LEARN – How Do Asteroids Get Provisional Designation?

LEARN – How Do Asteroids Get Provisional Designation?

LEARN – How Do Asteroids Get Provisional Designation? https://asteroidday-uploads.s3.eu-central-1.amazonaws.com/wp-content/uploads/2022/12/01165305/asteroids-designation.jpg 480 270 Asteroid Day Asteroid Day https://asteroidday-uploads.s3.eu-central-1.amazonaws.com/wp-content/uploads/2022/12/01165305/asteroids-designation.jpg

Learning Objective:
Learn about the history of provisional asteroid designation and how the currently adopted system works.

With the growing number of asteroids discovered in the 19th century, astronomers quickly ran out of symbols for primary asteroid designations. They had to come up with a more elaborated system of numbers and letters that indicate when an asteroid was discovered and its discovery order in a given period. In this blog post, you will find out how the provisional asteroid designation system evolved.


Use of symbols

In the first half of the 19th century, asteroids were only referred to by name (see how in this lesson). At that time, astronomers did not imagine that there would be a large number of these objects in the solar system. Initially, astronomers decided to assign symbols to asteroids just like to planets. It was because the first asteroids discovered had been considered planets and later minor planets before they actually were considered asteroids.

The symbols used for designation were inspired by mythological characters. For example, Ceres was designated with the symbol of a stylised sickle, related to the Roman goddess of agriculture. Pallas, an asteroid discovered soon after, got the symbol of a stylised spear because its name was derived from the Roman goddess of battles.

Designation table of celestial bodies from the “Berliner Astronomisches Jahrbuch” (Berlin astronomical yearbook, in German) with symbols of classical planets and first discovered minor planets, which would later be called asteroids. (Johann Franz Encke – 1850)

After a few decades of hiatus with new asteroids discovered, additional new symbols were introduced to represent them. However, these symbols were difficult to draw and astronomers couldn’t come to a consensus regarding their use. This pushed scientists to come up with simpler and more practical alternatives.

List of asteroids taken from the 1864 edition of Doctor Webster’s complete dictionary with their respective symbols.

Use of numbers

In 1851, astronomer Johann Franz Encke proposed a system of circled numbers for asteroids. It was quickly adopted by the astronomical community. At this time with no modern technology yet in place, the information about asteroids and other celestial bodies was published in almanacks and magazines.

In the first publication editions after the adoption of this new system, the first four asteroids discovered (Ceres, Pallas, Juno and Vesta) were still designated only by their symbols, while Astraea, the fifth asteroid discovered in history, was assigned the number 1.

Shortly thereafter, astronomer Benjamin Apthorp Gould improved this system by assigning the number 1 to Ceres and keeping the numerical sequence within circles in later publications of the almanacks. Later, the circles were replaced by a pair of parentheses – (1) Ceres, (2) Pallas… – until they were completely omitted. Currently, only a number is placed next to the official name of an asteroid. For example 6 Hebe, 9 Metis, 14 Irene, etc.

Table with the new system using numbers for asteroids published by Benjamin Gould in 1852 in the Astronomical Journal.

In the first decades of the 20th century, publications using the old system of symbols for asteroids still circulated, however, the use of numbers together with names grew very popular. Finally, International Astronomical Union made this designation system official.

Designation with numbers and letters

Before a discovered asteroid gets its definitive number, it is necessary to employ a provisional designation for the time of its orbital data confirmation. The provisional designation currently used includes the year of discovery, letters, and numbers as the main reference.

This system first appeared in 1892 in the German astronomical journal Astronomische Nachrichten, still published today. The designation consists of the year of discovery followed by a letter indicating the discovery sequence. These letters belong to the Latin or Roman alphabet, but the letter “I” is omitted to avoid confusion due to the similarity with “1”.

3D model of the asteroid Badenia based on light curve observations. Discovered by Max Wolf in 1892, it was the first asteroid to receive a provisional designation (DAMIT – Database of Asteroid Models from Inversion Techniques)

In August 1892, German astronomer Max Wolf discovered the asteroid and named it Badenia, in honour of Baden, the Grand Duchy of the German Empire that existed until 1918. This was the first asteroid to receive a provisional designation in history: 1892 A.

With the next designation, the first coincidence took place. Wolf discovered another asteroid in September that was given the provisional designation 1892 B. Orbital calculations for this object led to the conclusion that this asteroid had been previously discovered by another astronomer, making that specific designation out of use.

The number of asteroids discovered was growing rapidly and in the following year, the new system already had to be revised, with the inclusion of double letters (AA, AB, AC…) regardless of the year of discovery (1894 AQ was a sequence of 1893 AP, for example ). In 1916, all possible combinations of double letters (keeping the exception of the letter I) were used, and the sequence had to be restarted. The asteroid Stateira, discovered in September of that year, was given the provisional designation 1916 AA.

Undoubtedly, this naming system was useful, but it had some limitations, which caused a lot of confusion. Finally, in 1924, astronomer Ernest Clare Bower from the Washington Naval Observatory suggested an adaptation that allowed a chronological ordering of the discovered asteroids. His idea was officially accepted the following year and is used to this day.

How and when an asteroid receives its provisional designation?

The first detection of an asteroid is made through a sequence of images of the same sky area that astronomers take within a short time interval. This makes it easier to identify the asteroid because it will move in the image sequence when compared to the stars and galaxies that appear in the background.

After a verification to filter out known objects, a report is sent to the Minor Planet Center (MPC), an entity linked to the International Astronomical Union responsible for information on asteroids and comets. An observatory or astronomer has its designation for monitoring purposes. This designation can be up to 7 characters, defined by the observer.

The MPC receives the information, sets the orbital parameters of the new object, confirms them, and then announces the object in the circulars for other observers to track. It is at this stage that the asteroid receives its provisional designation improved by Bower and officially adopted almost 100 years ago.

The first part of this designation is the year of discovery, always with four digits. Then the first capital letter indicates the half-month of the year in which the discovery was made. The letter A corresponds to the first half-month of the year, which runs from January 1 to 15. The letter B indicates the half-month that corresponds to the period from January 16 to 31 and so on. This way, we can divide the year into 24 half-months. The letter I continues to be omitted to avoid confusion with the number 1 and the letter Z is of no use. Therefore, the letters corresponding to the half-months are distributed as follows:

Then another capital letter is added to the designation. This time, the letter indicates the order of discovery in that half-month. The letters “A” to “Z” are used, also excluding the letter “I”, which provides 25 options for discoveries in that period.

Let’s use the example of the asteroid Didymos, a small moon targeted by the DART mission. Didymos was given the provisional designation 1996 GT, meaning that the asteroid was discovered in 1996, in the seventh half-month of that year (between April 1st and 15th) and was the 19th object discovered in that period.

In recent decades, the increase in the number of astronomers and observatories dedicated to asteroids together with the use of new technologies has caused a boom in discoveries, making the 25-letter limit for each half-month of the year insufficient.

To work around this problem, a number is placed after the second letter, indicating the cycle of letters used. So if a 26th asteroid is discovered in that period, the designation A1 is used. For the 27th asteroid it will be B1 and so on until Z1. This allows to designate up to 50 asteroids, but if it is still insufficient, a new cycle (A2 to Z2) starts and so on.

For example, when the asteroid Ryugu was discovered, it was given the provisional designation 1999 JU3. It means that Ryugu was discovered in 1999, in the first half-month of May, and it was the 95th asteroid discovered in that period.

Asteroid Bennu was also discovered in 1999, and given the provisional designation 1999 RQ36. We can understand that this asteroid was discovered in the first half-month of September and was the 916th object discovered in that period.

When discovered, the asteroids Didymos, Ryugu and Bennu were designated as above before being given their definitive numbers and names (below) (NASA-DART/JAXA-Hayabusa2 / NASA-OSIRIS-REx)

Learn more about this subject by visiting these websites:
Asteroid Numbers and Names (ESA)
How Are Asteroids Discovered? – Scott Manley
Minor Planet Center Circulars – Latest asteroid discovered