C/2018 C2 (Lemmon)

Lore & Background

While near perihelion, A/2018 C2's heliocentric orbit is not bound to the Solar System, but unlike ʻOumuamua, it is not an interstellar object. The heliocentric eccentricity drops below 1 starting with an epoch of January 2023, when it is 13.8 AU from the Sun, making the orbit bound to the Sun before it leaves the planetary region. The more stable barycentric orbit shows it is only a very distant small Solar System body, approaching as far as 50,000 AU from the Sun, around the distance of the Oort Cloud.

It had an orbital period of millions of years until the current approach to the Solar System, where perturbations will shorten the orbit drastically to about 5,200 AU, with an orbital period of 130,000 years. Hui Man-To calculated that the comet would have its next perihelion after 130 to 140 thousand years, at a distance of about 2 AU. A/2018 C2 made its closest approach to the Sun on 2 June 2018 at a distance of 1.9 AU, outside the orbit of Mars. Since this object outgassed generating a cometary coma while near the inner Solar System, it was reclassified as a hyperbolic comet.

Reader's Guide

C/2018 C2 (Lemmon) is notable as a hyperbolic comet that was initially mistaken for a hyperbolic asteroid, highlighting the challenge of distinguishing distant small bodies before they develop cometary activity. Its discovery alongside A/2017 U7 on 4 March 2018 underscored the Mount Lemmon Survey's role in detecting unusual Solar System objects. The comet's orbit is significant because, while its heliocentric path is unbound near perihelion, it is not an interstellar object like ʻOumuamua; its barycentric orbit shows it originates from the Oort Cloud region, approaching as far as 50,000 AU from the Sun. The object's orbital evolution—from a period of millions of years to a drastically shortened 130,000-year orbit—illustrates the gravitational perturbations that reshape long-period comet trajectories. Hui Man-To's calculation of its next perihelion in 130,000 to 140,000 years at about 2 AU provides a rare long-term orbital forecast. The reclassification from asteroid to comet on 22 March 2018, based on observed outgassing, reinforces the dynamic nature of small Solar System bodies and the importance of continuous observation.

Did You Know?

• C/2018 C2 was first observed on 5 February 2018 by the Mount Lemmon Survey.
• It was initially classified as a hyperbolic asteroid (A/2018 C2) before being reclassified as a hyperbolic comet on 22 March 2018.

The Systematic Designation Framework

The designation C/2018 C2 is not an arbitrary string of characters; it is a product of the systematic naming architecture that the International Astronomical Union has constructed to guarantee every celestial body can be identified without confusion. As the catalogue of known astronomical objects has swelled from mere hundreds in antiquity to well beyond one billion in the modern era, with new entries arriving every single year, the demand for an unambiguous identification system became critical. The IAU, acting as the sole internationally recognized authority for assigning designations to celestial bodies, developed a suite of systematic naming schemes tailored to different categories of objects. The comet's code follows this broader philosophy: a structured sequence of characters encoding the object's class, the year of its discovery, and its sequential position within that period. This mirrors the same principle underlying the Bayer designations for stars, where a Greek letter paired with a constellation name uniquely pins down a specific star among thousands, applied here to an entirely different class of celestial body.

Honoring the Discoverer

The surname Lemmon appended to the comet's designation reflects a long-standing tradition in astronomy of attaching a discoverer's name to a newly identified object. Among stars, roughly two dozen carry historic names honoring the astronomers who first catalogued or studied them, such as Barnard's Star and Kapteyn's Star. The IAU's Working Group on Star Names has also approved commemorative names like Cervantes and Copernicus for specific stars, extending the practice of recognition beyond pure science into broader cultural tribute. For comets and other minor bodies, the discoverer's name serves a similar dual purpose: it credits the individual who first brought the object to scientific attention, and it provides a human, memorable handle alongside the purely systematic code. In a field where the catalogue of known objects grows continuously and where most entries are identified only by strings of letters and numbers, the discoverer's name becomes the thread connecting a cold designation to the person who first noticed the object against the night sky.

From Hundreds to Billions: The Naming Imperative

In the ancient world, the sky was a manageable inventory. Only the Sun, the Moon, a handful of stars, and the most obvious wandering planets carried names at all. The total number of objects any culture could track with the naked eye was limited to perhaps a few thousand bright stars, bounded by the physiological ceiling of human vision at an apparent magnitude of six. That world has been irrevocably transformed. Telescopes and modern sky surveys have multiplied the catalogue of known objects by orders of magnitude, pushing the count past one billion and climbing further every year. The IAU's response has been to construct layered, systematic naming schemes for each major class of celestial object, ensuring no two entries in the global registry can be confused. The designation C/2018 C2 is a small but essential node in that vast architecture: a compact code that, without ambiguity, tells any astronomer on Earth exactly which comet is being discussed, regardless of language, culture, or the particular survey in which it was first recorded.

The IAU's Unifying Role

The International Astronomical Union occupies a unique position in the global scientific landscape: it is the single, internationally recognized body empowered to assign designations to celestial bodies and to surface features on them. This monopoly on naming authority exists for one overriding reason—to guarantee that every name or code is unambiguous across every language, every research institution, and every country. The IAU maintains globally unique initialisms for star catalogues, oversees the approval of proper star names through bodies like the Working Group on Star Names, and extends its jurisdiction to planets, minor planets, and comets alike. It also explicitly rejects the commercial practice of selling fictitious star names, reinforcing that celestial nomenclature is a matter of public scientific record rather than private branding. For an object like C/2018 C2 (Lemmon), this means its designation is not a local convention or a journal's house style; it is a globally standardized identifier, vetted and maintained under the IAU's framework, ensuring that a researcher in Tokyo and one in São Paulo are referring to the very same comet when they write those characters.