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SYFY WIRE Bad Astronomy

What’s In a Comet Name?

By Phil Plait
Comet Lovejoy with the Very Large Telescope (firing a laser into the sky to help it focus better on stars). Credit: ESO/ G. Brammer

I’ve been writing about comets a lot lately, as they’ve been making the news — most notably C/2012 S1 (ISON), as well as comet C/2013 R1 (Lovejoy).

I’ve been reading a lot about them, and saw a few people were chiding others who weren’t using the correct name for a comet. For example, I’ve been referring to C/2012 S1 (ISON) as just “ISON” in my posts, and technically, that’s incorrect, even if it saves time and makes things more human-brain-compatible.

But it got me thinking about comet names, so I thought I’d give a quick overview here.

First off, comets are hard to pin down as a category. They are icy objects that tend to have a lot of rock, gravel, and dust embedded in them. Some are really active when they get near the Sun, and some aren't. Some asteroids look an awful lot like comets, and vice versa. Some orbit the Sun on short orbits, some on very long orbits, and some drop down into the inner solar system, scream back out, and never return.

But we have to name them! So there’s a system.

Comet Halley, seen in 1910. Credit: NASA/JPL-Caltech

First comes a letter. P is used for a comet on a known, periodic orbit. C is for comets on paths that are open, that is, higher than the Sun’s escape velocity. Those usually only make a single pass before heading out into interstellar space. D is used (rarely) if a comet breaks up, disintegrates, or is lost after the initial discovery. Finally, X is used for comets where no orbit is known; usually ones seen long ago, before the mathematics of orbital mechanics was invented.

Next comes the year of discovery. Simple enough. Although for comets on periodic orbits where the discovery date is unknown (historic comets, usually), the last apparition (return of the comet) can be used for the date.

After that is a letter/number pair. The letter represents the part of the year the comet was discovered, divided up by half-months (or, more accurately, from the 1st to the 15th, then the 16th to the end of the month). A is the first half of January, B the second half, C the first half of February, and so on. I is not used (since it can be confused with the number 1), and this means Z never will either (think it through …). The number represents the order in which the comet was discovered in that period. So the fourth comet discovered in the first half of January would be A4.

Finally, in parentheses, there’s the last name of the discoverer (up to three can be listed, separated by hyphens). Many comets are now found in automated surveys, so the observatory or survey name is used there.

Got it?

Comet C/2012 S1 (ISON), or just ISON to its friends. Credit: Waldemar Skorupa, via

So C/2012 S1 (ISON) is a comet that made one pass before heading out forever, discovered in 2012 in the second half of September (on the 21st, to be exact), the first such comet found in that period, and was made at the Russian International Scientific Optical Network (or ISON) observatory.

Simple, right?

Well, not really. I think the half-month designation is unnecesssary nowadays; it’s clunky and (as far as I can see) not needed since all that info can be found easily in a database. Comets used to be numbered sequentially in order of discovery, but sometimes a comet might be discovered and not reported until later, messing up the sequence. Understandable, but that's still true of the current system, where the second comet found in the first week in January 2014 might not get reported until after the third. Perhaps the order of actual discovery isn't terribly important anymore, but just when it's reported. That would resolve that issue either way.

Comet P/2013 S3 (Plait). Click to get the joke. Drawing by Zach Weinersmith, modified by Phil Plait.

However, these are the rules, and there are reasons for them. I’ll note that some comets are exceptions, like 1P/1982 U1 Halley. It was the first periodic comet ever determined—Halley didn’t discover it; he figured out that it was the same comet reported historically every 75 or 76 years, so we put his name on it. Note that it has the year listed as 1982, when it was first observed coming back toward the Sun on its last pass.

So yeah, this “convention” is somewhat flexible, and could use some improvement in my opinion. Still, I can understand why officials might want scientists and journalists to stick with it. There is some sense there; after all, Pan-STARRS will find more comets, and there are (so far) four comets Lovejoy. It can get confusing.

However, it’s usually pretty obvious which one I mean when I write about them. To avoid confusion, I tend to use the full name the first time I mention the comet, then the discoverer/observatory name thereafter. So it’s C/2012 S1 (ISON) the first time, and then just ISON thereafter. And if there are two such comets in the sky at the same time, I’ll probably use the letter/number pair to distinguish them, silly as it is.

I do think naming things is important, and a convention is a good idea. But I also think it should be simple and easy to use. That’s not always possible; there are millions of faint galaxies seen in images, so astronomers use their coordinates to distinguish them. That’s fine, but if so the odds are good I’ll wind up shortening the name in an article. I’m trying to communicate to the public, not publish a scientific journal article. I’ve never been all that obedient at hoop-jumping, and I’m far more likely to go the other way when I feel like someone’s pedanting me.

After all, the important thing here is to convey the excitement and beauty of the phenomenon, and not necessarily the weird and generally confusing conventions behind naming them.

As Shakespeare himself said (though it was changed later in edits): A comet, by any other name, would look as cool.

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