Comet groups (i.e. more than two comets showing the almost same orbital elements or can be reduced to such elements of an earlier period) are a still not so common phenomenon. Until around 1990 only one comet group was known for sure: The Kreutz group of sungrazing comets, named after the man who described it in three papers.
Separation of comets is known almost since comets were observed by mankind. Especially in recent years spectacular disintegration and splitting events were seen. In the seventies and eighties of the last century two spacecrafts, designed to observe the Sun, showed that the Kreutz group consists of much more members than were seen previously. In 1995 another spacecraft, SOHO, was launched and made the astonishing number of more than 850 comet discoveries (as of 2004), most of them very small members of the Kreutz group.
About 90% of all SOHO comets were members of the Kreutz group. The remaining comets were "sproradic" ones. Sometimes these comets came in pairs, most of the time only some days apart. At the end of 2001 there were only confirmed pairs - no other group of more than two members was known.
On Jan. 16, 2002, the orbit for comet C/2001 X8 (SOHO) was published in MPEC 2002-B01. I checked the orbital elements of this comet (as usual with new comets) and found that comet C/1997 L2 (SOHO) had almost the same orbital elements. I reported this to Brian Marsden at the MPC and was confident to have identified another pair - but here about 4.5 years apart! By further checking of my orbit database I found an interesting object - C/2001 E1 (SOHO) - which showed almost the same orbital elements as C/1997 L2 and C/2001 X8 except the for the inclination. Interestingly the inclination was exactly the retrograde of the inclination of the other two. So I tried to fit the observations with a direct solution. The direct solution was able to fit the observations slightly better than the retrograde one. Brian Marsden decided then - in light of the other two comets - that it is quite reasonable to assume the membership of C/2001 E1 to the other two comets and the revised orbit was published in MPC 44504. The first comet group besides the Kreutz group was now confirmed!
One may ask why there was a wrong solution adopted for the first orbit of comet C/2001 E1. The problem with all SOHO comets is the short arc of observation. This makes the orbit calculation sometimes uncertain, leaving several possibilities for a solution. For the Kreutz sungrazers this problem is solved by assuming the membership to the Kreutz group, thus restricting the parameters for the orbital solution.
So it seemed quite possible that C/2001 E1 was not the only comet for which a wrong orbital solution was adopted. I then analysed the astrometric data of all SOHO non-Kreutz comets and discovered that at least three of these comets can be represented by the orbital elements of the new found group. This was finally confirmed in IAUC 7832, where also another group, called the Marsden group, was announced (discovered by B. G. Marsden), overlooked in a similar way like the Meyer group.
The next logical step was to re-check the archival SOHO images for members of the new groups. This was promising because previous searches concentrated mainly on Kreutz comets. Especially the German SOHO comet hunter Rainer Kracht reported lots of archival SOHO non-Kreutz comets which were added to both groups. R. Kracht was also the one who sugested that a small group of four comets are constituting another group. B. G. Marsden confirmed this and pointed out that these four comets - called the Kracht group - are most likely an extension of the Marsden group. In the last years all three groups have grown considerably.Back