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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