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Re: (meteorobs) NASA News articles



I enjoyed the piece on the upcoming Leonids.  It fueled by enthusiasm
which has hardly dimmed since the amazing show last year.  There are a
couple of points that I felt might be worthy of note.

1) 5 minute uncertainty
The predictions by David Asher and I have been checked
by comparison with all reasonably well observed outburst maximum times and
ZHRs (1866, 1867, 1869, 1966 and 1969).  In every case, using ONLY the
comet orbit, the standard model of dust ejection and the rigorous
application of gravitational perturbations, we predict a time which fits
all the above outbursts with residuals of around 5 minutes or better. It
is thus reasonable to expect this sort of accuracy in our future
predictions, as we didn't use any historic Leonid data in creating
the predictions.  All the ZHRs derived by Peter Brown for these
outbursts in his re-analysis of historic Leonid showers, fit our dust
trail density model very closely.

2) Confirmation of Soviet research.
The times we calculate confirm work done by Kondrat'eva and Reznikov in
1985, that has been basically ignored.  Their more recent work in 1997
with Murav'eva is again confirmed by our work.  Reading the Transactions
of the IAU, Commission 22 reports in 1985, 1988 and 1991, there are more
references to Leonid work by these astronomers from Kazan than from all
other groups put together.  Some of these studies are in English and can
easily be obtained through the library services:
  E.D. Kondrat'eva and E.A. Reznikov, "Comet Tempel-Tuttle and the Leonid
      Meteor Swarm",  Solar System Research, Vol 19 pp96-100, (1985)
  E.D. Kondrat'eva, I.N. Murav'eva and E.A. Reznikov, "On the Forthcoming
      Return of the Leonid Meteoric Swarm", Solar System Research, Vol 31,
      pp 546-549, (1997)

David Asher and I were ourselves unaware of the research done at Kazan
when we started looking at the rigorous, continuous, evolution of dust
from  ejection from the comet to passage close to the Earth.  It seemed to
us that the IRAS dust trails demonstrated that the main component of
recent comet activity would be a dust trail, and calculation of each young
dust trail's position in any given year would indicate the location of the
densest regions within the Leonid shower.  Each time the comet returns, a
new dust trail is created, but differential perturbations soon separate
these out from each other the further one goes from the comet.

Most of the visual meteors are caused by meteoroids of a size that tends
to lag the comet by about 6 months per orbit due to the effect of solar
radiation pressure, so passage through the appropriate part of a dust
trail after the comet has passed perihelion will give the highest rates.
Dust trails become depleted with time, but the overall structure
stretches, rather than dissipates in three dimensions.  However, the
portions of the dust trails that are in the same resonance with Jupiter as
the comet, do not become depleted with time.

The ability to predict the time of a Leonid maximum to within several
minutes accuracy has a very specific consequence for the satellite threat.
At the predicted time of maximum, should a suitable orbital geometry
exist, the satellite can be located within the Leonid shadow of the
Earth, or at the side of the orbit furthest from the center of the dust
trail (or a combination of the two).  This would minimise any threat to
that satellite.

Details of this work, which includes times and rates of maxima for
approaches to dust trails over the next several years and at the next
return of 55P/Tempel-Tuttle, and details of the threat to satellites,
appears in:

R.H. McNaught and D.J. Asher "Leonid Dust Trails and Meteor Storms", WGN, 
    the Journal of the IMO, Vol 27, pp85-102, (1999).

and the basis of the work, in more technical form, in

D.J. Asher "The Leonid meteor storms of 1833 and 1966", MNRAS, 1999 in
    press.

We have a number of other articles, in press, submitted and in 
preparation.

Surprising as it may seem, we are not saying that the work by other
researchers who look at the dynamics of ejected dust have made errors in
their studies.  The problem is one of temporal resolution.  We looked ONLY
at the dust that was emitted at each perihelion and crossed its node at
the time the Earth was close to the node.  Dust that has a different
orbital period will arrive at its node before or after this time and
could have no bearing on Leonid meteors in the Earth's atmosphere.  If we
had looked at the orbits that crossed the node over a spread of time, the
narrow dust trails would have become computationally diluted.  It would be
like trying to assess the distribution of quartz in an area, without
knowing the specific location each specimen was collected from.  You know
from the collected specimens that quartz must be common in the general
area, but not that there are several narrow veins crossing it.

Cheers, Rob

Robert H. McNaught
rmn@aaocbn.aaodot gov.au


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