Alert from the IAU C-22 Pro-Am Working Group:


Peter Jenniskens      
Chairman - IAU C-22 Pro-Amat Working Group
e-mail: peter@max.arc.nasa.gov      


                                              **Beta Cygnids Nov. 1-8**

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. [Note Harold Ridley calculated a radiant of RA = 299, Dec = +14, with maximum on November 17th - J Shanklin]. 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 can be found at the astroarts website. [See also Periodic comets

Distributed by:


James Richardson
Graceville, Florida
richardson@digitalexp.com

Operations Manager / Radiometeor Project Coordinator
American Meteor Society (AMS)
http://www.serve.com/meteors/

Minor edits by J Shanklin