There are a few known mechanisms which determine the physical lifetime of comets. Most comets have their orbits perturbed by gravitational forces and are either ejected from the Solar System or driven onto orbits with small perihelion distances where they are eventually destroyed. Those on more stable orbits are believed to either gradually lose their activity until they become dormant comets, or, alternatively, to experience catastrophic comet-splitting events (see Jewitt, 2004; Boehnhardt, 2004, and see Section 6.3). One of the possible mechanisms leading to comet splitting is activitydriven spin-up. This mechanism takes place when outgassing produces torques which bring the rotation rates of the nuclei up to a critical limit. Above this limit, the centrifugal force exceeds the nucleus’ gravity and the material strength, and the comet nucleus starts to shed mass and disintegrates (e.g. Davidsson, 1999). The accompanying fragmentation substantially increases the surface-area-to-volume ratio, accelerating the depletion of remaining volatiles and often producing debris streams and swarms of fragments (Farnham et al, 2001; Combi et al, 2014).
If cometary nuclei do not undergo significant mass-loss and disruption events during the peak of their activity, their evolution is instead governed by the progressive depletion or sequestration of volatiles. Continued sublimation can lead to the formation of an insulating mantle, as devolatilized material accumulates to a thickness exceeding the thermal skin depth. This mantling thermally isolates the interior, effectively sealing substantial volatile reservoirs while rendering the nucleus observationally inactive.

Such objects are classified as dormant comets, whose volatiles remain present but shielded from solar insolation. With continued thermal processing, some nuclei may ultimately become dead comets, having lost their accessible volatiles entirely while retaining structural integrity (Jewitt, 2004). Both dormant and dead comets are observationally indistinguishable from asteroids placed on comet-like orbits (see Fern´andez et al, 2001). Dynamical and physical surveys estimate that dormant comets may constitute on the order of a percent of the near-Earth object population, with size-limited estimates suggesting that 0.3–3.3 % of near-Earth asteroids with diameters larger than 1 km are dormant short-period comets (Mommert et al, 2015). Discriminating between these intact end states, and characterizing their physical differences, remains challenging due to the small sizes of cometary nuclei, their variable observing geometries, and their extremely low or absent activity (Jewitt and Hsieh, 2024).
These evolutionary outcomes give rise to a variety of dynamically and physically distinct small-body populations (Jewitt and Hsieh, 2024). Asteroids in Cometary Orbits (ACOs) are objects on comet-like orbits (typically low Tisserand parameter with respect to Jupiter) that show no detectable coma or tail; they are interpreted as a mix of dormant/extinct comets and asteroidal interlopers (Fern´andez et al, 2001). Damocloids inhabit long-period or Halley-type orbits and are generally regarded as inactive cometary nuclei that have lost observable activity (Jewitt, 2005, 2015). Manx comets are bodies on orbits like those of Oort Cloud comets that exhibit little or no activity but have asteroidal (rocky) spectral characteristics, indicating inner Solar System origins followed by emplacement in distant reservoirs (Meech et al, 2016; Knight et al, 2017). In the inner Solar System, main belt comets reside on dynamically stable asteroid-like orbits yet exhibit transient activity due to mechanisms such as sublimation, impacts, or rotational disruption (Hsieh and Jewitt, 2006); when measured, their CO2 to H2O ratios are extremely depleted compared to classical comets (Kelley et al, 2023). Dark comets are morphologically inactive near-Earth objects that nonetheless show non-gravitational accelerations consistent with low levels of outgassing, implying volatile release below traditional detection thresholds (Micheli et al, 2023; Hui and Jewitt, 2017). These populations illustrate the continuum between comets and asteroids, modulated by dynamical history and volatile content.