SOHO's 500 Comets: A Triumph of Amateur Astronomy

(An expanded version of an article from the September 2002 Eyepiece )

Comet SOHO-367, discovered by XingMing Zhou
on October 22, 2001, became one of the brightest
Kreutz comets ever found on SOHO images.
[Image courtesy of the SOHO/LASCO consortium.
SOHO is a project of international cooperation
between ESA and NASA.]

When the SOHO (Solar and Heliospheric Observatory) mission was launched in late 1995, no one could have anticipated the phenomenal trove of comets that its images would provide. Previous solar observatory spacecraft, SOLWIND and Solar Max, had revealed a handful of comets on images taken by special cameras called coronagraphs that mask the sun's disk to reveal the corona, but SOHO has far outstripped them. On August 12, the 500th comet was found on SOHO images.

Some 87% of SOHO's comets belong to the Kreutz group of sungrazing comets, and are fragments of some of the brightest comets ever seen; the Kreutz family includes the Great Comets of 1843, 1882 and 1965, all of which were visible in daylight when near the Sun. The first SOHO comets were found by professionals from NASA and the European Space Agency, joint sponsors of the mission, but in August, 1999, shortly after SOHO images were made public on the project's website-- Lovejoy of Australia became the first amateur to find a SOHO comet (actually, two on the same day). Soon a trickle of amateur discoveries became a torrent, and over the next three years amateurs found over 400.

Michael Oates, an British amateur astronomer, pioneered the technique of using advanced photo-processing techniques to search SOHO's archives for previously missed comets, particularly from the early years before the images were adequately patrolled. Working both with archival and current (real-time) images, Oates has found 136 comets, roughly a tenth of all comets in the historical record.

It is perhaps fitting that Germany's Rainer Kracht was the one to find SOHO-500. Kracht has been the most prolific SOHO comet hunter over the past year, finding 63 since August 2001, many of them archival comets belonging to three new families of sungrazing comets that were discovered earlier this year.

Just as fitting, SOHO-500 belongs to one of these non-Kreutz groups. It is the 31st member of the Meyer group, first identified by Maik Meyer of Germany, in January. Meyer, who created the Catalogue of Comet Discoveries website-- contains discovery information on all known comets, noticed similarities in the orbits of three non-Kreutz SOHO comets. He tried recalculating the orbits of other "stray" comets and found three more with nearly identical orbital elements--thus the Meyer group.

Meyer says of his discovery, "The essential thing is that amateurs can still make big contributions to science, even from the computer desk. In this case it was possible because the SOHO team has a free distribution policy of their data (as well as the Minor Planet Center with the astrometric data). I don't want to promote myself. I was just lucky to find the first new group. It could have been easily another person, but I am an amateur, and this is the main point."

On the same International Astronomical Union circular in which the Meyer group was announced, Brian Marsden of the Harvard/Smithsonian Center for Astrophysics reported another comet group that he had discovered. Soon after, Kracht identified a new comet group that now bears his name. The Marsden group has 12 known members, the Kracht group 8. The Marsden and Kracht groups may be loosely related. Unlike the Kreutz group, no members of the Meyer, Marsden or Kracht groups have been seen other than on SOHO images.

The comets of all three new groups tend to appear stellar, tailless and faint. However, there is evidence that the daytime Arietid meteor stream may be related to the Marsden group. Comet 96P/Machholz and the Quadrantid meteor shower also show some orbital elements similar to the Marsden group, so they may also be loosely connected (probably originating from the same parent body). Orbits of all three groups place the comets south of the Sun around perihelion, which may help explain why no comets belonging to these groups appear in the historical record.

Analysis of SOHO's trove of Kreutz comets has given astronomers a clearer idea not only of the nature and history of the group, but also of the way that comets split apart. Over the centuries, various comets have been observed to split into two or more pieces. Usually this happens close to perihelion, when the heat and tidal forces are at their greatest. This is particularly true of Kreutz sungrazers, whose orbits can take them less than 100,000 miles from the Sun's seething disk. (Some are even believed to collide with the Sun.)

Careful analysis of pairs of Kreutz fragments, however, has shown that such fragmentation can take place at any part of the comet's orbit, even near aphelion. None of SOHO's Kreutz comets have been observed to survive perihelion; the fragmentation of the Kreutz group has been progressive and swift.

Kreutz comets can be grouped into two subgroups, each with slightly different orbital characteristics. The main representative of Subgroup 1 is the Great Comet of 1843; of Subgroup 2 it is the Great Comets of 1882 and 1965. The latter two comets are believed to be the largest components of a daylight comet that was observed to split in 1106 AD. It is possible that the brilliant comet of 372 BC that was observed by the Greek astronomer Ephorus was the parent of all the Kreutz comets we see today; its main split into the two Kreutz subgroups may have taken place as recently as the 3rd Century AD, when the comet was far from the Sun.

SOHO has another instrument called SWAN (Solar Wind Anisotropy) that can detect comets. It produces a small image of the entire sky in which bright points in ultraviolet light, some of them comets, can be seen. Although only three comets have been discovered with SWAN, it was recently determined that many of the comets found visually by amateurs could have been seen earlier on SWAN images, which will undoubtedly get much more attention in the future. A recent SWAN comet (2002 O6) is currently visible in the morning sky at about sixth magnitude.

The advent of automated sky surveys such as LINEAR, designed to detect near-Earth asteroids but which also find numerous comets, has seriously cut into the number of visual comet discoveries by amateurs in recent years. There has been a resurgence this year, with four visual discoveries by amateurs since February, but with new and more powerful surveys planned, it may be harder still for backyard astronomers to be the first to discover comets in their telescopes.

But just as technology is closing one door, it has opened another in allowing amateurs to detect some of the smallest comets ever seen, which nonetheless are relatives of the most spectacular comets to grace our skies, and to find relations between hitherto unconnected comets. The contributions (and enthusiasm) of amateurs have been critical in establishing SOHO's place in comet hunting, and amateurs will play a key role as the spacecraft's comet totals continue to grow.

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