Comets

Where they come from

Jupiter-family comets have short periods and low inclinations, and arrive via the Centaurs from the Kuiper Belt. They have been heated many times and are relatively depleted.

Long-period comets come from the Oort cloud, a roughly spherical reservoir tens of thousands of AU out. They arrive on near-parabolic orbits from any direction, often on their first approach in millions of years, which makes them the most pristine material we can observe.

That distinction shows up in the data as a modelling problem: long-period comets frequently have no meaningful semi-major axis at all. Our orbit solver handles them with Barker's equation, using perihelion distance and time of perihelion instead.

Three visitors from outside

3 objects in this catalog are not from here. 1I/'Oumuamua (2017) was inert and oddly elongated; 2I/Borisov (2019) was an unmistakable comet with an eccentricity of 3.36; and 3I/ATLAS (2025) is the most extreme yet at eccentricity 6.14 — a value no solar-system body can reach.

High eccentricity alone is not enough to claim interstellar origin. Two comets in the catalog exceed eccentricity 1.05 without qualifying: they are ordinary solar-system comets that a planetary encounter kicked onto an escape trajectory outward. Our statistics keep the two cases separate.

Comets that come apart

Cometary nuclei are fragile. 73P/Schwassmann-Wachmann 3 broke into dozens of pieces in view of modern telescopes, and about 73 objects in this catalog follow that orbit closely enough to belong to the break-up. Sorting genuine fragments from coincidental neighbours takes more than a similar orbit, so read that as an estimate.

The extreme case is the Kreutz group, which has its own page: over a thousand fragments of a single enormous comet that disintegrated centuries ago, still following the same orbit.

A famous catalogue of things that were not comets

Comets were the prize long before anyone cared about asteroids. A new one carried your name, and in the 18th century hunting them was a recognised way to make a reputation — which is why the most quoted list in amateur astronomy exists as a by-product of the hunt.

Charles Messier found or shared in the discovery of a dozen or so comets from Paris in the second half of the 18th century, and while sweeping for them he kept running into fuzzy patches that refused to move against the stars. A comet moves; these did not. His catalogue, first published in 1774, was compiled largely so that he and others would stop re-checking the same impostors — and it is now the most used list in amateur astronomy: M1 the Crab Nebula, M31 the Andromeda Galaxy, M42 the Orion Nebula, M45 the Pleiades. Not every entry was a near-miss for a comet; some, the Pleiades among them, were added for completeness. But the catalogue's organising idea was what is not a comet.

The irony has outlasted the intent. Messier's comets are remembered by specialists, while the deep-sky objects he listed to get them out of his way — 103 in his own editions, 110 in the modern list after later astronomers added entries from his and Pierre Méchain's notes — are the backbone of amateur observing to this day. Beginners still work through the "Messier list" without knowing it began as a record of false alarms.

Best-known members

The lowest-numbered periodic comets, which are the longest-known — a D/ prefix means the comet has been destroyed or lost. Almost no comet has a published absolute magnitude, so they cannot be ranked by brightness. Queried live from the catalog.