[BAA Comets] Hubble Space Telescope imaging of a 1996 outburst of 29P/SW1: A proposal

[Richard Miles]

Thanks to Hal Weaver and Paul Feldman, I have now made a preliminary 
investigation of the HST images of 29P/Schwassmann-Wachmann 1.

The SW1 observing program for the HST back in 1996 was primarily designed as 
a spectroscopic program and in that light, the WFPC2 images were largely 
taken for purposes of context.  The exposures were bracketed (120, 400 and 
600 sec duration) not knowing what the comet brightness would be. The 
schedule was arranged weeks in advance and the first exposures began at 
07:38 UT on 1996 March 11. It turns out that 29P happened to go into 
outburst about 8 hours prior to this time (my estimate - see below), the 
outburst being a relatively energetic one reaching 12th magnitude so the 
March 11th images were saturated near the position of the comet's nucleus. A 
second series were taken about 20 hours later, the first of which did not 
saturate the pixel where the nucleus was located.

I have applied a rotational gradient filter using IRIS software (courtesy of 
Christian Buil) to bring out the fine structure within the expanding inner 
coma as shown here:

http://britastro.org/~rmiles/29P/hst_05829_29P_15deg_labelA.png

Since the Hubble images were taken, our understanding of this enigmatic 
comet has grown and we now know that the outbursts suddenly eject material 
which then travels at roughly uniform velocity. By extrapolating the motion 
of the main condensations back to the position of the nucleus, it is 
possible to estimate the time of outburst. At the following weblink , I have 
adjusted the scale of the first-epoch image so that discrete features are 
comparable in size to the related features in the second-epoch stacked 
image. The two versions differ in scale by a factor of about 4.2, which 
equates to a time of outburst of 1996 March 11.0. See:

http://britastro.org/~rmiles/29P/29P_HST_expansion.png

I must say there is an impressive amount of detail to be seen in the 
first-epoch HST image, which subtends a mere 1.1 arcseconds!


My Proposal:

One feature that is commonly observed in 29P outbursts is the virtual lack 
of an expanding front of material at some characteristic position angle. 
This feature can be clearly seen in the HST imaging by way of an apparently 
shielded cone subtending p.a. 50-130 deg. What the origin of this 
'shielding' is yet to be explained but may be related to the polar 
orientation of the spin-axis of the nucleus. Alternatively, the 
non-isotropic expansion feature may be linked to the morphology of the 
outburst site and the fact that if the outburst is typically very localised 
at the surface of the relatively large nucleus, the trajectory angles are 
largely restricted to one hemisphere. This is the most likely explanation in 
my view.

Further support for this interpretation can be found from the shape of the 
leading front of the expansion shell, which appears to be spherically 
symmetric for a range of p.a. spanning a little over 180 deg. This is 
reminiscent of comet 17P/Holmes but in that latter case, the spherical 
symmetry spanned virtually the entire 360 deg. It should be remembered 
though that in the case of 17P, the nucleus is relatively small (~2 km) and 
a large portion of the crust (~1% of the total nucleus mass) was ejected 
into space at a speed of 135 m/s. With 29P, we do not see any long-lived 
debris surviving after an outburst and its nucleus is much more massive 
(possibly ~40 km across) so the hemispherical shape of the expansion front 
suggests that the area over which most of the volatiles expanded and 
accelerated the finest dust particles following its explosive outburst is 
quite small compared to the total surface area of the nucleus.

Given this interpretation then the direction  in which the 'shielded cone' 
points must be opposite the position on the nucleus of the active site 
responsible for the outburst. Now that could be a very useful parameter 
whereby the various active sites could be mapped provided we know the 
rotation rate and direction/obliquity of the polar axis.

Richard Miles
British Astronomical Association

P.S. Cassini has a best resolution of about 1.2 arcsec and is currently very 
well placed to observe 29P. From its closer, almost opposite vantage point 
to 29P than us here on Earth it would be invaluable in solving the 3D riddle 
of these expanding streams of material thereby locating the position of each 
active site on the nucleus. Such data would really help to solve the 
orientation and spin rate of this object thereby mapping the site of some 
outbursts. Usually it is best to observe between 1.5-6 days post-outburst to 
visualise the motion of fine structure within the inner coma. If a Cassini 
project were launched, the probe could be used to observe 29P following any 
of the prompt alerts from the amateur community who closely monitor this 
favourite object. The HST could provide the view from the Earth in 
unsurpassed detail.

What a thought - Cassini and HST working in tandem made possible by 
monitoring of 29P by amateurs!

P.P.S. Observations using the Faulkes Telescopes suggest that this object 
appears to occasionally undergo some less energetic outbursts, which are not 
picked up in small telescopes. So that means that there could be more 
opportunities to attempt coordinated observations than past outburst 
statistics would indicate.