(meteorobs) Meteor Activity Outlook for October 12-18, 2007

[meteoreye at comcast.net]

Yes, joe, from October's NAMN notes:

Last year's Orionids were exceptional, and it is possible there may be a
repeat this year. When I saw that I had counted hourly rates of 25 an hour
under my suburban skies, I knew I had reached into the box of chocolates and
pulled out a chocolate covered cherry! Normally I would expect maybe a dozen
per hour with the unexceptional skies (LM +5.6). The IMO (International
Meteor Organization) received reports of 12,012 Orionids from 58 visual
observers in 389 hours of observing time. The analysis clearly showed what
an exceptional year it was, with the ZHR greater than the normal 23 per hour
peak for nearly 5 full days.

There were 3 distinct peaks.

The first, at Solar Longitude 207.88 (1245 UT Oct 21) with ZHR 53 +/-3 of
bright meteors. The second, at Solar Longitude 209.79 (1030 UT Oct 23) with
ZHR 58 +/- 7, also bright meteors. The last, at Solar Longitude 211.79 (1100
UT Oct 25) with ZHR 47 +/-9 of mostly faint meteors.

The population index ("r") for the 3 peaks were 1.6, 1.9, and 2.9
respectively, compared to the normal Orionid value of 2.5. The population
index is the ratio of meteors in one magnitude range (+1,+2, etc.) compared
to the next, so values below 2 consist of an excess of bright meteors,
normally only seen in major shower peaks. Sporadic meteors are faint, with
an r ~ 3.2, which is why light pollution has such a large effect on meteor
rates between major showers. These unusually low r values (bright meteors)
for the first two peaks suggest that the structure we passed through was not
part of the normal Orionid stream we sample. Jurgen Rendtel, in his analysis
in WGN 35:2 (April 2007) suggest the cause might be particles trapped in 1:6
orbital resonance with Jupiter. That means for every 6 of Jupiter's orbits
(71.17 years), meteoroids released from Halley's comet will make 1
revolution and therefore be influenced by Jupiter's gravity in the same
way each orbit. This keeps them confined in space, rather than slowly
spreading out as usually happens.

This is confirmed by Mikaya Sato and Jun-ichi Watanabe of Japan's National
Astronomical Observatory in a paper released by the Astronomical Society of
Japan, also in April. In addition to verifying the 1:6 resonance with
Jupiter as the cause, with their modeling they showed that the particles we
ran into last year were ejected from Halley's comet almost 3000 years ago.
This provides an explanation for the brightness; over the millennia the
smaller meteoroids would have been removed by solar effects leaving only
larger particles, similar to the 1999 Leonids fireball outburst. This
resonance may explain enhanced Orionid counts from 1936-1938 from the
previous orbit of this concentration (~ 72 years). The later peak of smaller
meteors would appear to be fresher than that, so further modeling will need
to take place.

Both of these suggest that over the next few years (through 2009) we have
the potential to pass through the same structure which might lead to
enhanced rates. While it's not a New Moon this year, early in the peak
window the moon sets before midnight, which is when the radiant rises far
enough above the horizon to record meteor data. Unfortunately, by the 25th
moonset will be very close to twilight (depending on your location) and
there will only be a short moon free period. Still, it seems wise to make it
a priority to take your evening nap, and plan on spending some time outside
between midnight and dawn from the 19th to the 25th.

Meteor Wayne
writer; NAMN Notes :)

> ------------------------------------------------------------------------- 
> Has anyone come up with an explanation concerning the unusual and unexpected 
> enhancement of last year's Orionids; and are there any thoughts as to 
> whether last year was a "one shot deal" or possibly carry over through this 
> year? 
> 
> -- joe rao 
> 
> 
> 
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