| Newsgroups: sci.astro
From: [email protected]
Subject: Perseid information
Organization: University of Western Ont, London
Date: Wed, 14 Jul 1993 01:15:19 GMT
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Press release from the International Meteor Organization (IMO).
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Exceptionally high meteor activity on August 11-12?
Contact addresses are given at the end.
Astronomers predict the possibility of exceptionally high meteor
activity on the night of August 11-12, 1993. If the prediction
proves to be true both astronomers and the general public might
be treated to a spectacular rain of shooting stars of an
intensity that is seen only a few times in a century. If it
actually occurs, the event can be easily observed with the naked
eye.
The possibility that sky-watchers will be treated to a
spectacular celestial pyrotechnics show in the night
of August 11-12 during the annual Perseid meteor shower has
greatly increased since the re-discovery last September of the
parent comet of the meteor stream, named Swift-Tuttle.
An attentive watcher may see several meteors or, as they are
commonly called, shooting stars per hour under dark skies during
any clear, moonless night. Meteors are the luminous phenomena
produced by tiny dust particles called meteoroids when the
meteoroid enters the Earth's upper atmosphere at high speeds.
On a few nights of the year, however, more meteors than usual
will be seen because the Earth then crosses through the orbit of
a stream of meteoroids as opposed to merely meeting "individual"
meteoroids. One such event occurs every year around August 12
when night watchers can see up to about one meteor per minute
from the stream which is known as the
Perseids as the meteors radiate from the constellation
Perseus. Such an event is called a meteor shower; the Perseids
are the most-watched annual meteor shower in the northern
hemisphere.
The meteoroids in the Perseid stream are derived from
the dust emitted by Comet Swift-Tuttle as it moves in its orbit
about the Sun. This dust is dispersed along the entire orbit of
the comet, but the concentration is the greatest in the vicinity
of the comet itself, where recently ejected material is still
present. Since Comet Swift-Tuttle only "just" passed through the
inner Solar System (late in 1992) the Earth will actually
encounter the main dust-belt located behind the comet on August
11-12, 1993, as a consequence of which we may see a very strong
meteor shower.
The first indications that the Perseid stream might produce
something unusual this year were found by the International
Meteor Organization when an analysis of world-wide observations
in 1988 and 1989 revealed slightly increased Perseid activity.
In 1991, the shower produced a rate of many hundreds of meteors
per hour for about an hour for Japanese stargazers,
while an even more spectacular display was observed in
Asia and Eastern Europe in 1992. This year, the passage of the
Earth through the comet's orbit only a few months after the comet
passed the same location leaves open the chance that we might
encounter a very dense collection of dust and even be witness to
an outburst stronger than in 1991 or 1992. This geometry might
even lead to a "storm" of shooting stars.
Meteor storms are characterized by meteors occurring at the rate
of one every few seconds and sometimes as high as several per
second. Real meteor storms are very rare events.
In the last two centuries, strong meteor storms have taken
place on only a handful of nights: November 13, 1833 (America),
November 14, 1866 (Europe, America), November 27, 1872 (Europe),
October 9, 1933 (Europe), October 9, 1946 (America) and November
17, 1966 (America).
Contrary to most other rare astronomical events, such as
eclipses, a meteor storm may be seen only once in a lifetime. It
can be observed only on the night side of Earth. Also, unlike
eclipses, meteor storms cannot be predicted in advance with
certainty as it is impossible to know precisely the dust
distribution around the parent comet's orbit. However, the
conditions that MAY lead to a meteor storm can be calculated
beforehand. The encounter geometry in 1993 is similar to that of
another meteor shower, the Leonids, in 1833 when a most
spectacular meteor storm occurred.
The present calculations suggest that meteor activity will
be at its climax near 1 Universal Time (formerly often called
Greenwich Mean Time) on August 12, 1993. This timing favors
Europe and may include Eastern North America, as observers there
might witness the event during the night of August 11 - 12. The
exact time is also somewhat uncertain; storm activity could occur
as much as 6 hours on either side of this time. Even if storm
activity does not occur, a spectacular outburst as in 1991 and
1992 remains very likely. Hence it is more than worth-while
to watch the Perseids during the night of August 11-12.
Higher than normal activity might furthermore extend to the
nights of August 9, 10, 12, and 13 as well. The best
period to watch the shower on these other nights is from midnight
until dawn local time, though, because the position of the
point in the sky where the meteors appear to radiate from is
higher at those times.
To see the shower, one should look up or make oneself comfortable
on an arm-chair and watch a large area the sky for some time
without concentrating on a particular star or other celestial
object. The shower can be seen anywhere north of 30 degrees
South, but north of the equator is best.
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Background information
Meteoroids are small bodies that populate the Solar System.
They vary from micrometer-sized dust particles of low-density
that are debris from the decay of comets, to solid chunks of rock
meters across that come from broken-up minor planets. Comets,
such as Comet Halley, leave behind belts of meteoroidal debris
along their orbit after being heated by the Sun and emitting
gasses which drag the meteoroidal particles off the comet. These
belts may become meteor showers on the Earth if the Earth
encounters this stream of debris. All the planets and their
satellites are continuously bombarded by meteoroids which either
evaporate in the atmospheres or hit the surface and create
craters, the same process which formed the craters on the lunar
surface. As the Earth has a fairly thick atmosphere, the chances
of our planet being hit by a large meteoroid which causes damage
on the ground is extremely small. Nevertheless, such events do
happen when looking over large time scales. Currently,
scientists believe that the Earth is hit by a significantly-sized
meteoroid over time scales on the order of tens of millions of
years.
Scientists believe, however, that cometary meteoroids (such as
the Perseids) are not structurally strong enough to survive
atmospheric flight and so they cannot reach the surface of the
Earth as a meteorite. Most Perseid meteoroids enter the
atmosphere at about 60 km/s and totally evaporate in the Earth's
atmosphere at heights around 100 km. A Perseid meteor that is
roughly as bright as the brightest stars, results from a
meteoroid that is only a few milligrams in mass, comparable to
a sand grain.
The Perseids have been observed for a long time: ancient Chinese,
Japanese, and Korean recordings of this shower date back 2000
years. The parent comet of the Perseids, Swift-Tuttle, orbits the
Sun on average once every $130.5$ years. During the previous
return of the comet, high Perseid activity was seen during
1861-1863. Unfortunately, our information about 1863, the year
most comparable with 1993, is still scanty.
The Leonid meteor shower was responsible for the meteor storms of
1833, 1866, and 1966. Some witnesses compared the 1833 event to a
snow storm. During the meteor storm of 1966, meteor activity
reached 40 meteors per second. Some observers were left with the
impression that the Earth was traveling through a tunnel of
shooting stars at an incredible speed.
Suggested references for further background information:
Brandt J.C., Rendezvous in Space: The Science of Comets, 1992,
W.H. Freeman and Co., New York.
Dodd R.T., Thunderstones and Shooting Stars, 1986, Harvard
University Press, Cambridge, Mass.
Kronk G.W., Meteor Showers: A Descriptive Catalogue, 1988,
Enslow Publishers, Hillside, N.J.
Roggemans P., The International Meteor Organization
Handbook for Visual Meteor Observations, 1989, Sky
Publishing Corporation, Cambridge, Mass.
*****************************************************************
International Meteor Organization
The International Meteor Organization is a global, non-profit,
scientific organization which coordinates meteor research. It can
be contacted through the following of its Council members:
P. Brown , Dept. of Physics, Univ. of Western Ontario, London,
Ont., N6A 3K7, Canada.
phone: +1-519-661-3283 ext.~6458, fax: 661-2033,
email: [email protected]
R. Hawkes, Dept. Physics, Mt.Allison Univ., Sackville, N.B.,
E0A 3C0, Canada.
phone: +1-506-364-2582, fax: 364-2216.
e-mail: [email protected]
Masahiro Koseki, 4-3-5 Annaka, Annaka-shi, 379-01 Gunma-ken,
Japan.
phone: +81-27-38248-23
D. Steel, Anglo-Australian Obs., Private Bag, Coonabarabran,
N.S.W., 2357, Australia.
phone: +61-68-426-314, fax: 842-298
e-mail: [email protected]
J. Wood, 126 Lansdowne St., Kensington, W.A. 6151, Australia,
phone: +61-9-367-1121.
A. Terentjeva, Astron. Council, Pjatnitskaja 48, Moscow,
109 017, Russia.
phone: +7-095-231-5461, fax: 230-2081.
e-mail: [email protected]
J. Rendtel, Gontardstr. 11, D-14471 Potsdam, Germany.
phone: +49-331-960-727, fax: 762-308.
e-mail: [email protected]
P. Roggemans , Pijnboomstr. 25, B-2800 Mechelen, Belgium,
phone: +32-15-41-12-25
M. Gyssens, Heerbaan 74, B-2530 Boechout, Belgium,
phone: +32-3-455-68-18, fax: 454-22-97.
e-mail: [email protected]
A. McBeath, 25 West Pk., Morpeth, Northumberld., NE61 2JP, U.K.
phone: +44-670-503-379
==============================================================================
Newsgroups: sci.astro
From: [email protected]
Subject: More Perseid information
Organization: University of Western Ont, London
Date: Wed, 14 Jul 1993 01:15:52 GMT
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.
The International Meteor Organization would be interested to
receive your observations, whether you see unusual numbers of
meteors or not. Please follow the techniques outlined in the
August, 1993 issue of Sky and Telescope and send the completed
summaries to the addresses given therein.
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