[HSMS]




From:  Peter Jenniskens        


                       **Beta Cygnids Nov. 1-8** 

Below is another alert to possible unusual meteor activity. Please
observe this event if possible in the hope of documenting a meteor
outburst.

On November 3, 1997, the Earth will pass close to the orbit of comet
103P/Hartley 2. That orbit used to pass just within the Earth's orbit
back in the return of 1991 and now will pass just outside the Earth's
orbit. At face value, the minimum distance (+0.040 AU) will be too
large to have any reasonable expectation of meteor activity associated
with this comet. However, comet and meteoroids do not change their
orbit in the same manner. There is a chance, perhaps, that the
meteoroids 49 days in front of the comet are less perturbed than the
comet itself and intersect the Earth's orbit. However, there is no
indication that we might expect high rates. Instead, we hope for a
recognizable signature of the stream, some level of activity that can
provide information as to how meteoroid streams are perturbed when the
comet orbit changes strongly.

The Earth will pass closest to the comet's orbit near perihelion on
Nov. 3 at 0.9 hours UT (solar longitude 220.681 -J2000) rather than at
the comet's descending node on Nov. 2 at 7.1 hours UT. Conditions are
excellent, with only a sliver of a Moon early in the evening. Don
Yeomans at JPL calculated an apparent radiant position at
approximately RA = 295.6, DEC = +31.3 degrees (J2000), not far from
beta-Cygnus. That is quite a bit higher in declination than a previous
radiant calculated by Hasegawa for the 1985 orbit: RA = 290, DEC = +7
[4], illustrating the change in orbit. The present orbit extends the
interval that northern hemisphere observers can view the stream. The
meteors should enter at an apparent velocity of only about 17 km/s.  

What are the chances that there will be a meteor outburst (even if
only one meteor per hour)? Yeomans calculated that the Earth will lead
the comet to this close approach point by 49 days. That is very
little. But the separation distance will be some 0.04 AU with the
Earth just outside the comet's orbit, which is very large. It will
depend on how the dust in front of the comet is perturbed whether we
will see any meteor activity at all.  

Moreover, calculations by Mark Matney of Johnson Space Center show
that the most recent dust may not have spread far enough along the
comet orbit for Earth to be able to meet it 49 days in front of the
comet. This is not a certain conclusion, because the difference is
only a factor of two in time. However, it is clear that we should not
put our expectations too high.

Also, the problem is that the time of nearest passage does not need to
be the time that we cross the dust sheet. The comet has an orbit at a
shallow 13.6 degree angle with the ecliptic (was 9.3 degrees before
the disturbance). A small deviation of the main dust sheet relative to
the comet orbit can lead to a significantly different time of the
event.

In principle, the window of opportunity stretches from the time of
passing the comet node on November 2 at 07.1h UT, until at least the
point of nearest passage to the comet orbit on November 3 at 00.9h UT,
while probably extending several days after that until a point in time
closer to the node of the previous orbit (Nov. 8).

The duration of the outburst depends on the thickness of the dust
sheet and the path of the Earth through the stream. If we only account
for the inclination of the orbit, then the duration of the event, the
time between activity levels of 14% of peak activity, will be of order
5 - 8 hours, based on the thickness of the dust sheets responsible for
the Andromedids, iota-Draconids, October Draconids and Puppids, which
are all very similar. It is clear that such relatively short period of
activity coupled with a very uncertain time of maximum needs global
monitoring. 

There may also be some low level activity that extends over many days.
Any meteor activity of slow meteors from the beta-Cygnid radiant is an
interesting clue to learning how meteor streams are perturbed when the
comet orbit is strongly changed.  

The comet itself may reach magnitude +8 shortly after passing
perihelion on December 21. Orbital elements calculated by Kenji
Muraoka, finding charts and a predicted brightness evolution can be
found at the website:
  
     Comet Catalog etc 

----------------------------------------------------------------------
Mark Davis,
Mt. Pleasant, South Carolina, 
USA Coordinator, North American Meteor Network Assistant Coordinator, 
ALPO & AMS
======================================================================= 

     NAMN home page:

      Mark Davis 


============================================================================
Jason K. Baack 
Cutler Health Center
Electronic Communications Coordinator
Voice: 207.581.4140 
Email:baack@maine.edu







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Comments:  Rein, W6/PA0ZN 



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