up with significantly different miss distances? How can
these
calculations differ so greatly if they use exactly the same data?
-------------------
AWH:
This is another excellent question, but again may take a bit of
space
to explain thoroughly. Recall my analogy of a couple weeks ago to
train
tracks. The key point is that all of the various solutions you
mention
fall "on the tracks", and therefore are concordant with
each other,
even though numerically (in one dimension) they sound
significantly
different. I'll draw a little picture below, which is only
intended to
be schematic, not quantitative.
---------------------------------------------------------------------------
1
2
3
---------------------------------------------------------------------------
X
_
|_| Earth
In the above sketch, the parallel lines represent the "error
ellipse"
that I have likened to a pair of railroad tracks. For the
Chodas
solution, the "ellipse" extends out a few meters
(around 10 feet) in
both directions from your computer screen. The error ellipse
defines
the range of space within which the real solution (the actual
encounter, in this case) might occur. Any solution which falls
essentially in the center of this range of uncertainty can be
considered "concordant" with any other solution that
also does, even if
they are not precisely coincident.
Thus solutions 1, 2, and 3 are concordant with one another,
especially
when we remember that the error envelope extends many meters out
of the
range plotted. But the solution X is not concordant, even
though it
would appear to predict a miss distance in the same range as 1,
2, or
3. The fact that the various orbit computers obtained
solutions like
1, 2, and 3 which are mutually concordant even if a bit different
gives
us confidence that all are essentially correct. There were no
solutions
like "X".
You may reasonably ask why there should be any difference at all
among
solutions using exactly the same data. One reason is that the
problem
is quite complex, and is of a class known as
"non-linear," so that the
solution is obtained by successive approximations. To solve such
a
problem, one makes an initial guess, then computes the
"covariance
matrix" which describes how the errors in fit to the
observations vary
with each (and all together) of the solution parameters, in this
case
the orbital elements. This covariance matrix allows one to derive
a
next level approximation to the solution parameters that will
better
fit the data. But because the problem is non-linear (in a sense,
you
are taking a straight line step down a curved path), you don't
quite
end up at the exact solution, just a better approximation of
it.
This same covariance matrix allows you to estimate when a
solution is
"good enough," that is, when the difference of
the present solution
from the optimum one is so small that the residual difference has
no
physical significance or predictive value. In our example above,
solutions 1, 2, and 3 are all physically equivalent; one is no
more
likely than the other to be a better estimate of what nature will
actually do. So one possible explanation of why the solutions are
different may be that the different computers simply chose
different
thresholds of "good enough" in deciding to quite making
successively
finer improvements in their orbits.
Different choices of initial orbits can lead to different
"trajectories" of convergence on the final solution, so
that it is not
surprising that various computations end up at slightly different
spots
in the error envelope. As long as a solution is sensibly in the
center
of the envelope, then they are all equally valid and concordant.
In
physical terms, for the example above, it is impossible to
predict from the
data available whether the true answer will be 1, 2, 3, or
something else
within the error envelope, so all can be taken as equally valid.
------------
BJP:
How much observational data (days, weeks, months, years?) is
required
to calculate a NEO-orbit with such accuracy that all
estimations do no
longer differ significantly?
---------------------
AWH:
This is bound to vary from case to case. At the outset, I would
like to
emphasize that the early solutions for XF11 did not "differ
significantly" from one another, which is the point of my
answer to the
previous question. As to when an orbit solution is good enough to
rule
out a collision, that is a more difficult question. The easiest
case is
when the Minimum Orbit Intersection Distance (MOID) can be shown
to be
greater than some distance such that we can be sure that a
collision is
impossible, no matter where in their respective orbits the Earth
and
the asteroid are.
To make another analogy, this is like verifying that two
different
aircraft are flying at different elevations such that no
collision is
possible. This turned out to be the case for 1997 XF11, but only
barely. This is why we could rule out a collision, but still not
define
very precisely just how close the encounter would be. Imagine two
aircraft flying in crossing paths but 1000 feet different in
elevation.
they might come as close as 1000 feet to each other, but then
again,
they might not get closer than miles apart, depending on where
along
their courses they are with respect to the other.
But suppose the orbits are intersecting, or in the above analogy,
the
aircraft are flying at the same altitude. Then it becomes a much
more
time consuming task to rule out a collision. 1997 XF11 provides a
good
lesson in how things might proceed, if we "tickle" the
orbit a bit in
the computer (or our mind) so that it is on a collision course.
The
first test is the MOID: if you're not on the tracks, you're not
going
to get hit. In the case of 97 XF11, we could determine reliably
that we
were "not on the tracks" from only the 1997-8
data.
The next question is, if you are "on the tracks," are
you within the
range of uncertainty along the orbit? From the 1997-8 data alone,
the
answer was, yes, we are in the range of uncertainty along the
track, so
if the solution had put the Earth "on the tracks," the
1997-8 data
would not have been sufficient to rule out a collision. It would
still
be improbable, because the range of uncertainty was several
hundred
times the width of the Earth. When we added in the 1990 data, the
uncertainty range along the track shrank to a much smaller
distance,
and in particular, the Earth was no longer included within that
range.
That is, the Earth not only was laterally displaced from the
error
ellipse, it was far off from the end of the ellipse, like so:
______________
<______________>
error ellipse
_
|_|
Earth
But it may not have been so. Suppose it had turned out not only
that
the Earth was "on the track", but also "within the
range" of the new
error ellipse using the 1990 data. Note that the new error
ellipse is
still longer than the Earth, so if we were within that box, we
would
still not know whether we are going to be hit or not. I believe I
have
drawn it correctly; the error ellipse, last time I checked, still
has a
long dimension longer than the Earth, so if we were in it, we
would
need to improve the orbit further in order to rule out (or in) an
impact.
In about two years, in 2000, 1997 XF11 will be easily observable
for
most of the year, but far away from the Earth. It is possible
that
observations at that time would suffice to shrink the error
ellipse so
that its longest dimension is less than the size of the Earth. I
tend
to doubt it, or if so only barely. In 2002, the asteroid will
pass
within radar range, and if radar observations are obtained, the
error
ellipse can be expected to shrink by two or three orders of
magnitude.
Following that, we would be able to say with quite absolute
certainty
whether or not an impact would occur. So, for 1997 XF11, which I
believe is a fairly typical example, if it had been on a
collision
trajectory, it probably would be until 2002 when we get radar
observations before we could say for sure that it would (or would
not)
impact. But that is still 26 years warning. LET ME
EMPHASIZE THAT THIS
IS ALL BY WAY OF ILLUSTRATION -- 1997 XF11 IS NOW KNOWN TO BE
NOWHERE
NEAR ON A COLLISION TRAJECTORY.. We are neither "on the
track" nor
"within the range" of a collision path.
The good news is, the above scenario is extremely unlikely, and I
can
show simply why that is so. There are a thousand or so
"XF11's" out
there to be found. Using XF11 as an example, after a month or two
of
tracking, we can project ahead for a century whether the orbit
intersects the Earth's (MOID less than the Earth's dimension),
and we
can define the position in the orbit to within a few million km.
Of the
100 or so already known, none have a MOID smaller than the
Earth's
dimension, although a couple (like XF11) come close enough that
one
might need to become concerned about specific close encounters,
that
is, detailed location in the orbit. Although the statistics of
this are
poor, we can estimate that of the thousand or so left to
discover, only
of the order of 10 will be found to have MOIDs close enough to
zero
that we need to consider specific encounter circumstances, that
is,
position in orbit.
But that position is known to within a couple million km (for a
century
or so), and a typical NEA orbit track is several billion km long
(circumference, or whatever that is for an ellipse). So the
chance that
there will be an encounter with the Earth such that the MOID
point in
the orbit is contained within the error ellipse is only around
one in
1000, per Earth orbit, for a given asteroid, or only around 10%
in
a century. For all the asteroids (about 10) for which the MOID is
low
enough to matter, that's still only a chance of about one
asteroid.
This is a very rough argument, and because the assumptions used
are
very generous, the bottom line is that it is unlikely (not highly
unlikely, but somewhat unlikely) that even one asteroid which
will be
discovered by a Spaceguard Survey will be found in an orbit which
cannot be quickly resolved, within a month or two, to be harmless
for
at least a century. This is a fundamentally different perspective
than
has been repeated in much of the impact hazard literature, and
one
which should affect how we approach the question of checking and
announcing a claim of a hazardous encounter.
----------------------
BJP:
How big has to be the miss distance in order to "rule
out" collision?
How big, to make it highly unlikely? How big, to make it
impossible to
rule it out?
-----------------
AWH:
The short answer here is, one Earth radius from the center of the
Earth, if you are sure of the trajectory. We have targeted
spacecraft
to swing by the Earth only a few hundred km up, with no ill
effects on
either the Earth or the spacecraft. But I am being a bit
facitious.
What counts, of course, is the uncertainty.
As I have indicated above several places, the uncertainty in the
"cross
track" direction, as we say in spacecraft navigation, is
typically a
thousand km or so for a near-Earth asteroid. So if that track
(cf. the
first schematic diagram above) misses the Earth by a few thousand
km
(well, maybe 10,000), we can be comfortable enough. However the
along-track uncertainty can easily be a million km. So if we had
a case
with a MOID of zero, we would do well to keep a very close eye on
the
object even if the nominal miss distance in some future encounter
is
estimated to be a million km or more. Thus you really can't state
a
minimum or maximum distance for "worry" vs. "no
worry," because the
uncertainly envelope is so terribly elongated.
----------------
BJP:
What errors and inaccuracies can effect the estimation of
orbit-calculations and how should they be taken into account?
-----------------
AWH:
A typical error source is catalog errors in the reference stars
used to
derive positions from the astrometric observations. For this
reason, we
often see errors of as much as an arcsecond, even when the
measurement
precision is a small fraction of that. Now and then gross errors
occur,
such as misidentifying an image (of a reference star or the
asteroid).
For that reason one needs more than just a few observations to be
sure
everything is OK. The theoretical minimum needed to compute an
orbit is
three positions. It's pretty dangerous to do anything with less
than
about 10, so that any wrong measurements will jump out. 100 is
better.
Obviously, the more observations you have from the more
independent
observers, the better your chances are that there are no obvious
errors, or that if there are they will jump out at you and can be
eliminated. The things to check in a solution are whether there
are any
outliers in the distribution of residuals (individual bad
observations), if there systematic trends in the residuals
(indicating
unmodelled effects or an inaccurate solution), or a non-Gaussian
distribution of residuals (suggesting a bad fit or faulty
solution),
and finally, the "devil's advocate" test I described in
answer to your
first question, where you force the trajectory to do what you are
testing against, namely a collision, and see what that does to
the fit
to the observations.
----------------
BJP:
In view of the fact that there is no general agreement about the
actual
impact probability (due to the biases in the impact crater record
and
the asteroidal/cometary flux), why does background probability
play
such an important role in the assessment of the impact
hazard?
-------------------
AWH:
The background probability is important only to as an
order-of-magnitude gauge of importance of individual events. I
think
your statement that there is "no general agreement" is
a bit strong. I
think within an order of magnitude, there is pretty general
agreement.
If you don't like "background level," I could couch the
same concept a
bit differently. I think we can agree upon the a priori impact
probability of a single object, given only that it exists and its
orbit
crosses the Earth's (that is, perihelion inside of, and aphelion
outside of, the Earth's distance). The instant an object is
discovered,
that may be all we know, and the intrinsic hazard from that one
object
can only be stated as being at that level, which happens to be
about
10^-8 per year, since the random collision lifetime of a single
object
is about 10^8 years (a bit less, actually). As the orbit becomes
better
known, what we are learning is where the object will go for the
next
century or so, and as an obvious corollary, where it WON'T go.
That is,
certain bits of space have an increased probability of being
intersected, and other bits of space must therefore have a
decreased
probability of being intersected (the sum total is constant of
course,
the asteroid actually will be one place or another, all of the
time).
You can think of the space we occupy in terms of a probability
function. If we happen to be sitting on a peak of that function,
we
have a better (or worse?) than random chance of being hit; if in
a
valley, a lower than average probability. After a very short
amount of
observation of a given object, that probability function already
becomes a very peaked affair, with most of the "chance"
concentrated
along a very narrow track, and the rest of the space left far
below
"background" level. So in almost every case, we can
expect that as soon
as we start to gather enough observations to get even a rough
preliminary orbit, the probability of an impact will go down, not
up.
It is only in a very rare case that we should see an increasing
probability at all. This is the same point I was making a couple
questions above.
To return to the point of your question, I would say that a well
authenticated impact probability of even 10^-6, which is way
below the
"background level" in 30 years, would suffice to raise
my eyebrows, but
the chance that any one object out there would have such a high
probability, after even a month or two of observations, is very
small.
---------------
BJP:
If 1997 XF11 would have been on a potential collision course,
would
that have altered your view that impact would be highly unlikely
due to
the background impact probability? In other words, what is more
important for any future risk assessment - orbit calculations or
impact
probability statistics?
------------------
AWH:
I'm not sure I understand the point of your question. If one
finds that
something is going to happen, then it's going to happen, and the
probability of its occurrence becomes moot. However, we must be
mindful
of the admonition, "extraordinary claims require
extraordinary proof."
This brings me back a third time to the point that it is
intrinsically
unlikely that even one object will be found for which an impact
cannot
be ruled out from a properly analyzed set of observations of only
a
couple months duration. Thus a claim of such a hazard, based on
such a
set of observations, must be regarded as
"extraordinary," and treated
accordingly. That doesn't mean disbelieved out of hand, but it
does
mean it demands careful scrutiny and checking.
--------------
BJP:
These are just some of the many questions which have been
occupying my
mind for the last four or five weeks. Perhaps you are happy to
answer a
couple of them so that we can continue our debate on a more
advanced
level.
------------
AWH:
Whew! Done! Sorry to have carried on so long. I think this has
done me
good in terms of clarifying some of the points in my own mind
(you
learn more from teaching than you do from studenting, it's well
known).
I hope the above dissertation has similarly clarified some points
for
you.
----------------------
BJP: Thanks for your time and effort.
*
CCNet DIGEST, 28 April 1998
---------------------------
(1) OCEANIC IMPACT MIGHT HAVE INSPIRED (Australian) ABORIGINAL
LEGEND: AN ADDENDUM
Duncan Steel <dis@a011.aone.net.au>
(2) THE SKY & TELESCOPE IMPACT WEB SITE
Phil Burns <pib@nwu.edu>
(3) DEEP IMPACT & ARMAGEDDON WEB SITES
Phil Burns <pib@nwu.edu>
(4) METEOR STORMS AHEAD
Phil Burns <pib@nwu.edu>
(5) METEOR STORMS THREATEN SATELLITES
Jane E Allen, AP
(6) IMPACT ORIGIN OF THE SERPENT MOUND, OHIO?
Phil Burns <pib@nwu.edu>
(7) NEW RESEARCH ON THE MJOLNIR IMPACT CRATER
H. Dypvik & R.E. Ferrell, University of
Oslo
========================
(1) OCEANIC IMPACT MIGHT HAVE INSPIRED (Australian) ABORIGINAL
LEGEND: AN ADDENDUM
From Duncan Steel <dis@a011.aone.net.au>
Dear Benny,
With regard to the interesting item Bob Kobres sent you, given a
heading OCEANIC IMPACT MIGHT HAVE INSPIRED (Australian)
ABORIGINAL
LEGEND, here is an addendum.
There is another legend retold in the mythology of the Paakantji
people
of the Darling River (from around Wilcannia in western New South
Wales). This tells of a foreseen falling star which brought fire
and a
following flood, killing many people and leaving behind strange
stones.
Sound familiar? The story is told in the (picture) book
'The Story of
the Falling Star' by Elsie Jones, Aboriginal Studies Press,
Canberra,
1989 (ISBN 0-85575-199-1). A speach bubble from a lady's
picture on
the back cover (perhaps Elsie Jones) says "This story is so
old we
don't even know how old it is...Malkarra was a special kind of
person...He told the Paakantji people something bad was going to
happen...They didn't trust him...If only they had listened to
Malkarra
they would've been gone when the star fell..." Various
places in the
book feature drawings of an incoming bolide which would not be
out of
place in the forthcoming Hollywood movies.
This legend and book is mentioned as a postscript to:
D. Steel & P. Snow, 'The Tapanui region of New Zealand: A
'Tunguska' of
800 years ago?' pp.569-572 in Asteroids, Comets, Meteors 1991
(eds. A.
Harris & E. Bowell), Lunar and Planetary Institute, Houston,
Texas,
U.S.A. (1992).
...which is largely concerned with legends of the Maoris of New
Zealand
which, it was suggested, might involve a Tunguska-type event.
That
paper was featured in a half-page article in New Scientist (5
October
1991, page 19).
Returning to the content of the newspaper article sent by Bob
Kobres,
it is well-recognized that the east coast of Australia shows
signs of
one or more large tsunamis during the Holocene. I am not
sure
how/whether the dating is done. The major sand dune formations
are
aligned so as to be side-on to the north-east (towards Hawaii,
say) and
ocean-bottom boulders have been found over 40 metres above sea
level (I
am informed; I am no geomorphologist & I have not gone
looking myself
to verify these things).
Duncan Steel
=====================
(2) THE SKY & TELESCOPE IMPACT WEB SITE
From Phil Burns <pib@nwu.edu>
SKY & TELESCOPE magazine now has a web site about impact
events
to complement the cover story in the June 1998 issue:
http://impact.skypub.com/
The site includes an article by Gerrit L. Verschuur and an
annotated list of
web pages about the impact threat by Stuart J. Goldman.
-- Phil "Pib" Burns
Northwestern University, Evanston, IL. USA
http://pibweb.it.nwu.edu/~pib/
=========================
(3) DEEP IMPACT & ARMAGEDDON WEB SITES
From Phil Burns <pib@nwu.edu>
Two movies with plotlines about large impact events are scheduled
for
release soon here in the U.S. Their release will probably
result in
a new round of questions about impact events from the general
public.
DEEP IMPACT from Paramount is written by Michael Tolkin and Bruce
Joel Rubin and stars Robert Duvall, Tea Leoni, Elijah Wood,
Vanessa
Redgrave, Maximilian Schell, Leelee Sobieski, and Morgan
Freeman.
Steven Spielberg is one of the executive producers. This movie
has
its own web site at:
http://www.deep-impact.com/
describing the story. The Discovery Channel here in the States
will
air a special entitled DEEP IMPACT NIGHT on May 4, 1998. I
assume
this is some kind of tie-in with the movie.
ARMAGEDDON stars Bruce Willis. This movie also appears to have
its
own web site:
http://www.armageddon.com/
I have not been able to access this web site successfully so I
don't
have any more information about the movie.
-- Phil "Pib" Burns
=========================
(4) METEOR STORMS AHEAD
From Phil Burns <pib@nwu.edu>
Several folks on the Cambridge list have been warning folks for
some
time of the dangers to satellites posed by a possible Leonid
meteor
storm next November. The mainstream press here in the U. S. has
finally started paying attention. Jane E. Allen of The Associated
Press offers a report dated April 27 entitled "Meteoroids
Threaten
Satellites." You can read this at:
http://www.abcnews.com/sections/science/DailyNews/satellites980427.html
The report stems from the Leonid Meteoroid Storm and Satellite
Threat
Conference.
-- Phil "Pib" Burns
=========================
(5) METEOR STORMS THREATEN SATELLITES
By JANE E. ALLEN
MANHATTAN BEACH, Calif. (AP) - In November, the Earth's
atmosphere will
be hit with the most severe meteor shower in 33 years, a
bombardment of
debris that could damage or destroy some of the nearly 500
satellites
that provide worldwide communications, navigation and
weather-watching.
The debris consists only of particles - some thinner than a hair
and
most no larger than a grain of sand - but they are hurtling
through
space so fast that they can have the destructive power of a
.22-caliber
bullet.
As a result, about 200 commercial and military satellite
operators,
insurers and scientists began brainstorming here Monday about
what they
can do to prepare, such as turn off spacecraft or turn them away
from
the stream of particles. The two-day gathering is called the
Leonid
Meteoroid Storm and Satellite Threat Conference.
"The consequences are still virtually unknown. There has not
been a
meteor storm since the onset of the modern space age. Nobody
planned
for it," said Peter Brown, a physics and astronomy graduate
student at
the University of Western Ontario who advises satellite
operators.
The particles, known as meteoroids, are vastly smaller than the
asteroids that could one day slam into Earth, and none are
expected to
come anywhere near the surface of the planet when they strike
this
November and again in November 1999.
But before the particles burn up in Earth's atmosphere, they
could poke
holes in solar panels, pit lenses, blast reflective coating off
mirrors, short out electronics with a burst of electromagnetic
energy,
even reprogram computers, said Edward Tagliaferri, a consultant
to the
Aerospace Corp., a nonprofit organization.
In 1993, for example, a meteor struck the European Space Agency's
Olympus satellite and destroyed its directional control,
rendering it
useless.
"What if you get unlucky?" Delbert Smith, a Washington
lawyer who
represents international networks and satellite operators, asked
at the
conference. "Who's going to explain to the major
corporations your
satellites aren't there anymore?"
While only a couple of satellites might get disabled - and some
cost as
much as $500 million - all of them will suffer surface damage,
said
David Lynch, a scientist with the Aerospace Corp.
Military satellites are better shielded because most are built to
withstand nuclear assault. But unlike commercial spacecraft that
can be
turned off temporarily, military satellites "can't afford to
be off the
air," Tagliaferri said.
The Hubble Space Telescope - which suffered minor surface damage
in the
1993 shower - will move to protect itself against Leonid damage
by
turning away from the stream of particles, an option being
considered
by many satellite owners.
First reported by Chinese astronomers back in 902, the Leonid
meteoroid
storms - so-named because they are found in front of the
constellation
Leo - become intense every 33 years. They occur when Earth passes
through a trail of dust left behind by the comet Tempel-Tuttle.
Scientists aren't sure when the heaviest showers will occur -
Nov. 17,
1998, or Nov. 18, 1999.
The spectacular showers will be visible this year across the
Western
Pacific and Eastern Asia; the 1999 showers will be visible in the
Middle East, Eastern Europe and Central Asia. Storms last 90
minutes to
two hours.
Back in 1966, when fewer than 100 satellites circled the Earth,
the
comet produced peak showers of 144,000 meteors each hour and no
major
damage. This year, with more than five times the number of
circling
spacecraft, some experts think the rate could be 5,000 to 100,000
an
hour.
But astronomer Donald Yeomans of NASA's Jet Propulsion Laboratory
in
Pasadena put the rate as low as 500 to 2,000 particles per hour.
And
Brown agreed that the rate won't be as high as it was in 1966.
(C) 1998 The Associated Press.
==============
(6) IMPACT ORIGIN OF THE SERPENT MOUND, OHIO?
From Phil Burns <pib@nwu.edu>
The Columbus (Ohio) Dispatch for April 26, 1998 contains a report
by
David Lore reporting on the controversy surrounding the origin of
Serpent Mound in Adams County, Ohio (USA). A volcanic
origin for this
feature has been the orthodox viewpoint, although a few suggested
an
impact origin. Geologist Mark Baranoski now suggests that there
is
"unequivocal evidence" for an impact origin.. His
conclusions are
disputed by other geologists who have examined Serpent Mound.
The full story may be found at
http://www.dispatch.com/pan/localarchive/lore26nws.html
-- Phil "Pib" Burns
Northwestern University, Evanston, IL. USA
http://pibweb.it.nwu.edu/~pib/
=================
(7) NEW RESEARCH ON THE MJOLNIR IMPACT CRATER
H. Dypvik*) & R.E. Ferrell: Clay mineral alteration
associated with a
meteorite impact in the marine environment (Barents Sea). CLAY
MINERALS, 1998, Vol.33, No.1, pp.51-64
*) UNIVERSITY OF OSLO, DEPARTMENT OF GEOLOGY, POB 1047, N-0316
OSLO, NORWAY
More than 50 samples from a Barents Sea borehole near the Mjolnir
Structure (an extraterrestrial impact feature) were used to
investigate
changes in the clay assemblage associated with the submarine
impact.
Seismic evidence, the presence of shocked quartz and a prominent
Ir
anomaly restricted the potential impact affected zone to a 10 m
interval, straddling the Jurassic/Cretaceous boundary. Increased
abundance (up to 30 wt%) of a smectite, a randomly
interstratified
smectite-illite with 85% smectite layers, forms the basis for a
two-layer oceanic impact clay model that differs from published
terrestrial cases. The smectite is assumed to represent
seawater-altered impact glass from the ejecta blanket material
that was
mixed with resuspended shelf sediments by the collision generated
waves. The smectite-rich interval is almost 5 m thick. It is
overlain
by a coarser unit (similar to 2 m thick) containing abundant
smectite,
shocked quartz grains, and anomalous Ir contents at its base. The
smectite-rich interval may have originated as a density/turbidity
current, generated by the impact and the collapse and erosion of
the
crater rim. Seawater alteration of volcanic glass and changes in
the
tectonic regime of the provenance area, or changing oceanic
current
circulation patterns could produce similar variations in the clay
mineral assemblage. The most compelling evidence for the possible
impact derivation of this clay assemblage is the direct
association
with the Mjolnir Impact Structure and associated mineralogical
and
geochemical anomalies. Copyright 1998, Institute for Scientific
Information Inc.
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