How Asteroids and Comets Get Their Names and Designations

Three layers: designation, number and name

A newly found object that cannot be matched to a known one gets a provisional designation from the MPC once enough observations exist to compute a reliable orbit. The path from first sighting to a recorded discovery is its own story. The code that results looks like 2004 MN4. Later, when the orbit is well established, the object gets a permanent number. Last, and only sometimes, it gets a name.

The IAU requires two identifiers for each object. For most numbered objects those are the number and the principal provisional designation, so a name is a third. According to the IAU's naming rules, a name is not required for identification at all. Objects discovered before 1892 have no principal provisional designation, so for them the name serves as the second identifier.

One object can end up with more than one provisional designation if it is found twice before anyone connects the two sightings. The person who makes the link picks which code becomes the principal one, and the MPC says that choice does not change who the discoverer is.

Reading a provisional designation

A provisional designation has a four-digit year, a letter for the half-month, a second letter for the order within that half-month, and an optional number that counts how many times the second letter has cycled. In the examples below, the year and half-month letter match the date of the discovery observation.

The year is split into 24 half-months, and each gets a letter. The letters skip the one between H and J, and Z is not used for half-months.

The second letter runs from A for the first object to Z for the 25th, again skipping the letter between H and J. When the 25 letters are used up, the letters start over and a number is added. The MPC's examples are 1995 SA for the first object of the second half of September 1995, 1995 SZ for the 25th, and 1995 SA1 for the 26th.

Apply that to 2004 MN4. The M means June 16 to 30. N is the 13th letter in the cycle. The 4 means four full cycles of 25 have passed. That makes 4 times 25 plus 13, so Apophis was the 113th object designated in that half-month. JPL's database lists its discovery date as June 19, 2004, at Kitt Peak, which fits the M.

Another case is 2001 CC21. The C means February 1 to 15, and the second C is the third letter. With 21 full cycles, that works out to the 528th object of that half-month. JAXA says this asteroid was discovered on February 3, 2001, which fits.

From a number to a name

Numbering is automatic once the orbit is good enough. The MPC says an object usually receives a permanent number after observations at four or more oppositions, while unusual objects such as near-Earth asteroids may be numbered after three or even two. An opposition is the time of year when an object sits opposite the Sun in our sky and is best placed for observing, so collecting four of them takes years.

Only after numbering can a name be proposed. The IAU's Working Group Small Body Nomenclature (WGSBN) runs the process. Discoverers, whether individuals or survey teams as the MPC recognizes them, have the privilege of proposing a name for ten years after the object is numbered. After that the WGSBN may assign a name itself. A name becomes official when it is published in the WGSBN Bulletin, and a name is accepted only after enough members of the working group vote for it.

The rules are specific. A name can be at most 16 characters, must use the modern Latin script, and must not contain numbers. Commercial names are not allowed. Names of political or military figures or events are not allowed until 100 years after the death or the event. A citation of up to 360 characters explains the choice. Discoverers cannot propose a name that contains their own surname.

Some groups need themed names. Near-Earth asteroids with a perihelion distance under 1.15 AU must have mythological names, except those tied to creation or the underworld. An AU, or astronomical unit, is the average Earth-Sun distance, about 150 million km (93 million miles). Jupiter Trojans take names from the Trojan War or from Olympic and Paralympic athletes, and Plutinos take mythological names tied to the underworld. Dwarf planets and their moons are named jointly by the WGSBN and a second IAU group that handles planetary features. The long dispute behind one of them is told in the story of Eris and the planet debate.

JAXA's asteroid 2001 CC21 shows the process in full. Hayabusa2 was headed there for a flyby, so JAXA ran a public naming campaign from December 6, 2023 to May 9, 2024 and received 3,082 applications. A committee of nine children, together with members of the mission team, chose Torifune, short for Ame-no-torifune, a god's ship in Japanese mythology. The LINEAR team, which discovered the asteroid, proposed the name to the IAU, and the WGSBN announced it in its Bulletin on September 23, 2024. The mythological theme was no accident, because the IAU requires it for near-Earth asteroids. Hayabusa2's flyby is part of the timeline of missions to small bodies.

Naming comets

Comet codes start with a prefix. According to the MPC, P/ marks a periodic comet, defined as one with a period under 200 years or with confirmed observations at more than one perihelion passage. C/ marks a comet that is not periodic in that sense. D/ marks a periodic comet that no longer exists or is thought to have disappeared. X/ marks a comet with no meaningful orbit, and A/ goes in front of a code that turned out to belong to an asteroid.

The rest of the code follows the same plan as asteroids: the year, a letter for the half-month and a number for the order of announcement. The third comet announced in the second half of February 1995 would be 1995 D3. Comets that return get a number in front, as in 1P/1682 Q1 for Halley. A comet that splits gets letters added, such as -A and -B. The return and history of Halley's Comet show how a numbered comet is tracked across centuries.

Names are simpler. Comets are named for their individual discoverers when possible, using last names, and a team or survey name is allowed. The IAU guidelines prefer no more than two names per comet and only one name per observing program. Examples include C/1999 S4 (LINEAR) and C/1997 B3 (SOHO). Comets found in publicly posted data usually get no personal names. A very bright comet seen by many people at once may get a generic name such as Great Comet. Names are announced only after a reasonable orbit exists, to avoid renaming comets that were lost.

Common mistakes and myths

"You can buy or register a name." The MPC says minor planets cannot be bought, and its advice to someone who wants one named is to go out and discover one. A name sold by a company does not go through that process.

"The MPC names asteroids." The MPC issues designations and numbers. The WGSBN approves names.

"The first person to see an asteroid names it." Only the recognized discoverer can propose a name, and only after numbering.

"A designation tells you the size or type." It tells you only when and in what order the object was reported.

"Every comet is named for a person." Many carry survey or spacecraft names, and some very bright ones get generic names.

"The year in the code is the year of the announcement." It follows the discovery observation, which can be earlier than the announcement. The guide to the JPL database shows where to find an object's discovery details.

Frequently asked questions

Can an object have a name but no number?

No. Names go only to numbered objects, because the number comes first and the name is optional.

Why do some names look odd or mythological?

Special groups follow themes. Near-Earth asteroids, Plutinos and objects beyond Neptune use mythological names, and Jupiter Trojans use Trojan War characters or Olympians.

Can the discoverer name an asteroid after themselves?

Not with their own surname. The rules bar discoverers from proposing a name that contains their own surname, though close relatives' names are allowed under conditions.

What happens if nobody proposes a name?

The object keeps its number and provisional designation. Ten years after numbering, the WGSBN may assign a name itself.