SOHO-620: A Comet on the Right(hand) Track

Comet SOHO-620, the faint splotch between
the hashmarks, is a probable cousin to Comet Ikeya-Seki,
though perhaps a million times fainter. (Image courtesy
the SOHO/LASCO consortium. SOHO is a project of
international cooperation between ESA and NASA.)

On the night of May 28, 2003, I adjusted the cropping of the large (1024 x 1024 pixel) SOHO C2 images on which I look for comets, in order to reveal the extreme right edge of the field. (The images are too large to fit on my computer screen, so I can only view part of them at a time.) One area I always want to look at is the path of Kreutz comets—which make up over 85% of the comets found on SOHO images. Because of the changing geometry of the Earth, Sun, and the orbit of these comets over the course of a year, their paths appear to sweep out an arc resembling that of a giant pendulum. In early May, Kreutz comets approach the Sun from the bottom left, but by the end of the month, the track has already shifted well to the right, so it seemed time to start checking that edge of the photos.

My instinct proved correct; starting at the image for 0250 UT May 29, I picked up an object moving upward from the lower right side of the field. I first saw it in the smaller black-and-white images; when I determined that it moved consistently in four images, I filed a report, which was soon confirmed by XingMing Zhou in China. The object later became SOHO-620, my fifth comet.

Even for this time of year, the comet came from considerably to the right of where most Kreutz comets emanate. I surmised that mine was a Kreutz Subgroup II comet, closely akin to Comet Ikeya-Seki as well as the Great Comet of 1882. I did an orbital simulation that seemed to verify this; SOHO-620 is probably my first Subgroup II comet (no orbits nor official announcements of my past three discoveries have been released as of December, 2003). Subgroup I, whose most prominent known member is the Great Comet of 1843, outnumbers Subgroup II by nearly 4 to 1. For much of the year, the tracks of the two subgroups are more or less overlaid upon each other, but around May, Subgroup II tracks are considerably (around 20 degrees) to the right of Subgroup I tracks, while by November, the situation is reversed.

The Kreutz sungrazing comet group is believed to be the remnants of a large (~150 km) comet that several millennia ago was deflected into an orbit that brings it unusually close to the Sun. Subjection to the intense radiation and tidal forces around perihelion have caused it to progressively fragment, though Zdenek Sekanina has demonstrated that the splitting can take place at any point in the comet’s orbit. Sekanina has theorized that the division into the two main subgroups may have occurred late in the third century AD. One component of that split, with a period of around 700 years, has been linked to the Great Comet of 1106, which was seen in daylight and itself observed to have split in two (perhaps the 1882 object and 1965’s Ikeya-Seki, both of which themselves were seen to split into several pieces near perihelion). SOHO-620, as well as the other Kreutz Subgroup II fragments seen in SOHO’s images, were likely part of the same split in 1106.

The Kreutz Subgroup I members are the detritus of a second component of the initial split, having a period of around 400 years; it has been linked to the Great Comet of 1843. (That would mean that several of its appearances have remained unaccounted for in the historical record. Kreutz comets coming to perihelion between mid-May and mid-August approach and recede from behind the Sun, and are never visible in a dark sky, and even as “daylight comets” could easily be overlooked due to their proximity to the Sun. It has only been with the advent of coronagraphs, carried by SOHO and its predecessors SOLWIND and Solar Max, that such comets could be readily detected.)

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