Comet Names

Comet P/Halley as taken March 8, 1986 by W. Liller, Easter Island, part of the International Halley Watch (IHW) Large Scale Phenomena Network.  Credit: NSSDC's Photo Gallery (NASA)

Comet P/Halley as taken March 8, 1986 by W. Liller, Easter Island, part of the International Halley Watch (IHW) Large Scale Phenomena Network. Credit: NSSDC’s Photo Gallery (NASA)

The way comets are named tells us a little about them, so let’s take a look.

First, the way comets are named was changed in 1994 at the International Astronomical Union at its General Assembly in The Hague. The following methodology started in 1995.

For clarity, I’m only going to present an overview and if you want more visit the IAU Cometary Designation System page.

Looking at these three comet names:

C/2012 S1 (ISON)
P/2013 O2 (PANSTARRS)
C/2013 O3 (MCNAUGHT)

The very first thing that stands out is they start with either a P or a C. If the letter is a P the comet is called a periodic and has an orbit of less than 200 years or has had more than one observed perihelion passage (closest approach to the Sun). Conversly a C is a non-periodic comet with a 200-year or more orbit.

It is possible to see a few other designations:
A means the body was actually an asteroid or minor planet and incorrectly classified as a comet.
X means an orbit could not be reliably determined for the comet.
D means a dead, broken up or lost comet.

A notable comet with the D designation would be: D/1993 F2 the famous comet which impacted Jupiter in 1994.

You will sometimes also see a number before the prefix. The number is a sequential number assigned and maintained by the IAU Minor Planet Center to comets that have been followed through aphelion or otherwise positivily identified for more than one orbit. The very first comet to be followed is very famous: 1P/1682 Q1 (Halleys), we are in the mulitple hundreds now.

Then the year of discovery, easy enough, in the list above ISON was discovered in 2012, and Halleys in 1682.

The next part is a way to tell which part of what month the comet was discovered AND the nth comet discovered in that period. In the examples above:

C/2012 S1 (ISON) has S1, the S meaning it was discovered in the second half of September (see table below) and it was the first comet disovered in that period.

The next two: P/2013 O2 (PANSTARRS) and C/2013 O3 (MCNAUGHT) were the second and third comets discovered in the second half of July.

Here’s the table (courtesy of the IAU):

Letter Dates Letter Dates
A – Jan. 1-15       B – Jan. 16-31
C - Feb. 1-15      D – Feb. 16-29
E – Mar. 1-15       F - Mar. 16-31
G – Apr. 1-15       H -  Apr. 16-30
J – May 1-15        K – May 16-31
L – June 1-15      M – June 16-30
N – July 1-15       O – July 16-31
P – Aug. 1-15      Q – Aug. 16-31
R – Sept.1-15      S – Sept.16-30
T – Oct. 1-15       U – Oct. 16-31
V – Nov. 1-15       W – Nov. 16-30
X – Dec. 1-15      Y – Dec. 16-31

I is omitted and Z is unused

Finally the person(s) or organizations/scientic efforts ex: PANSTARRS, LINEAR etc discovering the comet.

Again this is just an overview and not meant to capture all the nuances, you can find out everything else you might want to know at the link included at the beginning of the post and be sure to visit our Comet page.

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