JuanCastillo29/space-mission-intelligence-data huggingface dataset:
reservoirs

So far this work has described the mechanisms that can generate comets from small body reservoirs, and how those reservoirs evolve over time. Since we can detect both exocomets and these reservoirs in the form of debris discs, it is pertinent to ask whether we can determine the specific sources for individual observed comets. This is frequently attempted for Solar System comets. For example, the Kreutz family of Sun-grazing comets are best modelled as originating in the Oort Cloud and then undergoing significant orbital evolution, based on dynamical arguments (Fern´andez et al, 2021). Composition arguments can also be used; for example, Comet 96P/Machholz has an unusual composition that may point towards a Kuiper Belt origin, although an interstellar origin is also possible (Schleicher, 2008). C/2016 R2 (PanSTARRS) has high abundances of CO and N2 (Biver et al, 2018; Cochran and McKay, 2018) which may indicate formation in a cold environment (Mousis et al, 2021) or that it is a fragment of a Pluto-like body (Biver et al, 2018).
However, major challenges here are that the storage and release of volatiles in comets is poorly understood and that surveys of chemical inventories are limited in the number of comets, number of species, and range of heliocentric distances over which observations are taken, though even the limited sample available to date indicates a complex taxonomy (Russo et al, 2016; Eistrup et al, 2019). The Rosetta results on 67P/Churyumov–Gerasimenko show an even more complex picture with adsorption and release processes affecting observed abundances (Luspay-Kuti et al, 2022; Mandt et al, 2024). These Rosetta results have not yet been fully linked to ground based observations (Saki et al, 2024).
The spin of a small body can be affected by several factors, one of which is the absorption and re-radiation of stellar radiation, following redistribution of the incident energy within the body. When a body is aspherical, this can generate asymmetric torques on it, altering its spin, causing the body to enter a tumbling spin state, or even accelerating it to rotational fission (Paddack, 1969; Rubincam, 2000). This physical effect is named the Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect, reviewed by Vokrouhlick´y et al (2015). YORP has been directly measured through long-baseline observations of asteroids’ phase curves, where a YORP torque induces a quadratic drift in the timing of phase curve maxima (Lowry et al, 2007; Taylor et al, 2007). The spin of a cometary nucleus (or other body with significant volatile content) can also be affected by sublimation, also known as outgassing, which occurs when stellar radiation heats up the nucleus. Outgassing can occur in two broad flavours: either localised to a particular region of the nucleus, or across the entire body. The latter is similar to YORP, but driven by outgassing rather than radiation; it also leads to net torques on asymmetric bodies. Since sublimating gas molecules carry orders of magnitude more momentum than photons, this “SYORP” can generate much stronger torques than YORP, and hence spin up much larger bodies or result in large changes in the rotation period in a relatively short amount of time; one example is the increase of the rotation period of comet 41P by around 30 hr during its apparition in 2017 (Bodewits et al, 2018). Since comets have relatively low strength, sub-kilometer nuclei of small comets are likely rapidly destroyed through torque-induced rotational instability, and this effect is even more pronounced for sun-grazing comets (Jewitt, 2021). YORP and outgassing are both effects of irradiation from the host star, and therefore become stronger for more luminous stars. All of these effects have been primarily studied for Solar System comets. The physics for exocomets orbiting Sun-like stars would be similar; the effects should be stronger for A-type stars, with luminosities that are up to a few 10s of solar luminosities, and stronger still for giant branch stars, with luminosities that are orders of magnitude greater than the Sun (see Section 10).
Similar to rotational spin-up, orbital changes of small bodies can occur due to the action on the orbit of radiative torques or/and outgassing. If the body is small enough and the incident radiation is strong enough, then these torques can noticeably shift the orbit of the body (¨Opik, 1951; Beekman, 2006). This physical effect is named the Yarkovsky effect, which is reviewed by Vokrouhlick´y et al (2015). For main-sequence stars this effect is negligible on timescales well under a Myr, and certainly over the course of a single periapsis passage.

In the Solar System it primarily affects bodies smaller than 30–40 km. As with the YORP torque, this has been directly detected for some asteroids, through small deviations from their purely gravitational orbits (Chesley et al, 2003). In the Solar System, the Yarkovsky effect acts on Main Belt asteroids to slowly move them into unstable resonances and subsequently be delivered to the terrestrial planet region to become Near Earth Objects, guaranteeing a long-term source of such objects (Vokrouhlick´y et al, 2015). For comets, outgassing and perturbations from the planets may change their orbits during each perihelion passage. An often-used formulation to model the acceleration due to these changes was created by combining functional forms from Delsemme and Miller (1971) and Marsden et al (1973) into 


 , (2)
�⃗r·⃗v r
�
¨⃗r = −GM⊙
⃗r r ����
⃗r + N(r)
+A3 ⃗r × ⃗v |⃗r × ⃗v| ����
A1
+A2 r⃗v −⃗r
r3
|⃗r × ⃗v| � �� �
Radial ≡ˆ R
Transversal ≡ˆT
Normal ≡ˆ N
where
�ξ�−ζ
�−η �
N(r) ≡α �r
1 + �r
, (3)
r0
r0
(e.g., Szutowicz, 2000; Sosa and Fern´andez, 2011). Here, the vector ⃗r represents the distance between the centre of the comet and the centre of the star. The first term represents the star’s Keplerian potential, and the second the additional acceleration due to outgassing. The function N(r) quantifies the strength of this perturbation as a function of distance to the star; if N(r) ≡0, the comet experiences no perturbations and remains on a fixed orbit. This is only possible if α = 0, where α represents the extent of the outgassing, or more specifically the magnitude of the sublimation rate. The constants r0, η, ξ and ζ are empirically derived, with typical values of r0 = 2.808 au, η = 2.15, ξ = 5.093, and ζ = 4.6142 (Sosa and Fern´andez, 2011). The values of A1, A2 and A3 are best treated as time-dependent. The radial, transversal and normal contributions to the motion are labelled in Equation 2. A general formulation such as the one above may represent a useful starting point for representing the motion of exocomets which are observed to be periodic. However, observations of individual main-sequence exo-systems have yet to definitively show periodic exocometary behaviour. Nevertheless, this formulation is general enough to model a comet’s motion over any length of time, and special cases can reduce to models such as assuming that sublimation occurs only on the starlit side of an exocomet (Veras et al, 2015a).
Small bodies can disrupt partially or fully, and through a variety of mechanisms (Jewitt, 2004; Boehnhardt, 2004). The physical details of the disruption process have been described extensively for Solar System comets, with examples shown in Figure 4.

These processes are, however, less well-studied for extrasolar systems.

[Figure: Fig. 4 Disruption of Solar System bodies. Left: partial disintegration of comet 73P/Schwassmann-Wachmann 3 producing a swarm of nuclei, as seen by the Hubble Space Telescope (NASA – ESA – H. Weaver: APL/JHU – M. Mutchler and Z. Levay: STScI, https://science.nasa.gov/ image-detail/comet-73p/). Right: the aftermath of an impact on asteroid (596) Scheila, as seen by the Neil Gehrels-Swift observatory (Bodewits et al, 2011, Image credit: NASA, https://stsci-opo.org/ STScI-01EVT4S5ZKVNHPZTD5AXSETYT1.jpg).]

One way that objects can be disrupted is via interactions with other bodies. Total disruption is guaranteed if an object’s periapsis lies within the Roche radius for tidal disruption around the star, where the object fragments as the differential tidal force across its volume exceeds its own self-gravity and material strength. The Roche radius typically lies at around 1 R⊙, depending on the mass and luminosity of the star, and the composition of the object. Thus, on a close approach, the object could impact the star directly, fragment due to tidal forces, or sublimate entirely (Brown et al, 2011, 2015, 2017). The mechanics of these processes are a function of the body’s shape (McDonald and Veras, 2021). Even outside a star’s Roche radius, disruption (total or partial) of one body by another can still occur. A small body could cross the Roche radius of an exoplanet (analogous to Shoemaker–Levy 9 and Jupiter; e.g. Harris 1996) and be disrupted. Disruption is also possible somewhat outside the Roche radius, as an exchange of spin and orbital angular momentum can lead to rotational fission (e.g. Richardson et al, 1998; Makarov and Veras, 2019; Makarov et al, 2020; Veras et al, 2020a). Several other channels exist for the disruption of small bodies. Thermal processes can destroy nuclei through complete sublimation during repeated periapsis passages; the sublimation lifetime of these comets is a strong function of stellar type, even on the main sequence (Marboeuf et al, 2016). Structural failure is another route: comets are sometimes seen to disintegrate, for example 73P/Schwassmann–Wachmann 3 (Figure 4, left panel). In addition, the YORP effect or anisotropic outgassing can spin a body up to centrifugal break-up speed (Bottke et al, 2006; Jewitt, 2021), as described above. Spin rates below the break-up rate can still lead to modest periodic mass shedding as a time-dependent function of a body’s structural deformation along its orbit, as modelled for asteroid (3200)
6). Structural failure is another route: comets are sometimes seen to disintegrate, for example 73P/Schwassmann–Wachmann 3 (Figure 4, left panel). In addition, the YORP effect or anisotropic outgassing can spin a body up to centrifugal break-up speed (Bottke et al, 2006; Jewitt, 2021), as described above. Spin rates below the break-up rate can still lead to modest periodic mass shedding as a time-dependent function of a body’s structural deformation along its orbit, as modelled for asteroid (3200)

Phaethon (Nakano and Hirabayashi, 2020), which is not only the likely progenitor of the Geminid meteor stream (Gustafson, 1989; Williams and Wu, 1993), but also has the closest perihelion of any named asteroid, at 0.14 au.
As described above in Section 3.3, collisional processes also disrupt small bodies. In the Solar System, collisions have been directly observed, for example the impact on asteroid (596) Scheila (e.g. Bodewits et al 2011), as shown in Figure 4 (right panel).
To date, no direct evidence of an exocometary break-up around a main-sequence star has ever been observed5. As a result, active asteroids in the Solar System represent the primary observational guide for understanding how exocometary nuclei might break up. Typical examples which have prompted relevant related discussions are 311P/PANSTARRS (Jewitt et al, 2015) and P/2013 R3 (Jewitt et al, 2017), the latter of which spun up to break-up. A more atypical, but particularly important example, is (3200) Phaethon. With its extremely small perihelion (0.14 au), it provides a useful testbed for the physics of the survival and destruction of small bodies close to their star (see Vrignaud et al, 2026). While we have no direct evidence of exocometary break-up, there is indirect evidence. The long duration of exocometary signatures in the β Pic spectrum (sometimes greater than several days) indicates that those signatures may not be produced by single objects, but rather by the transit of chains of exocomets originating from the break-up of a parent body. A similar hypothesis was proposed to explain the unusual shape of a cometary transit observed in the light curve of KIC 8462852 (“Boyajian’s star”, Boyajian et al 2016), with at least 7 objects transiting the star within a few days (Kiefer et al, 2017). The unusual shape of 1I/‘Oumuamua could also be consistent with tidal disruption that occurs in some cases as exocomets are ejected from their systems, though erosion or evaporation from an initially much less extreme aspect ratio may offer a better explanation (Jackson and Desch, 2021, and see Section 8). Returning to β Pic, the “cat’s tail” feature may be evidence of a recent collisional break-up.
Comets in the Solar System have been studied for decades with the expectation that their properties will reveal the conditions and physical properties during the epoch of planet formation, because they have spent most of their lives at large heliocentric distances, less affected by insolation and the solar wind than bodies closer to the Sun. This has motivated a series of successful space missions to comets, including Giotto, Stardust, Deep Impact, and Rosetta/Philae (see Snodgrass et al, 2024, for a history). In combination with remote observations, as well as modelling and laboratory efforts, these missions have revealed that comets are in fact geological objects which have undergone complex evolution since their formation. Moreover, dynamical links have been established that connect comets in the Solar System with other populations: mainly their reservoirs in the Oort Cloud and trans-Neptunian region, but also more
5Although several disintegrating planets have been detected, such as KIC12557548b (Rappaport et al, 2012) and K2-22b (Sanchis-Ojeda et al, 2015; Tusay et al, 2025).
[Figure: Fig. 5 Schematic overview of the evolution processes acting on small bodies around solar-mass stars throughout the system’s evolution, from the main sequence, through the giant branch (GB), to the white dwarf (WD) phase. The diagram includes the most likely mechanisms driving the evolution for a given stage, and the properties which are most likely to evolve as a result. The symbols of the properties from left to right are: size, shape, spin state, material properties, surface topography, composition, surface reflectance. The last column indicates the observed populations which are used to extract information about the given evolutionary stage. Populations outside the Solar System are indicated in pink with dashed borders in this column.]
satellites of the giant planets (c.f. Jewitt and Hsieh, 2024).
The properties of each population and the comparison between these different
populations allow us to constrain the main processes believed to act on the objects.
Stages of the evolution and the dominant processes identified are illustrated in Fig. 5.
However, by considering objects in the Solar System, we are limited to direct evi-
dence only about the evolutionary processes taking place around a main-sequence
solar-type star with a system having the architecture of the Solar System. Evidence
from exocometary systems allows us to expand this study to further evolutionary
stages and more diverse systems. Observations of debris discs and WD systems enable
us to study the evolutionary processes taking place at early and later evolutionary
stages, while the exocomets detected through photometry and spectroscopy reveal the
properties of small bodies showing cometary activity around different main-sequence
stars. For observations of individual cometary nuclei beyond the Solar System, we
are limited at present to our three confirmed interstellar visitors 1I/‘Oumuamua,
2I/Borisov and 3I/ATLAS (Meech et al, 2017; Williams et al, 2017; Borisov et al, 2019; Jewitt and Luu, 2019; Denneau et al, 2025; Seligman et al, 2025), discussed in Section 8.
A major benefit of studying the evolutionary processes acting on small bodies throughout their evolution is that with a better understanding of the physical processes and the properties they affect, we can link better the current properties of comets to the primordial planetesimals, and we can provide better constraints on planetesimal formation models, disc composition, mixing of small body populations, and early enrichment of the planets with organic material and water. At any given moment, the Solar System provides only a snapshot of the current evolutionary stage of individual objects and populations. To understand the connections between different populations, we must identify similarities in object properties and patterns within populations. Following this approach, the main dynamical and genealogical links between small-body groups which were initially studied as separate populations have been identified (see Dones et al, 2015; Fraser et al, 2023). Below, we provide an overview of the five stages of the comet life cycle, along with the associated populations, evolutionary processes, and properties subject to alteration. This current view on cometary evolution is summarized in Fig. 5, to which we add processes acting on comets around giant branch and white dwarf stars, relevant for other, more evolved, planetary systems.
Today’s comets formed in the primordial Kuiper Belt (PKB) - a progenitor population of planetesimals beyond the orbits of the giant planets (see Nesvorn´y, 2018, and references therein). Within the first 100 Myr of the formation of the Solar System, the PKB underwent an extensive dispersal (see Section 3, and Nesvorn´y, 2018, for a review). This instability ejected most objects from the PKB into interstellar space. The remaining objects were emplaced into today’s Oort cloud, the dynamically-excited component of the Trans-Neptunian region, irregular satellites of the giant planets, Jupiter and Neptune Trojans, and the outer Main asteroid belt. Assessing the early evolution of cometary progenitors remains elusive due to the scarcity of direct evidence from that epoch. Many estimates are model-dependent and are limited by the uncertainty on the planetesimal formation mechanism, location, time and timescales (see Guilbert-Lepoutre et al, 2024; Bannister et al, 2025). Even with these limitations, three main mechanisms are believed to have contributed at least to some extent to early cometary evolution: 1) Thermal processing due to radiogenic heating from to the decay of short-lived radiogenic isotopes (such as 26Al; see Malamud and Perets 2016; Davidsson 2021; Malamud et al 2022); 2) Thermal processing from solar irradiation leading to sublimation of the hypervolatile species (Steckloff et al, 2021; Parhi and Prialnik, 2023; Gkotsinas et al, 2024); and 3) collisional processing, including cratering, fragmentation and catastrophic breakup, which could have altered the physical properties of the individual objects as well as the size distribution of the whole population (Morbidelli and Rickman, 2015; Bottke et al, 2023).
(Nesvorn´y et al, 2017; Vokrouhlick´y et al, 2019; Raymond et al, 2020). Counterintuitively, DNCs are predicted in these models to be more volatile-depleted than dynamically old comets, owing to their different early dynamical evolution. Harrington Pinto et al (2022) indeed found such a difference, though it may also be attributed to cosmic ray processing of
6Note that Weissman et al (2020) favour “primitive” over “pristine”, as the former implicitly allows for some evolution since formation.

the surface layers (Gronoff et al, 2020; Maggiolo et al, 2020; Harrington Pinto et al, 2022).
The upcoming mission Comet Interceptor (CI; Snodgrass and Jones, 2019; Jones et al, 2024) is uniquely designed to visit a DNC. This design, however, requires CI to be built and launched potentially before its target is discovered, and the task of identifying the most suitable object7 to accomplish the mission’s goals is still pending by design (Snodgrass et al, 2026). Comet Interceptor may even be able to visit an Interstellar Object, but a future, more capable, mission would offer greater possibilities here (Snodgrass et al, 2025): ISOs are more challenging than bound comets owing to their higher velocity.
If the orbits of Centaurs evolve to cross Jupiter’s orbit, they become dynamically dominated by Jupiter and (if they are not ejected) typically have orbital periods reduced to < 20 years, becoming Jupiter-Family Comets (JFCs). These objects get increasingly warm as they approach the Sun and are expected to begin sublimating water ice around 3–5 au. Active JFCs are significantly better understood than comets at any other stage of evolution. This is largely due to the spacecraft missions which have provided in-situ studies of six JFC nuclei (see Snodgrass et al, 2023). In combination with Earth-based observations, they have allowed us to characterize JFCs in terms of their shapes, rotations, physical properties (density, porosity, strength), compositions, surface morphologies, surface properties and activity mechanisms. In particular, Rosetta’s continuous observations of the nucleus of 67P/Churyumov–Gerasimenko throughout its perihelion provided an amazing leap in revealing the links between the nucleus properties and activity (Keller et al, 2015a,b). It led to a breakthrough in our understanding of the effects that sublimation of near-surface ices has on comet erosion (e.g., Vincent et al, 2016a,b). Comparing Rosetta data to observations from other missions, Vincent et al (2017) found evidence that dynamically young JFCs have surfaces dominated by steep cliffs and pits, while more evolved comets are characterised by smooth terrains covered by pebbles and dust. These differences in morphology can also be linked to different photometric properties of comet nuclei from spacecraft observations (Longobardo et al, 2017; Vincent, 2019) and ground photometric observations (Kokotanekova et al, 2017, 2018). According to this hypothesis, young JFCs can be distinguished from evolved comet nuclei as they have higher albedos, and their phase functions have a steeper gradient with the phase angle α. LPCs and DNCs entering the inner Solar System are believed to undergo similar evolution in the water-sublimation region. The main difference compared to JFCs is that, despite their larger orbital distances, they still show activity out to distances of up to a few tens of au (Jewitt, 2021); in contrast, typical JFC aphelion distances are ≲15 au (e.g. Shober et al 2024). Comets that pass sufficiently close to the Sun lose parts of their outer layers, exposing fresher, relatively unprocessed material beneath, as seen with comet 67P (Bockel´ee-Morvan et al, 2016). Additionally, a comet’s evolution is influenced by its rotational state. For instance, the high obliquity of comet 67P results in pronounced
7The preferred target is a dynamically new comet, or even an interstellar object, but if a suitable target is not found another long-period comet will be selected.