How Are Asteroids and Comets Designated?

Do you know how minor planets are identified and named after they are discovered? Let’s learn about the designation of asteroids and comets in this article.

Before crafting the object’s designation, we first have to consider whether it has a prefix or not. There is no prefix for ordinary asteroids, and the object’s bare representation is displayed. But if the telescopes see cometary activity or if it’s traveling at very high speeds, it will be assigned a prefix.

For comets, they will first bear a prefix of C. This means the designation will start with “C/”, following the rest of it. An example is the long-period comet C/2014 UN271, one of the largest comets ever observed. If there is enough data for a reliable orbit determination and it’s a short-period comet (i.e., period < 200 years), the prefix changes to P, such that the designation starts with “P/”. An example is 1P/Halley, which is the first periodic comet ever identified. However, if it gets lost or disintegrates, which sometimes does occur, the prefix changes to D (“D/”), like 3D/Biela, which disintegrated in the 1850s.

For interstellar objects, the designation prefix is I. This happened to 1I/’Oumuamua, for example, which is well-known for being the first object spotted flying past the Solar System from interstellar space. On the other hand, if no cometary activity was detected for an object initially identified as a comet, even at perihelion, its prefix changes to A, which is the first letter of the word “asteroid”. This happened to a few objects, like A/2021 X2, classified as a hyperbolic asteroid by the JPL small-body database.

Now, after talking about prefixes, we move on to the provisional designation. This is the name given to an object after it has been spotted moving in the sky. You may think it is best to number the asteroid directly, but that is not the case. First, if we see a dot moving on telescope images, we often see something already known. Therefore, before even the provisional designation is given, the astrometry is compared to that of known asteroids, and it’s submitted for further analysis only after there’s no match. Secondly, even after the asteroid’s existence is confirmed, it could be too small for follow-up observations and disappear from view, having large uncertainties in the nominal trajectory. Therefore, they could be lost, and it’s not good to see a numbered asteroid disappear from view altogether.

The designation of the object is split into two parts, separated by whitespace. Let’s take 2017 YE5 as an example. The first part, “2017”, is the year of discovery, showing that the object was found in 2017. The second part, “YE5”, is more complicated, so we’re explaining it character by character. The first character indicates the half-month in which the object was located. Here’s a table showing the dates corresponding to each letter:

Letter	A	B	C	D	E	F	G	H
Dates	Jan 1-15	Jan 16-30	Feb 1-15	Feb 16-28/29	Mar 1-15	Mar 16-30	Apr 1-15	Apr 16-30

Letter	J	K	L	M	N	O	P	Q
Dates	May 1-15	May 16-31	Jun 1-15	Jun 16-30	Jul 1-15	Jul 16-31	Aug 1-15	Aug 16-31

Letter	R	S	T	U	V	W	X	Y
Dates	Sep 1-15	Sep 16-30	Oct 1-15	Oct 16-31	Nov 1-15	Nov 16-30	Dec 1-15	Dec 16-31

Note that I and Z are omitted from the table.

As you can see from the above list, 2017 YE5 was found in the second half of December. But what does “E5” mean? The discovery in the half-month is sorted by alphabet. The first discovery is “A”, the second is “B”, and so on. Note that “I” is omitted, so the 25th discovery is “Z”. Then, it loops back to “A” again, this time with a “1” after it so that it becomes “A1”. And then, it completes a cycle again, and the suffix number increases by 1, and so on. For example “E5” is the 25 * 4 + 5 = 105th designation of that half-month. So 2017 YE5 is the 105th object found in the second half of December 2017.

Alternatively, referring to the above table, 2014 UN271 was found in the second half of October 2014. The last letters of the designation, “N271”, refer to the 25 * 271 + 13 = 6788th object in the list. Again, “I” is not included in any part of the designation because it resembles the number 1. Therefore, even though “N” is the 14th letter in the alphabet, it is the 13th letter in the list of asteroid designations.

The provisional designations of comets are a lot simpler. The first parts of the designations of asteroids and comets are the same. For example, like 2014 UN271, 2014 U1 belongs to an object found in the 2nd half of October, and 2013 A1 is an object discovered in the 1st half of January 2013. And you may have already noticed the pattern on the second part of the designation, which is essentially just a number reflecting the number of comets discovered before it. For instance, 2013 A1 is the first comet found in that half-month period, while 2014 U5 is the 5th in the specified period.

Note that, however, comets have prefixes in their designations, complicating things a little bit. However, we have already mentioned it in the first section of this article. To recap, C/2013 A1 has a prefix of “C” because its period is much longer than 200 years. On the other hand, P/2010 E4 has a prefix of “P” because it is a short-period comet, having an orbital period of fewer than eight years and, obviously, much less than 200 years.

Once an accurate orbit is determined, the object can be assigned a number. As of December 31, 2022, there are 619999 asteroids and 452 comets on the numbered list. They are usually assigned a number based on the dates of orbital determination. This can result in asteroids with earlier discoveries but with higher designations. For example, 1861 Komensky was discovered in 1970, 38 years after the next numbered asteroid, 1862 Apollo. An exception is 134340 Pluto, a dwarf planet with a very high minor planet number for its 1930 discovery. While its orbit was known for a very long time before it was numbered, that exception was caused by its classification as a planet until 2006, where it was reclassified as a dwarf planet.

Comets use a different numbering system, however, separate from those of asteroids. This system is also based on the date of orbit determination. For example, comet 288P/2006 VW139 was discovered in 2006, 187 years after the original discovery of 289P/Blanpain in 1819. As you would notice, the number of numbered comets comes before the prefix. For example, Halley’s Comet is 1P, not P1. Occasionally, comets get lost and unobservable after calculating the orbit, changing their prefixes to D. That has happened to a few comets, like 3D/Biela and 5D/Brorsen. Sometimes, they are also observed to fragment into multiple pieces. The fragments will be sorted by alphabet, where the letter is placed after the primary designation with a hyphen separating the two parts. For example, the first fragment of comet 73P is 73P-A, the second one is 73P-B, and so on.

Finally, after the asteroids get numbers, some also get names. These names are very diverse, with asteroids getting names from different sources, but they all undergo a process in which an IAU committee decides whether it should take that official name. After that, the name replaces the provisional designation in the usual designation of asteroids. For example, after asteroid 101955 was named Bennu, it is usually referred to as 101955 Bennu instead of 101955 (1999 RQ36).

On the other hand, comets use a much simpler naming system. They are usually named after the discoverer of the comet. The name of the discoverer is included when a human or a team was the first to notice the comet, while the name of the sky survey or observatory is used if it’s autonomously discovered in telescopic images.

In this article, we have introduced how asteroids and comets are designated and named. Remember the principles where we identify these small bodies so that you fully understand what you’re writing when you are referring to an asteroid from a provisional designation or a number. If you would like us to include more in this article, please comment with your opinions below.