How Asteroids are Named

Amateur astronomers around the world search through their telescopes night after night, hoping to spot a new astronomical object which has not already been charted. Dedicated surveys map tiny parts of the sky in excruciating detail. If an anomaly shows up in 2 successive nights of observation, it could be a new asteroid, and that means its discoverer could get to name it.

Provisional designation

When an asteroid is first discovered, it is immediately given a provisional designation which consists of 4 parts: the year in which it was discovered, the half-month of discovery, the sequence of discovery during that half-month, and the number of times the entire sequence of discovery cycle has already been used for the sequence of discovery during that half-month. This combination of year, letters, and number identifies the possible asteroid until its orbit has been confirmed by other astronomers.

The year is written using all 4 numbers. The half-month of discovery is assigned a letter of the alphabet in order of progression. The sequence of discovery also goes through the entire alphabet in order. The last number is usually written in subscript, but can be written flattened out.

For example, the near-Earth asteroid 2005 YU55, which passed nearly 200,000 miles from the Earth in November 2011, was first discovered in 2005 during the first half of December (Y). So many other asteroids were discovered before it in that same half-month that the cycle of 25 letters had already been completed 55 times. On its 56th use, 2005 YU55 was the 19th asteroid (U) to be discovered.

The letter “I” is not used at all to avoid similarity with the number “1”. The letter “Z” is not needed for identifying half-months, because each year only has 24 half-months. However, the letter “Z” is used for the sequence of discovery, so that sequence has 25 letters.

Half-months are always measured up to the 15th day of each month. This is the case for all months, regardless of how many days they have.

The discovery of a new asteroid sometimes leads to the discovery of an entire group of asteroids which follow the same orbital behavior. In one case, the provisional name assigned to the original asteroid (1992 QB1) became the basis for naming all the objects in that category “cubewanos.”

Survey designations

Asteroids which have been discovered during one of the 4 surveys of the 1960s and 1970s also have a special survey designation. Each survey has been assigned its own identifier: Palomar-Leiden (P-L), First Trojan Survey (T-1), Second Trojan Survey (T-2), and Third Trojan Survey (T-3). The survey designation combines the order of discovery within that survey with the survey identifier.

Permanent name

By long-standing tradition, the discoverer of a new asteroid gets to name it. The new name consists of the sequential number in which the asteroid’s orbit has been confirmed, followed by a name of the discoverer’s choosing. The sequential number is often dropped in casual use when it is clear which asteroid is being referred to.

The provisional designation continues to be used until the new name is registered with the Minor Planet Center. Even after a sequential confirmation number has been assigned, the provisional designation is used until the asteroid is also named.

The discoverer is slightly restricted in the types of names he may choose. Asteroids which belong to a particular group of asteroids are traditionally named with related mythological names, although this is no longer common practice. Asteroids can be named after people, cities, or even fictional characters, but they may no longer be named after pets.

For example, most of the Apollo asteroids, which cross Earth’s orbit on a regular basis, do not have permanent names. These are still referred to by their provisional designation. An example of these is 2005 YU55. However, by convention, most of the Apollo asteroids which have been named have mythological names related to the Greco-Roman sun god Apollo.

Unlike the usual practice with comets, the discoverer may also not name the asteroid after himself. This may be because most comets don’t last very long after being discovered, but the asteroid will probably keep orbiting for centuries to come.

Unofficial names

Many organizations will let you name an asteroid and give you an official-looking certificate. These organizations are not affiliated with the Minor Planet Center, which is the only organization which can assign an official name to an asteroid. The certificate may look pretty and be a fun gift, but it has no official status.