A: shows the location of the comet in a HI1-B image taken on Dec 13,2008 at 00:49UT. Also of note in the image is a beautiful coronal mass ejection.
B: shows the track of the comet through the data. This image is more useful to be viewed along-side one of the following movies.
View Larger Image December 17,2008: You would think that, after 13-years of historic comet discoveries with SOHO and two years of amazing STEREO/SECCHI observations and discoveries, we had put a check-mark in most of the boxes for comet-related achievements. But last week, Australian comet-hunter Alan Watson helped us with yet another historic achievement -- the recovery of a comet! Here's how it unfolded.
While diligently scouring the latest STEREO/SECCHI Heliospheric Imager ("HI") HI-1B data for comets and asteroids, Alan noticed the appearance of an object rising up from the lower-edge of the images. Immediately recognizing it as a "non-group comet" (not belonging to any known population), he reported it to the popular 'stereohunter' chat group page. It was soon established that this was not one of the known bright comets currenty gracing the skies and so we assumed it to be a new discovery for SECCHI. Meanwhile, veteran comet hunter Rainer Kracht (Germany) recorded a few positions of the comet in the data and produced a set of very approximate orbital elements for it. Maik Meyer, Rainer's fellow countryman and another of our most esteemed comet hunters, saw these orbital elements and noticed their similarity to those of a comet that was discovered back in 2003, but lost in the depths of space sometime thereafter. This theory was confirmed when Dr. Brian Marsden released the electronic circular designating it P/2008 X4 and linking the SECCHI observations to the comet P/2003 K2 (Christensen). And so it was official -- SECCHI had made a historic recovery of a comet!
P/2003 K2 (Christensen) was discovered by Eric Christensen and then observed from the ground for a few weeks in May/June 2003. The 2003 observations indicated that the comet was a short-period object with period of ~5.7yrs. The angular elements of its orbit were determined to a reasonable accuracy, but there was inherent ambiguity as to exactly where along its orbital path the comet was. Regardless, a prediction was set that it would return close to the Sun in January 2009. In late 2008, knowing that it was getting closer to the Sun, astronomers began searching for the comet again, but observations were turning up negative. In the course of email discussion about this object, Maik Meyer remarked:
"About three weeks ago ... I searched remotely from Tahiti for the comet. I used at first the nominal orbit with T=2009-Jan-8 and covered the line of variation of Â±10 days. Nothing was found down to 18 mag. I then recalculated the orbit without the last set of observations, which lead to a T at the end of January. I again covered about Â±10 days. Alas, nothing found down to 18 mag. This also suggests that the comet was on a different part of its orbit (although the magnitude could still have been too faint)."
The appearance of P/2003 K2 in HI-1B data on December 8, 2008 confirmed Maik's suspicions -- namely that the comet was indeed on a different part of its orbit.
This latest STEREO/SECCHI discovery ("recovery") is noteworthy for a couple of reasons. First, it means that we now know much more accurately where the comet is in space, so now ground-based observers can find it when it moves away from the Sun in January as a fading 10th magnitude object. Secondly, it means the comet can be declared "officially" periodic, and redesignated as such, instead of bearing just the provisional "P/2003 K2" designation. And finally, as a major coup for the STEREO/SECCHI project, this is the very first recovery of a single-apparition comet by a spacecraft! ("Single-apparition" means it has only been observed to pass the Sun once.) Not even SOHO has managed to do this in over 13-years of observing the near-Sun environment!
So this is a wonderful achievement for the STEREO/SECCHI mission and the fantastic group of international comet hunters that volunteer so much of their time to this work. Of course, particular congratulations must go to Alan Watson, Maik Meyer and Rainer Kracht for their work on identifying this object. And now the best bit -- movies and pictures!
Finally, there is also a bonus comet in the extended movie for the sharp-eyed among you. At the very start of the sequence, a Kreutz-group comet (SOHO-1583) can be seen flying into the lower-left edge of the images, about halfway between the solar outflow and the bottom of the image. (Can't see it? Here's a hint.) The images used here have been highly processed in such a way that solar features (CMEs, outflow, etc) are enhanced, and the stars are subdued. From time to time, the images may 'jump' or 'flash'. That is due to both instrumental and image processing affects. You can download more 'traditional' HI-1 images from the SECCHI Images web page.