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Cambridge Conference Digest - January 23, 1998

CAMBRIDGE-CONFERENCE DIGEST, 23 January 1998

The cambridge-conference-list is a scholarly electronic network 
organised by Dr Benny J Peiser at Liverpool John Moores 
University, United Kingdom). For further information, please 
contact . 

Information circulated on this network is for scholarly use only. 
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(1) WHY SELLING PLANETARY DEFENSE IS STILL HARD GOING

(2) SCANNING THE SKIES FOR METEOR STREAMS: TECHNIQUES & PROBLEMS

(3) BRITISH SCIENTISTS DEVELOP METEOR SCATTER PREDICTION MODEL

(4) HOW TO DETERMINE PERIODS OF METEOR STREAM ACTIVITY 

(5) OBSERVING METEORIC BOLIDES FROM THE GROUND AND FROM SPACE

(6) METEOROID ORBITS: IMPLICATIONS FOR NEAR-EARTH OBJECT SEARCH 
    PROGRAMS

(7) ORBITAL EVOLUTION OF METEOROIDS FROM SHORT PERIOD COMETS

(8) HOW AMATEUR ASTRONOMERS CAN ENHANCE OUR KNOWLEDGE OF METEORIC 
    ACTIVITY AND CONTRIBUTE TO PLANETARY DEFENSE EFFORTS


==============================
(1) WHY SELLING PLANETARY DEFENSE IS STILL HARD GOING

From: Bob Kobres 

As one who has been trying to 'sell' the idea of defending our planet 
for about seventeen years now I can say that receptivity to the idea 
is by far greater today than it ever has been. A problem that 
remains, however, is that there is still no sense of urgency 
associated with implementing such a defense system. This leisurely 
approach to the situation is in large part due to comforting
statements made over the years by individuals within the academic 
community who are seen as authorities and periodically questioned 
about the risk we actually face. It once seemed to me that it was a 
requirement to conclude an article on this subject with a `not to 
worry though' paragraph before the paper would be published--mustn't 
frighten the public you know.  Perhaps there was a bit of social 
concern in not wanting to be an alarmist, however it is less than 
honest to say that we KNOW anything about how soon another event 
could occur. I mention this because there is still a good deal of 
confusion among people, who have only a cursory interest in this 
subject, about the frequency or contemporary likelihood of impact 
occurrence.  
 
If we want adequate funding and an ambitious agenda for mitigating 
this threat we need to speak more forcefully.  I noticed in Duncan 
Steel's note on the demise of the Australian NEO program that he 
said: "The probability of such an impact catastrophe occurring soon 
is small, but the stakes are too high to ignore."  Now there is 
nothing factually wrong with this statement, however a casual reader 
might take comfort while skimming this text that the probability
of an impact catastrophe occurring soon is small. A more effective 
way to convey our situation would be to say:  

THOUGH WE DO NOT KNOW WHEN THE NEXT IMPACT EVENT WILL HAPPEN, THERE 
IS REASON TO FEAR THAT EVEN A RELATIVELY SMALL IMPACT--THE TYPE THAT 
DO OCCUR MORE OFTEN--COULD BE DEVASTATING AT PRESENT BECAUSE THE 
DAMAGE OF THE COLLISION WOULD BE SUPERIMPOSED ON THE STRESS WE AS A
SPECIES HAVE, OF LATE, PLACED ON THE ENVIRONMENT. IF WE WISH TO 
ENSURE THAT WE ARE DOING ALL WE CAN TO AVOID A CIVILIZATION OR 
SPECIES CRUSHING EVENT THEN WE NEED...  

If more people who are perceived as authorities in this area of
research would adopt a rhetoric similar to above I think that the 
'green slime' would begin to flow more easily to where it is urgently 
needed.
 
The point to emphasize is that this is a one shot deal.  Potentially 
it's one strike and we're out and we do not know if time is on our 
side. What we do know is that unless we develop a viable defense 
system for our biosphere we will get smacked and there is absolutely 
no reason to suspect that such an eventuality is somehow safely 
remote in time from our contemporary situation.
 
For some of my earlier verbiage see:
http://abob.libs.uga.edu/bobk/sdanger.html
and links from there.
 
Also, with Benny's permission, I've started an archive of Cambridge 
Conference correspondence.  

The menu is at: http://abob.libs.uga.edu/bobk/cccmenu.html
 
Heads up.  bobk
Bob Kobres
 
bkobres@uga.cc.uga.edu
http://abob.libs.uga.edu/bobk
706-542-0583

=====================================
(2) SCANNING THE SKIES FOR METEOR STREAMS: TECHNIQUES & PROBLEMS

T. J. Jopek & C. Froeschle: A stream search among 502 TV meteor 
orbits. An objective approach. ASTRONOMY AND ASTROPHYSICS, 1997, 
Vol.320, No.2, pp.631-641

*) ADAM MICKIEWICZ UNIVERSITY OF POZNAN, OBSERWATORIUM ASTRON, 
SLONECZNA 36, PL-60286 POZNAN, POLAND

In the first part of the paper a short review of the computer 
meteor stream searching techniques is given. Different 
fractions of the stream component obtained amongst radio, 
photographic and TV data are partially due to the use of 
different methods applied in the stream search. A new objective
approach is proposed in order to obtain the threshold value D-c
of the orbital similarity corresponding to the probability of 
chance occurrence of the stream. The 502 Canadian TV data has 
been used to test this approach. It appears, that the values of
D-c given by Southworth & Hawkins (1963), and Lindblad (1971b) 
formulae are too high to warrant sufficient reliability of the 
identified streams. Indeed using these formulae the result is 
that the probability to obtain by chance at least one stream of
4-5 members goes from 21% to 68%. The effect of the three 
different distance functions used to measure the orbital 
similarity (Southworth & Hawkins (1963), Drummond (1979, 1981),
and Jopek 1993) has been investigated. For all functions taking
the same 95% reliability level, the number of streams detected 
is considerably less than in the case of the traditional 
approach (Jopek 1993a). However, results are different using 
different distances. Finally, we give the list of the orbits of
eight streams identified at the reliability level W-M = 95% 
using the Jopek distance.

====================
(3) BRITISH SCIENTISTS DEVELOP METEOR SCATTER PREDICTION MODEL

A. Akram*) & P. S. Cannon: A meteor scatter prediction model and its 
application to adaptive beam steering. RADIO SCIENCE, 1997, Vol.32, 
No.3, pp.1023-1035

*) DEF RES AGCY,TACT COMMUN,RADIO SCI & PROPAGAT GRP,MALVERN WR14 
   3PS, WORCS, ENGLAND

A computer model to predict the underdense meteor arrival rate 
over a forward meteor scatter communications link is presented.
The model incorporates important effects such as major shower 
streams, a nonuniform radiant distribution, and antenna 
polarization coupling. A particularly useful aspect of the 
model is its capacity to predict the passage of sporadic and 
shower hotspot regions across the sky and thereby provide 
directional information to drive an adaptive beam steering 
system. Directional data from a phased array reception system 
in the United Kingdom has been used to determine the diurnal 
arrival distribution of sporadic meteors. This diurnal 
variation is broadly reproduced by the model and suggests that 
just two daily changes in the direction of a high-gain beam 
would offer considerable advantage over a fixed beam system. 
This paper shows that further improvements in system 
performance can be achieved with accurate predictions of the 
time of appearance and location of shower streams. A 
comparative study between monthly predictions and experimental 
data from a high-latitude link in Greenland over a year shows 
reasonable agreement but highlights the need for higher-
resolution radiant data.

========================
(4) HOW TO DETERMINE PERIODS OF METEOR STREAM ACTIVITY 

J. Svoren*), L. Neslusan & V. Porubcan: Determination of the period 
of activity of meteoroid streams. PLANETARY AND SPACE SCIENCE, 1997, 
Vol.45, No.5, pp.557-562

*) SLOVAK ACADEMY OF SCIENCE, INSTITUE OF ASTRONOMY, TATRANSKA LOMNIC 
   05960, SLOVAKIA

A method for determination of the period of activity of 
meteoroid streams from photographic orbits is presented and 
discussed. It is also aimed at identifying stream members from 
marginal regions of activity. The search utilizes the 
Southworth-Hawkins D criterion. The primary separation of 
meteoroid stream members is based on the cumulative 
distribution of D. Variations of the orbital elements q, e, 
omega and i with the solar longitude were derived from a set of
selected orbits and applied to a search for the stream members 
from marginal regions of activity. Since the radiant is one of 
the most reliable parameters derived from observation, 
deviations of individual radiants from the stream radiant were 
taken as an additional criterion for a final stream membership 
classification. An application of the procedure to nine major 
streams led in the case of four streams to a probable 

assignment of meteors to the stream before or after the so far 
known 'classical' period of activity: delta Aquarids (1), 
Perseids (8), Leonids (3) and Geminids (1). The significant 
number of Perseids found prior to July 23 (the onset of the 
stream activity) and the relatively small dispersion of their 
angular elements suggest that their assignment to the stream is
rather reliable. 

========================
(5) OBSERVING METEORIC BOLIDES FROM THE GROUND AND FROM SPACE

Z. Ceplecha*), C. Jacobs & C. Zaffery: Correlation of ground- and 
space-based bolides. ANNALS OF THE NEW YORK ACADEMY OF SCIENCES, 
1997, Vol.822, pp.145-154

*) ACADEMY OF SCIENCE OF THE CZECH REPUBLIC, ONDREJOV 25165, CZECH 
   REPUBLIC

Data on large meteoroid impacts into the Earth's atmosphere are
available up to over ten meter sizes from global satellite 
observations by optical sensors, while data from ground-based 
photographic observations of meteoric fireballs (bolides) are 
available only up to meter sizes. However, the ground-based 
observations yield very precise data on the motion and ablation
of these bodies, as well as their light curves. The space-based
observations yield very precise light curves. Using data of the
brightest Prairie Network bolides, ablation coefficients are 
determined for 48 bolides brighter than magnitude -10 and 
compared to relative time changes of their brightness. This way
the known classification scheme for photographic bolides (based
mostly on observed ablation coefficient) is related to maximum 
increase and decrease of their brightness. The same 
classification scheme is applied to 16 light curves of somewhat
larger bodies observed globally by DOD satellites. Size range 
of these bodies is found to be from 2 to 15 m with median value
of 3 to 4 m. Only separation of type I + II (38%) from type 
IIIA + IIIB (62%) is possible. Increase of relative strength of
populations of cometary meteoroids (namely of the IIIB type) 
observed for photographic meteors up to a size of 5 m continues
to larger sizes up to 15 m. Majority of meteoroids in the size 
range from 2 to 15 m are cometary bodies with the weakest known
structure.

========================
(6) METEOROID ORBITS: IMPLICATIONS FOR NEAR-EARTH OBJECT SEARCH 
    PROGRAMS

D. Steel: Meteoroid orbits: Implications for near-earth object search 
programs. ANNALS OF THE NEW YORK ACADEMY OF SCIENCES, 1997, Vol.822, 
pp.31-51

SPACEGUARD AUSTRALIA PL, POB 3303, ADELAIDE, SA 5000, AUSTRALIA
dis@a011.aone.net.au

The available orbital database on macroscopic potential 
impactors of our planet (asteroids and comets, collectively 
near-Earth objects or NEOs) numbers less than one thousand, 
whereas there have been some hundreds of thousands of orbits of
Earth-impacting meteoroids determined in various surveys, 
mostly using meteor radars. If one assumes that NEOs have 
orbital characteristics broadly similar to meteoroids, then the
orbits of the latter can give important indications concerning 
the conduct of search programs designed to discover large NEOs 
well ahead of any catastrophic impact, allowing ameliorative 
action to be taken. For smaller NEOs that cannot be 
telescopically detected until the day or so before impact, the 
radiant distribution of observed meteors shows the regions of 
the sky from which impactors are most likely to emanate. It is 
shown that the vast majority of meteoroids striking the Earth 
have geocentric (apparent) radiants within two near-ecliptic 
regions a few tens of degrees wide and centered on longitudes 
+/- 90 degrees from the apex of the Earth's way (the so-called 
helion and anti-helion sources). These are bodies with low 
inclinations, large eccentricities (e = 0.7 - 0.9) and quite 
small semimajor axes (mostly a = 1.3 - 2.5 AU). After allowing 
for the terrestrial motion about the Sun (conversion to the 
true radiant), the longitudes are around +/- 120 degrees from 
the apex. For a ground-based search on the nightside, the best 
search region is that within similar to 20 degrees of the are 
joining the geocentric and true radiants (longitudes 90 degrees
and 120 degrees). On the dayside, proximity to the solar 
direction argues for a space-based surveillance program, if 
small NEOs are to be found just prior to impact.

============================
(7) ORBITAL EVOLUTION OF METEOROIDS FROM SHORT PERIOD COMETS

G. Cremonese*) , M. Fulle, F. Marzari & V. Vanzani: Orbital evolution 
of meteoroids from short period comets. ASTRONOMY AND ASTROPHYSICS, 
1997, Vol.324, No.2, pp.770-777

*) OSSERV ASTRON PADOVA,VIC OSSERVATORIO 5,I-35122 PADUA,ITALY

We perform an accurate modelling of orbital evolution of dust 
grains taking into account both the ejection parameters derived
from the analysis of the dust tail of each considered parent 
comet (Fulle 1989), and the integration of the Newton equations
in the context of a nine-body problem (Sun, seven major planets
and the dust particle) plus solar radiation and wind forces. 
Among Short Period Comets (SPC) we have selected P/Schwassmann-
Wachmann 1 (P/SW1) and P/Griegg-Skjellerup (P/GS), which 
represent two significantly different objects from a dynamical 
point of view. Dust from P/SW1 is dominated by Jupiter 
perturbations: after 2 10(4) years, about 7% of the grains are 
ejected in hyperbolic orbits, 80% of the grains have the 
perihelion out of 4 AU from the Sun, and only 1% of them 
reaches the Sun distance of 1 AU, thus contributing to the 
inner zodiacal cloud. Dust from P/GS is dominated by the P-R 
drag, although large,grains, due to their longer collapse 
lifetime, are sensitive to Jupiter perturbations. Therefore the
Tisserand criterion represents a useful tool both to estimate 
the orbital evolution of,grains larger than 100 mu m (i.e. the 
most likely canditates to replenish the zodiacal dust cloud, 
Grun et al. 1985). and in distinguishing the parent sources of 
meteoroids collected with near Earth space experiments able to 
measure the impact velocity vectors. Jupiter perturbations 
oppose to the P-R drag forces and reduce significantly the 
contribution of SPC to the inner zodiacal dust: the simple sum 
of the dust mass contribution from each SPC may be an 
overestimate of their actual supply.

============================
(8) HOW AMATEUR ASTRONOMERS CAN ENHANCE OUR KNOWLEDGE OF METEORIC 
    ACTIVITY AND CONTRIBUTE TO PLANETARY DEFENSE EFFORTS

S. Molau*) & R. Arlt: Meteor shower radiant positions and structures 
as determined from single station video observations. PLANETARY AND 
SPACE SCIENCE, 1997, Vol.45, No.7, p.857

*) DLR, INST OF PLANETARY EXPLORATION, RUDOWER CHAUSSEE 5, D-12489 
BERLIN, GERMANY

Single station video observations of meteors can be used for 
the precise determination of radiant positions of major and 
minor meteor showers. Video systems combine large quantities of
recorded events with high accuracies, and are therefore applied
in the investigation of fine structures within the radiant. In 
the past four years, amateurs from the Archenhold-Obervatory 
Berlin have been operating the wide angle video system MO VLE. 
The maxima of several major meteor showers could be recorded. 
The video tapes were digitized and analysed on PCs. Using the 
Radiant software developed within the International Meteor 
Organization, the position was determined of the Quadrantid 
(alpha = 229.4 degrees +/- 1.5 degrees, delta = + 49.7 degrees 
+/- 1.5 degrees), Lyrid (alpha = 271.6 degrees +/- 1.5 degrees,
delta = + 32.9 degrees +/- 1 degrees), Perseid (alpha = 46.0 
degrees +/- 2 degrees, delta = + 57.7 degrees +/- 2 degrees), 
Orionid (alpha = 93.6 degrees + 1 degrees, delta = + 14.9 
degrees +/- 1 degrees) and Leonid (alpha = 154.5 degrees +/- 2 
degrees, delta = 21.4 degrees +/- 1 degrees) meteor shower 
radiants and possible radiant fine structures searched for. The
obtained radiant positions are in good agreement with the 
common values in the literature; no significant fine structures
could be found. From the video records of the 1995 alpha-
Monocerotid outburst, a radiant at alpha = 117 degrees +/- 3 
degrees and delta = + 1 degrees +/- 2 degrees was derived, 
which proves older results to be in error by several degrees.