In astronomical records, the total number of cataloged comets that have been physically discovered, tracked, and had their orbital paths calculated exceeds 4,000 objects. Within this catalog maintained by the Minor Planet Center and NASA’s Jet Propulsion Laboratory, comets are categorized into distinct dynamical groups based on their orbital characteristics. Over 500 of these are officially numbered periodic comets (designated with the prefix “P/”), which means they have been observed through at least two distinct perihelion passages, confirming their calculated orbital return. Several hundred additional objects are recognized as periodic comets with periods under 200 years but have only been tracked through a single apparition, while the remaining bulk of the catalog—over 3,000 bodies—consists of long-period or non-periodic comets on parabolic or near-parabolic trajectories that have swept through the inner observation zone only once in recorded human history. Prior to the invention of the telescope, human tracking was limited entirely to naked-eye apparitions, with ancient Babylonian, Chinese, and Greco-Roman annals recording only about 50 to 100 distinct bright cometary apparitions across several millennia.

Several prominent periodic comets have well-documented trajectories and calculated arrival intervals. The most famous is 1P/Halley, which travels on a highly elongated, retrograde elliptical trajectory inclined at  to the ecliptic plane. Halley has a semi-major axis of approximately  and an orbital eccentricity of , bringing its perihelion to  inside the orbit of Venus and its aphelion out to  beyond Neptune. Halley completes an orbit every 75 to 76 years, having last passed perihelion in February 1986, with its next appearance projected for July 28, 2061. Comet 2P/Encke holds the shortest known orbital period of any recognized comet at approximately 3.30 years. Encke moves along a low-inclination path () with an eccentricity of , carrying it from a perihelion of  near Mercury out to an aphelion of  in the main asteroid belt, with its next perihelion calculated for February 2027.

Other extensively monitored periodic comets include 67P/Churyumov–Gerasimenko, a Jupiter-family comet with an orbital period of 6.44 years, a perihelion of , an aphelion of , and an inclination of , which was studied in close proximity by the Rosetta mission and next returns in April 2028. Comet 109P/Swift–Tuttle, the parent body of the annual Perseid meteor stream, moves on a steep retrograde trajectory with an inclination of , an eccentricity of , a perihelion of , and an aphelion of , yielding an orbital period of approximately 133.3 years with its next return predicted for roughly the year 2126. Comet 12P/Pons–Brooks follows a Halley-type orbit with a period of 71.3 years, an inclination of , a perihelion of , and an aphelion of , having reached perihelion in April 2024 and scheduled to return around 2095.

Regarding the physical survival of these objects, the mathematical odds that an active cometary body could survive continuous inner-system cycling over deep time are effectively zero. When a comet approaches the Sun within approximately  to , the solar radiation flux—governed by the inverse-square relation —rapidly drives the sublimation of volatile water ice, carbon dioxide, and carbon monoxide. At perihelion, comets routinely shed tens to hundreds of tons of mass every second through explosive outgassing jets. For a typical cometary nucleus measuring only a few kilometers across, thermodynamic mass-loss models indicate that an actively cycling comet loses significant outer layers per pass, completely exhausting its volatile inventory or disintegrating within  to  years (representing only a few hundred to a few thousand orbits).

Furthermore, comets exhibit very low bulk densities (typically ) and structural porosities exceeding , making them fragile conglomerates rather than solid rock. They face extreme mechanical destruction mechanisms, including tidal disruption when passing within the Roche limit of the Sun or massive bodies like Jupiter—exemplified by Comet Shoemaker–Levy 9 breaking into over twenty fragments before impacting Jupiter in 1994, and Comet 3D/Biela splitting in two in 1846 before dissolving entirely into a meteor shower. Additionally, asymmetric outgassing generates severe non-gravitational torques that spin nuclei beyond their tensile bursting limits, while thermal shock from temperature gradients exceeding hundreds of degrees causes rapid structural fracturing.

Because physical observation proves that comets are rapidly depleting systems with short empirical lifespans, standard astrophysics acknowledges that none of the active comets observed today could have been cycling in their current orbits for millions or billions of years. To reconcile their ongoing existence within the conventional timeline, the mainstream model relies on the theoretical existence of distant, cold holding reservoirs—namely the Kuiper Belt/Scattered Disc and the unobserved, hypothetical Oort Cloud located thousands of astronomical units away. Under this theoretical framework, comets are treated not as long-term inner-system survivors, but as pristine, cryogenic bodies that remain dormant until gravitational perturbations from passing stars or giant planets disrupt their orbits and send them plunging inward as new, short-lived visitors. Without positing these replenishment reservoirs, the observed destruction rates and rapid mass dissipation of comets would present an immediate physical contradiction to long-age cosmological models.