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