Thanks for this plausible explaination and for Peters confirmation of the
outer coma! Keep on watching this nice object (though the moon is interfering
for the next few days).

> Am Tue, 24 Nov 2020 23:14:04 -0000
> schrieb Richard Miles - rmiles.btee at btinternet.com (via baa-comet list) <baa-comet@simplelists.com>:
>
> Thomas Lehmann wrote:
> > At this low resolution (about 4") I cannot get much detail in the inner
> > outburst coma. But I'd like to draw your attention to the huge outer coma,
> > extending at least 4 arcmin away from the nucleus. Its asymmetric shape
> > does follow the inner coma to some degree so it might be related
> > to the current outburst and not be a leftover of some previous one.
> > Is it expected to have some material ejected at a much higher velocity
> > than what you derive from the size of inner coma?
> > Or is the comet sitting within a longer living cloud of dust?
>
> The extended outer coma of 29P is the sum total of many outbursts in which
> debris, dust and other particles have been accelerated by gas created from
> each outburst such that their velocities exceed the escape velocity from the
> gravitational attraction of the large (60-70 km) nucleus. The range of net
> velocities involved (after slowed by gravity) range from almost zero to
> around 200-250 m/s. So the extended coma will be an integrated quantity from
> the last few years of outbursts.
>
> It is relatively straightforward to calculate the maximum velocity of
> sub-micron dust in this way as it is related to the Maxwell-Boltzmann
> distribution of speeds exhibited by gas molecules at the particular
> temperature of the gas. For 29P we can assume that the gas has a molecular
> weight of CO and N2. The temperature of the gas is very low because it is
> produced when solar radiation warms CO, etc. ice that is ejected by the
> cryo-eruption - around 25-30K if my memory serves me correctly - sometimes
> this is called the sublimation temperature of the solid ice in question. I
> can refer you to the section in one of my Icarus papers from 2016.
>
> Don't forget that at a distance of around 5 AU, 4 arcmin equates to about
> 900,000 km. Of course solar radiation presure and the solar wind shape the
> size of the outer coma. It would be a good project for amateurs to try to
> map the intensity of the outer coma by stacking several hours of images when
> the comet is far from the Milky Way and see how it relates to previous
> strong outbursts. There is a paper from several decades ago which quoted
> something like one-third of the Zodiacal (interplanetary) dust can be
> accounted for by 29P's outbursts. Indirect evidence suggests that the object
> we call 29P has been active for at least 800 years.
>
> Richard
>
>
>
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