SOHO-564: A December Surprise

Comet SOHO-564's motion through the LASCO C2 field
(from bottom to top) on December 3, 2002.
The sun is out of the field to the upper right
(in the direction of the bright ray).

(Image courtesy SOHO/LASCO consortium. SOHO is a project
of international cooperation between ESA and NASA.)

On December 2, 2002, I attended a lecture by Robert Kirshner, author of "The Extravagant Universe," at the Hayden Planetarium/Rose Center at the American Museum of Natural History in New York, with no inkling that I had plenty of astronomical excitement in store for me afterwards. I got home a little after 10 p.m. and looked at the latest SOHO images, the large, blue-filtered pictures from the LASCO C3 camera. There had been a break of several hours in SOHOís stream of images; the images had resumed at 0142 UT Dec. 3, and maybe three new images were available. I didnít notice anything unusual then, but when two new images were posted soon after and I viewed them sequentially, I noticed a rather faint spot beneath the sun, which from frame to frame moved upwards and a little to the left. To me it clearly seemed to be a comet because of its consistent appearance and motion, even though it was quite faint. I quickly measured its position in two frames and filed a report; I then measured three more positions and posted a follow-up:

Dec 02 2002 23:35:27

Update on C3 comet
Comet in C3, 1024 x 1024, 0,0 in upper left

Dec 3
0342 477,622
0318 478,626
0242 479,631
0218 480,635
0142 480,641

Fairly faint, condensed, globular


The comet was soon confirmed by Alexander Mimeev, a Moscow comet hunter. The comet was harder to see in the ensuing images, as it sailed into a coronal streamer (a bright stream of material in the sunís atmosphere) and was soon lost to view. I was hoping, though, that the comet would later appear in the more sensitive, narrower-field C2 camera when it got closer to the sun. I stayed up late, waiting for it to appear in C2; I was very excited and energized by finding the comet; itís always a thrill to find one, but in this case it had been over five months since my last find, while other people had found fully 90 SOHO comets during that period, so Iíd been feeling I was overdue. SOHO comet hunting is highly competitive, and there are some very skilled comet hunters who have had far more success than I. Iíve found three comets in a year and a half; Sebastian HŲnig found that many in one 50-minute period a few days ago, while XingMing Zhou has found five comets in the past week. In the C2 image for 06:06 UT, there was a spot on the bottom that I was pretty sure was the comet, but no new images were posted that night. It was almost a day later when the C2 images that followed it were posted; indeed, the spot was the comet, and it moved almost straight upwards as it steadily faded over a series of close to 10 images. I have created an animation of the cometís motion through C2, which I have included at the top of this page.

There has been a marked increase in the number of Kreutz comets found in SOHO images since around November 10. From November 9 to December 11, fully 32 small Kreutz sungrazers were foundóan average of one a day! (This compares to 5 comets found during the same period in 2001.) Several times, these comets have arrived in groups of twos or threes, obviously related fragments that recently split. It will be interesting to examine the orbital elements of all 31, once they become available. (The glut of comets has created a backlog of orbits for Brian Marsden to calculate, and an unusually long delay in their release.) If most have very similar orbits, itís likely that they came from a single, relatively recent fragmentation event. If so, it is possible that we are seeing debris in the vicinity of a larger sungrazer that is on its way. (There hasnít been a Kreutz comet bright enough to be visible from the ground since 1970; the brightest, such as 1965ís Ikeya-Seki, can be seen naked-eye in broad daylight, even when very near the sun.) It could also turn out to just be a random clumping of less-related Kreutz fragmentsónobody really knows what this increase in comet flux means. SOHO has only monitored the solar vicinity for 6 years, less than a hundredth of the orbital period of a Kreutz comet (which average around 1000 years).

--Tony Hoffman

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