Provisional Designation in Astronomy - Minor Planets

Minor Planets

The current system of provisional designation of minor planets (asteroids, centaurs and trans-Neptunian objects) has been in place since 1925, and superseded several previous conventions, each of which was rendered obsolete by the increasing numbers of minor planet discoveries.

The first element in a minor planet's provisional designation is the year of discovery, followed by two letters and, optionally, a number.

The first letter indicates the half-month of the object's discovery within that year —"A" denotes discovery in the first half of January, "D" is for the second half of February, "J" is for the first half of May ("I" is not used), and so on until "Y" for the second half of December. The first half is always the 1st through to the 15th of the month, regardless of the numbers of days in the second "half".

The second letter and the number indicate the order or discovery within that half-month. The 8th minor planet discovered in the second half of March 1950, for example, would be provisionally designated 1950 FH. But since modern techniques typically yield far more than 25 objects (again, "I" is not used) in a half-month, a subscript number is appended to indicate the number of times that the letters have cycled through. Thus, the 28th minor planet discovered in the second half of March 1950 would be 1950 FC₁. For technical reasons, such as ASCII limitations, the subscript is sometimes "flattened out", so that this could be written 1950 FC1. The subscripts were first used with 1926 GA₁.

An idiosyncrasy of this system is that the second letter is listed before the number, even though the second letter is considered "least-significant". This is in contrast to most of the world's numbering systems. This idiosyncrasy is not seen, however, in online and electronic documents, where the number is frequently written before the final letter in so-called "packed form". For example, in electronic communications regarding minor planets 1926 GA₁ would usually be written 1926G01A.