Thursday, July 23, 2020

deciphering comet names

Got to thinking. Dangerous, I know. 

I remembered Chris saying, during our comet presentation, that C/ was used on comets that were new to us. If orbital data indicated a return trip, they would be designated with the P/ prefix. But seeing numbers in front of P and new designations for interstellar comets, I thought I'd do a deeper dive.

I read information at the amazing wikipedia on naming conventions and at the Minor Planet Center, which is the official source, part of the International Astronomy Union. Still, some things remained unclear and the MPC information was very technical. I found an old article by Phil Plait. He's usually brilliant at clarifying things but he did not address the prefix number on P comets. A piece at Harvard University was helpful.

I think this is the nomenclature rule or format:


  • # is the sequential observation or discovery number, for periodic comets
  • T is type 
  • YYYY year of discovery, often omitted for periodics
  • (the space is required between the year and period)
  • Z is the period within the year
  • 9 is discovery number within the Z period
  • DISCNAME name of discoverer(s), shown parenthetically
Further, the types are:
  • A: re-classified as an asteroid
  • C: non-periodic comet
  • D: broken up, lost, crashed
  • I: interstellar
  • P: periodic comet (with a relatively short period, say 100 years)
  • S: satellite to a parent comet
  • X: unreliable data
And for the period within the year and month, with each month broken into two parts:
  • A: first part of January
  • B: last half of January
  • C: first part of February
  • ...
  • I and Z are not used
Regarding the discoverer naming. If one person is associated with the comet, their surname may be noted. When there are multiple discoverers, up to two or three names may be shown, with hyphens. It is common for human names to be shown with an initialised letter. e.g. Churyumov-Gerasimenko. Robotic or automated systems are discovering comets in which case the system name is noted, usually with capitals. e.g. ATLAS or Pan-STARRS.

Make sense? Sheesh... When I looked at some examples, even when "the rules" was being closely followed, I found a bewildering array of names. Let's have a look, shall we?

C/2020 F3 (NEOWISE). In the news of late. Maybe you've heard of it? Nice and easy. I try to use the proper name when referring to it, sometimes shortened to C/2020 F3, or the full designation. Many casually call it "comet NEOWISE" which is acceptable, for now...

C/1995 O1 (Hale-Bopp). Many are comparing "NEOWISE" and "Hale-Bopp," with C/2020 F3 putting on a comparable show in the sky. Officially, "Hale-Bopp" is known as C/1995 O1. This is a comet on a non-return path, discovered in 1995, in the bottom half of July, by astronomers Alan Hale and Thomas Bopp.

D/1993 F2 (Shoemaker-Levy 9). A rather famous comet 27 years ago that made all the headlines was "Shoemaker-Levy." Discovered by Carolyn and Eugene Shoemaker and David Levy in 1993, this comet was captured by Jupiter's tremendous gravity well and ultimately crashed into the planet. Given it no longer exists, the D/ code is applied. This comet is no more! It has ceased to be! This is an ex-comet!

Now here's a good example of where we shouldn't casually refer to this as simply "Shoemaker-Levy" as the Shoemaker duo with Levy discovered many comets such as Shoemaker-Levy 1 (from 1990), Shoemaker-Levy 2 (also 1990), or say Shoemaker-Levy 3 (from 1991).

OK. So far, so good. 

Still with me?

But then I thought about other comets I've seen (or imaged) like 103P/Hartley 2 or 46P/Wirtanen. Wait. What? This convention seems completely different! Periodic comets are preceded by a number and this seems to be a simple sequential number. So 1 would be the first discovered periodic, 2 would be the secondary short-period returning comet, and so on. Begs the question, who keeps track? I suppose the IAU.

46P/Wirtanen. This appears to be slightly casual description but wikipedia shows a dozen alternate designations! I believe the official IAU tag is 46P/1948 A1. It seems to me that these comets are expressed often in a casual way without the discovery year and without parenthesis about the discoverer's name.

1P/1682 Q1 (Halley). Ah. The famous Halley's comet. 1P indicates it is the first known regularly returning comet. Initially discovered in the 17th century. Now what can be further confusing about periodics is that the quoted year number may appear different. For example, 1P/1682 may be shown as 1P/1986. That's because the famous 75-year naked-eye comet was last seen in '86.

1I/'Oumuamua. Previously C/2017 U1 (PANSTARRS). Discovered by Weryk using the Pan-STARRS system in Hawaii. Ugh. Certainly a most amazing object but it's bending the rules. The 1 indicates the first of its kind. Capital I is for interstellar as it has been determined from it's hyperbolic path that it ain't from 'round here. I found this moniker widely used but again I believe it is a contraction. It appears the official IAU label is: 1I/2017 U1 ('Oumuamua). OK, good. By the way, Weryk's name is not used; the IAU decided on the Hawaiian word for "scout." Also note the first character is not an apostrophe; it is an 'okina and is pronounced as a glottal stop or plosive. 

Note, the second configured interstellar interloper is 2I/Borisov.

Confused? I think the nomenclature I show above works. But it is certainly clunky. It's better than the past systems for sure but still, it ain't easy.

I just read the IAU page on the Naming of Astronomical Objects. Even their initial description does not seem to address numbered objects. But later, they said, "When a periodic comet is observed after its second apparition, the MPC gives it a permanent number indicating the order of the discovery." Ah. 

So, it strikes me that there's a fork, a split, at the high level. That is you have periodic comets and... others. As Chris noted, C or P types. But it seems the P comets have numbers. It was the Harvard information that emphasised this division but in a way that made sense given the numbering. I think the right way to think about it is we have numbered comets and unnumbered comets.

I made an infographic. I show the hierarchy, with subordinates, and examples. I also show "migration" arrows, for re-designations or reclassification.

comet hierarchies and re-designations

There are two main classes, unnumbered or numbered.

Under unnumbered, we have C/ class, the non-returning comet. This appears to be the initial designation applied to all new discoveries.

Also within unnumbered, we have the dead, deceased comets, the D/ class.

Under numbered, we have the P/ class with the preceding IAU assigned sequential number.

And finally we have the I/ class, interstellars.

The dotted arrows indicate how things might change. A non-return comet can disintegrate or fall into a gas giant. Bye bye! Could happen to a periodic too.

A comet initially discovered, after we get some good orbital data, or after perturbations, might turn into a periodic. (I supposed the opposite could happen too... now that I think about it.)

I think I've got it. Does this help? It works for me, demystifying all the variants in comet designations.


A subtext of all this is to remind people to be careful only using the discoverer's name. For example, don't say you shot an image of comet PANSTARRS last night. We won't know which comet you're talking about.

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