(meteorobs) 1972 Draconids [was 2005 Draconids]

[Mikhail Maslov]

Let me introduce some remarks. This is easily shown by models considering only
gravitational forces. In 1972 as I know the predictions were based
mainly on the favorable geometry of the comet orbit - its VERY close
distance to Earth's one and small time interval between perihelion and
supposed Draconids activity. It was like a guessing. The main problem is that the particles
are ejected with some velocities relative to the comet, so from the
very beginning they have an orbit slightly different from the comet's
one. It is the reason of lengthy trails formation. The comet could
also be considered as a particle with ejection velocity equal to zero.
We didn't "encounter" such a particle even in case of very favorable
perihelion, what was the reason to suppose encounters with other
particles not knowing their orbits?
This tells us that an outburst occurrence is mainly a matter of
luck - how the trails will be formed by previous perturbations. I can
say that in 20 centure every year of or around perihelion of 21P the
set of trails intersected Earth's orbit, but mainly in the absence of
Earth itself. 21P has a very short period, so its trails are much
shorter and they can be in the vicinity of Earth's orbit close enouth to
give an outburst only for very little period of time - from several
days to month. In case of, say, the Leonids, situation is different.
Much longer orbital period cause much more longer trails so they can
produce activity for several years around the comet perihelion. Or, we
can look at the Lirids. Esko Lyytinen explains all their outbursts
over the last two centures to be caused by a single 1st rev. trail.
Their parent comet Tatcher has long period, from several thousands
years, I don't remember exactly, so even 1 rev. trail is very lonf
and all these outbirsts are "around" the single comet apparition in 19
centure.
Speaking of luck I mean that comet's orbit geometry doesn't define if
an outburst would occur. It didn't happened in 1972 because the bulk
of trails intersected the Earth's orbit around 10 August. If the Earth
was at this date in its 7-9 October place, a strong outburst
certainly would happen. In 1998 the comet's orbit itself was much
farther, but particles of certain ej. velocities (1) had their des. node at
1 AU, and (2) they passed this node around 8 Ocrober. In
2005 comet's orbit geometry is relatively good, its perihelion is
around 2 months before the Draconids, but no trails encounters are
expected. In 2011 the geometry is not favorable, but some level
encounters follow from the computations.

Best regards, Mikhail Maslov


MJC>> > Do the models explain what happened in 1972, when the possibility of a
MJC>> > storm was touted, but almost no visual activity was present.

HL>> I did some trail computing of the Draconids years ago. Yes - the lack in
HL>> activity in 1972 is easily explainable,

MJC> Right, using today's models there was no encounter with a concentration
MJC> of particles.  I'm wondering why _at the time_ there was an expectation
MJC> of at least abnormal activity.  There was the closeness of the orbits
MJC> crossing only a couple of months after the comet.  So my question is
MJC> more, what physics has been introduced to our models which move the
MJC> particles away from the comet orbit, thus making the 1972 Draconids a
MJC> disappointment.  Poynting-Robertson was well known.  YORP, ejection
MJC> velocity distributions, planetary perturbations?

MJC> Malcolm

MJC> ---
MJC> Mailing list meteorobs
MJC> meteorobs at meteorobs.org
MJC> http://lists.meteorobs.org/mailman/listinfo/meteorobs