The two main comet reservoirs are in the Oort cloud and in the dynamically excited component of the Trans-Neptunian region consisting of objects with large orbital inclinations and eccentricities (see Dones et al, 2015, for a review). During their prolonged residency in the outer Solar System, comet progenitors experienced minimal processing. They were mainly subject to space weathering: objects in the reservoirs experienced long-term irradiation by the solar wind, UV photons, and cosmic rays, as well as micrometeorite bombardment, and potentially the interstellar medium, if at a great enough distance from the star (Stern, 1990; Cooper et al, 2003), as well as occasional passages with massive stars and even supernovae (Stern and Shull, 1988). These processes likely affected the surface composition, as well as the colour, albedo, and the surface regolith structure (e.g., Hapke, 2001; Cooper et al, 2003; Hudson et al, 2008; Pieters and Noble, 2016; Zhang et al, 2023). Additionally, recent models show that at the heliocentric distances of the Trans-Neptunian region, a combination of solar radiation and radionuclide decay would significantly deplete hypervolatiles like CO, N2 and CH4, though some may survive trapped in amorphous ice (Parhi and Prialnik, 2023). At larger distances, galactic cosmic rays would also deplete hypervolatiles (Desch and Jackson, 2021).
Due to their large distances from the Sun (≳2000 au), Oort Cloud objects cannot be observed directly. TNOs, however, are accessible to telescopes based on or near Earth and have even been visited by the New Horizons mission (Stern et al, 2015, 2019). Remote observations of TNOs have revealed interesting properties of the population which provide important constraints on the cometary life cycle. Notably, the TNO region hosts objects with a remarkable diversity of surface colours ranging from objects with neutral (solar-like) colours to objects with very red surfaces, known as ultrared objects (Jewitt, 2002). Recent spectroscopic observations with JWST have helped to break previous degeneracies in surface composition, allowing bodies to be classified based on their surface ices and other molecules (De Pr´a et al, 2025; Pinilla-Alonso et al, 2025). Another peculiar characteristic of the TNO region is the high abundance of binary and contact-binary objects, which has been used as a constraint on planetesimal formation mechanisms (Nesvorn´y, 2018; Showalter et al, 2021). Unfortunately, as of now, the findings about TNOs provide only indirect evidence for comet evolution. The TNOs typically observed with telescopes have radii > 100 km and are therefore much larger than typical comet nuclei (1-10 km), due to the detection distances and the steep size distribution of both populations. The target of New Horizons, (486958) Arrokoth, is of the same size range as the larger comets; however, it is a Cold Classical Kuiper Belt Object which is believed to have formed in situ. Both Oort cloud objects and ecliptic comets are thought to have formed closer to the Sun than the Cold Classicals, in the portion of the primordial planetesimal disc subject to strong scattering from the gas and ice giant planets (see Nesvorn´y, 2018; Gladman and Volk, 2021, for recent reviews). Thus, Arrokoth may have formed further out than the comets (Stern et al, 2019; Nesvorn´y et al, 2017).