Official labeling scheme for newly discovered minor planets

A newly discovered minor planet is named by a label (designation) that indicates the date of  its discoveries in relation to recently (during the same semi-month period) made discoveries. The standard designation consists of a sequence of three parts, describing and ordering the date of the first observation of a new object [1,2]:

1. four-digit number indicating the year,
2. an uppercase letter identifying the half-month (also called semi-month),
3. an uppercase letter identifying the numerical sequence of discovery, followed by one or more digits (representing a counter) if more than 25 previous discoveries were made within the same half-month.

The half-month designations are listed in the right-side table below. For a given month, the first letter refers to days 1 to 15 and the second letter to any of the remaining days of that month. Letters I and Z are not used.

Within any half-month, a discovery is ordered by the 25 letters of the alphabet, excluding letter I. From the 26th discovery onward, the letters are recycled and a numeral, counting recycling repetition, is appended—when possible as subscript [1]. For example, D₂ represents 54th; 50 + 4, since 2 repetitions formally consume 2⋅25 = 50 letters and D is the 4th letter of the alphabet.

The four digits for the year are separated from the following parts by small white space.

Let's look at some complete designations:

• 1992 QB₁ is the 27th (1⋅25 + 2) object discovered in the second half of August in the year 1992: this was the first trans-Neptunian object (TNO) to be discovered after Pluto and Charon [3]; 
• 2003 UB₃₁₃ is the 7,827th (313⋅25 + 2) object discovered in the second half of October in 2003: this object is now called Eris, a TNO and a Kuiper belt object (KBO) in particular;
• 2005 YU₅₅ is the 1,395th (55⋅25 + 20) object discovered in the second half of December in 2005: an asteroid as big as an aircraft carrier recently zipping by Earth [4], narrowly avoiding collision.

Such designations are systematic, but often provisional: as the example of Eris shows, objects may later be named by figures or spirits from mythology as shown for TNOs named after creation deities.

Designations of minor bodies and natural satellites in the solar system are managed by the Minor Planet Center (MPC) in conjunction with the Central Bureau for Astronomical Telegrams (CBAT). The MPC is also responsible for the efficient collection, computation, checking and dissemination of astrometric observations and orbits for minor planets and comets by issuing various types of circulars [5].

Keywords: astronomy, planetary science, asteroids, dwarf planets, trojans, centaurs, nomenclature.

References with further details
[1] IAU Minor Planet Center > New- and Old-Style Minor Planet Designation: www.minorplanetcenter.net/iau/info/OldDesDoc.html.
[2] Neil deGrasse Tyson: The Pluto Files. W. W. Norton & Company, New York and London, 2009; fottnote on page 53.
[3] NASA Science News > What Lurks In The Outer Solar System?: science.nasa.gov/science-news/science-at-nasa/2001/ast13sep_1/.
[4] Alice Chang: Big asteroid passes Earth. Reno Gazette-Journal November 9, 2011, page 5C.
[5] IAU Minor Planet Center: www.minorplanetcenter.net/iau/mpc.html.