IAU Minor Planet Center

Cometary Designation System

The following resolution concerning designations and names of comets was adopted by the International Astronomical Union at its General Assembly in The Hague on Aug. 24:

1. Commission 20 of the IAU, considering that:
   • there is essentially a 1:1 correspondence between the provisional (year/letter) and definitive (year/Roman numeral) designation systems for comets;
   • the procedure for interpolating old discoveries of comets into the existing designation systems is unsatisfactory, particularly when orbit determinations are not available;
   • the application of a new designation at each return of a periodic comet to perihelion is an unnecessary complication, particularly when the comet's recovery can be described as ``routine'', or for the rapidly increasing number of periodic comets that are followed all around their orbits; and
   • there can be confusion whether a newly-discovered object is a comet or a minor planet,
   proposes to replace the present designation systems for comets with a system that closely resembles, but is not identical to, the designation system for minor planets.
2. Specifically, it is resolved that the year/letter and year/Roman numeral systems be replaced by one in which each cometary discovery is given a designation consisting of the year of observation, the upper-case code letter identifying the halfmonth of observation during that year according to the procedure used for minor planets, and a consecutive numeral to indicate the order of discovery announcement during that halfmonth. Each new designation shall be supplied by the IAU Central Bureau for Astronomical Telegrams when the discovery is announced in one of its Circulars. For example, the third comet reported as discovered during the second half of February 1995 would be designated 1995 D3.
3. The nature of an object can further be indicated by an initial prefix. In particular, such prefixes should be applied in cases where comets have possibly been misdesignated as minor planets, or vice versa. If necessary, the prefix A/ would precede a comet designation that actually refers to a minor planet (or asteroid). For comets the acceptable prefixes are P/ for a periodic comet (defined to have a revolution period of less than 200 years or confirmed observations at more than one perihelion passage) and C/ for a comet that is not periodic (in this sense), with the addition of X/ for a comet for which a meaningful orbit can not be computed and D/ for a periodic comet that no longer exists or is deemed to have disappeared.
4. If a comet is observed to return (or have its periodicity established by observation through aphelion or from identifications), the P/ (or D/) shall be preceded by an official sequential number (e.g., 1P/1682 Q1 = Halley), the list to be maintained by the Minor Planet Center and published in the Minor Planet Circulars. Subsequent recoveries shall be acknowledged with further designations only when the predictions are particularly uncertain.
5. The practice of providing future predictions for the returns to perihelion of all periodic comets for which there is a reasonable chance of future observations will continue. While this currently means, for example, the publication of predictions for the comets for the year n in the batch of Minor Planet Circulars for May of the year n-3, the elements being for the 40-day date closest to perihelion passage, it is to be expected that this process will be supplemented--and perhaps eventually supplanted--by one that provides the orbital elements for these comets routinely at epochs 200 days apart, as in the case of minor planets.
6. In the case of a comet that has separated into discrete components, those components should be distinguished by appending -A, -B, etc., to the designation (or to the P/ or D/ periodic comet number).
7. Noting that some redundancy of nomenclature is desirable, it is proposed to retain in general terms the tradition of naming comets for their discoverers. In this framework, a committee has been formed to establish more precise procedures to ensure fairness and simplicity.
8. It is proposed that comet names be announced in the IAU Circulars only following consultation between the Central Bureau for Astronomical Telegrams and the Commission 20 Small Bodies Names Committee.
9. Whereas the new designation system for comets implies the possibility of confusion (if incorrect spacing is used) with that for new planetary satellites, it is proposed to indicate satellites with the prefix S/.
10. It is proposed that the new designation system for comets be introduced at the beginning of the year 1995. In the interests of avoiding confusion and maintaining continuity, Roman numeral designations will be published in the Minor Planet Circulars for pre-1995 comet discoveries/recoveries passing perihelion in 1993 and 1994, and new-style designations will be supplied for pre-1995 comets, together with lists of correlations with both the year/letter and the year/Roman numeral

The Small Bodies Names Committee is now the Committee on Small Body Nomenclature.

See the current list of periodic comet numbers in numerical order or in alphabetical order (by name).