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1994-01-23
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Perseids 1993 - Shower or Storm?
From: KD2BD@NN2Z.NJ.USA.NA
To : METEOR@WW
The following was copied from Usenet's sci.astro newsgroup by KD2BD.
Perseids 1993 - Shower or Storm?
by Peter Brown
After many years of speculation, the parent comet of the Perseid meteor
stream returned to the neighborhood of the sun in the last months of 1992.
The original predictions for the comet placed its perihelion passage in the
first years of the 1980's. Much anticipation surrounded this event and many
people reported noticeable increases in Perseid activity, particularly in
1980. In all likelihood, the returns around 1980 were ordinary, the few
observers who noted high activity became the "standard" quoted ZHR's for many
years and therefore a self-fulfilling prophecy developed with respect to high
Perseid activity. In addition, these years still saw vastly different
methods of reduction and analysis of visual data so that comparisons between
different groups and even individuals with varying perceptions were
unrealistic.
The returns after 1980/1981 were generally quoted as weaker in activity in
direct proportion to the interest in the stream and the belief that P/Swift-
Tuttle had arrived unseen or not at all. Beginning in 1988, the
International Meteor Organization implemented global analysis of the stream
using standardized reduction techniques and from data with uniform collection
parameters. Additionally, the IMO introduced computer calculated ZHR
procedures allowing flexibility to researchers in choosing the methods of
reduction and permitting accuracy checks of the final results. This initial
global analysis produced a surprise - a double maxima!
The result was widely criticized as the statistical significance of the new
structure could not be objectively determined and the reduction procedures,
though based on the best available techniques at the time, were still
somewhat new. The new peak appeared some 12 hours before the "normal"
Perseid peak. Initial explanations ranged from differences in perception
between different groups of observers to a simple statistical "blip" in the
data. Considering more than 53,000 meteors had been used in the analysis the
latter explanation seemed doubtful.
Then the same double peak structure was found in the 1989 data at the same
location, again separated by 12 hours from the primary "stable" maximum.
The data from 1989 were even higher quality than in 1988 with about the same
number of meteors. The analysis techniques had been refined through
experience with other shower global analysis and the conclusion seemed
inescapable - a double peaked structure for the Perseids existed. The double
peak profile had not been conclusively observed previously and a few
explanations for the structure were given. The newer peak was felt to
consist of younger particles than the main peak some 12 hours later (a
conclusion that was to ultimately prove true) and the authors of the 1989
analysis speculated that the material might be from a passage of P/Swift-
Tuttle in the early 1980's.
The 1990 return was destroyed by the full moon and no reliable analysis could
be attempted with such bad data. In 1991, observers around the world had
been alerted to the possibility of enhanced activity due to the new peak
some 12 hours before the main maxima. In 1991 the new peak would favor
observers in Japan - and favor them it did! The Japanese observers witnessed
one of the strongest displays of the Perseids in the last century with ZHR's
over 400. This was clearly stronger activity than had been witnessed in the
past few returns and it seemed that the stream was changing. Shortly after
the Japanese announced the heightened activity, Brian Marsden pointed out
that in a paper he published in 1973 he discussed the possibility that
P/Swift-Tuttle might actually return in 1992 if it was the same comet
observed in 1737. While he had ranked the possibility as slight that the
1737 comet was P/Swift-Tuttle in 1973, the enhanced Perseid display in 1991
revived the remote chance that the comet might return in 1992.
The telling sign would be Perseid activity in 1992, unfortunately a full
moon would compete with the meteor show and make data analysis very tricky.
As the data from the previous returns showed that Europe would be the best
place to observe the early peak, much preparation was made there to capture
the event. Unfortunately, meteor showers, unlike eclipses, have an inherent
unpredictability resulting from our lack of knowledge regarding the dust
distribution about the parent comet. The 1992 display showed this maxim
perfectly; the new peak shifted some 2-3 hours earlier than what had been
observed in past years. As a result, Asian and Russian observers were in
the best locations to witness the display. After much analysis of the
available observations it appears that the 1992 activity was higher than in
1991 - perhaps with a peak ZHR of order 500, though this peak value will
remain highly uncertain due to the effects of lunar interference.
This brings us to the next logical stage of the "act"; the 1993 display.
With P/Swift-Tuttle recovered shortly after the 1992 display (and with
elements close to those predicted by Marsden in his 1973 paper) it became
apparent that the geometry between the comet and the Earth could make the
1993 display very strong. Indeed, our geometry with the comet is very
similar to that between the Earth and P/Tempel-Tuttle in 1833. This is
suggestive that a strong return is in store for observers in 1993. However,
P/Tempel-Tuttle is NOT P/Swift-Tuttle and the dust distribution about the
latter is unknown. While there is much circumstantial evidence favoring a
storm, nothing can be certain.
Keeping these cautionary notes in mind, what might be predicted for 1993?
Based on the node of the comet and the maximum activity in 1992 one would
expect peak activity to be at 1 UT on August 12, 1993. Some have suggested
that the shift in activity between the 1991 and 1992 displays suggest that
we can expect another 0.1 day advancement of activity in 1993, closer to
22 UT on Aug. 11. While this is possible, I consider the shift unlikely -
meteor storms usually occur very close to the node of their parent comet as
the 1992 display did relative to P/Swift-Tuttle. Basing an estimate of this
sort on two data points (the 1991 and 1992 maxima) is a bit questionable
the least so I see little reason to suppose a further 0.1 day shift will
occur.
What sort of display are we likely to encounter? The past meteor storms for
which reliable observational data exist suggest that newly ejected cometary
material is rich in faint meteors - this seems to be the best guess of what
will be seen in 1993. That is not to say that there will be little or no
large particles encountered, but the proportion of faint meteors to bright
meteors will be higher than in regular Perseid displays. The central
questions, how long will the display last and what will be the maximum
activity are very difficult to predict. Meteor storms generally last for a
few hours at most - some historical records suggest that large displays can
carry on for days, but these records are very open to interpretation. Data
from more recent storms seems to suggest that several hours (2-6) is a good
guess for the longest time for which unusually high activity might be
observed. The 1833 Leonids, for example, showed strong activity for nearly
6 hours.
The peak rates are complete unknowns. The largest meteor storms on record
for the last few centuries produced activity on the order of 100,000 meteors
per hour for intervals shorter than about 1 hour. Ancient records do little
to pin down peak rates of meteor storms earlier than about 1800. Everyone's
guess is equally valid in this instance.
Whatever the 1993 display produces it will go down in history as one of the
most waited for showers ever.
<end of message>
___________________________________________________________________________
Peter Brown works at the Department of Physics at the University of Western
Ontario, Canada and is a member of the IMO.
---------------------------------------------------------------------------
Comment by OH5IY: Take note that that was written before the much waited
(but much less than ancticipated activity) arrival of the perseids in 1993
and keep this in mind when using my software and its PEAK PREDICTION part.