MRO HiRISE Sees Comet ISON

This post was written by CIOC Chair Casey Lisse, and posted on his behalf by Karl Battams

I just wanted to say a few words concerning this week’s MRO/HiRISE observations of Comet ISON. The current understanding is that what you see in the published Sept 29 images is the nucleus (or the nucleus and directly associated coma, IMHO), and some nearby coma nebulosity. The last message we have from the observers concerning the 29 Sept 2013 images suggests that at HiRISE’s very fine 10 - 20 km resolution per pixel (projected at the comet), they are imaging the nucleus and inner coma out to ~10^4 km.

This means that the inner coma is likely filling the entire HiRISE FOV, and they need to figure out how not to remove the coma as false background. I.e., they have to worry about understanding their CCD background, something that is usually not important when imaging the very bright Martian surface. Some help may come from the standard star observations that went along with the Comet ISON pointings, because these should have only star plus dark sky plus background in them. So it is reasonable to expect that some more coma structure should come out as the image processing improves, and as the Oct 1-2, 2013 images, with the comet about a factor of 2 closer to Mars, are published.

The HiRISE images are also important for a different reason. There have been some more recent claims (not peer-reviewed) that ISON is again fizzling; this might not be wrong, but I wouldn't say it is dying. For example, the CIOC’s Matthew Knight’s observations from Lowell Observatory of the last few nights doesn't see these decreases, and we have received similar statements from other well-respected professional observers. And there is also Meech et al.'s (2013) prediction that what we are seeing with ISON is merely the return to normalcy for a small comet after it suffered a big outburst at the end of 2012.

I would dearly love to see an image of the comet's dust morphology, to see if it growing a "fat waist", full of large chunks and fragments of the nucleus, that follow along the comet's orbit and not its small particle anti-solar tail. Comet C/1996 Q1 (Tabur) showed this very well before it disappeared, and comet 73P/SW-3 B showed this as it flared in May 2006 and fragmented (Sitko et al. 2011, Weaver et al. 2006). The new MRO images do not show any evidence for multiple nuclei and thus fragmentation, so either it has not fragmented, or any fragments that may be there haven't separated very much. Next week Hubble will be looking at ISON using imaging and spectroscopy, and these images will be very interesting to search for any changes from the Apr - May 2013 HST Comet ISON imagery.

I welcome the continuing discussion, predictions, and observations of Comet ISON. While I naturally have some preference for seeing the comet continue a coherent existence, the amount of attention and scrutiny ISON is receiving will teach us a lot about dynamically new comets regardless of the outcome. E.g., if the comet does disrupt soon, we will know much more about how ISON lived and dies than we learned about Comet C/2010 X1 (Elenin) in 2011 or even C/1996 Q1 (Tabur) in 1997, which for many was bright and exciting one month, and simply gone the next.