The Language of the Cosmos:
Understanding Comet Nomenclature and Classification
REVIEW
© NASA — Comet Hale Bopp seen from Space Shuttle Columbia on STS-83.
Among the most spectacular celestial phenomena visible from Earth, comets have captivated humanity for millennia. These icy visitors from the outer reaches of our Solar System have evolved from objects of superstition and fear into subjects of rigorous scientific study. As our understanding of these cosmic wanderers has grown, so too has the complexity of naming and classifying them. The nomenclature system that astronomers employ today represents centuries of refinement, reflecting not only advances in observational technology but also our expanding knowledge of orbital mechanics and Solar System dynamics.
The story of comet nomenclature is one of continuous evolution, driven by the exponential increase in comet discoveries and the need for a systematic approach that serves both professional astronomers and the wider scientific community. From the simple year-based designations of antiquity to the sophisticated alphanumeric codes employed by the International Astronomical Union (IAU) today, the way we name comets reveals much about how science adapts to accommodate new knowledge whilst preserving connections to history and tradition.
Historical Naming Practices
The Early Years: Year-Based Designations
Before the development of modern astronomical practices, comets were identified in the simplest possible manner: by the year of their appearance. References to "the Comet of 1702" or "the Great Comet of 1680" were commonplace throughout the 17th and 18th centuries (IAU, n.d.). Particularly bright comets that captured public attention were distinguished by the adjective "great," leading to designations such as "the Great Comet of 1882." When multiple spectacular comets appeared in a single year, astronomers would add the month to avoid confusion, resulting in names like "the Great January Comet of 1910" (Wikipedia, 2025).
This simple system worked adequately when comets were relatively rare observations and when the scientific community had limited means to track and study them systematically. However, it lacked the precision required for more detailed astronomical work and offered no insight into the nature of the comet's orbit or whether it might return.
The Halley Revolution
The landscape of comet nomenclature changed dramatically in 1758 when a prediction made by English astronomer Edmond Halley came true. In 1705, Halley had published a catalogue of orbits for 24 comets and noted that the comets observed in 1531, 1607, and 1682 possessed remarkably similar orbital characteristics, appearing at approximately 76-year intervals (Britannica, 1999). He boldly suggested these were not three different comets but rather a single periodic comet, and predicted its return in 1758.
Although Halley did not live to see his prediction verified—he died in 1742—the comet was recovered on Christmas Day 1758 and passed perihelion on 13th March 1759, the first successful prediction of a comet's return (Britannica, 1999). The comet was subsequently named in Halley's honour by French astronomer Nicolas-Louis de Lacaille, marking the first instance of a comet being named after a person associated with its study (Observatory of Paris, n.d.). This established a precedent that would gradually become standard practice.
The 19th Century: A Transitional Period
Throughout the 19th century, comet-naming practices remained somewhat inconsistent. Following the example of Halley's Comet, names were generally used for short-period comets only after their second apparition, whilst single-apparition short-period comets and long-period comets continued to be referred to primarily by designations, with names occasionally given parenthetically (IAU Committee on Small Body Nomenclature, 2003). This distinction reflected the scientific understanding that periodic comets—those that returned predictably—were of particular interest and worthy of commemoration.
During this period, astronomical publications typically placed the comet designation first, with the discoverer's name in parentheses if mentioned at all. This practice demonstrated that scientific precision took precedence over personal recognition, though it would gradually shift as the century progressed.
The convention of naming comets after their discoverers rather than those who calculated their orbits began to gain traction during this era. Comet Faye, discovered by Hervé Faye in 1843, is often cited as the first comet to be named after its discoverer rather than an orbit computer (Wikipedia, 2025). However, this convention did not become widespread until the early 20th century.
The 20th Century: Provisional and Permanent Designations
By the 20th century, improvements in technology and dedicated search programmes led to a massive increase in comet discoveries, necessitating a more formal system. The adopted approach involved two stages: a provisional designation and a permanent designation (Observatory of Paris, n.d.).
Under this system, when a comet was discovered, it received a provisional designation consisting of the year of discovery followed by a lowercase letter indicating the order of discovery within that year. For example, the ninth comet discovered in 1969 was designated "Comet 1969i" (Bennett). Once the comet had been observed through perihelion and its orbit established, it was assigned a permanent designation comprising the year of perihelion passage followed by a Roman numeral indicating the chronological order of perihelion passages for that year (IAU, n.d.). Thus, Comet West, discovered in 1975 and passing perihelion on 25th February 1976, was first designated "1975n" and later "1976 VI" (Observatory of Paris, n.d.).
Whilst this dual-designation system represented a significant improvement, it had notable shortcomings. Comets discovered after perihelion passage created inconsistencies, and the system struggled to accommodate short-period comets observable throughout their entire orbits, making their provisional designations seem arbitrary (Observatory of Paris, n.d.).
The Modern IAU System: Established 1995
Resolution and Rationale
Recognising the limitations of the existing system, the IAU adopted a comprehensive new designation scheme that came into effect on 1st January 1995 (WGSBN, 1994). This system was designed to closely resemble the minor planet designation scheme already in use, providing consistency across small body nomenclature. The new system addressed several key concerns:
The virtual one-to-one correspondence between provisional and definitive designations in the old system made it unnecessarily complex; interpolating old discoveries into existing designation systems proved problematic, particularly when orbit determinations were unavailable; applying new designations at each perihelion passage for periodic comets added unnecessary complication, especially for routinely recovered comets or those followed throughout their entire orbits; and confusion often arose regarding whether newly discovered objects were comets or minor planets (WGSBN, 1994).
The Working Group on Small Body Nomenclature (WGSBN), formerly the Committee on Small Body Nomenclature, now oversees these guidelines in collaboration with the Minor Planet Center (MPC), which handles all cometary designations (WGSBN, n.d.).
Structure of Modern Comet Designations
The current IAU nomenclature system consists of three principal components:
1. The Prefix
The prefix indicates the nature and orbital characteristics of the comet:
P/ denotes a periodic comet, defined as any comet with an orbital period of less than 200 years or confirmed observations at more than one perihelion passage (IAU, n.d.).
C/ indicates a non-periodic comet, meaning any comet not meeting the definition of periodic.
X/ designates a comet for which no reliable orbit could be calculated, typically historical comets with insufficient observational data.
D/ marks a periodic comet that has disappeared, broken up, or been lost. Examples include 3D/Biela and the famous D/1993 F2 (Shoemaker-Levy 9).
I/ indicates an interstellar object, a category added in 2017 following the discovery of 1I/ʻOumuamua (ESA, 2024).
A/ denotes an object initially mistakenly identified as a comet but actually a minor planet, or an object on a hyperbolic orbit showing no cometary activity (Wikipedia, 2025).
2. The Year and Discovery Code
Following the prefix, the designation includes the year of discovery followed by an uppercase letter identifying the half-month of observation. The letters run from A (first half of January) through Y (second half of December), excluding the letter I to avoid confusion (IAU, n.d.). This half-month system provides more granularity than a simple annual count whilst remaining manageable.
3. The Sequential Number
The final component is a number indicating the order of discovery announcement during that specific half-month period. For instance, P/2013 B3 would denote the third periodic comet announced during the second half of January 2013 (IAU, n.d.).
Periodic Comets and Permanent Numbers
After a periodic comet has been observed through at least two perihelion passages—confirming its periodic nature—it receives an additional permanent number prefix indicating the order of discovery or recognition as a periodic comet (Observatory of Paris, n.d.). Halley's Comet, being the first periodic comet identified, bears the designation 1P/1682 Q1, whilst Encke's Comet is 2P/Encke.
This numbering system provides an efficient way to uniquely identify each periodic comet, particularly important given that many discover programmes now find multiple comets, leading to numerous objects sharing the same discoverer name. The systematic designation thus becomes the primary identifier, with the name serving a more traditional or commemorative function (ICQ, n.d.).
Name Assignment
Separately from the systematic designation, comets routinely receive names, almost always honouring their discoverers. The comet-naming guidelines, adopted in March 2003 by the Committee on Small Body Nomenclature and updated to reflect current organisational structures, govern this process (IAU Committee on Small Body Nomenclature, 2003).
Key Naming Principles:
Single vs. Multiple Discoverers: A comet may be named after up to three independent discoverers or a single team. When multiple independent discoveries occur, names are listed chronologically and separated by hyphens (using en dashes in formal publications). For example, Comet Swift-Tuttle was found first by Lewis Swift and then independently by Horace Parnell Tuttle several days later (Wikipedia, 2025).
Team and Programme Names: In recent decades, many comets have been discovered by large-scale automated surveys conducted by professional teams. These comets are named after the survey programme or instrument, such as 160P/LINEAR (Lincoln Near-Earth Asteroid Research) or Comet NEOWISE, discovered by NASA's Near-Earth Object Wide-field Infrared Survey Explorer (ESA, 2024).
Surname Complications: For discoverers with hyphenated surnames, the hyphen is replaced with a space in the comet name to avoid confusion with multiple discoverers. For instance, 105P/Singer Brewster commemorates Stephen Singer-Brewster (Wikipedia, 2025).
Temporal Priority: Discovery is recognised based on information available to what was formerly the Central Bureau for Astronomical Telegrams (CBAT) at the time of first official announcement. Once confirmation has been widely issued, the comet is generally no longer available for additional recognised discovery claims (IAU Committee on Small Body Nomenclature, 2003).
Historical Exceptions: Whilst comets are typically named after discoverers, some historic comets bear the names of those who calculated their orbits. The two most famous examples are 1P/Halley and 2P/Encke, honouring Edmond Halley and Johann Encke respectively for their groundbreaking orbital calculations rather than their discovery of these objects (Wikipedia, 2025).
Elimination of Numerical Suffixes: Prior to 1995, when multiple comets shared the same discoverer name, they were distinguished by adding Arabic numerals (e.g., Shoemaker-Levy 1 through 9). This practice was discontinued with the 1995 reforms, as the robust alphanumeric designation system rendered such suffixes unnecessary (IAU Committee on Small Body Nomenclature, 2003). Today, the systematic designation alone provides unambiguous identification.
Classification by Orbital Characteristics
Beyond nomenclature, comets are classified according to their orbital properties, which provide insight into their origins and dynamical histories within the Solar System.
The 200-Year Dividing Line
The primary classification distinguishes between short-period and long-period comets, with an orbital period of 200 years serving as the threshold (Britannica, 1999). This somewhat arbitrary boundary reflects historical observational practice: comets with periods shorter than 200 years could conceivably be observed multiple times within human lifespans, enabling confirmation of their periodic nature.
Short-period comets (less than 200 years) receive the P/ prefix and are further subdivided into two dynamically distinct groups:
Jupiter-Family Comets (JFCs): These comets typically have orbital periods of less than 20 years, relatively low orbital inclinations (up to about 35°), and aphelia near Jupiter's orbit. They are thought to originate from the Kuiper Belt, a flattened disc of icy bodies beyond Neptune's orbit (Britannica, 1999). The defining characteristic is their Tisserand parameter (T_Jupiter) value between 2.0 and 3.0, a quantity that remains approximately constant for a given comet's orbit and helps identify returning periodic comets (Britannica, 1999).
Jupiter-family comets are often patchily observed, as orbital interactions with Jupiter frequently perturb their orbits, causing them to appear at unexpected positions and sometimes resulting in their loss (Wikipedia, 2025). They are also known as "ecliptic comets" due to their concentration near the ecliptic plane. Famous examples include 2P/Encke (orbital period 3.3 years) and 67P/Churyumov-Gerasimenko, target of the European Space Agency's Rosetta mission.
Halley-Type Comets (HTCs): Named after the archetypal member, these comets have orbital periods between 20 and 200 years. They orbit between Jupiter and Pluto and can approach the planetary system from various angles. Unlike Jupiter-family comets, Halley-type comets have Tisserand parameter values less than 2.0 and display much higher and more diverse orbital inclinations, including retrograde orbits (ESA, 2024). They are thought to originate from the Oort Cloud, an enormous spherical cloud of icy bodies lying hundreds of billions of kilometres beyond the outermost planets.
Long-period comets (more than 200 years) receive the C/ prefix and have completely random orbital inclinations approaching from all directions, characteristics consistent with origin in the spherical Oort Cloud (Britannica, 1999). Their orbital periods can extend to millions of years, making them effectively one-time visitors from a human perspective. Famous examples include C/1995 O1 (Hale-Bopp) and C/2020 F3 (NEOWISE).
The Physical Rationale for Classification
These orbital classifications are not arbitrary but reflect fundamental aspects of Solar System structure and cometary origins. The flattened Kuiper Belt naturally produces comets with low-inclination orbits that remain near the ecliptic plane—the Jupiter-family comets. In contrast, the spherical Oort Cloud can supply comets arriving from any direction with random inclinations—the long-period comets. Halley-type comets likely represent an intermediate population, possibly long-period comets whose orbits have been modified through planetary interactions (ScienceDirect, n.d.).
The 200-year threshold for the P/C prefix distinction has less physical significance and more practical utility. It roughly separates comets whose returns can be observed and predicted within human timescales from those making effectively unique passages through the inner Solar System from an observational standpoint.
Special Cases and Edge Conditions
Interstellar Visitors
The discovery of 1I/ʻOumuamua in October 2017 represented a watershed moment for Solar System nomenclature. Initially classified as comet C/2017 U1, then reclassified as asteroid A/2017 U1 when it showed no signs of cometary activity, the object's interstellar origin became apparent through precise trajectory analysis. The IAU responded by creating an entirely new classification category: I/ for interstellar objects (ESA, 2024). The object's formal designation thus became 1I/2017 U1 (ʻOumuamua), with the Hawaiian name meaning "scout" or "messenger from afar arriving first."
The second confirmed interstellar visitor, 2I/Borisov, discovered in 2019, did show clear cometary activity, demonstrating that interstellar objects can exhibit the full range of characteristics seen in Solar System bodies (ESA, 2024). The I/ prefix now provides a clear designation for these exotic visitors, whilst the sequential number (1I, 2I, etc.) tracks the order of their discovery.
Comet-Asteroid Ambiguity
The boundary between comets and asteroids is not always clear-cut. Classification traditionally depends on whether an object displays cometary activity—the development of a coma and tail from outgassing as it approaches the Sun. However, some objects defy easy categorisation.
Objects initially reported with asteroidal designations may later develop cometary characteristics, leading to dual designations that incorporate the original minor planet designation into the cometary one. This produces designations such as 227P/2004 EW38 (Catalina-LINEAR), derived from the provisional minor planet designation 2004 EW38 (Wikipedia, 2025).
Conversely, some objects that never approach close enough to the Sun to develop observable cometary activity may nonetheless have compositions consistent with comets. Active asteroids or main-belt comets further blur these boundaries, displaying comet-like outgassing whilst residing in near-circular orbits among the asteroids (ESA, 2024).
Lost and Destroyed Comets
The D/ prefix serves an important archival function, marking comets that have disappeared from observation or are confirmed to have disintegrated. Classic examples include 3D/Biela, which was observed to split into two pieces during its 1846 apparition and was never seen again after 1852—though a spectacular meteor shower appeared in 1872 from the direction where the comet should have returned, demonstrating its complete disintegration (Britannica, 1999).
The most famous D/ comet is undoubtedly D/1993 F2 (Shoemaker-Levy 9), which approached Jupiter closely in the late 1960s or early 1970s and was captured into orbit around the planet. When discovered in 1993, it had already fragmented into at least 21 pieces. Between 16th-22nd July 1994, these fragments collided with Jupiter in a series of spectacular impacts observed by astronomers worldwide (Wikipedia, 2025).
Historical Comets: The X/ Designation
Many historical comets, particularly those observed before modern astrometric techniques, lack sufficient data for reliable orbit calculations. The X/ prefix acknowledges these observations whilst indicating the uncertainty in their orbital parameters. For example, X/-341 denotes a comet described by Aristotle in 341 BCE, whilst X/1106 C1 marks the Great Comet of 1106 (Wikipedia, 2025).
These designations preserve the historical record whilst honestly communicating the limitations of the available data.
Practical Examples and Case Studies
C/1995 O1 (Hale-Bopp)
This spectacular comet exemplifies the modern nomenclature system in practice. The designation C/1995 O1 breaks down as follows:
C/ = non-periodic comet
1995 = year of discovery
O = second half of July (15th-31st July)
1 = first comet discovered in that half-month
The comet was discovered independently by Alan Hale and Thomas Bopp on 23rd July 1995, hence the dual name separated by a hyphen (ESA, 2024). Its actual passage through perihelion occurred on 1st April 1997, when it became one of the brightest comets of the 20th century. With an orbital period of approximately 2,533 years, it falls firmly into the long-period category (Britannica, 1999).
1P/Halley
The systematic designation of Halley's Comet illustrates the permanent numbering system for periodic comets:
1P = the first periodic comet to be recognised as such
/1682 Q1 = discovered in the second half of August 1682
The "1P" designation takes precedence in formal astronomical databases, though "Halley's Comet" remains the name by which it is universally known. Its most recent perihelion passage occurred in February 1986, and it is not expected to return until 2061 (Britannica, 1999).
67P/Churyumov-Gerasimenko
This Jupiter-family comet serves as an excellent example of a modern short-period comet designation:
67P = the 67th periodic comet to receive a permanent number
/Churyumov-Gerasimenko = named after Soviet astronomers Klim Churyumov and Svetlana Gerasimenko who discovered it in 1969
The comet orbits the Sun every 6.45 years with relatively low inclination, quintessentially Jupiter-family characteristics. It achieved fame as the target of the European Space Agency's Rosetta mission, which successfully placed a spacecraft into orbit around the comet in 2014 and deployed the Philae lander to its surface—the first time such feats had been accomplished (BBC, 2024).
C/2020 F3 (NEOWISE)
This recent bright comet demonstrates the naming of comets discovered by automated survey programmes:
C/ = non-periodic comet
2020 = year of discovery
F = second half of March (16th-31st March)
3 = third comet discovered in that half-month
Discovered by NASA's NEOWISE mission on 27th March 2020, it became the brightest comet visible in the northern hemisphere since Hale-Bopp, peaking in July 2020 (ESA, 2024). The designation uses the survey programme name rather than individual discoverers.
The Role of the Working Group on Small Body Nomenclature
The current authority for comet nomenclature resides with the IAU's Working Group on Small Body Nomenclature (WGSBN), which comprises 15 members—11 voting members and 4 ex-officio representatives from related IAU working groups and the Minor Planet Center (WGSBN, n.d.). This body is responsible for reviewing and approving all comet names, maintaining and updating nomenclature guidelines, resolving disputes or ambiguities in naming, and ensuring fairness and consistency in the naming process.
The Minor Planet Center, operating under the auspices of the IAU, handles the practical assignment of designations and maintains the authoritative database of all known comets and their orbital elements (IAU Committee on Small Body Nomenclature, 2003).
Looking Forward: Challenges and Adaptations
As astronomical technology continues to advance, comet nomenclature faces ongoing challenges. Large-scale automated surveys now discover hundreds of comets annually, many of which are faint and may never develop the spectacular tails that captured humanity's imagination for millennia. The proliferation of surveys—LINEAR, NEAT, Catalina Sky Survey, Pan-STARRS, and others—means many comets share identical names, making the systematic designation increasingly important for unambiguous identification.
The discovery of interstellar objects has already prompted one significant addition to the system. Future discoveries—perhaps of objects from even more exotic sources—may necessitate further refinements. The boundary between active and inactive comets, comets and asteroids, continues to blur as our observational capabilities improve and our understanding of small body composition and activity mechanisms deepens.
Climate change poses an unexpected challenge to historical continuity in comet observation. Many important astronomical facilities, particularly those in mountainous or high-latitude regions, face threats from changing weather patterns and increasing light pollution from expanding human settlements. Preserving the long-term observational records essential for refining orbital calculations and recognising returning comets requires sustained commitment and international cooperation.
Conclusion
The nomenclature system for comets represents far more than a mere cataloguing exercise. It embodies centuries of astronomical tradition, accommodates the exponential growth in comet discoveries enabled by modern technology, and provides a framework for understanding these objects' places within the broader structure of our Solar System. From the simple year-based designations of antiquity to the sophisticated alphanumeric codes of the contemporary IAU system, comet nomenclature has evolved to meet the changing needs of the astronomical community whilst preserving connections to history and honouring those who expand our knowledge of these celestial wanderers.
The seemingly arcane prefixes, letters, and numbers in a designation like C/2020 F3 (NEOWISE) actually convey a wealth of information: the comet's periodicity (or lack thereof), when it was discovered, in what order, and by whom or what survey programme. For periodic comets, the permanent number provides a unique identifier independent of the potentially shared discoverer names.
As we continue to explore our Solar System and beyond, comet nomenclature will undoubtedly continue to evolve. The addition of the I/ prefix for interstellar objects demonstrates the system's capacity to accommodate genuinely new categories of discovery. Yet through all these changes, the fundamental goal remains constant: to provide clear, systematic, and meaningful identifiers for these remarkable objects that have stirred human curiosity and imagination since the dawn of civilisation.
The IAU's nomenclature system, refined through decades of experience and international collaboration, stands as a testament to science's ability to develop elegant solutions to complex problems—creating order from the apparent chaos of the cosmos whilst honouring both tradition and innovation.
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© /Clara — NeurAstra 2025