[CraigS]

Originally Posted by Outbackmanyep

Hi Craig,

There's a few factors as to why we can't see the nucleus.
As you know comets are only visible due to either scattering of reflected sunlight from dust particles or flourescence from ionised gas.
The coma is split up into 3 main parts, the hydrogen envelope which is invisible at visual wavelengths, the outer coma (which is the part i measure) and the inner coma which is what we refer to as the central condensation or "false" nucleus. The nucleus is hidden within this central condensation. Ground based telescopes are able to image the inner coma and using a special computer program, the Larson-Sekanina filter (which was produced to study coma morphology), they can carefully produce maps of jets emanating from the nucleus which are projected into the coma.

http://users.libero.it/mnico/comets/ls.htm

A similar thing was done here:
http://www.mpg.de/english/illustrationsDocumentation/documentation/pressReleases/2010/pressRelease20100426/

So we know the jets are there but what constitutes the jets will be unknown until spectroscopic observations from in-situ spacecraft can do that for us. We can't really see them as the reflected sunlight from dust particles coming from the nucleus itself creates the false nucleus which shrouds the true nucleus from observation.

My observations describing "large and diffuse" means that through binoculars or the eyepiece the outer coma has a large angular diameter comparable with something such as the moon, i view many fainter comets with coma's between 0.5' to 5', so in regards to 103P it has a coma dia of around 20' which is larger than "normal".

Hope this helps.

Hi OBMYp;

Thanks for your considered words .. and the links.
I've have read so many papers over the last 3 days or so that I think my eyes are about to drop out !

From what I've read, there's quite a bit involved in the issue of remote detection of CO2 from a distant comet. It seems that the Keck scopes are regarded as having some of the best IR sensing capabilites of Earth bound scopes at the moment - in the 0.8 to 5.4 micrometre band, however, these are mainly set up for exoplanet observations (perhaps not dust emitting comets). The distance to the comet and the resolving power of the scopes is one issue, as getting visibility close to the nucleus surface is tricky - even perhaps, not possible.

Deep Impact/EPOXI detected CO2 at about 4.25 microns, directly emitted at the surface source of the jets.

The space bound scopes are set up to detect at smaller IR wavelengths than Hartley's jets have proven to be. (Eg: The Hubble/NICMOS spectrometer operates at about 1.5 to 2.5 microns).

As you say, with comets, the dust causes unique issues amongst which is polarisation of the light in different parts of the absorption/emission spectra. This causes all sorts of problems for which they've developed corrective measures which in turn, rely on theoretical models. (As distinct from direct observation/measurements). As the gas moves away from the nucleus, it may also get zapped by UV etc (depending on its distance from the Sun), which may cause different atomic oscillation modes, and chemical recombination with accompanying H2O, which can then disguise the initial plain old CO2 gas signatures. Remote scopes may miss the CO2 gas because they may not be able to resolve the light close to the nucleus.

I've also found out that there's controversy about IR measurements for exoplanets as well (mainly due to the different interpretations upon which the theoretical models are based, and in evolving IR sensing technologies themselves.

Putting all this together, I can now see why they've sent up the comet probes. It removes any ambiguities because of theoretical/corrective measures and allows the creation of a comet detecting specialised set of instrumentation, which has now proven itself !!

Great stuff .. very exciting.

Apologies for hijacking your observation report .. I tapped into it to gain more knowledge of it all - thanks kindly for that .. I really appreciate the efforts you guys/gals go to, and I just wanted to let you know that others do look at your reports ... and the efforts you contribute do help others in their respective quests !!

Cheers & Regards