ESA Science & Technology05-Jul-2005 16:41:55

Background Science

Classification of Comets

How to name a comet

A new cometary designation system was adopted by the International Astronomical Union (IAU) in August 1994, and took effect at the beginning of 1995.

The principal designations for discoveries of comets, according to the Catalogue of Cometary Orbits, consist of the year and an upper-case letter to indicate the half-month of discovery in that year, so:

A = Jan 1-15

B = Jan 16-31

C = Feb 1-15, ..., Y = Dec 16-31

(I is omitted to avoid confusion)

followed by a numeral showing the order of announcement of discovery in that half-month.

The half-months are as used for discoveries of minor planets, the difference being that minor-planet designations have a second letter (as well as possible numerals) to indicate the order of discovery announcement.

'Discovery' refers to the time when the discovery observation is actually made (and in the case of a photographic observation, for example, it is usually the time of mid-exposure), even though the comet might not actually be recognized until long afterward. Unlike that for minor planets, the cometary system is precisely the same for comets discovered before and after 1925 (Universal Time is used in either case), with the usual astronomical convention of using the Julian calendar before October 1582 and the Gregorian calendar afterward.

The new cometary designations are mainly prefixed by:

  • P/ for 'short' period comet
  • C/ otherwise

More rarely:

  • D/ considered defunct - i.e. it would be ill-advised seriously to consider a prediction for a future return, because either it is known no longer to exist, or it has failed to show at several expected returns, or because its orbit is poorly known.
  • A/ would be used to denote an object given cometary designation but deemed to be a minor planet (no examples so far).
  • X/ is used for comets for which it is not possible to compute orbits - and that in some cases may never even have existed.

Why Comet 46 P/Wirtanen?

When the periodicity of a comet is well-established, either because of a recovery or an identification at a second passage through perihelion, this is shown by assigning a sequential periodic-comet number, which normally appears in front of the P/ (or D/) prefix.

There is also the possibility of assigning such a number to a comet, if and when it is observed through its first aphelion passage after discovery. An attempt has been made to assign these periodic comet numbers in a historically meaningful manner (e.g. 1 P/Halley, 2 P/Encke, etc), to allow continuity between past and future returns. Thus Comet Wirtanen was the 46th periodic comet to be assigned a number in this way.

Components of comets that are observed to split are indicated by -A, -B, etc after the P/ (or D/) number.

Why a new system?

The new system abandons the tradition, introduced by the Astronomische Nachrichten in 1846, of applying Roman numerals to comets in the order they were observed to pass perihelion in a particular year. In practice this proved troublesome. A subsidiary system, adopted by the Astronomische Nachrichten in 1870, which assigned lower-case letters to comets in order of announcement of discovery in a particular year, was applied too erratically, with different letters, or no letter at all, being assigned to the same comet. In reality, a substantial fraction of both Roman numeral and letter designations had come to be applied not to discoveries, but to recoveries of periodic comets on subsequent perihelion passages.

It was felt that a modern designation system should generally restrict acknowledgement of recoveries to those comets making their first predicted returns (when the error is expected to be greatest).

Finally, the distinction between a comet and a minor planet is often quite unclear, whether considered in terms of particular observations or of the general evolution of the solar system. It was therefore felt that any new cometary designation system should be more similar to that used for minor planets.

Taken from the tenth edition of the Catalogue of Cometary Orbits, by B.G. Marsden and G.V. Williams, available from the International Astronomical Union's Minor Planet Center:

Minor Planet Center
Smithsonian Astrophysical Observatory
60 Garden Street
Cambridge, MA 02138

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