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(meteorobs) Excerpts from "CCNet 113/2001 - 2 November 2001"
Dr. Peter Jenniskens quoted in Wired! I love the "Leonid time of year": it's
almost like an early Christmas/Hannukah/Eid-al-fitr for meteor observers. :)
Clear skies,
Lew Gramer
------- Forwarded Message
From: Benny Peiser <B.J.Peiser@livjm.acdot uk>
To: cambridge-conference <cambridge-conference@livjm.acdot uk>
Subject: CCNet 113/2001 - 2 November 2001
Date: Fri, 2 Nov 2001 11:32:40 -0000
CCNet 113/2001 - 2 November 2001
================================
[...]
(4) COMING SOON: SPECTACULAR METEORS
Wired News, 31 October 2001
[...]
(6) SMALL IMPACTOR DETECTION
Hermann Burchard <burchar@mail.math.okstatedot edu]
[...]
(10) TUNGUSKA STUDY IN GERMAN NEWS
Christian Gritzner <christian.gritzner@mailbox.tu-dresdendot de>
[...]
==================
(4) COMING SOON: SPECTACULAR METEORS
>From Wired News, 31 October 2001
http://www.wired.com/news/technology/0,1282,48016,00.html
By Elan Lohmann
The United States can look forward to the most spectacular meteor show since
1966 -- and it might be another 98 years before anything so sensational will
be seen again.
The Nov. 18 Leonid meteor shower will be "very impressive, rare and
something that you'll want to see," said Peter Jenniskens, a research
scientist specializing in the study of meteors at the NASA/Ames Research
Center at California's Moffett Field.
"The August Perseids meteor shower, which normally gets the most annual
astronomer attention, records a rate of about 80 meteors an hour, but this
November's Leonids will record a rate over 2000," Jenniskens said.
Viewing conditions in the United States are expected to be sublime this
year. One reason for this is the new moon, which falls on Nov. 18, when the
sky will be its darkest.
A typical Leonid shower yields about 10 to 15 meteors per hour, but this
year Jenniskens estimates the meteor shower will have as many as 4,200 an
hour at its peak. Viewers along the East Coast will likely see the meteors
fall directly from above, while in the West they will shoot across the sky
at an angle.
The perfect viewing time is estimated to be between 4 and 6 a.m. EST, on
Nov. 18.
"It is a naked-eye event. All one needs is a clear dark sky away from the
city lights to enjoy the phenomena," Jenniskens said.
For a sneak preview, a good resource is the Leonid Flux Estimator, produced
by the Search for Extraterrestrial Intelligence/NASA Ames center. Tools on
the site will calculate the best locations for viewing, the optimal spots
from any town and how active the shower is expected to be in that area.
The whole show should last 2 hours and create the effect of Earth moving
through a trail of dust, Jenniskens said.
An ordinary meteor showers occurs when Earth passes through debris left
behind by comets. But this year, the Earth will be passing through
particularly dense ribbons of comet debris.
The Leonid storm will occur when the Earth passes through a trail of tiny
dust particles left behind by Comet Tempel-Tuttle during its passage in
1767.
Tempel-Tuttle orbits the sun every 33.25 years, shedding dust particles as
it is warmed by sunlight. It first crossed the Earth's orbit in 860 A.D. The
earth passes through some of the trail every year, but this year it will be
particularly close.
Jenniskens said the next major Leonid storm will occur again in 2099, which
will be one of its last tours. "The comet will then leave the Earth's orbit
for good," he said.
In November 1833, the show was so spectacular many eyewitnesses feared the
world was coming to an end. In 1966, Americans viewed another excellent
stellar show, while in 1999, Europe witnessed an epic series of showers.
Jenniskens will be participating in the NASA-sponsored 2001 Leonid Multi
Instrument Aircraft (MAC) mission, to be launched out of Edwards Air Force
Base.
The 2001 Leonid MAC campaign follows a highly successful airborne campaign
during the 1999 storm visible throughout Europe, when more than 4,000
meteors rained through the sky at its peak. It was the first to be observed
by modern observing techniques.
"Only an airborne mission can bring scientists to the right place at the
right time to view the Leonids, and guarantee clear weather," Jenniskens
said.
) Copyright 2001, Lycos, Inc. All Rights Reserved.
============================
* LETTERS TO THE MODERATOR *
============================
(6) SMALL IMPACTOR DETECTION
>From Hermann Burchard <burchar@mail.math.okstatedot edu]
Dear Benny,
risks from potential SMALL NEO (SNEO) impactors < 1 km down to 50 m were
considered too great to be disregarded by all CCNet contributors who in the
last several issues (Oct 29-31) have commented on this matter. The
cost-effectiveness question was raised and suggests a need to look for
alternative schemes with a more practical scope than cataloging all SNEOs.
Here, I am inquiring of the NEO community about the practicality and utility
of a program of short-term SNEO detection vs. exhaustive cataloging. Only
those SNEOs that threaten impact immediately, within a few weeks of
detection, need to be found. This time frame should provide
a chance to launch some well-prepared deflection mechanism (that clearly is
not yet in place, at present).
Some SNEOs can be discovered with plenty spare time for preparations using
visual telescopes. A large fraction of total hazard from SNEOs originates
from a fairly small number of discrete meteor streams -- how large a
fraction I am not sure. Further narrowing down SNEO search to these few
streams should allow for cost-effectiveness while still providing
substantial protection.
Again, chief among risk makers are primarily the alpha- and beta-Taurids,
according to the prevailing Giant Comet hypothesis of Clube-Napier. Of these
only the day-time beta-Taurids which "come out of the Sun" cannot be
detected visually. Very likely the Tunguska impactor belonged to this group.
For such SNEOs, I believe, long-baseline radar interferometry could be used,
either Earth- or satellite-based, out to a distance of a fraction of an AU.
[See my CCNet notes 21 Mar and 23 Mar (footnote), 2001, a reaction to
remarks in THE TIMES (Higher Ed Suppl 16 Mar): "One problem is a vast
population of objects that would not wipe out humankind on impact, but are
capable of devastating a city and are too small for practical detection."]
The day-time meteor streams were first detected at Jodrell Bank using radar.
Existing radio telescopes have been combined before for long-baseline
interferometry. From my crude estimates, a 4000 km aperture should be
sufficient using Ku-band radar for one week's notice. I defer to experts who
already understand these things (e.g. see Duncan Steel's book "Rogue
Asteroids.."):
(a) Orbital dynamics
(b) Wavelength and baseline (aperture) design
(c) Radar emission power
(d) Echo signal reception and decoding
If this is at all workable, how difficult would it be to conduct an
experiment to see how many SNEOs (if any) can be detected in the
beta-Taurids, approaching the general vicinity of planet Earth (out to the
distance of the moon or larger) next June from the direction of the Sun?
Admittedly, a practical scheme like this would have little theoretical
value, neither would it be as attractive a proposal as a general NEO
cataloging effort.
Regards,
Hermann
================
(10) TUNGUSKA STUDY IN GERMAN NEWS
>From Christian Gritzner <christian.gritzner@mailbox.tu-dresdendot de>
Hi Benny,
I just found that about the new Tunguska study at www.yahoodot de.
Best wishes,
Christian
Mittwoch 31. Oktober 2001, 19:25 Uhr
Neue Studie: Asteroiden-Einschlag in Tunguska
(ExpeditionZone) - Was die mysterivse Verw|stung im sibirischen Tunguska
Anfang des letzten Jahrhunderts verursacht hat, blieb bis heute ungekldrt,
da keinerlei Spuren der Ursache entdeckt wurden.
Wie vor drei Monaten im Rahmen einer fantastischen Theorie |ber
Spiegelmaterie ausf|hrlich berichtet, vernichtete am 30. Juni 1908 ein grell
leuchtender Feuerball die Region um den sibirischen Fluss Tunguska, rund
hundert Kilometer nvrdlich des Stddtchens Vanavara.
Ein unidentifizierbares Objekt krachte mit der tausendfachen Wucht einer
Hiroshima-Atombombe auf die Erde. \ber 6.000 Quadratkilometer Kiefernwald
wurden dabei verw|stet, weltweit wurden seismische Ersch|tterungen
registriert und ein bis nach Europa sichtbares Auroren-dhnliches
Nachgl|hen erhellte drei Tage lang den Nachthimmel.
Seit 1927 wurde das Areal von zahlreichen wissenschaftlichen Expeditionen
genauestens untersucht, doch es wurden weder ein entsprechend gro_er
Einschlagskrater noch f|r das Gestein von Himmelskvrpern typische
Bestandteile entdeckt. Dieses Rdtsel war seit Jahrzehnten Anlass f|r
verschiedenste Spekulationen, die bis hin zum Zusammensto_ au_erirdischer
Raumschiffe oder einem Schwarzen Loch reichten.
Italienische Geologen und Astophysiker der Universitdt von Bologna haben
sich seit Anbeginn besonders verdient bei den Forschungen um Tunguska
gemacht. Sie unternahmen nach der Erstexpedition von Prof. Dr. Kulik - dem
ein Grossteil des vorhandenen Dokumentarmaterials zu verdanken ist - immer
wieder Expeditionen in die abgelegene Gegend.
In jahrzehntelanger Kleinarbeit sammelten sie alle verf|gbaren Informationen
und Studien |ber die Katastrophe von 1908 zusammen. Als besonders wertvoll
erwiesen sich dabei auch die in verschiedenen Archiven russischer Behvrden
recherchierten Berichte von Augenzeugen. Aber auch die seismischen Daten,
Untersuchungen von Bodenproben und die Fallrichtung der Bdume gaben gute
Hinweise.
Nun prdsentiert ein interdisziplindres Team aus Astronomen, Physikern und
Geologen mehrerer italienischer Institute und der Universitdt von Bologna
den bislang detailliertesten Forschungsbericht |ber Tunguska in der
aktuellen Ausgabe des Fachmagazins Astronomy and Astrophysics (Vol. 377
Oktober III).
"Wir haben eine genaue Analyse zusammengestellt aus allen wissenschaftlichen
Publikationen und bislang unvervffentlichten Augenzeugenberichten," erkldrt
dazu Dr Luigi Foschini vom TeSRE-Institut gegen|ber BBC. "Dies erlaubte uns
erstmals, den Orbit des Himmelskvrpers zu errechnen." Wobei viele Anzeichen
- - wie Orbit, niedriger Einflugswinkel und Geschwindigkeit - eher auf einen
Asteroiden deuten w|rden.
Demzufolge kam ein rund 4 km durchmessender Asteroid sehr geringer Dichte
mit einer Geschwindigkeit von 11 km pro Sekunde aus dem s|dvstlichen Himmel
geschossen. Aus der Richtung und allen anderen Faktoren wurden 886 mvgliche
Orbits er- und berechnet.
Die Abgleichung mit den Flugbahnen bekannter Objekte mit passendem Orbit
ergab dann eine "Endauswahl" von 2 Kometen und 7 NEAs (NearEarthAsteroids),
Objekten die der Erde gefdhrlich nahe kommen kvnnen. Sie entstehen zumeist
im Asteroideng|rtel zwischen Mars und Jupiter, wenn gro_e Brocken
zusammenkrachen und kleine Bruchst|cke weggesprengt werden.
Laut Foschini konnte es sich um ein Objekt gehandelt haben, dhnlich dem 1997
entdeckten Asteroiden Mathilde, dessen Dichte nicht viel grv_er als jene von
Wasser ist. Der Eintritt in die Atmosphdre w|rde es soweit zerstvren, dass
nichts anderes mehr den Boden erreicht, als die Schockwelle.
Die Forschergruppe um L. Foschini, P. Farinella, Ch. Frvschlhe, R. Gonczi,
T.J. Jopek, G. Longo und P. Michel meint jedoch auch, dass dies noch nicht
der Weisheit letzter Schluss sein muss, denn letzten Endes handelt es sich
um Wahrscheinlichkeitsmodell - allerdings das bislang Genaueste.
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