|Comet C/2012 S1 ISON in the field of view of the STEREO COR2B imager (lower left). Jupiter is the bright dot center left. Animation of 30 STEREO images. Click to embiggen.||Comet C/2012 S1 ISON in the field of view of the SOHOLASCO C3 imager (lower left). Animation of 30 SOHO images. Click to embiggen.|
Comet ISON may not have reached unaided eye visibility, but it is certainly putting on a show in the bevy of spacecraft observing it. Today ISON entered the field of view of the STEREO COR2B imager and the SOHO LASCO C3 imager. We only have the low resolution images from the STEREO COR2B imager at the moment, but it appears to have caught a dramatic moment.
When a Coronal Mass Ejection blasts out of the Sun and slams into the comet, causing great disruption to the tail.
As you can see, at the time the time (around 4:40 UT) when the CME apparently slams into the comet in COR2B, it is nowhere near the comet in LASCO C3.
If you look at the image to the left, this is the NASA Goddard solar wind/CME model, you may need to click on it and embiggen to see in detail, but the CME blasts out bottom right and misses ISON (top right) by a great distance.
The apparent CME impact is just a line of sight effect.
As ISON makes its final plumet into the Sun tomorrow, it may encounter, or appear to encounter, other CME's. Interpreting these will require a good understanding of where the CME actually is, and avoid confounding by line of sight effects.