# Atmospheric response to seasonal changes in sea surface temperature during the boreal summer in the Tropical Atlantic

Dahirou Wane, Gaëlle de Coëtlogon, Alban Lazar, Malick Wade, Amadou T Gaye

► To cite this version:

Dahirou Wane, Gaëlle de Coëtlogon, Alban Lazar, Malick Wade, Amadou T Gaye. Atmospheric response to seasonal changes in sea surface temperature during the boreal summer in the Tropical Atlantic. Climate Dynamics, 2024, 62 (3), pp.1597 - 1612. 10.1007/s00382-023-06968-4. insu-04288175v2

HAL Id: insu-04288175

https://insu.hal.science/insu-04288175v2

Submitted on 28 Feb 2024

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<figure><img src="image_1.png" /></figure>

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# Atmospheric response to seasonal changes in sea surface temperature during the boreal summer in the Tropical Atlantic

Dahirou Wane, Gaëlle de Coëtlogon, Alban Lazar, Malick Wade, Amadou T Gaye

<figure><img src="image_1.png" /></figure>

## To cite this version:

Dahirou Wane, Gaëlle de Coëtlogon, Alban Lazar, Malick Wade, Amadou T Gaye. Atmospheric response to seasonal changes in sea surface temperature during the boreal summer in the Tropical Atlantic. Climate Dynamics, 2024, 62 (3), pp.1597 - 1612. 10.1007/s00382-023-06968-4. insu-04288175v2

HAL Id: insu-04288175

https://insu.hal.science/insu-04288175v2

Submitted on 28 Feb 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_2.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

# The Comet Interceptor Mission

Geraint H. Jones · Colin Snodgrass · Cecilia Tubiana · Michael Küppers · Hideyo Kawakita · Luisa M. Lara et al. [full author details at the end of the article]

Received: 14 October 2022 / Accepted: 29 November 2023

© The Author(s) 2024

## Abstract

Here we describe the novel, multi-point Comet Interceptor mission. It is dedicated to the exploration of a little-processed long-period comet, possibly entering the inner Solar System for the first time, or to encounter an interstellar object originating at another star. The objectives of the mission are to address the following questions: What are the surface composition, shape, morphology, and structure of the target object? What is the composition of the gas and dust in the coma, its connection to the nucleus, and the nature of its interaction with the solar wind? The mission was proposed to the European Space Agency in 2018, and formally adopted by the agency in June 2022, for launch in 2029 together with the Ariel mission. Comet Interceptor will take advantage of the opportunity presented by ESA's F-Class call for fast, flexible, low-cost missions to which it was proposed. The call required a launch to a halo orbit around the Sun-Earth L2 point. The mission can take advantage of this placement to wait for the discovery of a suitable comet reachable with its minimum $ΔV$ capability of 600 $ms^{-1}$. Comet Interceptor will be unique in encountering and studying, at a nominal closest approach distance of 1000 km, a comet that represents a near-pristine sample of material from the formation of the Solar System. It will also add a capability that no previous cometary mission has had, which is to deploy two sub-probes - B1, provided by the Japanese space agency, JAXA, and B2 - that will follow different trajectories through the coma. While the main probe passes at a nominal 1000 km distance, probes B1 and B2 will follow different chords through the coma at distances of 850 km and 400 km, respectively. The result will be unique, simultaneous, spatially resolved information of the 3-dimensional properties of the target comet and its interaction with the space environment. We present the mission's science background leading to these objectives, as well as an overview of the scientific instruments, mission design, and schedule.

**Keywords** Comets · Spacecraft · Instruments – spaceborne and space research

## 1 Introduction

Comets are the surviving remnants of the original building blocks of the Solar System. A significant amount of pristine material from the formation of the Solar System persists in the

<footnote>Note by the Editor: This is a Special Communication. In addition to invited review papers and topical collections, Space Science Reviews publishes unsolicited Special Communications. These are papers linked to an earlier topical volume/collection, report-type papers, or timely papers dealing with a strong space-science-technology combination (such papers summarize the science and technology of an instrument or mission in one paper).</footnote>
<figure><img src="image_3.png" /></figure>

# The Comet Interceptor Mission

Geraint H. Jones · Colin Snodgrass · Cecilia Tubiana · Michael Küppers · Hideyo Kawakita · Luisa M. Lara et al. [full author details at the end of the article]

Received: 14 October 2022 / Accepted: 29 November 2023

© The Author(s) 2024

## Abstract

Here we describe the novel, multi-point Comet Interceptor mission. It is dedicated to the exploration of a little-processed long-period comet, possibly entering the inner Solar System for the first time, or to encounter an interstellar object originating at another star. The objectives of the mission are to address the following questions: What are the surface composition, shape, morphology, and structure of the target object? What is the composition of the gas and dust in the coma, its connection to the nucleus, and the nature of its interaction with the solar wind? The mission was proposed to the European Space Agency in 2018, and formally adopted by the agency in June 2022, for launch in 2029 together with the Ariel mission. Comet Interceptor will take advantage of the opportunity presented by ESA's F-Class call for fast, flexible, low-cost missions to which it was proposed. The call required a launch to a halo orbit around the Sun-Earth L2 point. The mission can take advantage of this placement to wait for the discovery of a suitable comet reachable with its minimum $ΔV$ capability of 600 $ms^{-1}$. Comet Interceptor will be unique in encountering and studying, at a nominal closest approach distance of 1000 km, a comet that represents a near-pristine sample of material from the formation of the Solar System. It will also add a capability that no previous cometary mission has had, which is to deploy two sub-probes - B1, provided by the Japanese space agency, JAXA, and B2 - that will follow different trajectories through the coma. While the main probe passes at a nominal 1000 km distance, probes B1 and B2 will follow different chords through the coma at distances of 850 km and 400 km, respectively. The result will be unique, simultaneous, spatially resolved information of the 3-dimensional properties of the target comet and its interaction with the space environment. We present the mission's science background leading to these objectives, as well as an overview of the scientific instruments, mission design, and schedule.

**Keywords** Comets · Spacecraft · Instruments – spaceborne and space research

## 1 Introduction

Comets are the surviving remnants of the original building blocks of the Solar System. A significant amount of pristine material from the formation of the Solar System persists in the

<footnote>Note by the Editor: This is a Special Communication. In addition to invited review papers and topical collections, Space Science Reviews publishes unsolicited Special Communications. These are papers linked to an earlier topical volume/collection, report-type papers, or timely papers dealing with a strong space-science-technology combination (such papers summarize the science and technology of an instrument or mission in one paper).</footnote>
Oort Cloud (OC), which extends out to at least 1 light year from the Sun, unmodified or barely modified since the earliest days of the Solar System. All other material to which we have access – asteroids, meteorites, lunar and planetary surface samples and atmospheres – has been significantly or heavily modified, both physically and chemically since its formation. While multiple comets have been studied *in situ*, all, with the exception of 1P/Halley, have arguably been low-activity, short-period, highly-evolved comets, which have changed radically since their formation, having spent considerable time in the inner Solar System. Even Comet Halley, with its longer period and high activity is thought to have made several thousand returns to perihelion. Such objects are highly depleted in volatiles (A'Hearn et al. 1995), particularly low-temperature volatiles, at least in their outer layers. While all the comets studied *in situ* to date share certain characteristics (e.g., low albedo, jet activity, ...), the observed nucleus morphology shows considerable differences between objects, suggesting that they have experienced radically different evolutionary processes, possibly due, at least in part, to their orbital instability that leads to considerable variations of perihelion distance and thus insolation over timescales of decades and centuries. These highly evolved objects also show significant morphological differences with the only small Kuiper-Belt object to be observed to date, 2014 MU69 (Arrokoth), studied by the New Horizons mission during a ~3540 km flyby at a relative speed of 14.3 km s⁻¹, around 3 times further and likely slower than the proposed Comet Interceptor encounter.

Dynamically new comets arriving from the OC have, by definition, never visited the inner Solar System before. They are expected to be rich in the low-temperature volatiles retained from their formation and close to their pristine state, particularly in the case of a pre-perihelion encounter. Given the long lead-up times required to plan and launch a space mission, it has been impossible prior to Comet Interceptor to contemplate encounter missions with such objects, to observe a pristine, or minimally evolved nucleus and study its morphology, its activity, and its interactions with the interplanetary medium. The opportunity to observe, *in situ*, a dynamically new comet that is entering the inner Solar System for the first time will allow the data from previous comet encounter missions and from ground-based campaigns to be placed in its proper context. Such an encounter will allow the composition, both chemical and isotopic, of the protosolar nebula to be studied. This will offer valuable insights into the chemical and isotopic evolution of the Solar System since its formation.

It has been known for many years that there is a population of dynamically new objects that are lost to the Solar System. These objects' paths are perturbed in the inner Solar System into hyperbolic orbits and, thus, escape solar influence. It has long been assumed that there is a similar population of objects external to the Solar System that enter it from interstellar space: these objects have been expelled from the stellar system where they formed and thus present the opportunity of sampling primordial material formed around other stars, in different conditions to the formation of our Solar System. Such an interstellar wanderer manifests itself by having a sufficiently large hyperbolic excess of velocity to demonstrate that it could not have originated in the OC. Two such objects are now known: 1I/'Oumuamua and 2I/Borisov, discovered in 2017 and 2019 respectively. While the probability of detecting such an interstellar object that satisfies the targeting conditions for Comet Interceptor is low, it is non-zero: such an object would be potentially an extremely high-value target scientifically. It is expected that the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) will increase considerably the detection capability, not just of dynamically new objects, but also of this population of interstellar comets.

Comet Interceptor was proposed to the European Space Agency by an international consortium of scientists and engineers, in response to a call for mission proposals issued by
Oort Cloud (OC), which extends out to at least 1 light year from the Sun, unmodified or barely modified since the earliest days of the Solar System. All other material to which we have access – asteroids, meteorites, lunar and planetary surface samples and atmospheres – has been significantly or heavily modified, both physically and chemically since its formation. While multiple comets have been studied *in situ*, all, with the exception of 1P/Halley, have arguably been low-activity, short-period, highly-evolved comets, which have changed radically since their formation, having spent considerable time in the inner Solar System. Even Comet Halley, with its longer period and high activity is thought to have made several thousand returns to perihelion. Such objects are highly depleted in volatiles (A'Hearn et al. 1995), particularly low-temperature volatiles, at least in their outer layers. While all the comets studied *in situ* to date share certain characteristics (e.g., low albedo, jet activity, ...), the observed nucleus morphology shows considerable differences between objects, suggesting that they have experienced radically different evolutionary processes, possibly due, at least in part, to their orbital instability that leads to considerable variations of perihelion distance and thus insolation over timescales of decades and centuries. These highly evolved objects also show significant morphological differences with the only small Kuiper-Belt object to be observed to date, 2014 MU69 (Arrokoth), studied by the New Horizons mission during a ~3540 km flyby at a relative speed of 14.3 km s⁻¹, around 3 times further and likely slower than the proposed Comet Interceptor encounter.

Dynamically new comets arriving from the OC have, by definition, never visited the inner Solar System before. They are expected to be rich in the low-temperature volatiles retained from their formation and close to their pristine state, particularly in the case of a pre-perihelion encounter. Given the long lead-up times required to plan and launch a space mission, it has been impossible prior to Comet Interceptor to contemplate encounter missions with such objects, to observe a pristine, or minimally evolved nucleus and study its morphology, its activity, and its interactions with the interplanetary medium. The opportunity to observe, *in situ*, a dynamically new comet that is entering the inner Solar System for the first time will allow the data from previous comet encounter missions and from ground-based campaigns to be placed in its proper context. Such an encounter will allow the composition, both chemical and isotopic, of the protosolar nebula to be studied. This will offer valuable insights into the chemical and isotopic evolution of the Solar System since its formation.

It has been known for many years that there is a population of dynamically new objects that are lost to the Solar System. These objects' paths are perturbed in the inner Solar System into hyperbolic orbits and, thus, escape solar influence. It has long been assumed that there is a similar population of objects external to the Solar System that enter it from interstellar space: these objects have been expelled from the stellar system where they formed and thus present the opportunity of sampling primordial material formed around other stars, in different conditions to the formation of our Solar System. Such an interstellar wanderer manifests itself by having a sufficiently large hyperbolic excess of velocity to demonstrate that it could not have originated in the OC. Two such objects are now known: 1I/'Oumuamua and 2I/Borisov, discovered in 2017 and 2019 respectively. While the probability of detecting such an interstellar object that satisfies the targeting conditions for Comet Interceptor is low, it is non-zero: such an object would be potentially an extremely high-value target scientifically. It is expected that the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) will increase considerably the detection capability, not just of dynamically new objects, but also of this population of interstellar comets.

Comet Interceptor was proposed to the European Space Agency by an international consortium of scientists and engineers, in response to a call for mission proposals issued by
<figure><img src="image_3.png" /><figcaption>**Fig. 1** Sketch of the Comet Interceptor flyby, not to scale. Spacecraft A will pass furthest from the nucleus, with probes B1 and B2 passing closer. Both probes will relay their data in real time to be stored on spacecraft A for later transmission</figcaption></figure>

ESA in July 2018. The project was selected in June 2019 as the prime candidate for the agency's first F-class mission in its Cosmic Vision Programme. Phase A started in 2020, with two parallel industrial studies. In April 2021, Comet Interceptor moved into Phase B1. During Phase A and B1, the Science Study Team and Science Steering Committee studied and validated the science case, supported by the ESA study team and members of the proposal consortium, organized into working groups. The mission was formally adopted by ESA in June 2022.

The mission aims to intercept a long-period comet, LPC, ideally a dynamically-new comet, DNC, one of a subset of LPCs that is approaching the inner Solar System for the first time, or even an interstellar body. The intercept will involve a close-approach flyby scenario using three elements: a mother spacecraft, spacecraft A, and smaller probes named B1 and B2 that are carried as payloads until the flyby, and delivered to different flyby trajectories. This will allow the gathering of remote and *in situ* multi-point observations of the comet and its coma (Fig. 1).

Comet Interceptor follows the successful history of European exploration of comets, following in the footsteps of ESA's Giotto and Rosetta missions, as well as the substantial contribution of scientists from European countries to the two Soviet-led VEGA missions. A preliminary description of the mission as proposed to ESA was published by Snodgrass and Jones (2019). In this work, we describe the scientific rationale for the mission, as well as the mission itself, in much greater depth. This includes a description of the project's mature payload and mission design as they stand shortly after mission adoption.

# 2 Scientific Context

## 2.1 Introduction

While the Giotto flyby of 1P/Halley was the first mission to provide good resolution images of a cometary nucleus, the Rosetta rendezvous was conceived, e.g. Glassmeier et al. (2007),
<figure><img src="image_4.png" /><figcaption>**Fig. 1** Sketch of the Comet Interceptor flyby, not to scale. Spacecraft A will pass furthest from the nucleus, with probes B1 and B2 passing closer. Both probes will relay their data in real time to be stored on spacecraft A for later transmission</figcaption></figure>

ESA in July 2018. The project was selected in June 2019 as the prime candidate for the agency's first F-class mission in its Cosmic Vision Programme. Phase A started in 2020, with two parallel industrial studies. In April 2021, Comet Interceptor moved into Phase B1. During Phase A and B1, the Science Study Team and Science Steering Committee studied and validated the science case, supported by the ESA study team and members of the proposal consortium, organized into working groups. The mission was formally adopted by ESA in June 2022.

The mission aims to intercept a long-period comet, LPC, ideally a dynamically-new comet, DNC, one of a subset of LPCs that is approaching the inner Solar System for the first time, or even an interstellar body. The intercept will involve a close-approach flyby scenario using three elements: a mother spacecraft, spacecraft A, and smaller probes named B1 and B2 that are carried as payloads until the flyby, and delivered to different flyby trajectories. This will allow the gathering of remote and *in situ* multi-point observations of the comet and its coma (Fig. 1).

Comet Interceptor follows the successful history of European exploration of comets, following in the footsteps of ESA's Giotto and Rosetta missions, as well as the substantial contribution of scientists from European countries to the two Soviet-led VEGA missions. A preliminary description of the mission as proposed to ESA was published by Snodgrass and Jones (2019). In this work, we describe the scientific rationale for the mission, as well as the mission itself, in much greater depth. This includes a description of the project's mature payload and mission design as they stand shortly after mission adoption.

# 2 Scientific Context

## 2.1 Introduction

While the Giotto flyby of 1P/Halley was the first mission to provide good resolution images of a cometary nucleus, the Rosetta rendezvous was conceived, e.g. Glassmeier et al. (2007),
as the first mission to monitor the changing activity of a cometary nucleus before and after
perihelion, which it achieved (Taylor et al. 2017). Both missions revolutionized cometary
science. Rosetta showed that the composition of comet 67P/Churyumov-Gerasimenko (re-
ferred to as 67P) is rich in organics and has a different water isotope ratio to Earth's (Altwegg
et al. 2015); that its surface structure and morphology (Sierks et al. 2015) are controlled by
seasonal activity (Hässig et al. 2015), with starkly contrasting areas dominated by erosion
and by dust fall-back (El-Maarry et al. 2015, 2016); and that the comet's inner coma is
highly dynamic, changing in time and space, e.g., Della Corte et al. (2015), Feldman et al.
(2015), Lee et al. (2015). Rosetta also raised important new questions:

- What properties are primordial, and reflect the process of comet formation, and what are evolutionary features?

- How do these properties control cometary activity?

- Are the differences in composition seen in the coma and solar wind interaction spatial or temporal in origin?

We detail these problems in the following Sects. 2.2 and 2.3, divided into two themes that address Nucleus and Coma science, respectively, and present the Comet Interceptor mission to address them by making unique multi-point measurements at a much more pristine type of comet. The exact sources and mechanisms driving activity in comets remain a puzzle after Rosetta. Due to the flyby nature of the mission, Comet Interceptor will not be able to monitor changes in activity. However, detailed observations of the surface and the coma will be combined to address this point (i.e., potentially linking the coma structures to nucleus surface features). These are described within Sect. 2.3, in the Coma theme, below.

## 2.2 Theme 1: Comet Nucleus Science

### 2.2.1 Introduction

This science theme focuses on the nucleus, looking to answer the following questions. What are the:

- surface composition,

- shape,

- morphology, and

- structure

of the target object?

These questions can only be answered by *in situ* (as opposed to remote sensing from the Earth) measurements, in particular given the relative size of the nucleus with respect to the coma which hides the nucleus for observations from Earth and near-Earth based observatories.

Characterisation of the nucleus by Comet Interceptor will provide information on its bulk properties (shape, rotation rate, surface structure, etc.), which will in turn provide constraints for surface features' formation timescales. It is unknown whether there might be craters, depressions, layers, regolith, or boulders present – these could provide unique insights into early surface evolution. Do primordial small bodies display a singular primordial surface type, or do they show surface diversity at different size scales? Any impact crater population might provide an unmodified record of early bombardment in the Solar System, as few impacts are expected in the OC. This will give a unique insight into the early Solar System's accretion processes and the characteristics of primordial small planetesimals.
as the first mission to monitor the changing activity of a cometary nucleus before and after
perihelion, which it achieved (Taylor et al. 2017). Both missions revolutionized cometary
science. Rosetta showed that the composition of comet 67P/Churyumov-Gerasimenko (re-
ferred to as 67P) is rich in organics and has a different water isotope ratio to Earth's (Altwegg
et al. 2015); that its surface structure and morphology (Sierks et al. 2015) are controlled by
seasonal activity (Hässig et al. 2015), with starkly contrasting areas dominated by erosion
and by dust fall-back (El-Maarry et al. 2015, 2016); and that the comet's inner coma is
highly dynamic, changing in time and space, e.g., Della Corte et al. (2015), Feldman et al.
(2015), Lee et al. (2015). Rosetta also raised important new questions:

- What properties are primordial, and reflect the process of comet formation, and what are evolutionary features?

- How do these properties control cometary activity?

- Are the differences in composition seen in the coma and solar wind interaction spatial or temporal in origin?

We detail these problems in the following Sects. 2.2 and 2.3, divided into two themes that address Nucleus and Coma science, respectively, and present the Comet Interceptor mission to address them by making unique multi-point measurements at a much more pristine type of comet. The exact sources and mechanisms driving activity in comets remain a puzzle after Rosetta. Due to the flyby nature of the mission, Comet Interceptor will not be able to monitor changes in activity. However, detailed observations of the surface and the coma will be combined to address this point (i.e., potentially linking the coma structures to nucleus surface features). These are described within Sect. 2.3, in the Coma theme, below.

## 2.2 Theme 1: Comet Nucleus Science

### 2.2.1 Introduction

This science theme focuses on the nucleus, looking to answer the following questions. What are the:

- surface composition,

- shape,

- morphology, and

- structure

of the target object?

These questions can only be answered by *in situ* (as opposed to remote sensing from the Earth) measurements, in particular given the relative size of the nucleus with respect to the coma which hides the nucleus for observations from Earth and near-Earth based observatories.

Characterisation of the nucleus by Comet Interceptor will provide information on its bulk properties (shape, rotation rate, surface structure, etc.), which will in turn provide constraints for surface features' formation timescales. It is unknown whether there might be craters, depressions, layers, regolith, or boulders present – these could provide unique insights into early surface evolution. Do primordial small bodies display a singular primordial surface type, or do they show surface diversity at different size scales? Any impact crater population might provide an unmodified record of early bombardment in the Solar System, as few impacts are expected in the OC. This will give a unique insight into the early Solar System's accretion processes and the characteristics of primordial small planetesimals.
<figure><img src="image_4.png" /><figcaption>**Fig. 2** Classifications of comets by Levison (1996). T refers to the Tisserand parameter, whilst parameter *a* refers to orbital semi-major axis</figcaption></figure>

All comets already visited by spacecraft have undergone shape changes through sublimation and specific surface evolution processes during repeated perihelion passages. Recent results suggest that bi-lobate nuclei are common (e.g., Giotto, Rosetta, and other Jupiter Family Comet (JFC) missions). A comparison between a Long-Period Comet, LPC, and New Horizons studies of 486958 Arrokoth will be particularly instructive, as both these volatile-rich bodies are largely unaltered since formation, but have likely formed and subsequently resided in different Solar System contexts. An LPC (and even more-so a Dynamically-New Comet, DNC) would have experienced few changes due to insolation in recent times, when compared to JFCs, but could have had significant processing earlier in its lifetime. Characterisation of the difference and similarities will be invaluable, examining surface morphology and comparing to comets imaged by previous missions. In addition to this, composition measurements of a primordial surface, in comparison with Rosetta results, may tell us about chemical processing as comets evolve.

### 2.2.2 The Origin of Different Classes of Comet Nuclei

Comets of all kinds formed in the outer part of the Solar System's protoplanetary disc, where ices could condense, and the giant planets also formed. As these planets settled into their final orbits, the small bodies in this region were scattered to form the various comet reservoirs that we know today: the Scattered Disc of the Kuiper Belt (KB), the source of low-inclination JFCs; the OC at the edge of the Solar System, which supplies the population of LPCs; and the recently identified probable reservoir of icy bodies in the main asteroid belt that occasionally show activity as Main Belt Comets. The taxonomy of Levison (1996) splits comets into low-inclination 'Ecliptic' comets from the KB, of which JFCs are the dominant subset, and 'Nearly Isotropic Comets' (NICs), which can have any inclination, including highly retrograde orbits, from the OC (Fig. 2). NICs are subdivided into the LPCs (orbital periods >200 years) and the Halley Type Comets (HTCs) with shorter periods. There is some debate on the origin of HTCs, with competing models suggesting that these can also come from the Scattered Disc (e.g., Levison et al. 2006; Wang and Brasser 2014; Fernández et al. 2016, and Nesvorný et al. 2017). LPCs can be further divided into DNCs and returning comets, depending on whether or not a previous perihelion distance that an object had was thought to be within the planetary region.

Planetary systems naturally scatter and eject most of the planetesimals that they form from their primordial discs of dust and gas. These planetesimals travel the Galaxy as interstellar objects (ISOs).
<figure><img src="image_5.png" /><figcaption>**Fig. 2** Classifications of comets by Levison (1996). T refers to the Tisserand parameter, whilst parameter *a* refers to orbital semi-major axis</figcaption></figure>

All comets already visited by spacecraft have undergone shape changes through sublimation and specific surface evolution processes during repeated perihelion passages. Recent results suggest that bi-lobate nuclei are common (e.g., Giotto, Rosetta, and other Jupiter Family Comet (JFC) missions). A comparison between a Long-Period Comet, LPC, and New Horizons studies of 486958 Arrokoth will be particularly instructive, as both these volatile-rich bodies are largely unaltered since formation, but have likely formed and subsequently resided in different Solar System contexts. An LPC (and even more-so a Dynamically-New Comet, DNC) would have experienced few changes due to insolation in recent times, when compared to JFCs, but could have had significant processing earlier in its lifetime. Characterisation of the difference and similarities will be invaluable, examining surface morphology and comparing to comets imaged by previous missions. In addition to this, composition measurements of a primordial surface, in comparison with Rosetta results, may tell us about chemical processing as comets evolve.

### 2.2.2 The Origin of Different Classes of Comet Nuclei

Comets of all kinds formed in the outer part of the Solar System's protoplanetary disc, where ices could condense, and the giant planets also formed. As these planets settled into their final orbits, the small bodies in this region were scattered to form the various comet reservoirs that we know today: the Scattered Disc of the Kuiper Belt (KB), the source of low-inclination JFCs; the OC at the edge of the Solar System, which supplies the population of LPCs; and the recently identified probable reservoir of icy bodies in the main asteroid belt that occasionally show activity as Main Belt Comets. The taxonomy of Levison (1996) splits comets into low-inclination 'Ecliptic' comets from the KB, of which JFCs are the dominant subset, and 'Nearly Isotropic Comets' (NICs), which can have any inclination, including highly retrograde orbits, from the OC (Fig. 2). NICs are subdivided into the LPCs (orbital periods >200 years) and the Halley Type Comets (HTCs) with shorter periods. There is some debate on the origin of HTCs, with competing models suggesting that these can also come from the Scattered Disc (e.g., Levison et al. 2006; Wang and Brasser 2014; Fernández et al. 2016, and Nesvorný et al. 2017). LPCs can be further divided into DNCs and returning comets, depending on whether or not a previous perihelion distance that an object had was thought to be within the planetary region.

Planetary systems naturally scatter and eject most of the planetesimals that they form from their primordial discs of dust and gas. These planetesimals travel the Galaxy as interstellar objects (ISOs).
It was thought that the OC was primarily populated by bodies that formed near to Jupiter and Saturn, while the objects that would populate the KB formed more or less *in situ* at their current distances from the Sun (e.g., Dones et al. 2004; Duncan and Levison 1997), but modern thinking suggests a broader mixing of comets from different original locations in the protoplanetary disc being scattered to the various reservoirs (e.g., Dones et al. 2015). Results from the Stardust mission showed that material that must have formed in the warmer regions in the inner Solar System was incorporated into comets, suggesting widespread mixing within the disc (Brownlee 2014). It is therefore likely that all Solar System comets have broadly similar initial properties (although perhaps differing in composition, especially at an isotopic level, depending on where the majority of their component ices condensed from gas; see Sect. 2.2.4 and Rubin et al. 2020 for details). What we can expect to be different between JFCs (and perhaps HTCs) and LPCs is the degree to which they have subsequently evolved: the more distant OC stored comets in a colder environment (~10 K vs. ~40 K in the KB; Weissman et al. 2020), and LPCs (and especially DNCs) enter the inner Solar System directly from this cold reservoir, while JFCs have evolved through a period (~10^4 years) in the Centaur region (Volk and Malhotra 2008), with orbits between the giant planets, where significant activity can be expected to modify at least the surface layers. As such, even a 'new' JFC can be expected to be significantly modified from its primitive state, while LPCs should retain largely similar properties from the time that they were first ejected into the OC (discussed in more detail in Sect. 2.2.5).

The motivation for a mission to a less evolved comet is clear: new comets retain, to some degree, the properties of the building blocks of planets, and are at least highly primitive bodies, essentially unaltered since their ejection into their respective reservoirs, expect for cosmic ray processing, e.g. Garrod (2019). How much processing comets underwent during their formation in the disc, prior to ejection, is the subject of considerable debate (see, e.g., Weissman et al. 2020) and another reason to visit a comet from the OC, which retains the surface properties from the last interactions it had before ejection. There are two main theories of comet formation still under consideration following Rosetta, both of which are supported by evidence from the mission, but neither is entirely satisfactory. These are hierarchical accretion (e.g., Davidsson et al. 2016) and accretion of 'pebbles' by streaming instabilities (e.g., Blum et al. 2017). Both make predictions for the size scale of typical constituent building blocks of comets, and both sets of scale in features can be found in Rosetta data. Whether or not these features are primordial or evolutionary is the key question to advance this debate. There is also considerable interest in the question of how many collisions cometary nuclei have undergone, and the effects that these collisions would have on the properties of their ices, focussing on the period before the comets were ejected from the disc to their reservoirs. While the OC is so vast that collisions between bodies after ejection are virtually impossible, Jutzi et al. (2017) and Schwartz et al. (2018) both show that collisions large enough to alter the bulk shape of nuclei (perhaps forming the typical bilobed structure seen in many comets) were almost impossible to avoid in the disc prior to ejection. Investigating a comet that has not been significantly further altered since its last pre-ejection collision would be of great value to better constrain these models, and reveal how much these collisions locally alter material, compared with the (supposed) unaltered nucleus ices further from the collision site(s).

### 2.2.3 Bulk Properties

The current understanding of the bulk properties of comet nuclei is a result of both remote telescope observations and spacecraft visits. Spacecraft *in situ* observations provide precise
It was thought that the OC was primarily populated by bodies that formed near to Jupiter and Saturn, while the objects that would populate the KB formed more or less *in situ* at their current distances from the Sun (e.g., Dones et al. 2004; Duncan and Levison 1997), but modern thinking suggests a broader mixing of comets from different original locations in the protoplanetary disc being scattered to the various reservoirs (e.g., Dones et al. 2015). Results from the Stardust mission showed that material that must have formed in the warmer regions in the inner Solar System was incorporated into comets, suggesting widespread mixing within the disc (Brownlee 2014). It is therefore likely that all Solar System comets have broadly similar initial properties (although perhaps differing in composition, especially at an isotopic level, depending on where the majority of their component ices condensed from gas; see Sect. 2.2.4 and Rubin et al. 2020 for details). What we can expect to be different between JFCs (and perhaps HTCs) and LPCs is the degree to which they have subsequently evolved: the more distant OC stored comets in a colder environment (~10 K vs. ~40 K in the KB; Weissman et al. 2020), and LPCs (and especially DNCs) enter the inner Solar System directly from this cold reservoir, while JFCs have evolved through a period (~10^4 years) in the Centaur region (Volk and Malhotra 2008), with orbits between the giant planets, where significant activity can be expected to modify at least the surface layers. As such, even a ‘new’ JFC can be expected to be significantly modified from its primitive state, while LPCs should retain largely similar properties from the time that they were first ejected into the OC (discussed in more detail in Sect. 2.2.5).

The motivation for a mission to a less evolved comet is clear: new comets retain, to some degree, the properties of the building blocks of planets, and are at least highly primitive bodies, essentially unaltered since their ejection into their respective reservoirs, expect for cosmic ray processing, e.g. Garrod (2019). How much processing comets underwent during their formation in the disc, prior to ejection, is the subject of considerable debate (see, e.g., Weissman et al. 2020) and another reason to visit a comet from the OC, which retains the surface properties from the last interactions it had before ejection. There are two main theories of comet formation still under consideration following Rosetta, both of which are supported by evidence from the mission, but neither is entirely satisfactory. These are hierarchical accretion (e.g., Davidsson et al. 2016) and accretion of ‘pebbles’ by streaming instabilities (e.g., Blum et al. 2017). Both make predictions for the size scale of typical constituent building blocks of comets, and both sets of scale in features can be found in Rosetta data. Whether or not these features are primordial or evolutionary is the key question to advance this debate. There is also considerable interest in the question of how many collisions cometary nuclei have undergone, and the effects that these collisions would have on the properties of their ices, focussing on the period before the comets were ejected from the disc to their reservoirs. While the OC is so vast that collisions between bodies after ejection are virtually impossible, Jutzi et al. (2017) and Schwartz et al. (2018) both show that collisions large enough to alter the bulk shape of nuclei (perhaps forming the typical bilobed structure seen in many comets) were almost impossible to avoid in the disc prior to ejection. Investigating a comet that has not been significantly further altered since its last pre-ejection collision would be of great value to better constrain these models, and reveal how much these collisions locally alter material, compared with the (supposed) unaltered nucleus ices further from the collision site(s).

### 2.2.3 Bulk Properties

The current understanding of the bulk properties of comet nuclei is a result of both remote telescope observations and spacecraft visits. Spacecraft *in situ* observations provide precise
measurements of the physical properties of individual nuclei. On the other hand, telescopes both in the optical and in the thermal infrared allow the coarse characterization of numerous nuclei, therefore enabling population studies. One of the easiest to constrain nucleus properties is size. Since the effective radius of comet nuclei (the radius of a sphere having the same volume as the comet nucleus) can in principle be derived from single-epoch observations, the sizes of over 200 comets have been determined (Knight et al. 2023). This large database reveals a broad diversity of comet sizes: from hundreds of metres to a few tens of km (see Fig. 3) and has been used to derive the size-frequency distribution (SFD) of comets. The SFD of comet nuclei is believed to bear evidence of the processes involved in their formation and subsequent collisional and/or activity-driven evolution. De-biased Comet SFD studies show interesting trends indicating that the average size of LPCs is larger than that of JFCs (Bauer et al. 2017). Yet important questions such as whether there is a paucity of objects smaller than 2 km (see Bauer et al. 2017) and whether comets are a collisionally evolved population remain unresolved (see Weissman et al. 2020).

Another parameter broadly used to analyse the formation and evolution of comet nuclei is shape. The shapes of six nuclei have been studied in detail from *in situ* observations. The observations from the Rosetta mission to 67P stand out with their unprecedented resolution, which enabled a global 3D shape model with resolution down to metre scale (Preusker et al. 2017). In at least eight exceptional cases in which the comets approached the Earth sufficiently and could thus be observed by radar, shape models could be constructed (see Knight et al. 2023). For other comets, however, nucleus shapes can only be studied in terms of elongation. Measuring the peak-to-peak amplitude of the rotational lightcurve, Δm, provides a lower limit of the axial ratio of a/b of the comet nucleus. In principle it is possible to derive convex shape models of comet nuclei, provided that the comets are observed at a wide variety of different observing geometries, but this technique has so far only been applied to 67P in preparation for Rosetta (Lowry et al. 2012). Compiling elongation estimates from the different methods, Kokotanekova et al. (2017) determined a median axial ratio of a/b = 1.5 for JFCs, similar to the previous estimates from Lamy et al. (2004) and Snodgrass et al. (2011). As it can be seen in Fig. 3, the measured minimum a/b spans a range of ~1.0 to >3.0. The largest known axial ratio belongs to 103P (Thomas et al. 2013) which is one of the four comet nuclei visited by spacecraft that has a bilobate shape (the others are 1P Keller et al. 1986, 19P Britt et al. 2004; Oberst et al. 2004 and 67P Sierks et al. 2015). Moreover, the radar observations of 8P are also best modelled by assuming a contact binary shape (Harmon et al. 2010). This noticeable overabundance of highly elongated/bilobate objects in comparison to other small-body populations suggests some important difference in formation and/or evolutionary processes, which has not yet been fully explained. Whether or not the first LPC nucleus to be imaged *in situ* also shows a contact binary morphology will provide an important constraint on this question, and point to whether it is more likely attributable to a formation or an evolutionary process.

Rosetta's observations were also the first to enable the characterization of 67P's opposition effect; the non-linear increase of its phase function close to $\alpha = 0°$ (Fornasier et al. 2015; Masoumzadeh et al. 2017; Hasselmann et al. 2017). Few of the other comets visited by spacecraft, or observed remotely from the ground, have been observed close to opposition and all other comet phase curves are well described by linear phase functions. It is possible that the phase function slope at moderate phase angles ($\alpha \sim 5–60°$) can be used to reveal the level of erosion of the comet surface (see Longobardo et al. 2017; Kokotanekova et al. 2018; Vincent 2019). It remains to be seen if this also applies to LPCs, with very different surface evolutions, as all phase function data so far is on JFCs.

It is important to note that the average colours of JFCs, LPCs and dormant comets are indistinguishable within the uncertainties for each class (Jewitt 2015). Finally, Rosetta has
measurements of the physical properties of individual nuclei. On the other hand, telescopes both in the optical and in the thermal infrared allow the coarse characterization of numerous nuclei, therefore enabling population studies. One of the easiest to constrain nucleus properties is size. Since the effective radius of comet nuclei (the radius of a sphere having the same volume as the comet nucleus) can in principle be derived from single-epoch observations, the sizes of over 200 comets have been determined (Knight et al. 2023). This large database reveals a broad diversity of comet sizes: from hundreds of metres to a few tens of km (see Fig. 3) and has been used to derive the size-frequency distribution (SFD) of comets. The SFD of comet nuclei is believed to bear evidence of the processes involved in their formation and subsequent collisional and/or activity-driven evolution. De-biased Comet SFD studies show interesting trends indicating that the average size of LPCs is larger than that of JFCs (Bauer et al. 2017). Yet important questions such as whether there is a paucity of objects smaller than 2 km (see Bauer et al. 2017) and whether comets are a collisionally evolved population remain unresolved (see Weissman et al. 2020).

Another parameter broadly used to analyse the formation and evolution of comet nuclei is shape. The shapes of six nuclei have been studied in detail from *in situ* observations. The observations from the Rosetta mission to 67P stand out with their unprecedented resolution, which enabled a global 3D shape model with resolution down to metre scale (Preusker et al. 2017). In at least eight exceptional cases in which the comets approached the Earth sufficiently and could thus be observed by radar, shape models could be constructed (see Knight et al. 2023). For other comets, however, nucleus shapes can only be studied in terms of elongation. Measuring the peak-to-peak amplitude of the rotational lightcurve, Δm, provides a lower limit of the axial ratio of a/b of the comet nucleus. In principle it is possible to derive convex shape models of comet nuclei, provided that the comets are observed at a wide variety of different observing geometries, but this technique has so far only been applied to 67P in preparation for Rosetta (Lowry et al. 2012). Compiling elongation estimates from the different methods, Kokotanekova et al. (2017) determined a median axial ratio of a/b = 1.5 for JFCs, similar to the previous estimates from Lamy et al. (2004) and Snodgrass et al. (2011). As it can be seen in Fig. 3, the measured minimum a/b spans a range of ~1.0 to >3.0. The largest known axial ratio belongs to 103P (Thomas et al. 2013) which is one of the four comet nuclei visited by spacecraft that has a bilobate shape (the others are 1P Keller et al. 1986, 19P Britt et al. 2004; Oberst et al. 2004 and 67P Sierks et al. 2015). Moreover, the radar observations of 8P are also best modelled by assuming a contact binary shape (Harmon et al. 2010). This noticeable overabundance of highly elongated/bilobate objects in comparison to other small-body populations suggests some important difference in formation and/or evolutionary processes, which has not yet been fully explained. Whether or not the first LPC nucleus to be imaged *in situ* also shows a contact binary morphology will provide an important constraint on this question, and point to whether it is more likely attributable to a formation or an evolutionary process.

Rosetta's observations were also the first to enable the characterization of 67P's opposition effect; the non-linear increase of its phase function close to $\alpha = 0°$ (Fornasier et al. 2015; Masoumzadeh et al. 2017; Hasselmann et al. 2017). Few of the other comets visited by spacecraft, or observed remotely from the ground, have been observed close to opposition and all other comet phase curves are well described by linear phase functions. It is possible that the phase function slope at moderate phase angles ($\alpha \sim 5–60°$) can be used to reveal the level of erosion of the comet surface (see Longobardo et al. 2017; Kokotanekova et al. 2018; Vincent 2019). It remains to be seen if this also applies to LPCs, with very different surface evolutions, as all phase function data so far is on JFCs.

It is important to note that the average colours of JFCs, LPCs and dormant comets are indistinguishable within the uncertainties for each class (Jewitt 2015). Finally, Rosetta has
<figure><img src="image_5.png" /><figcaption>**Fig. 3** Histograms showing the range of observed comet properties (JFCs in blue and Halley-type/LPCs in grey hashing). The figure provides an overview of the range of measured values and the sample size of each parameter. In case a comet has multiple measurements of the same property, the most recently reported sufficiently precise measurement is displayed. The effective radius histogram is limited to thermal-IR measurements from Fernández et al. (2013) and Bauer et al. (2017). The axis ratios plotted are lower limits for all comets except for those visited by spacecraft. Adapted from Knight et al. (2023)</figcaption></figure>

revealed that the surface spectrum of 67P changes as the comet passes through perihelion due to the comet's seasonal water-ice cycle: the nucleus' mean spectral slope changed by ~30% or 50% (Fornasier et al. 2016; Filacchione et al. 2020). Polarimetric studies of comet surfaces have not yet been performed during a spacecraft visit. There are only two JFCs for which polarisation studies from the ground have been published, 2P/Encke (Boehnhardt et al. 2008) and 209P/LINEAR (Kuroda et al. 2015). These data are too limited to extract any general conclusions about the bulk polarisation properties of comet nuclei besides pointing out the similarities of these two comets and known dark asteroids (Kiselev et al. 2015).

Thermal properties dictate the temperature distribution throughout the nucleus, and are thus key to describe physical and chemical processes occurring in response to solar illumination. The thermal inertia of a comet nucleus, for instance, drives its ability to adapt its temperature to a change in local insolation. A material with a large thermal inertia takes longer to adapt its temperature to changing illumination conditions compared to a material with low thermal inertia. Low thermal inertia – and therefore low thermal conductivity – means that the interior of nuclei remains cold when the surface is heated by the Sun, and can therefore
<figure><img src="image_6.png" /><figcaption>**Fig. 3** Histograms showing the range of observed comet properties (JFCs in blue and Halley-type/LPCs in grey hashing). The figure provides an overview of the range of measured values and the sample size of each parameter. In case a comet has multiple measurements of the same property, the most recently reported sufficiently precise measurement is displayed. The effective radius histogram is limited to thermal-IR measurements from Fernández et al. (2013) and Bauer et al. (2017). The axis ratios plotted are lower limits for all comets except for those visited by spacecraft. Adapted from Knight et al. (2023)</figcaption></figure>

revealed that the surface spectrum of 67P changes as the comet passes through perihelion due to the comet's seasonal water-ice cycle: the nucleus' mean spectral slope changed by ~30% or 50% (Fornasier et al. 2016; Filacchione et al. 2020). Polarimetric studies of comet surfaces have not yet been performed during a spacecraft visit. There are only two JFCs for which polarisation studies from the ground have been published, 2P/Encke (Boehnhardt et al. 2008) and 209P/LINEAR (Kuroda et al. 2015). These data are too limited to extract any general conclusions about the bulk polarisation properties of comet nuclei besides pointing out the similarities of these two comets and known dark asteroids (Kiselev et al. 2015).

Thermal properties dictate the temperature distribution throughout the nucleus, and are thus key to describe physical and chemical processes occurring in response to solar illumination. The thermal inertia of a comet nucleus, for instance, drives its ability to adapt its temperature to a change in local insolation. A material with a large thermal inertia takes longer to adapt its temperature to changing illumination conditions compared to a material with low thermal inertia. Low thermal inertia – and therefore low thermal conductivity – means that the interior of nuclei remains cold when the surface is heated by the Sun, and can therefore
retain more volatile ices (i.e., can be more ‘pristine’). Estimates for comets made from spatially unresolved observations of the nucleus allowed a value lower than $50 \text{ J K}^{-1} \text{ m}^{-2} \text{ s}^{-1/2}$ to be derived. Owing to spacecraft flybys in the 2000s, thermal inertia has since been derived from radiance measurements, from which temperature can be inferred, on the surface with spatially resolved maps of comets 9P, 103P and 67P. They point toward a low thermal inertia, between 50 and $200 \text{ J K}^{-1} \text{ m}^{-2} \text{ s}^{-1/2}$ for 9P (Davidsson et al. 2013), and less than $250 \text{ J K}^{-1} \text{ m}^{-2} \text{ s}^{-1/2}$ for 103P (Groussin et al. 2013). The suite of instruments on-board Rosetta indicates that 67P’s thermal inertia is between 5 and $350 \text{ J K}^{-1} \text{ m}^{-2} \text{ s}^{-1/2}$ (Leyrat and Blain 2015). It varies across the surface (Leyrat and Blain 2015), perhaps due to variations in material properties such as density or porosity, between consolidated and unconsolidated terrains. Additional estimates in the near-subsurface were possible, down to 1 and 4 cm below the surface: they point to a thermal inertia of the order of 10 to $60 \text{ J K}^{-1} \text{ m}^{-2} \text{ s}^{-1/2}$, and lower than $80 \text{ J K}^{-1} \text{ m}^{-2} \text{ s}^{-1/2}$ (Gulkis et al. 2015; Choukroun et al. 2015; Schloerb et al. 2015; Marshall et al. 2018). Finally, the MUPUS measurement at the landing site of Philae suggests a local thermal inertia of the order of $120 \text{ J K}^{-1} \text{ m}^{-2} \text{ s}^{-1/2}$ (Groussin et al. 2019). These thermal inertia estimates depend strongly on the model used to derive them: they should be taken only as an indication that cometary material close to the surface has a low thermal inertia.

Indeed, temperatures are not directly measured with remote sensing instruments, but rather the infrared or sub-mm flux is detected, or the brightness temperature is measured. The kinetic temperature, which gives information on thermal properties of a surface, must be retrieved through models. Measurements in the near infrared can be contaminated by reflected solar radiation (Keihm et al. 2012). Instruments detect a nonlinear average of potentially very different temperatures in the field of view, with large and small-scale topographic features and perhaps compositional heterogeneities. On Rosetta, MIRO measurements were affected by both the thermal and the optical properties of the material, which made the interpretation challenging. A yet larger caveat comes from the lack of a thermal infrared instrument on the Rosetta payload. Lower limits for the temperature derived from VIRTIS-M were effectively restricted to the dayside of the nucleus (Tosi et al. 2019). Kinetic temperatures over complete diurnal cycles and for the same layer could not be retrieved, and thermal inertia maps were derived with large error bars (Groussin et al. 2019). Resolved images of both the day and night sides in the thermal infrared are required to put stronger constraints on thermal models of cometary nuclei.

### 2.2.4 Morphology of Nuclei

As cometary surfaces are almost impossible to resolve in Earth-based observations, our information about surface morphology of comets is based on flyby or orbital missions (with Rosetta currently the only mission to orbit around a comet for a prolonged amount of time). Figure 4 shows the shape and general morphology of all of the comets that have been imaged *in situ*. It is useful to draw comparisons between the morphologies observed on comets (mostly JFCs) and Arrokoth, the cold classical KBO visited by NASA’s New Horizons Mission in 2019. Whereas Arrokoth signifies a nearly primordial body characterised by low colour diversity, uniform textures, lack of topographical complexity, and a general lack of putative impact craters, circular features overall, or evidence of tectonics (McKinnon et al. 2020), comets that have spent a significant amount of their life in the inner Solar System display clear evidence of surface evolution and overall weathering as is demonstrated by the higher degrees of surface roughness, topographical complexity, morphological diversity of terrains, and presence of smooth regions suggestive of weathered and eroded fine-scale
retain more volatile ices (i.e., can be more ‘pristine’). Estimates for comets made from spa-
tially unresolved observations of the nucleus allowed a value lower than 50 J K⁻¹ m⁻² s⁻¹/²
to be derived. Owing to spacecraft flybys in the 2000s, thermal inertia has since been de-
rived from radiance measurements, from which temperature can be inferred, on the surface
with spatially resolved maps of comets 9P, 103P and 67P. They point toward a low ther-
mal inertia, between 50 and 200 J K⁻¹ m⁻² s⁻¹/² for 9P (Davidsson et al. 2013), and less
than 250 J K⁻¹ m⁻² s⁻¹/² for 103P (Groussin et al. 2013). The suite of instruments on-board
Rosetta indicates that 67P’s thermal inertia is between 5 and 350 J K⁻¹ m⁻² s⁻¹/² (Leyrat and
Blain 2015). It varies across the surface (Leyrat and Blain 2015), perhaps due to variations
in material properties such as density or porosity, between consolidated and unconsolidated
terrains. Additional estimates in the near-subsurface were possible, down to 1 and 4 cm be-
low the surface: they point to a thermal inertia of the order of 10 to 60 J K⁻¹ m⁻² s⁻¹/², and
lower than 80 J K⁻¹ m⁻² s⁻¹/² (Gulkis et al. 2015; Choukroun et al. 2015; Schloerb et al.
2015; Marshall et al. 2018). Finally, the MUPUS measurement at the landing site of Philae
suggests a local thermal inertia of the order of 120 J K⁻¹ m⁻² s⁻¹/² (Groussin et al. 2019).
These thermal inertia estimates depend strongly on the model used to derive them: they
should be taken only as an indication that cometary material close to the surface has a low
thermal inertia.

Indeed, temperatures are not directly measured with remote sensing instruments, but rather the infrared or sub-mm flux is detected, or the brightness temperature is measured. The kinetic temperature, which gives information on thermal properties of a surface, must be retrieved through models. Measurements in the near infrared can be contaminated by reflected solar radiation (Keihm et al. 2012). Instruments detect a nonlinear average of potentially very different temperatures in the field of view, with large and small-scale topographic features and perhaps compositional heterogeneities. On Rosetta, MIRO measurements were affected by both the thermal and the optical properties of the material, which made the interpretation challenging. A yet larger caveat comes from the lack of a thermal infrared instrument on the Rosetta payload. Lower limits for the temperature derived from VIRTIS-M were effectively restricted to the dayside of the nucleus (Tosi et al. 2019). Kinetic temperatures over complete diurnal cycles and for the same layer could not be retrieved, and thermal inertia maps were derived with large error bars (Groussin et al. 2019). Resolved images of both the day and night sides in the thermal infrared are required to put stronger constraints on thermal models of cometary nuclei.

2.2.4 Morphology of Nuclei

As cometary surfaces are almost impossible to resolve in Earth-based observations, our in-
formation about surface morphology of comets is based on flyby or orbital missions (with
Rosetta currently the only mission to orbit around a comet for a prolonged amount of time).
Figure 4 shows the shape and general morphology of all of the comets that have been im-
aged *in situ*. It is useful to draw comparisons between the morphologies observed on comets
(mostly JFCs) and Arroloth, the cold classical KBO visited by NASA's New Horizons Mis-
sion in 2019. Whereas Arroloth signifies a nearly primordial body characterised by low
colour diversity, uniform textures, lack of topographical complexity, and a general lack of
putative impact craters, circular features overall, or evidence of tectonics (McKinnon et al.
2020), comets that have spent a significant amount of their life in the inner Solar System
display clear evidence of surface evolution and overall weathering as is demonstrated by
the higher degrees of surface roughness, topographical complexity, morphological diversity
of terrains, and presence of smooth regions suggestive of weathered and eroded fine-scale
<figure><img src="image_6.png" /><figcaption>**Fig. 4** A subset of the cometary nuclei that have been visited by spacecraft and on the right an image of Arrokoth, a Kuiper Belt Object, which is a member of the Cold Classical population which is not believed to be a significant source of JFCs. Objects are not shown to scale</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 5** Pits on the surface of comet 67P/Churyumov-Gerasimenko (left). Hathor cliff above the ‘neck’ region between the two lobes of 67P (right). ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM</figcaption></figure>

materials (e.g., Keller et al. 1986; Britt et al. 2004; A'Hearn et al. 2005, 2011; Veverka et al. 2013; Sierks et al. 2015; Thomas et al. 2015).

Rosetta demonstrated how activity caused by seasonal sublimation of volatiles (mostly water-ice but also other types of ice), as comets cross the snow-line during the perihelion part of their orbit, can account for various fine-scale morphology on cometary surfaces, with clear evidence of seasonal evolution, e.g., El-Maarry et al. (2017, 2019). However, it is unlikely that major landscape evolution occurs that way, at least on seasonal scales (El-Maarry et al. 2017): seasonal erosion causes changes on scales of order 1–10 m, while there are pits (Fig. 5, left) approximately 100 m in diameter, and the cliff (Fig. 5, right) above the ‘neck’ region between the two lobes of 67P is around a kilometre high.

Therefore, we can treat or consider JFCs, and KBOs such as Arrokoth, as two opposite end-member points in a spectrum of bodies in varying stages of evolution. Visiting a more
<figure><img src="image_7.png" /><figcaption>**Fig. 4** A subset of the cometary nuclei that have been visited by spacecraft and on the right an image of Arrokoth, a Kuiper Belt Object, which is a member of the Cold Classical population which is not believed to be a significant source of JFCs. Objects are not shown to scale</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 5** Pits on the surface of comet 67P/Churyumov-Gerasimenko (left). Hathor cliff above the ‘neck’ region between the two lobes of 67P (right). ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM</figcaption></figure>

materials (e.g., Keller et al. 1986; Britt et al. 2004; A'Hearn et al. 2005, 2011; Veverka et al. 2013; Sierks et al. 2015; Thomas et al. 2015).

Rosetta demonstrated how activity caused by seasonal sublimation of volatiles (mostly water-ice but also other types of ice), as comets cross the snow-line during the perihelion part of their orbit, can account for various fine-scale morphology on cometary surfaces, with clear evidence of seasonal evolution, e.g., El-Maarry et al. (2017, 2019). However, it is unlikely that major landscape evolution occurs that way, at least on seasonal scales (El-Maarry et al. 2017): seasonal erosion causes changes on scales of order 1–10 m, while there are pits (Fig. 5, left) approximately 100 m in diameter, and the cliff (Fig. 5, right) above the ‘neck’ region between the two lobes of 67P is around a kilometre high.

Therefore, we can treat or consider JFCs, and KBOs such as Arrokoth, as two opposite end-member points in a spectrum of bodies in varying stages of evolution. Visiting a more
pristine LPC would add a pivotal midpoint across that evolutionary path that can further
explain how primordial KBOs transition to heavily evolved comets, and at which stage major
landscape processes occur. JFCs go through a transitional “Centaur” phase as their orbits
dynamically evolve to shorter orbits that place them in the inner Solar System. It is possible
that comets undergo substantial changes during that time as hypervolatiles can sublimate
beyond the water ice snowline. However, the degree of that change is currently unknown.
A major question that can be answered by Comet Interceptor is where would LPCs, or
generally comets that have not gone close to the Sun many times in their lifetime, place in
that evolutionary path? Would they be closer in morphology to KBOs, or would they have
developed some morphological and textural variety already during their early formation
stages (see Sect. 2.2.6 below)? Do they show evidence for collisions before their ejection to
the OC, and if so, what record do they retain of the primordial impactor population (e.g.,
crater size distribution)? Therefore, the possibility to encounter and investigate either an
LPC, or especially a DNC, would offer more insights to the evolutionary path of comets and
the conditions in the early Solar System more generally.

2.2.5 Composition of Nuclei

Most of our knowledge on the composition of comets comes from measurements (either *in situ* by spacecraft, or remotely via spectroscopy) of the gas coma, which is discussed in detail in the coma science section below (see Sect. 2.3). Broadly, we understand the composition of nucleus ices indirectly, by working backwards from sublimated gasses in the coma, or from the products of further gas phase photochemistry. However, direct measurement of the nucleus composition is of great importance to comet science:

- to discover the starting point for these sublimation/ chemistry models,

- to understand how (in)homogeneous the nucleus is,

- and to assess the composition of the original building blocks of comets (and planets) independent of our (lack of) understanding of evolution and activity processes.

We are limited to measurements of the surface composition, as the only attempt to di-
rectly measure the interior composition of a comet nucleus, the SD2 drill on the Philae
lander, failed to collect a sample (Boehnhardt et al. 2017). Mass spectroscopy results for
surface dust returned by Philae were inconclusive, but did show a high proportion of or-
ganic compounds (Boehnhardt et al. 2017), consistent with remote sensing measurements.
As discussed in Sect. 2.2.3 above, spectroscopy, both unresolved from telescopic observa-
tions and resolved from spacecraft encounters, reveals comet nuclei to be largely featureless
in the visible and near infrared, with reddish slopes (but shallower than many KBOs) and
very low albedo (e.g., Quirico et al. 2016). A broad 3.2 µm absorption was identified by
the Rosetta VIRTIS instrument at 67P, which has been interpreted in a variety of ways, in-
cluding salts, organic compounds, and/or silicates (Poch et al. 2020; Raponi et al. 2020;
Mennella et al. 2020). Features due to water ice are, perhaps surprisingly, limited in their
presence, but detected at comet 9P (Sunshine et al. 2006). They are only seen in Rosetta
data in localised spots (Barucci et al. 2016; Fornasier et al. 2023), where fresh subsurface
layers (e.g., Fig. 6) have been uncovered (e.g., by cliff collapse; Filacchione et al. 2016a), or
as short-lived frosts deposited during the comet night time (De Sanctis et al. 2015)). A re-
gion of CO2 ice was also identified at 67P (Filacchione et al. 2016b). Nuclei are mostly too
faint to be detected at shorter and longer wavelengths from Earth-based observations, but
UV spectroscopy from the Alice instrument on-board Rosetta revealed a featureless blue
pristine LPC would add a pivotal midpoint across that evolutionary path that can further
explain how primordial KBOs transition to heavily evolved comets, and at which stage major
landscape processes occur. JFCs go through a transitional “Centaur” phase as their orbits
dynamically evolve to shorter orbits that place them in the inner Solar System. It is possible
that comets undergo substantial changes during that time as hypervolatiles can sublimate
beyond the water ice snowline. However, the degree of that change is currently unknown.
A major question that can be answered by Comet Interceptor is where would LPCs, or
generally comets that have not gone close to the Sun many times in their lifetime, place in
that evolutionary path? Would they be closer in morphology to KBOs, or would they have
developed some morphological and textural variety already during their early formation
stages (see Sect. 2.2.6 below)? Do they show evidence for collisions before their ejection to
the OC, and if so, what record do they retain of the primordial impactor population (e.g.,
crater size distribution)? Therefore, the possibility to encounter and investigate either an
LPC, or especially a DNC, would offer more insights to the evolutionary path of comets and
the conditions in the early Solar System more generally.

2.2.5 Composition of Nuclei

Most of our knowledge on the composition of comets comes from measurements (either *in situ* by spacecraft, or remotely via spectroscopy) of the gas coma, which is discussed in detail in the coma science section below (see Sect. 2.3). Broadly, we understand the composition of nucleus ices indirectly, by working backwards from sublimated gasses in the coma, or from the products of further gas phase photochemistry. However, direct measurement of the nucleus composition is of great importance to comet science:

- to discover the starting point for these sublimation/ chemistry models,

- to understand how (in)homogeneous the nucleus is,

- and to assess the composition of the original building blocks of comets (and planets) independent of our (lack of) understanding of evolution and activity processes.

We are limited to measurements of the surface composition, as the only attempt to di-
rectly measure the interior composition of a comet nucleus, the SD2 drill on the Philae
lander, failed to collect a sample (Boehnhardt et al. 2017). Mass spectroscopy results for
surface dust returned by Philae were inconclusive, but did show a high proportion of or-
ganic compounds (Boehnhardt et al. 2017), consistent with remote sensing measurements.
As discussed in Sect. 2.2.3 above, spectroscopy, both unresolved from telescopic observa-
tions and resolved from spacecraft encounters, reveals comet nuclei to be largely featureless
in the visible and near infrared, with reddish slopes (but shallower than many KBOs) and
very low albedo (e.g., Quirico et al. 2016). A broad 3.2 µm absorption was identified by
the Rosetta VIRTIS instrument at 67P, which has been interpreted in a variety of ways, in-
cluding salts, organic compounds, and/or silicates (Poch et al. 2020; Raponi et al. 2020;
Mennella et al. 2020). Features due to water ice are, perhaps surprisingly, limited in their
presence, but detected at comet 9P (Sunshine et al. 2006). They are only seen in Rosetta
data in localised spots (Barucci et al. 2016; Fornasier et al. 2023), where fresh subsurface
layers (e.g., Fig. 6) have been uncovered (e.g., by cliff collapse; Filacchione et al. 2016a), or
as short-lived frosts deposited during the comet night time (De Sanctis et al. 2015)). A re-
gion of CO2 ice was also identified at 67P (Filacchione et al. 2016b). Nuclei are mostly too
faint to be detected at shorter and longer wavelengths from Earth-based observations, but
UV spectroscopy from the Alice instrument on-board Rosetta revealed a featureless blue
<figure><img src="image_8.png" /><figcaption>**Fig. 6** A water ice-filled depression on the surface of 67P's nucleus. This image is a false-color composite, where the pale blue patches highlight the presence and location of water-ice. ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA</figcaption></figure>

slope (Feaga et al. 2015). At sub-mm wavelengths, Rosetta/MIRO could only constrain nu-
cleus thermal emission, not give compositional information. Unresolved mid-infrared spec-
troscopy with the Spitzer space telescope shows broad features attributed to silicates, with
similarities to D-type asteroids and Jupiter Trojans (Kelley et al. 2017). Spacecraft obser-
vations, made *in situ* at these wavelengths, have yet to be attempted, but are a promising
direction to take. These wavelengths contain a wide array of features seen in common min-
erals and organic ices, and could reveal both composition and its variation across the nucleus
surface, in resolved spectroscopy and/or imaging.

2.2.6 Evolution of Cometary Nuclei

Both the mechanical and thermal processing of comet nuclei (Weissman et al. 2020) occur
during distinct phases of a nucleus’s life, e.g.:

- during formation and early evolution in the protoplanetary disc.

- during the long period of storage in the comet reservoirs.

- and as an active comet upon return to the inner Solar System.

The active comet phase is the one that is best understood, following the results from Rosetta, as the evolution of the nucleus is driven entirely by the comet's activity and therefore by the energy it receives from the Sun. Erosion of the surface has long been understood as a consequence of sublimation of nucleus ices, with metres of the surface lost, on average, per perihelion passage (e.g., Whipple 1950; Britt et al. 2004; Veverka et al. 2013; Keller et al. 2015). However, a surprising result from the Rosetta mission was the importance of fall-back of material lifted into the inner coma, with some areas of the nucleus blanketed by deep layers of fine material (e.g., Thomas et al. 2015; Marschall et al. 2020; Cambianica et al. 2021). In models of the nucleus consisting of pebbles, fall-back of decimetre-sized chunks leads to both surface morphology changes (Fig. 7) and evolution of near-surface composition, as
<figure><img src="image_9.png" /><figcaption>**Fig. 6** A water ice-filled depression on the surface of 67P's nucleus. This image is a false-color composite, where the pale blue patches highlight the presence and location of water-ice. ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA</figcaption></figure>

slope (Feaga et al. 2015). At sub-mm wavelengths, Rosetta/MIRO could only constrain nu-
cleus thermal emission, not give compositional information. Unresolved mid-infrared spec-
troscopy with the Spitzer space telescope shows broad features attributed to silicates, with
similarities to D-type asteroids and Jupiter Trojans (Kelley et al. 2017). Spacecraft obser-
vations, made *in situ* at these wavelengths, have yet to be attempted, but are a promising
direction to take. These wavelengths contain a wide array of features seen in common min-
erals and organic ices, and could reveal both composition and its variation across the nucleus
surface, in resolved spectroscopy and/or imaging.

**2.2.6 Evolution of Cometary Nuclei**

Both the mechanical and thermal processing of comet nuclei (Weissman et al. 2020) occur
during distinct phases of a nucleus’s life, e.g.:

- during formation and early evolution in the protoplanetary disc.

- during the long period of storage in the comet reservoirs.

- and as an active comet upon return to the inner Solar System.

The active comet phase is the one that is best understood, following the results from Rosetta, as the evolution of the nucleus is driven entirely by the comet’s activity and therefore by the energy it receives from the Sun. Erosion of the surface has long been understood as a consequence of sublimation of nucleus ices, with metres of the surface lost, on average, per perihelion passage (e.g., Whipple 1950; Britt et al. 2004; Veverka et al. 2013; Keller et al. 2015). However, a surprising result from the Rosetta mission was the importance of fall-back of material lifted into the inner coma, with some areas of the nucleus blanketed by deep layers of fine material (e.g., Thomas et al. 2015; Marschall et al. 2020; Cambianica et al. 2021). In models of the nucleus consisting of pebbles, fall-back of decimetre-sized chunks leads to both surface morphology changes (Fig. 7) and evolution of near-surface composition, as
<figure><img src="image_9.png" /><figcaption>**Fig. 7** Surface morphology changes due to fall-back of dust on the surface of 67P/Churyumov-Gerasimenko. ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM</figcaption></figure>

subsequent activity from the fall-back material is driven by water ice retained within these ‘chunks’, which have a lower abundance of the more volatile ices whose activity lifted the chunks from other areas in the first place (Fulle et al. 2020a). The many cycles of activity seen by short period comets mean that their surfaces have undergone significant evolution, while an LPC encountered at 1 au will have relatively little recent evolution, depending on where exactly its activity began on the inbound leg (models range from 35 to 85 au; Jewitt et al. 2021; Fulle et al. 2020b), and on model-dependent levels of erosion and fall-back during this time.

The earliest phase in comet evolution, which the nuclei spent in their formation zone at 5–30 au, could last anywhere from 2.5 to several hundred million years (various models have different timing for the instabilities in the giant planet orbits that scattered comets into their reservoirs, e.g., Nesvorný et al. 2018; Morbidelli et al. 2018; Pirani et al. 2019, 2021). The OC could not have been populated while the Sun was still in its embedded star cluster phase – whose duration was perhaps a few Myr (e.g., Adams 2010; Pfalzner and Vincke 2020; Parker 2020) – because the Sun’s tidal radius was too small (Tremaine and Dones 1993; Wyatt et al. 2017). When it comes to the survival of volatile species, this phase is significant: Davidsson (2021) reports that nuclei in the protoplanetary disc with diameters ranging from 4 to 200 km can lose all their condensed CO ice on short timescales (smaller than the minimum time to eject them in the reservoirs), through a combination of protosolar and long-lived radionuclide heating. This may have been avoided if cometesimals formed late enough (Davidsson et al. 2016), or in a region of the disc with relatively low abundance of radionuclides, which has been suggested by some observations (e.g., analysis of four Stardust samples indicate that 81P never contained any appreciable $^{26}$Al; Levasseur-Regourd et al. 2018). The drivers of evolution in this phase can also be expected to include collisions (Jutzi et al. 2017; Schwartz et al. 2018), although models of the timescales for CO-driven cometary activity suggest that erosion of ice-rich cometesimals in this early environment could dominate evolution over collisions (Fulle et al. 2020b).

Subsequently, and because the time of residence in the OC or the KB is very long, a significant fraction of the bulk of most comet nuclei can be affected by superficial heat sources, even if the thermal diffusivity is extremely low. Davidsson (2021) reported that, for objects large enough to have any pure condensed CO ice remaining in their bulk when they are ejected in their reservoirs, the long-term survival of supervolatile ices largely depends on whether nuclei are scattered in the OC, or in the KB. Indeed, objects reaching the OC could get subsurface temperatures low enough for CO gas, if diffusing from the deep interior, where it was heated due to radiogenic decay, to condense near the surface. In the KB, the equilibrium temperature ranges from 30 to 50 K, so that, for objects typically smaller than 4 km, all hypervolatiles initially present as pure ices should sublimate during the time of
<figure><img src="image_10.png" /><figcaption>**Fig. 7** Surface morphology changes due to fall-back of dust on the surface of 67P/Churyumov-Gerasimenko. ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM</figcaption></figure>

subsequent activity from the fall-back material is driven by water ice retained within these ‘chunks’, which have a lower abundance of the more volatile ices whose activity lifted the chunks from other areas in the first place (Fulle et al. 2020a). The many cycles of activity seen by short period comets mean that their surfaces have undergone significant evolution, while an LPC encountered at 1 au will have relatively little recent evolution, depending on where exactly its activity began on the inbound leg (models range from 35 to 85 au; Jettitt et al. 2021; Fulle et al. 2020b), and on model-dependent levels of erosion and fall-back during this time.

The earliest phase in comet evolution, which the nuclei spent in their formation zone at 5–30 au, could last anywhere from 2.5 to several hundred million years (various models have different timing for the instabilities in the giant planet orbits that scattered comets into their reservoirs, e.g., Nesvorný et al. 2018; Morbidelli et al. 2018; Pirani et al. 2019, 2021). The OC could not have been populated while the Sun was still in its embedded star cluster phase – whose duration was perhaps a few Myr (e.g., Adams 2010; Pfalzner and Vincke 2020; Parker 2020) – because the Sun’s tidal radius was too small (Tremaine and Dones 1993; Wyatt et al. 2017). When it comes to the survival of volatile species, this phase is significant: Davidsson (2021) reports that nuclei in the protoplanetary disc with diameters ranging from 4 to 200 km can lose all their condensed CO ice on short timescales (smaller than the minimum time to eject them in the reservoirs), through a combination of protosolar and long-lived radionuclide heating. This may have been avoided if cometesimals formed late enough (Davidsson et al. 2016), or in a region of the disc with relatively low abundance of radionuclides, which has been suggested by some observations (e.g., analysis of four Stardust samples indicate that 81P never contained any appreciable $^{26}$Al; Levasseur-Regourd et al. 2018). The drivers of evolution in this phase can also be expected to include collisions (Jutzi et al. 2017; Schwartz et al. 2018), although models of the timescales for CO-driven cometary activity suggest that erosion of ice-rich cometesimals in this early environment could dominate evolution over collisions (Fulle et al. 2020b).

Subsequently, and because the time of residence in the OC or the KB is very long, a significant fraction of the bulk of most comet nuclei can be affected by superficial heat sources, even if the thermal diffusivity is extremely low. Davidsson (2021) reported that, for objects large enough to have any pure condensed CO ice remaining in their bulk when they are ejected in their reservoirs, the long-term survival of supervolatile ices largely depends on whether nuclei are scattered in the OC, or in the KB. Indeed, objects reaching the OC could get subsurface temperatures low enough for CO gas, if diffusing from the deep interior, where it was heated due to radiogenic decay, to condense near the surface. In the KB, the equilibrium temperature ranges from 30 to 50 K, so that, for objects typically smaller than 4 km, all hypervolatiles initially present as pure ices should sublimate during the time of
residence in this reservoir, even without any radiogenic heating (De Sanctis et al. 2001; Choi et al. 2002; Jewitt 2004; Davidsson 2021).

We emphasize again that, for OC comets, the thermal processing prior to the injection in this reservoir does not guarantee that a nucleus stored there can preserve a pristine inventory of hypervolatiles. In terms of internal structure expected for OC comets, Davidsson (2021) suggests that any nuclei exposed to the proto-Sun’s intense heating would lose not only hypervolatiles condensed as pure ices, but also CO₂, down to a depth of ~30 m. In addition, partial crystallization could occur in the upper ~200 m. As a consequence, the near-surface layers of a comet nucleus may be significantly processed: even DNCs cannot be completely pristine, and some might have lost a significant amount of hypervolatiles prior to their scattering into the OC. In addition, Stern and Shull (1988) suggest that up to 20% of comet nuclei stored in the OC could have been heated to at least 30 K down to several dozen metres below the surface, due to the passage of luminous stars during the history of the Solar System. Most of them may have been heated to 45 K in the uppermost 1 m-layer due to stochastic supernovae events. This would lead to the formation of a surface layer depleted in hypervolatiles. Stern (2003) further reports that passing stars and supernovae heating events could modify the primordial composition of comet nuclei down to 5 to 50 m (for heating due to passing stars), and to 0.1 to 2 m (for heating due to supernovae events).

However, an extremely significant difference between JFCs and DNCs coming from the OC lies in their subsequent orbital evolution, which brings them to the orbits on which we observe them. Indeed, an intermediate evolution phase exists almost exclusively for Centaurs, of which a fraction become JFCs, during which comet nuclei are perturbed into their final orbit through a chaotic orbital evolution in the giant planet region. Processing during this phase intensifies, due to increasing equilibrium temperatures, and close passages to massive planets. Because the time spent in the giant planet region is significant (typically 10 Myr, Levison and Duncan 1997; Tiscareno and Malhotra 2003) the resulting processing is also substantial. This phase is non-existent for nuclei coming from the OC, which come from this reservoir to the inner Solar System in a more rapid and direct pathway.

Huebner et al. (2006) discussed the outcomes of such different injection types, which may result in extensive changes of the internal composition and structure of comets. Similarities between the structures resulting from the two types of orbital evolution are:

1. Surface temperatures, driven by the surface heat balance, and

2. Water and CO₂ gas production, which are controlled by erosion, keeping both water and CO₂ ice close to the surface.

However, the significant differences resulting from the two orbital evolutions are mostly related to the location of the CO sublimation front (and hypervolatiles in general), and the amorphous/crystalline ice interface. The CO sublimation front for JFCs should be located hundreds of metres below the surface, while it remains very close to the surface for DNCs. In terms of activity, this is reflected in the fact that CO production is typically continuous for JFCs, or at least with a pattern which does not follow the water emission, and the production rate for the JFC nucleus is significantly smaller (ten times lower in their example) than for a DNC. Water and CO₂ sublimation remain, however, characterized by peak emissions towards the perihelion of each orbit. The simulations reported in Huebner et al. (2006) also showed the formation of a transient dust mantle, which was destroyed when the last orbital change occurred.

In conclusion, any comet nucleus coming from the OC would be significantly less altered and more informative of the processes that shaped the Solar System in its early phases, when compared to the JFCs explored in space thus far.
residence in this reservoir, even without any radiogenic heating (De Sanctis et al. 2001; Choi et al. 2002; Jewitt 2004; Davidsson 2021).

We emphasize again that, for OC comets, the thermal processing prior to the injection in this reservoir does not guarantee that a nucleus stored there can preserve a pristine inventory of hypervolatiles. In terms of internal structure expected for OC comets, Davidsson (2021) suggests that any nuclei exposed to the proto-Sun’s intense heating would lose not only hypervolatiles condensed as pure ices, but also CO₂, down to a depth of ~30 m. In addition, partial crystallization could occur in the upper ~200 m. As a consequence, the near-surface layers of a comet nucleus may be significantly processed: even DNCs cannot be completely pristine, and some might have lost a significant amount of hypervolatiles prior to their scattering into the OC. In addition, Stern and Shull (1988) suggest that up to 20% of comet nuclei stored in the OC could have been heated to at least 30 K down to several dozen metres below the surface, due to the passage of luminous stars during the history of the Solar System. Most of them may have been heated to 45 K in the uppermost 1 m-layer due to stochastic supernovae events. This would lead to the formation of a surface layer depleted in hypervolatiles. Stern (2003) further reports that passing stars and supernovae heating events could modify the primordial composition of comet nuclei down to 5 to 50 m (for heating due to passing stars), and to 0.1 to 2 m (for heating due to supernovae events).

However, an extremely significant difference between JFCs and DNCs coming from the OC lies in their subsequent orbital evolution, which brings them to the orbits on which we observe them. Indeed, an intermediate evolution phase exists almost exclusively for Centaurs, of which a fraction become JFCs, during which comet nuclei are perturbed into their final orbit through a chaotic orbital evolution in the giant planet region. Processing during this phase intensifies, due to increasing equilibrium temperatures, and close passages to massive planets. Because the time spent in the giant planet region is significant (typically 10 Myr, Levison and Duncan 1997; Tiscareno and Malhotra 2003) the resulting processing is also substantial. This phase is non-existent for nuclei coming from the OC, which come from this reservoir to the inner Solar System in a more rapid and direct pathway.

Huebner et al. (2006) discussed the outcomes of such different injection types, which may result in extensive changes of the internal composition and structure of comets. Similarities between the structures resulting from the two types of orbital evolution are:

1. Surface temperatures, driven by the surface heat balance, and

2. Water and CO₂ gas production, which are controlled by erosion, keeping both water and CO₂ ice close to the surface.

However, the significant differences resulting from the two orbital evolutions are mostly related to the location of the CO sublimation front (and hypervolatiles in general), and the amorphous/crystalline ice interface. The CO sublimation front for JFCs should be located hundreds of metres below the surface, while it remains very close to the surface for DNCs. In terms of activity, this is reflected in the fact that CO production is typically continuous for JFCs, or at least with a pattern which does not follow the water emission, and the production rate for the JFC nucleus is significantly smaller (ten times lower in their example) than for a DNC. Water and CO₂ sublimation remain, however, characterized by peak emissions towards the perihelion of each orbit. The simulations reported in Huebner et al. (2006) also showed the formation of a transient dust mantle, which was destroyed when the last orbital change occurred.

In conclusion, any comet nucleus coming from the OC would be significantly less altered and more informative of the processes that shaped the Solar System in its early phases, when compared to the JFCs explored in space thus far.
## 2.3 Theme 2: Comet Environment Science

### 2.3.1 Introduction

This theme focuses on the coma, looking to answer the following questions:

- What is the composition of the gas and dust in the coma?

- How does it connect to the nucleus (i.e., how does cometary activity work)?

- What is the nature of its interaction with the solar wind?

These topics are addressed through observations of the gas, dust and plasma that are described in the subsections below. They are linked to each other, and also to the studies of the nucleus described in the previous section, through the processes of cometary activity; ices sublimating to gas, and lifting dust, followed by the decoupling of these and the photochemistry that generates the products we observe on large-scales. Data from telescopic observations and previous space missions have given us clues to understand all of these steps, but many puzzles remain. Measuring the composition and distribution of volatile species in the coma will help in understanding the activity processes of an LPC compared with the more evolved objects visited to date:

- What are the relative abundances of molecules of high volatility such as CH$_4$, CO and CO$_2$ with respect to water in an LPC versus JFCs (67P) and HTCs (1P) at similar heliocentric distances?

- Is there evidence for or against hyperactivity, i.e., significant activity being driven by sublimation from icy 'chunks' in the coma, as seen at 103P by EPOXI?

- Are there differences in isotopic composition, e.g., in D/H, and other species if sufficiently abundant?

For each of these questions, Comet Interceptor will test whether or not the phenomenon is evolution-related, and will inform the interpretation of ground-based observations of other comets. Comet Interceptor will also perform unique observations of the coma from three different positions simultaneously, due to its multispacecraft configuration. This will allow gas, dust, and plasma distributions or boundaries to be described in a 3-D way. This was not possible with previous missions, which sampled only a single location at a time, and will allow separation of spatial and temporal variations. The multipoint *in situ* plasma measurements will be complemented by Energetic Neutral Atom (ENA) observations, which may provide a more continuous observation of the variability of the solar wind, giving a clearer picture of which plasma variations are due to external influences.

### 2.3.2 Bulk and Isotopic Composition, and Distribution of Gas and Dust

The bulk of our knowledge of the composition of comets comes from studying the gas coma. Remote observations give a comparatively less detailed picture but of a larger number of comets, while *in situ* measurements with mass spectrometers, in particular from Rosetta, have revealed a wealth of detail of just a few. Unfortunately, there have not been any good opportunities yet to link these approaches. Although a campaign of remote observations did support Rosetta (Snodgrass et al. 2017), 67P was not particularly bright around its 2015 perihelion and could not be studied with high resolution spectroscopy, for example. The comet also presented a poor observation geometry, being located on the other side of the Sun with respect to Earth. Much of our knowledge of comet composition (especially on an isotopic
## 2.3 Theme 2: Comet Environment Science

### 2.3.1 Introduction

This theme focuses on the coma, looking to answer the following questions:

- What is the composition of the gas and dust in the coma?

- How does it connect to the nucleus (i.e., how does cometary activity work)?

- What is the nature of its interaction with the solar wind?

These topics are addressed through observations of the gas, dust and plasma that are described in the subsections below. They are linked to each other, and also to the studies of the nucleus described in the previous section, through the processes of cometary activity; ices sublimating to gas, and lifting dust, followed by the decoupling of these and the photochemistry that generates the products we observe on large-scales. Data from telescopic observations and previous space missions have given us clues to understand all of these steps, but many puzzles remain. Measuring the composition and distribution of volatile species in the coma will help in understanding the activity processes of an LPC compared with the more evolved objects visited to date:

- What are the relative abundances of molecules of high volatility such as CH$_4$, CO and CO$_2$ with respect to water in an LPC versus JFCs (67P) and HTCs (1P) at similar heliocentric distances?

- Is there evidence for or against hyperactivity, i.e., significant activity being driven by sublimation from icy 'chunks' in the coma, as seen at 103P by EPOXI?

- Are there differences in isotopic composition, e.g., in D/H, and other species if sufficiently abundant?

For each of these questions, Comet Interceptor will test whether or not the phenomenon is evolution-related, and will inform the interpretation of ground-based observations of other comets. Comet Interceptor will also perform unique observations of the coma from three different positions simultaneously, due to its multispacecraft configuration. This will allow gas, dust, and plasma distributions or boundaries to be described in a 3-D way. This was not possible with previous missions, which sampled only a single location at a time, and will allow separation of spatial and temporal variations. The multipoint *in situ* plasma measurements will be complemented by Energetic Neutral Atom (ENA) observations, which may provide a more continuous observation of the variability of the solar wind, giving a clearer picture of which plasma variations are due to external influences.

### 2.3.2 Bulk and Isotopic Composition, and Distribution of Gas and Dust

The bulk of our knowledge of the composition of comets comes from studying the gas coma. Remote observations give a comparatively less detailed picture but of a larger number of comets, while *in situ* measurements with mass spectrometers, in particular from Rosetta, have revealed a wealth of detail of just a few. Unfortunately, there have not been any good opportunities yet to link these approaches. Although a campaign of remote observations did support Rosetta (Snodgrass et al. 2017), 67P was not particularly bright around its 2015 perihelion and could not be studied with high resolution spectroscopy, for example. The comet also presented a poor observation geometry, being located on the other side of the Sun with respect to Earth. Much of our knowledge of comet composition (especially on an isotopic
<figure><img src="image_10.png" /><figcaption>**Fig. 8** Typical comet spectrum from ground-based visible observations, with key emission features marked. Major components such as water or CO₂ are not observable and require space missions to be characterised. (Image courtesy of C. Opitom)</figcaption></figure>

level) from telescopes is based on LPCs, which are often more active and brighter. A mission to a bright LPC would present an opportunity to compare high resolution spectroscopy at a range of wavelengths with *in situ* mass spectrometry measurements, calibrating our understanding of the much wider observed population. Modern infrared and sub-mm facilities, such as the ESO Very Large Telescope and the other 8–10 m class telescopes, and the Atacama Large Millimetre/submillimetre Array, which did not exist at the time of Giotto, are suitable for observing molecular species thought to be directly released from the nucleus.

It is well established that within about 3AU from the Sun the main driver of cometary activity is the sublimation of water ice. At larger distances, more volatile species like CO or CO₂ are likely to play a major role (e.g., Meech et al. 2017). There remains a lot of uncertainty about where and how the transition between driving species takes place, and whether or not there are differences in this between new and returning comets. It has long been observed that DNCs tend to be brighter (more active) than periodic comets (the ‘fading problem’: Oort 1950; Dones et al. 2004); the most significant difference between DNCs and returning comets appears to be greater activity at larger distances in the former population, which implies different driving species (Meech and Svoren 2004). Comet Interceptor will measure the absolute and relative densities of the main neutral volatiles (H₂O, CO, and CO₂) along the flyby trajectory, allowing us to derive production rates from these measurements, and show which is dominant in an LPC at ~1 au.

Although ground-based composition observations do not rival the comprehensive information that can be provided by *in situ* spacecraft measurements, remote observations still provide extremely valuable data on bulk composition (e.g., Fig. 8). Differences of composition between JFCs and more pristine comets coming from the OC are starting to emerge from ground-based observations, with the most highly volatile species (such as CO, CH₄, C₂H₆, and C₂H₂) being depleted in JFCs compared to OCCs (Dello Russo et al. 2016). However, this has not been confirmed *in situ*. Ground-based observations of highly volatile molecules in infrared/radio domains are rarely simultaneous: for example, CO in the M-band and other organics in the L-band are not observed simultaneously, although most species are usually observed with H₂O, or its proxy OH, to derive mixing ratios with respect to H₂O. This makes the measurement and comparison of relative abundances difficult in some cases, especially for data taken by old-fashioned infrared spectrometers. A large dispersion of mixing ratios observed in ground-based observations might be partially explained by such non-simultaneous observations. Comet Interceptor will provide an unprecedented opportunity to measure gas composition *in situ* and compare the abundance of molecules of high volatility such as CO, CH₄, C₂H₆, and CO₂ with respect to water in an LPC versus JFCs (67P) and evolvedHTCs (1P).
<figure><img src="image_11.png" /><figcaption>**Fig. 8** Typical comet spectrum from ground-based visible observations, with key emission features marked. Major components such as water or CO₂ are not observable and require space missions to be characterised. (Image courtesy of C. Opitom)</figcaption></figure>

level) from telescopes is based on LPCs, which are often more active and brighter. A mission to a bright LPC would present an opportunity to compare high resolution spectroscopy at a range of wavelengths with *in situ* mass spectrometry measurements, calibrating our understanding of the much wider observed population. Modern infrared and sub-mm facilities, such as the ESO Very Large Telescope and the other 8–10 m class telescopes, and the Atacama Large Millimetre/submillimetre Array, which did not exist at the time of Giotto, are suitable for observing molecular species thought to be directly released from the nucleus.

It is well established that within about 3AU from the Sun the main driver of cometary activity is the sublimation of water ice. At larger distances, more volatile species like CO or CO₂ are likely to play a major role (e.g., Meech et al. 2017). There remains a lot of uncertainty about where and how the transition between driving species takes place, and whether or not there are differences in this between new and returning comets. It has long been observed that DNCs tend to be brighter (more active) than periodic comets (the ‘fading problem’: Oort 1950; Dones et al. 2004); the most significant difference between DNCs and returning comets appears to be greater activity at larger distances in the former population, which implies different driving species (Meech and Svoren 2004). Comet Interceptor will measure the absolute and relative densities of the main neutral volatiles (H₂O, CO, and CO₂) along the flyby trajectory, allowing us to derive production rates from these measurements, and show which is dominant in an LPC at ~1 au.

Although ground-based composition observations do not rival the comprehensive information that can be provided by *in situ* spacecraft measurements, remote observations still provide extremely valuable data on bulk composition (e.g., Fig. 8). Differences of composition between JFCs and more pristine comets coming from the OC are starting to emerge from ground-based observations, with the most highly volatile species (such as CO, CH₄, C₂H₆, and C₂H₂) being depleted in JFCs compared to OCCs (Dello Russo et al. 2016). However, this has not been confirmed *in situ*. Ground-based observations of highly volatile molecules in infrared/radio domains are rarely simultaneous: for example, CO in the M-band and other organics in the L-band are not observed simultaneously, although most species are usually observed with H₂O, or its proxy OH, to derive mixing ratios with respect to H₂O. This makes the measurement and comparison of relative abundances difficult in some cases, especially for data taken by old-fashioned infrared spectrometers. A large dispersion of mixing ratios observed in ground-based observations might be partially explained by such non-simultaneous observations. Comet Interceptor will provide an unprecedented opportunity to measure gas composition *in situ* and compare the abundance of molecules of high volatility such as CO, CH₄, C₂H₆, and CO₂ with respect to water in an LPC versus JFCs (67P) and evolved HTCs (1P).
Aside from the volatiles mentioned above, Comet Interceptor will make detailed com-
positional measurements that are only possible *in situ*. The Rosetta spacecraft revealed the
presence of complex and diverse organic molecules in the coma of 67P, including key species
for prebiotic chemistry, some being observed in the coma of a comet for the first time (see
Altwegg et al. 2017a). These organics cannot be detected through ground-based observations
with current technology, preventing us from assessing the variation of their abundances be-
tween comets and, in particular, between primitive LPCs and processed JFCs. Obtaining an
inventory of (complex) organic molecules and other species important in prebiotic chem-
istry in a primitive LPC will delve further into the role of comets in transporting organic
matter to the early Earth (Marty et al. 2016). A surprising result of Rosetta was the abundant
molecular oxygen observed in the coma (Bieler et al. 2015b), while circumstantial evidence
for O₂ was also found in comet 1P (Rubin et al. 2015). Several formation mechanisms have
been discussed, from radiation of water ice and cold temperature chemistry in the ISM, to
various *in situ* formation mechanisms. Comet Interceptor will assess how ubiquitous O₂ is
in comets: a more pristine LPC or DNC target will narrow down possible formation mech-
anisms (cf. Luspay-Kuti et al. 2018). Evidence of ammonium salts in 67P found by Rosetta
(Altwegg et al. 2020) was also unexpected and may change the traditional view of species
parentage and how molecules are stored in cometary ices; for example, NH₄CN (if present
in the coma) can produce NH₃ and HCN, which were previously believed to be "parent"
molecules released directly from the nucleus.

Isotopic ratios, being very sensitive to physico-chemical conditions, provide crucial in-
formation on the provenance of cometary material, and therefore provide information for
comet and planet formation models. For instance, the D/H ratio in cometary water has been
used to infer whether or not comets could be a source of the water on Earth (Hartogh et al.
2011). To date, all comets with known D/H either exhibit a terrestrial ratio or an elevated
ratio (Altwegg et al. 2017b). A recent study suggests two distinct sources of water with
different D/H: one source on the surface of the nucleus and the other in the form of subli-
mating icy grains in the coma (Lis et al. 2019). Fulle (2021) models these observations with
'pebbles' of different ice content. Depending on the type of the activity of the target comet,
Comet Interceptor may obtain measurements of both reservoirs during the flyby. The D/H
ratio of a less evolved object may furthermore give some insight into outgassing-related
fractionation processes. These measurements will also help in interpreting differences or
similarities between remote and *in situ* D/H measurements, in particular if simultaneous
measurements can be made from the ground, which is likely to be the case if Comet Inter-
ceptor's target is a relatively bright LPC, as expected. Measurement of the ¹⁸O/¹⁶O in H₂O
by Rosetta revealed an enrichment in ¹⁸O compared to the terrestrial value (Altwegg et al.
2019). Models of chemical evolution in the protoplanetary disc or natal molecular cloud
of our Solar System do not yet explain these results, and demonstrate the value of isotopic
measurements in furthering our understanding of the topic of planetary system formation in
general (Hily-Blant et al. 2017; Wiström and Charnley 2018; Furuya and Aikawa 2018).
Even though D/H is the primary objective, if the target is sufficiently active, Comet Inter-
ceptor will investigate isotopes in other species including, e.g.: ¹⁸O/¹⁶O in H₂O, ¹³C/¹²C in
CO₂, and ³⁴S/³²S in OCS and CS₂.

Many pre-solar signatures have been observed in the isotopes of volatiles in 67P (Hoppe
et al. 2018). Similar bulk abundances of volatile molecules were observed in Comet C/1995
O1 Hale-Bopp and 67P, and within objects in the ISM (Bockelée-Morvan et al. 2000; Droz-
dovskaya et al. 2019). The detailed *in situ* measurement of an LPC or a DNC will probe the
potential locations of origin of its ices at a molecular, elemental, and isotopic level.
Aside from the volatiles mentioned above, Comet Interceptor will make detailed com-
positional measurements that are only possible *in situ*. The Rosetta spacecraft revealed the
presence of complex and diverse organic molecules in the coma of 67P, including key species
for prebiotic chemistry, some being observed in the coma of a comet for the first time (see
Altwegg et al. 2017a). These organics cannot be detected through ground-based observations
with current technology, preventing us from assessing the variation of their abundances be-
tween comets and, in particular, between primitive LPCs and processed JFCs. Obtaining an
inventory of (complex) organic molecules and other species important in prebiotic chem-
istry in a primitive LPC will delve further into the role of comets in transporting organic
matter to the early Earth (Marty et al. 2016). A surprising result of Rosetta was the abundant
molecular oxygen observed in the coma (Bieler et al. 2015b), while circumstantial evidence
for O₂ was also found in comet 1P (Rubin et al. 2015). Several formation mechanisms have
been discussed, from radiation of water ice and cold temperature chemistry in the ISM, to
various *in situ* formation mechanisms. Comet Interceptor will assess how ubiquitous O₂ is
in comets: a more pristine LPC or DNC target will narrow down possible formation mech-
anisms (cf. Luspay-Kuti et al. 2018). Evidence of ammonium salts in 67P found by Rosetta
(Altwegg et al. 2020) was also unexpected and may change the traditional view of species
parentage and how molecules are stored in cometary ices; for example, NH₄CN (if present
in the coma) can produce NH₃ and HCN, which were previously believed to be "parent"
molecules released directly from the nucleus.

Isotopic ratios, being very sensitive to physico-chemical conditions, provide crucial in-
formation on the provenance of cometary material, and therefore provide information for
comet and planet formation models. For instance, the D/H ratio in cometary water has been
used to infer whether or not comets could be a source of the water on Earth (Hartogh et al.
2011). To date, all comets with known D/H either exhibit a terrestrial ratio or an elevated
ratio (Altwegg et al. 2017b). A recent study suggests two distinct sources of water with
different D/H: one source on the surface of the nucleus and the other in the form of subli-
mating icy grains in the coma (Lis et al. 2019). Fulle (2021) models these observations with
'pebbles' of different ice content. Depending on the type of the activity of the target comet,
Comet Interceptor may obtain measurements of both reservoirs during the flyby. The D/H
ratio of a less evolved object may furthermore give some insight into outgassing-related
fractionation processes. These measurements will also help in interpreting differences or
similarities between remote and *in situ* D/H measurements, in particular if simultaneous
measurements can be made from the ground, which is likely to be the case if Comet Inter-
ceptor's target is a relatively bright LPC, as expected. Measurement of the ¹⁸O/¹⁶O in H₂O
by Rosetta revealed an enrichment in ¹⁸O compared to the terrestrial value (Altwegg et al.
2019). Models of chemical evolution in the protoplanetary disc or natal molecular cloud
of our Solar System do not yet explain these results, and demonstrate the value of isotopic
measurements in furthering our understanding of the topic of planetary system formation in
general (Hily-Blant et al. 2017; Wiström and Charnley 2018; Furuya and Aikawa 2018).
Even though D/H is the primary objective, if the target is sufficiently active, Comet Inter-
ceptor will investigate isotopes in other species including, e.g.: ¹⁸O/¹⁶O in H₂O, ¹³C/¹²C in
CO₂, and ³⁴S/³²S in OCS and CS₂.

Many pre-solar signatures have been observed in the isotopes of volatiles in 67P (Hoppe
et al. 2018). Similar bulk abundances of volatile molecules were observed in Comet C/1995
O1 Hale-Bopp and 67P, and within objects in the ISM (Bockelée-Morvan et al. 2000; Droz-
dovskaya et al. 2019). The detailed *in situ* measurement of an LPC or a DNC will probe the
potential locations of origin of its ices at a molecular, elemental, and isotopic level.
<figure><img src="image_11.png" /><figcaption>**Fig. 9** Images showing different morphology of gas jets of different species from Rosetta/OSIRIS (from Bodewits et al. 2016)</figcaption></figure>

**Spatial Distribution and Structures of the Neutral Gas Coma** The spatial distribution of volatiles in the coma is of particular interest as it can provide information on how the ices are distributed in the nucleus. However, this aspect is a difficult issue to address. Ground-based observations only allow mapping of the distribution of volatiles on very large scales, missing the crucial transition region between the nucleus and the inner coma. As Rosetta orbited the comet for an extended period, it built up maps of coma composition of mother and daughter species at much smaller scales above different areas (Fig. 9). Rosetta confirmed that comets have heterogeneous comas dominated by large fluctuations in composition, often linked to diurnal and seasonal variations in the major outgassing species such as H₂O, CO₂, and CO (e.g., Hässig et al. 2015). A general large-scale anticorrelation between H₂O and CO₂ was observed by Rosetta (e.g., Mall et al. 2016; Migliorini et al. 2016). The gas density in the coma is strongly affected by nucleus concavities and sun illumination conditions, even when the distribution of ices on the nucleus surface is quite uniform (Bieler et al. 2015,b).

Separate but simultaneous measurements from spatially distributed sub-spacecraft will provide snapshots of different coma regions at the time of flyby. This method actually has advantages over Rosetta's approach, as it will allow, for the first time, separation of spatial and temporal variations. Interpretation of Rosetta results instead requires complex models to understand how the comet changed with time and space, because measurements of different coma areas were taken at different places and times along Rosetta's orbit. The unique multi-spacecraft architecture of Comet Interceptor will allow the addressing of important questions on how dynamic effects in the coma relate to each other: Which are due to the changing position of the spacecraft relative to the nucleus, and which to the comet's time-varying behaviour? (cf. Hansen et al. 2016).

**Dust Coma** The refractory component of the comet coma is referred to as 'dust'. It is made up of minerals and organic components lifted from the surface with sizes ranging from sub-micron to metre-scale chunks. The properties of the dust are important to understanding comet composition and formation. For example, the similarity in size of the dominant particles in 67P's coma and those predicted to form planets via streaming instability was used to argue in favour of that model, assuming that particles reflect the original size distribution (Blum et al. 2017). Measuring the dust size distribution down to nm size in an LPC will
<figure><img src="image_12.png" /><figcaption>**Fig. 9** Images showing different morphology of gas jets of different species from Rosetta/OSIRIS (from Bodewits et al. 2016)</figcaption></figure>

**Spatial Distribution and Structures of the Neutral Gas Coma** The spatial distribution of volatiles in the coma is of particular interest as it can provide information on how the ices are distributed in the nucleus. However, this aspect is a difficult issue to address. Ground-based observations only allow mapping of the distribution of volatiles on very large scales, missing the crucial transition region between the nucleus and the inner coma. As Rosetta orbited the comet for an extended period, it built up maps of coma composition of mother and daughter species at much smaller scales above different areas (Fig. 9). Rosetta confirmed that comets have heterogeneous comas dominated by large fluctuations in composition, often linked to diurnal and seasonal variations in the major outgassing species such as H₂O, CO₂, and CO (e.g., Hässig et al. 2015). A general large-scale anticorrelation between H₂O and CO₂ was observed by Rosetta (e.g., Mall et al. 2016; Migliorini et al. 2016). The gas density in the coma is strongly affected by nucleus concavities and sun illumination conditions, even when the distribution of ices on the nucleus surface is quite uniform (Bieler et al. 2015,b).

Separate but simultaneous measurements from spatially distributed sub-spacecraft will provide snapshots of different coma regions at the time of flyby. This method actually has advantages over Rosetta's approach, as it will allow, for the first time, separation of spatial and temporal variations. Interpretation of Rosetta results instead requires complex models to understand how the comet changed with time and space, because measurements of different coma areas were taken at different places and times along Rosetta's orbit. The unique multi-spacecraft architecture of Comet Interceptor will allow the addressing of important questions on how dynamic effects in the coma relate to each other: Which are due to the changing position of the spacecraft relative to the nucleus, and which to the comet's time-varying behaviour? (cf. Hansen et al. 2016).

**Dust Coma** The refractory component of the comet coma is referred to as 'dust'. It is made up of minerals and organic components lifted from the surface with sizes ranging from sub-micron to metre-scale chunks. The properties of the dust are important to understanding comet composition and formation. For example, the similarity in size of the dominant particles in 67P's coma and those predicted to form planets via streaming instability was used to argue in favour of that model, assuming that particles reflect the original size distribution (Blum et al. 2017). Measuring the dust size distribution down to nm size in an LPC will
provide some information towards testing the universality of this assumption: if it is similar to JFCs it would imply a link with primordial dust distribution, and support streaming instability models, whereas a difference would imply that comet material has been processed at µm scales and that dust properties reveal evolutionary processes.

Whether or not solid material lifted from the comet's surface also contains ice is important for our understanding of activity processes, as sublimation from dust in the coma could form a distributed source of water or other gasses. Rosetta found that little water is provided by sublimation from particles beyond a few nucleus radii, so there is no significant distributed source for water at 67P. That does not preclude distributed sources for minor constituents: for instance, evidence has been found for a distributed source for the hydrogen halides (De Keyser et al. 2017) and some of the lesser volatiles including organics (Altwegg et al. 2016, 2017a). This is in stark contrast to the earlier EPOXI flyby of comet 103P (A'Hearn et al. 2011) and remote observations of the innermost coma of comet 73P/Schwassmann-Wachmann 3, where a significant fraction of the outgassing occurred from a distributed source of icy grains (Fougere et al. 2012, 2013). Still, assessing the existence of a distributed source is difficult, even during a fast flyby where one can consider gas and dust production to be basically constant: cometocentric longitude, latitude, and distance all change simultaneously, and therefore it is not straightforward to extract purely radial density profiles from which one can ascertain the existence of a distributed source. To detect and identify distributed sources, it is necessary to scan a very large range of radial distances and, simultaneously, to assess the dust and gas abundances at a minimum of two points in the coma: again, the multi-point architecture of Comet Interceptor will give it a unique advantage in addressing this question. Constraining the dust-to-ice ratio in the coma (individually for icy particles, or in bulk by comparison of dust and gas production rates), and by inference in the nucleus, will also provide information for planet formation models. The question of how much ice is present in larger chunks in the coma is a critical parameter to understand the overall dust-to-ice ratio within a comet, which is a very important number to constrain the properties of the formation location of the comet, and one that is still the subject of intense debate following Rosetta (see, e.g., Fulle et al. 2019; Choukroun et al. 2020). Measurements at a less evolved LPC would be very valuable to understand this intrinsic ratio.

**Dust Reflectance Properties** Ground-based observations of the intensity, colour, and degree of linear polarisation of light scattered by cometary dust particles are used extensively for retrieving information on their physical properties, including their morphology and structure, as defined by Güttler et al. (2019) (Fig. 10). However, they are usually limited to certain observational geometries. Observations from spacecraft offer the opportunity to sample a wider range of geometries at a single time, from within the comet's coma. The analysis of the Rosetta dataset has exposed challenging contradictions between the properties of cometary dust as modelled from ground-based and from *in situ* observations. The Rosetta OSIRIS camera system provided unique observations of the intensity of light scattered by dust within 67P's coma (Bertini et al. 2017). The observed phase functions show a peculiar U-shape with a minimum at a phase angle around 100° (compared with the minimum at ~55° in the Halley-Marcus function. Those data would indicate the presence of large grains. Further, ground-based observations of the degree of linear polarisation of 67P show a negative polarisation branch (NPB) at small phase angles and a maximum observed DLP (Degree of Linear Polarisation) of ~8% at a phase angle of 32° obtained after the 2015 perihelion (Myers and Nordsieck 1984; Chernova et al. 1993; Hadamcik et al. 2016). Micronsized particles may reproduce the shape of the DLP curve, but certainly not the U-shaped
provide some information towards testing the universality of this assumption: if it is similar to JFCs it would imply a link with primordial dust distribution, and support streaming instability models, whereas a difference would imply that comet material has been processed at µm scales and that dust properties reveal evolutionary processes.

Whether or not solid material lifted from the comet's surface also contains ice is important for our understanding of activity processes, as sublimation from dust in the coma could form a distributed source of water or other gasses. Rosetta found that little water is provided by sublimation from particles beyond a few nucleus radii, so there is no significant distributed source for water at 67P. That does not preclude distributed sources for minor constituents: for instance, evidence has been found for a distributed source for the hydrogen halides (De Keyser et al. 2017) and some of the lesser volatiles including organics (Altwegg et al. 2016, 2017a). This is in stark contrast to the earlier EPOXI flyby of comet 103P (A'Hearn et al. 2011) and remote observations of the innermost coma of comet 73P/Schwassmann-Wachmann 3, where a significant fraction of the outgassing occurred from a distributed source of icy grains (Fougere et al. 2012, 2013). Still, assessing the existence of a distributed source is difficult, even during a fast flyby where one can consider gas and dust production to be basically constant: cometocentric longitude, latitude, and distance all change simultaneously, and therefore it is not straightforward to extract purely radial density profiles from which one can ascertain the existence of a distributed source. To detect and identify distributed sources, it is necessary to scan a very large range of radial distances and, simultaneously, to assess the dust and gas abundances at a minimum of two points in the coma: again, the multi-point architecture of Comet Interceptor will give it a unique advantage in addressing this question. Constraining the dust-to-ice ratio in the coma (individually for icy particles, or in bulk by comparison of dust and gas production rates), and by inference in the nucleus, will also provide information for planet formation models. The question of how much ice is present in larger chunks in the coma is a critical parameter to understand the overall dust-to-ice ratio within a comet, which is a very important number to constrain the properties of the formation location of the comet, and one that is still the subject of intense debate following Rosetta (see, e.g., Fulle et al. 2019; Choukroun et al. 2020). Measurements at a less evolved LPC would be very valuable to understand this intrinsic ratio.

**Dust Reflectance Properties** Ground-based observations of the intensity, colour, and degree of linear polarisation of light scattered by cometary dust particles are used extensively for retrieving information on their physical properties, including their morphology and structure, as defined by Güttler et al. (2019) (Fig. 10). However, they are usually limited to certain observational geometries. Observations from spacecraft offer the opportunity to sample a wider range of geometries at a single time, from within the comet's coma. The analysis of the Rosetta dataset has exposed challenging contradictions between the properties of cometary dust as modelled from ground-based and from *in situ* observations. The Rosetta OSIRIS camera system provided unique observations of the intensity of light scattered by dust within 67P's coma (Bertini et al. 2017). The observed phase functions show a peculiar U-shape with a minimum at a phase angle around 100° (compared with the minimum at ~55° in the Halley-Marcus function. Those data would indicate the presence of large grains. Further, ground-based observations of the degree of linear polarisation of 67P show a negative polarisation branch (NPB) at small phase angles and a maximum observed DLP (Degree of Linear Polarisation) of ~8% at a phase angle of 32° obtained after the 2015 perihelion (Myers and Nordsieck 1984; Chernova et al. 1993; Hadamcik et al. 2016). Micronsized particles may reproduce the shape of the DLP curve, but certainly not the U-shaped
<figure><img src="image_12.png" /><figcaption>**Fig. 10** Morphology of different types of dust particles, from summary by Güttler et al. (2019)</figcaption></figure>

OSIRIS phase function The major challenge is reconciling conclusions about the properties of cometary dust obtained from the analysis of both datasets, which demands a common framework to interpret consistently all the datasets available (Moreno et al. 2018; Markkanen et al. 2018; Levasseur-Regourd et al. 2019; Muñoz et al. 2020).

Rosetta did not provide *in situ* polarimetric observations, but these were obtained by Giotto's HOPE (Halley Optical Probe Experiment) instrument, which took observations along the line-of-sight of the spinning spacecraft at different wavelengths free of gas emission. These allowed an estimate to be made of the density of the cometary dust, which was found to be low (about $100 \text{ kg m}^{-3}$), and of its geometric albedo (0.04, matching that of the nucleus). The polarisation was highest when crossing dust jets and lower in the inner coma region. During the Giotto Extended Mission flyby of 26P/Grigg-Skjellerup, HOPE hinted at the presence of pebbles ejected from the nucleus. Since then, polarisation imaging of active comets have confirmed the existence of lower polarisation in the innermost coma and increased polarisation along large-scale dust jets. Interstellar comet 2I/Borisov presented unique polarimetric features: the polarisation increased steeply with phase angle, reaching values substantially higher than typically measured in small Solar System bodies (Bagnulo et al. 2021). These polarimetric properties distinguish 2I from dynamically evolved objects such as JFCs, and suggest that 2I was a highly pristine object, with a coma probably characterised by relatively small-sized aggregates. This also suggests that detailed *in situ* polarimetric observations at a DNC or LPC will be very valuable in revealing differences in dust properties between pristine and evolved comets.

**Dust Structure and Distribution** Numerical simulations based on the reflectance properties described previously conclude that dust particles are irregular, porous, built of smaller grains and possibly fractal in nature. Additionally, experimental simulations under different conditions (including microgravity) suggest the presence of fluffy aggregates with comparable amounts of minerals and organics (Levasseur-Regourd et al. 2015). This agrees with the results from Rosetta's *in situ* sampling, in particular microscopic imaging of grains by the COSIMA (Langevin et al. 2020; Kimura et al. 2020) and MIDAS instruments (Mannel et al. 2019).

*In situ* measurements (typically measuring dust flux from momentum transferred in collisions with a target on the spacecraft) naturally sample only a single coma location at a time. Rosetta's GIADA measured grains' cross-sectional area, velocity, and impact momentum. It could therefore separate fluffy and compact particles. The instrument also characterized the submicrometre- to micrometre-sized dust mass flux at 67P, finding a differential size distribution index of ≈ −3.0, which confirms that particles of size ≥0.1 mm dominate the dust coma cross-section during that comet's entire orbit (Della Corte et al. 2019). On flyby missions, the velocity of impacting particles is dominated by the relative spacecraft velocity to
<figure><img src="image_13.png" /><figcaption>**Fig. 10** Morphology of different types of dust particles, from summary by Güttler et al. (2019)</figcaption></figure>

OSIRIS phase function The major challenge is reconciling conclusions about the properties of cometary dust obtained from the analysis of both datasets, which demands a common framework to interpret consistently all the datasets available (Moreno et al. 2018; Markkanen et al. 2018; Levasseur-Regourd et al. 2019; Muñoz et al. 2020).

Rosetta did not provide *in situ* polarimetric observations, but these were obtained by Giotto's HOPE (Halley Optical Probe Experiment) instrument, which took observations along the line-of-sight of the spinning spacecraft at different wavelengths free of gas emission. These allowed an estimate to be made of the density of the cometary dust, which was found to be low (about $100 \text{ kg m}^{-3}$), and of its geometric albedo (0.04, matching that of the nucleus). The polarisation was highest when crossing dust jets and lower in the inner coma region. During the Giotto Extended Mission flyby of 26P/Grigg-Skjellerup, HOPE hinted at the presence of pebbles ejected from the nucleus. Since then, polarisation imaging of active comets have confirmed the existence of lower polarisation in the innermost coma and increased polarisation along large-scale dust jets. Interstellar comet 2I/Borisov presented unique polarimetric features: the polarisation increased steeply with phase angle, reaching values substantially higher than typically measured in small Solar System bodies (Bagnulo et al. 2021). These polarimetric properties distinguish 2I from dynamically evolved objects such as JFCs, and suggest that 2I was a highly pristine object, with a coma probably characterised by relatively small-sized aggregates. This also suggests that detailed *in situ* polarimetric observations at a DNC or LPC will be very valuable in revealing differences in dust properties between pristine and evolved comets.

**Dust Structure and Distribution** Numerical simulations based on the reflectance properties described previously conclude that dust particles are irregular, porous, built of smaller grains and possibly fractal in nature. Additionally, experimental simulations under different conditions (including microgravity) suggest the presence of fluffy aggregates with comparable amounts of minerals and organics (Levasseur-Regourd et al. 2015). This agrees with the results from Rosetta's *in situ* sampling, in particular microscopic imaging of grains by the COSIMA (Langevin et al. 2020; Kimura et al. 2020) and MIDAS instruments (Mannel et al. 2019).

*In situ* measurements (typically measuring dust flux from momentum transferred in collisions with a target on the spacecraft) naturally sample only a single coma location at a time. Rosetta's GIADA measured grains' cross-sectional area, velocity, and impact momentum. It could therefore separate fluffy and compact particles. The instrument also characterized the submicrometre- to micrometre-sized dust mass flux at 67P, finding a differential size distribution index of ≈ −3.0, which confirms that particles of size ≥0.1 mm dominate the dust coma cross-section during that comet's entire orbit (Della Corte et al. 2019). On flyby missions, the velocity of impacting particles is dominated by the relative spacecraft velocity to
the comet, which is well known in all cases, so particle masses can be directly inferred from momentum sensors, as achieved, for example, by instruments on the Giotto and the Stardust mission (the latter also delivering a sample of the more solid coma grains to Earth). Comet Interceptor will be the first mission to make such measurements at two coma locations simultaneously, which will be very valuable for understanding the evolution of particles (e.g., any evidence of fragmentation) flowing away from the nucleus.

The spatial dust distribution within the coma is important for understanding activity, i.e., how, and where the dust flows away from the nucleus (see next section), and can be divided into the inner coma, larger scale coma, and tails. Both remote sensing and in situ measurements reveal significant variation in the population in different regions. There are broad outflows and more collimated jets, which appear to be controlled by nucleus topography in the inner coma, but are also visible on 1000s of km scales in ground-based imaging. The large-scale features have yet to be conclusively linked to the inner coma structures and nucleus. At the very largest scales, dust is swept into the characteristic dust tail, where differences in acceleration due to solar radiation pressure differentiate the material largely by particle size – models of tail morphology can therefore be used to place constraints on particle properties.

The fast flyby will provide an instantaneous snapshot of the nucleus and coma. However, depending on the comet's spin state, pre- and post-closest approach observations will contribute additional information on the diurnal evolution of the near/far environment. Such observations, even with an unresolved nucleus, will be extremely valuable to characterize the gas and dust distribution, as well as the activity of different regions at multiple local times.

### 2.3.3 Activity

Cometary activity is a complex process: the exact sources and mechanisms driving activity in comets remain a puzzle, e.g. Keller and Kührt (2020). Rosetta revealed that cometary activity, jets, and outbursts (Fig. 11) are linked to distinct morphological nucleus features (Vincent et al. 2015, 2016). Working activity models have been recently provided in case of pebble-made nuclei (Fulle et al. 2020a,b; Gundlach et al. 2020; Fulle 2021; Ciarniello et al. 2021, 2022). The pebble model is however debated in the community, as the existing data is not sufficient to fully assess its validity. By characterizing an LPC, in particular a DNC, from a close distance, Comet Interceptor will obtain unique observations of dust and gas release from a type of object not studied before and provide a test case to evaluate all current cometary activity models. The surface of a DNC on its first approach to the Sun is expected to be little processed and Comet Interceptor will allow us to assess how this impacts activity. If activity can be attributed clearly to local areas in a DNC this would offer strong evidence that they are due to primordial shape or composition heterogeneities, and not evolution-driven features, as no processed surface crust is expected. Evidence for or against hyperactivity – significant activity being driven by sublimation from icy ‘chunks’ in the coma (Fig. 12) – would test also whether or not this phenomenon (seen in 103P but not in 67P) is evolution-related, and may be important in the interpretation of ground-based observations of other DNCs. Typically, the distribution of coma gas and dust structures in the near environment and their nucleus source locations are inferred from the spatial tracking of jet-like emissions across multiple images. The combined motion of the spacecraft and rotation of the nucleus usually provide sufficient geometric variation to reconstruct the jets’ 3D shape. This method, however, assumes that tracked features do not evolve between images, which may not always be the case. By providing multiple views of the same features
the comet, which is well known in all cases, so particle masses can be directly inferred from momentum sensors, as achieved, for example, by instruments on the Giotto and the Stardust mission (the latter also delivering a sample of the more solid coma grains to Earth). Comet Interceptor will be the first mission to make such measurements at two coma locations simultaneously, which will be very valuable for understanding the evolution of particles (e.g., any evidence of fragmentation) flowing away from the nucleus.

The spatial dust distribution within the coma is important for understanding activity, i.e., how, and where the dust flows away from the nucleus (see next section), and can be divided into the inner coma, larger scale coma, and tails. Both remote sensing and in situ measurements reveal significant variation in the population in different regions. There are broad outflows and more collimated jets, which appear to be controlled by nucleus topography in the inner coma, but are also visible on 1000s of km scales in ground-based imaging. The large-scale features have yet to be conclusively linked to the inner coma structures and nucleus. At the very largest scales, dust is swept into the characteristic dust tail, where differences in acceleration due to solar radiation pressure differentiate the material largely by particle size – models of tail morphology can therefore be used to place constraints on particle properties.

The fast flyby will provide an instantaneous snapshot of the nucleus and coma. However, depending on the comet's spin state, pre- and post-closest approach observations will contribute additional information on the diurnal evolution of the near/far environment. Such observations, even with an unresolved nucleus, will be extremely valuable to characterize the gas and dust distribution, as well as the activity of different regions at multiple local times.

### 2.3.3 Activity

Cometary activity is a complex process: the exact sources and mechanisms driving activity in comets remain a puzzle, e.g. Keller and Kührt (2020). Rosetta revealed that cometary activity, jets, and outbursts (Fig. 11) are linked to distinct morphological nucleus features (Vincent et al. 2015, 2016). Working activity models have been recently provided in case of pebble-made nuclei (Fulle et al. 2020a,b; Gundlach et al. 2020; Fulle 2021; Ciarniello et al. 2021, 2022). The pebble model is however debated in the community, as the existing data is not sufficient to fully assess its validity. By characterizing an LPC, in particular a DNC, from a close distance, Comet Interceptor will obtain unique observations of dust and gas release from a type of object not studied before and provide a test case to evaluate all current cometary activity models. The surface of a DNC on its first approach to the Sun is expected to be little processed and Comet Interceptor will allow us to assess how this impacts activity. If activity can be attributed clearly to local areas in a DNC this would offer strong evidence that they are due to primordial shape or composition heterogeneities, and not evolution-driven features, as no processed surface crust is expected. Evidence for or against hyperactivity – significant activity being driven by sublimation from icy ‘chunks’ in the coma (Fig. 12) – would test also whether or not this phenomenon (seen in 103P but not in 67P) is evolution-related, and may be important in the interpretation of ground-based observations of other DNCs. Typically, the distribution of coma gas and dust structures in the near environment and their nucleus source locations are inferred from the spatial tracking of jet-like emissions across multiple images. The combined motion of the spacecraft and rotation of the nucleus usually provide sufficient geometric variation to reconstruct the jets’ 3D shape. This method, however, assumes that tracked features do not evolve between images, which may not always be the case. By providing multiple views of the same features
<figure><img src="image_13.png" /><figcaption>**Fig. 12** Image from NASA's EPOXI mission shows part of the nucleus of Comet 103P/Hartley 2. The sun is illuminating the nucleus from the right. A distinct cloud of individual particles is visible, gas release from which is responsible for the high apparent activity level given this nucleus's size. Image Credit: NASA/JPL-Caltech/UMD</figcaption></figure>

<figure><img src="image_14.png" /></figure>

from different angles at the same time, Comet Interceptor will remove this uncertainty and allow for a more precise mapping of apparent active regions.

The most obvious observational evidence of nuclear activity is the collimated dust jets ejected from the surface. There is strong complementarity between *in situ* and ground-based imaging of coma structures, as was shown for 67P, where the analysis of ground based data revealed the presence of structures originating from certain latitude/longitudes on the nucleus (Vincent et al. 2010, 2013; Lara et al. 2011; Marschall et al. 2016). This allowed the nucleus's rotational period to be derived, based on previous apparitions, before the spacecraft arrived. These first results related to 67P's spin period and activity were confirmed by Rosetta, although the mission's findings go beyond the classical view of active areas on a nucleus surface producing jet-like features. Comet Interceptor's three viewpoints will provide the data necessary to link nucleus surface activity and topography with the large dust structures in the coma. Additionally, ground-based observations of the target's activity should be plentiful and high signal-to-noise, for a relatively bright LPC target; these will complement and connect to the multi-point spacecraft measurements.

### 2.3.4 Plasma Environment and Interactions

The neutral gas released from a comet eventually becomes partially ionised through exposure to solar UV radiation, and/or energetic particle impact bombardment. Ions born close to the nucleus interact with the cometary neutrals and, for a high enough outgassing, a rich range of chemical reactions can take place, producing new ion species, e.g. Beth et al.
**Fig. 11** Outburst on the surface of 67P/Churyumov-Gerasimenko observed on 3 July 2016 (left) and on 29 July 2015 (right). ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM

<figure><img src="image_14.png" /></figure>

**Fig. 12** Image from NASA's EPOXI mission shows part of the nucleus of Comet 103P/Hartley 2. The sun is illuminating the nucleus from the right. A distinct cloud of individual particles is visible, gas release from which is responsible for the high apparent activity level given this nucleus's size. Image Credit: NASA/JPL-Caltech/UMD

<figure><img src="image_15.png" /></figure>

from different angles at the same time, Comet Interceptor will remove this uncertainty and allow for a more precise mapping of apparent active regions.

The most obvious observational evidence of nuclear activity is the collimated dust jets ejected from the surface. There is strong complementarity between *in situ* and ground-based imaging of coma structures, as was shown for 67P, where the analysis of ground based data revealed the presence of structures originating from certain latitude/longitudes on the nucleus (Vincent et al. 2010, 2013; Lara et al. 2011; Marschall et al. 2016). This allowed the nucleus's rotational period to be derived, based on previous apparitions, before the spacecraft arrived. These first results related to 67P's spin period and activity were confirmed by Rosetta, although the mission's findings go beyond the classical view of active areas on a nucleus surface producing jet-like features. Comet Interceptor's three viewpoints will provide the data necessary to link nucleus surface activity and topography with the large dust structures in the coma. Additionally, ground-based observations of the target's activity should be plentiful and high signal-to-noise, for a relatively bright LPC target; these will complement and connect to the multi-point spacecraft measurements.

### 2.3.4 Plasma Environment and Interactions

The neutral gas released from a comet eventually becomes partially ionised through exposure to solar UV radiation, and/or energetic particle impact bombardment. Ions born close to the nucleus interact with the cometary neutrals and, for a high enough outgassing, a rich range of chemical reactions can take place, producing new ion species, e.g. Beth et al.
(2020). These fresh cometary ions come under the influence of the heliospheric magnetic field, join, as pick-up ions, the solar wind flow through and past the comet, and are carried downstream, forming the visible ion or plasma tail.

**Origin and Overall Structure of the Solar Wind Interaction Region** The ion cloud around a nucleus presents an obstacle to the solar wind (Alfven 1957; Biermann et al. 1967). A comet's plasma environment is heavily dependent on the number of cometary ions in the coma, and therefore the gas production rate. As the cometary pick-up ions are incorporated into the solar wind – a process known as mass-loading – the flow around the comet changes, which modifies the cometary and solar wind plasma. In the cometary reference frame, pickup ions are initially almost at rest, there is thus a large discrepancy in velocity between the two flows. Mass loading simultaneously slows the solar wind and accelerates cometary ions. A bow shock forms where the solar wind adjusts abruptly to the comet's presence. This also has consequences for the magnetic field, as it is largely frozen into the flow, and starts to drape around the obstacle.

**The Value of Multi-Point Measurements** A multi-spacecraft encounter would provide a significant advantage over previous missions in that it would sample different paths through the comet-solar wind interaction region. This includes both the largest scales, beyond comtocentric distances explored by Rosetta, and the smaller scales, nearer the nucleus. Rosetta showed how strongly structured and dynamic the cometary plasma environment is, and how different processes affect different parts of the comet magnetosphere for different activity levels. Charge separation, shocks, and other boundaries, as well as the excitation of waves and plasma heating, all affect the exchange of energy, momentum, and mass on multiple scales simultaneously. This in turn is important for processes such as the sputtering of the nucleus surface and coma dust with energetic solar wind and cometary ions (Wurz et al. 2015), for the excitation of coma constituents and associated emissions (Galand et al. 2020), for coma chemistry, and for the formation of the comet ion tail and its rich structure. Rosetta lacked a larger scale overview and understanding of the 3D structure of the induced magnetosphere and magnetosheath. Earlier flyby missions, such as Giotto, had the opposite issue: they provided measurements over very large scales, but only along a single path.

With Comet Interceptor we therefore expect to observe, along multiple chords (Fig. 13), a shock, the diamagnetic cavity, strong electron heating, penetrating solar wind ions, energetic cometary ions moving towards the nucleus, and strong wave excitation. These measurements are needed to assess the relative importance of different mechanisms and to understand how the solar wind affects the comet environment for different types of objects and levels of comet activity. The 3D picture is needed to distinguish between different solar wind-coma interaction models. This can only be properly investigated with a multi-spacecraft mission, in order to reduce the spatio-temporal ambiguities introduced by potentially changing solar wind conditions during the flyby.

**Specific Features of Interest** Bow shocks have been observed in several comets and modelled extensively, e.g., Koenders et al. (2013). In general, this shock moves outwards in the upstream solar wind, away from the nucleus, as the neutral gas production rate increases, with standoff distances up to millions of km upstream at high gas production rates (as for 1P). At low production rates, the critical point is never reached, and no bow shock forms. For example, Giotto's 1992 flyby of 26P revealed a bow shock during the outbound trajectory, but not inbound (Coates et al. 1997). Instead, it detected a bow wave (e.g., Scarf et al. 1986), a more gradual field increase rather than the jump-like classical shock. This was attributed to the low Mach number of the mass-loaded solar wind flow. At Halley, the bow
(2020). These fresh cometary ions come under the influence of the heliospheric magnetic field, join, as pick-up ions, the solar wind flow through and past the comet, and are carried downstream, forming the visible ion or plasma tail.

**Origin and Overall Structure of the Solar Wind Interaction Region** The ion cloud around a nucleus presents an obstacle to the solar wind (Alfven 1957; Biermann et al. 1967). A comet's plasma environment is heavily dependent on the number of cometary ions in the coma, and therefore the gas production rate. As the cometary pick-up ions are incorporated into the solar wind – a process known as mass-loading – the flow around the comet changes, which modifies the cometary and solar wind plasma. In the cometary reference frame, pickup ions are initially almost at rest, there is thus a large discrepancy in velocity between the two flows. Mass loading simultaneously slows the solar wind and accelerates cometary ions. A bow shock forms where the solar wind adjusts abruptly to the comet's presence. This also has consequences for the magnetic field, as it is largely frozen into the flow, and starts to drape around the obstacle.

**The Value of Multi-Point Measurements** A multi-spacecraft encounter would provide a significant advantage over previous missions in that it would sample different paths through the comet-solar wind interaction region. This includes both the largest scales, beyond comtocentric distances explored by Rosetta, and the smaller scales, nearer the nucleus. Rosetta showed how strongly structured and dynamic the cometary plasma environment is, and how different processes affect different parts of the comet magnetosphere for different activity levels. Charge separation, shocks, and other boundaries, as well as the excitation of waves and plasma heating, all affect the exchange of energy, momentum, and mass on multiple scales simultaneously. This in turn is important for processes such as the sputtering of the nucleus surface and coma dust with energetic solar wind and cometary ions (Wurz et al. 2015), for the excitation of coma constituents and associated emissions (Galand et al. 2020), for coma chemistry, and for the formation of the comet ion tail and its rich structure. Rosetta lacked a larger scale overview and understanding of the 3D structure of the induced magnetosphere and magnetosheath. Earlier flyby missions, such as Giotto, had the opposite issue: they provided measurements over very large scales, but only along a single path.

With Comet Interceptor we therefore expect to observe, along multiple chords (Fig. 13), a shock, the diamagnetic cavity, strong electron heating, penetrating solar wind ions, energetic cometary ions moving towards the nucleus, and strong wave excitation. These measurements are needed to assess the relative importance of different mechanisms and to understand how the solar wind affects the comet environment for different types of objects and levels of comet activity. The 3D picture is needed to distinguish between different solar wind-coma interaction models. This can only be properly investigated with a multi-spacecraft mission, in order to reduce the spatio-temporal ambiguities introduced by potentially changing solar wind conditions during the flyby.

**Specific Features of Interest** Bow shocks have been observed in several comets and modelled extensively, e.g., Koenders et al. (2013). In general, this shock moves outwards in the upstream solar wind, away from the nucleus, as the neutral gas production rate increases, with standoff distances up to millions of km upstream at high gas production rates (as for 1P). At low production rates, the critical point is never reached, and no bow shock forms. For example, Giotto's 1992 flyby of 26P revealed a bow shock during the outbound trajectory, but not inbound (Coates et al. 1997). Instead, it detected a bow wave (e.g., Scarf et al. 1986), a more gradual field increase rather than the jump-like classical shock. This was attributed to the low Mach number of the mass-loaded solar wind flow. At Halley, the bow
<figure><img src="image_15.png" /><figcaption>**Fig. 13** Multi-point measurements will determine the scale and shape of several structures in the comet-solar wind interaction. The time at which each of the three spacecraft/probes cross (or do not cross) the bow shock (green) and diamagnetic cavity (blue) will determine their shapes and scales. The magnetic field (red) will also be probed using magnetometers on all the three platforms</figcaption></figure>

shock as observed by Giotto outbound was quasi-parallel (Neubauer et al. 1990). At 67P, Rosetta's trajectory did not allow for the detection of a bow shock or wave. At lower gas production rates, a feature interpreted as an infant bow shock, a highly asymmetric structure behaving like a shock and confined to one side of the interaction region (Gunell et al. 2018) was observed in the plasma environment. Spectral breaks in the pick-up ion energy distributions have furthermore been interpreted as indirect proof of a bow shock far upstream of the spacecraft (Alho et al. 2019, 2021). Modelling also revealed that solar wind charge exchange, as well as asymmetric nucleus outgassing plays a major role in the dynamics, width, and extent of the shock (Simon Wedlund et al. 2017; Alho et al. 2021). As a consequence, measuring the specifics of the shock (or wave) at different locations, or finding that there is a shock- like feature at one spacecraft and a wave- like behaviour at another, brings invaluable information on the solar wind-comet interaction. Depending on the flyby velocity and measurements' sampling rate, unprecedented multi-point information could be obtained on the shock's internal structure.

The draped magnetic field threads through the comet's coma and ion tail. However, the solar wind magnetic field varies in both strength and direction. Therefore, layers of magnetic field with different directions are embedded in the induced magnetosphere; structuring referred to as "nested draping". This has been observed during the fast flybys of comet 1P, for which a tentative model of how the magnetic field lines on either side of the nucleus are connected could be made (Raeder et al. 1987). Performing a flyby with magnetome-
<figure><img src="image_16.png" /><figcaption>**Fig. 13** Multi-point measurements will determine the scale and shape of several structures in the comet-solar wind interaction. The time at which each of the three spacecraft/probes cross (or do not cross) the bow shock (green) and diamagnetic cavity (blue) will determine their shapes and scales. The magnetic field (red) will also be probed using magnetometers on all the three platforms</figcaption></figure>

shock as observed by Giotto outbound was quasi-parallel (Neubauer et al. 1990). At 67P, Rosetta's trajectory did not allow for the detection of a bow shock or wave. At lower gas production rates, a feature interpreted as an infant bow shock, a highly asymmetric structure behaving like a shock and confined to one side of the interaction region (Gunell et al. 2018) was observed in the plasma environment. Spectral breaks in the pick-up ion energy distributions have furthermore been interpreted as indirect proof of a bow shock far upstream of the spacecraft (Alho et al. 2019, 2021). Modelling also revealed that solar wind charge exchange, as well as asymmetric nucleus outgassing plays a major role in the dynamics, width, and extent of the shock (Simon Wedlund et al. 2017; Alho et al. 2021). As a consequence, measuring the specifics of the shock (or wave) at different locations, or finding that there is a shock- like feature at one spacecraft and a wave- like behaviour at another, brings invaluable information on the solar wind-comet interaction. Depending on the flyby velocity and measurements' sampling rate, unprecedented multi-point information could be obtained on the shock's internal structure.

The draped magnetic field threads through the comet's coma and ion tail. However, the solar wind magnetic field varies in both strength and direction. Therefore, layers of magnetic field with different directions are embedded in the induced magnetosphere; structuring referred to as "nested draping". This has been observed during the fast flybys of comet 1P, for which a tentative model of how the magnetic field lines on either side of the nucleus are connected could be made (Raeder et al. 1987). Performing a flyby with magnetome-
ters on three spacecraft at different distances will allow the first reconstruction of the shape of the draped magnetic field. Three chords through the upstream induced magnetosphere would deliver three lines of magnetic field strength and direction that can be integrated into a whole. If the flyby is slow, dynamic draping can also be observed, first shown by Rosetta (Volwerk et al. 2019). When the convection velocity of the magnetic field is faster than that of the spacecraft past the comet, then the time variation of the magnetic field between the spacecraft can show how the nested draping is moving towards the comet. It was also shown that the magnetic field draping is not confined to the solar wind magnetic field plane, but can be shifted (Koenders et al. 2016).

In strongly outgassing comets, the solar wind-comet interaction forms a region imme-
diately surrounding the nucleus called the diamagnetic cavity. This cavity is devoid of a
magnetic field and prevents the solar wind's full penetration into the region closest to the
nucleus. First observed in an artificial comet (through a barium release in the solar wind just
outside the Earth's magnetosphere (Bernhardt et al. 1987), this cavity was also found at 1P
by Giotto, extending up to 4000 km from the nucleus. At 67P, Rosetta repeatedly detected
the cavity, which extended only a few 10s to 100s km from the nucleus. The size of this
magnetic field-free region depends on the activity of the comet, and with 67P being much
less active than 1P, the cavity size was accordingly much smaller. Due to the long residence
time of Rosetta at 67P, it was found that the cavity boundary is very dynamic, moving in
and out but, with only one spacecraft only limited information on the boundary could be
obtained.

The exact mechanism that sustains the diamagnetic cavity is not understood and more
measurements are therefore needed; its boundary has been observed only at 1P and 67P.
The plasma in the cavity is much quieter than outside it and is more dominated by cold
electrons (Odelstad et al. 2018). The stability of the boundary appeared to differ between the
two comets (Neubauer 1987; Goetz et al. 2016), and only rough estimates of the boundary
velocity could be derived.

The boundaries between different regions are asymmetric and highly variable in both
space and time. They depend on both the comet's activity and variations in upstream solar
wind conditions. Edberg et al. (2016b,a), Hajra et al. (2018), and Goetz et al. (2018) showed
that drastic changes occurred in the plasma at 67P, when interplanetary coronal mass ejec-
tions or co-rotating interaction regions passed the comet. To advance our understanding of
the physics of these interactions, we need to go beyond what was possible with previous
missions, all of which could only measure at one location at one time, by measuring at
multiple positions simultaneously, allowing the separation of spatial and temporal changes.
Whether one, two, or all three of the Comet Interceptor platforms will enter the diamagnetic
cavity, will on its own give information on the cavity's size and shape, as well as indications
of its dynamics, and will be expanded upon by other measurements.

Solar wind charge exchange produces cometary ions as well as energetic neutral hydro-
gen and helium that can be observed remotely through ENA imaging (Simon Wedlund et al.
2016, 2017, 2019c,a,b): their presence testifies to the efficiency of energy and momentum
transfer between the solar wind and the cometary coma. Observations of ENAs are also usef-
ful to characterise plasma interactions in the region where the solar wind reaches deepest
into the neutral atmosphere (Ekenbäck et al. 2008), possibly partaking in the sputtering of
the nucleus's surface (Nilsson et al. 2015). Ion chemistry within the coma also depends on
neutral gas composition and, therefore, ion observations can shed light on the bulk compo-
sition of the nucleus, and the still poorly understood complex chemical and photo-ionisation
ters on three spacecraft at different distances will allow the first reconstruction of the shape of the draped magnetic field. Three chords through the upstream induced magnetosphere would deliver three lines of magnetic field strength and direction that can be integrated into a whole. If the flyby is slow, dynamic draping can also be observed, first shown by Rosetta (Volwerk et al. 2019). When the convection velocity of the magnetic field is faster than that of the spacecraft past the comet, then the time variation of the magnetic field between the spacecraft can show how the nested draping is moving towards the comet. It was also shown that the magnetic field draping is not confined to the solar wind magnetic field plane, but can be shifted (Koenders et al. 2016).

In strongly outgassing comets, the solar wind-comet interaction forms a region imme-
diately surrounding the nucleus called the diamagnetic cavity. This cavity is devoid of a
magnetic field and prevents the solar wind's full penetration into the region closest to the
nucleus. First observed in an artificial comet (through a barium release in the solar wind just
outside the Earth's magnetosphere (Bernhardt et al. 1987), this cavity was also found at 1P
by Giotto, extending up to 4000 km from the nucleus. At 67P, Rosetta repeatedly detected
the cavity, which extended only a few 10s to 100s km from the nucleus. The size of this
magnetic field-free region depends on the activity of the comet, and with 67P being much
less active than 1P, the cavity size was accordingly much smaller. Due to the long residence
time of Rosetta at 67P, it was found that the cavity boundary is very dynamic, moving in
and out but, with only one spacecraft only limited information on the boundary could be
obtained.

The exact mechanism that sustains the diamagnetic cavity is not understood and more
measurements are therefore needed; its boundary has been observed only at 1P and 67P.
The plasma in the cavity is much quieter than outside it and is more dominated by cold
electrons (Odelstad et al. 2018). The stability of the boundary appeared to differ between the
two comets (Neubauer 1987; Goetz et al. 2016), and only rough estimates of the boundary
velocity could be derived.

The boundaries between different regions are asymmetric and highly variable in both
space and time. They depend on both the comet's activity and variations in upstream solar
wind conditions. Edberg et al. (2016b,a), Hajra et al. (2018), and Goetz et al. (2018) showed
that drastic changes occurred in the plasma at 67P, when interplanetary coronal mass ejec-
tions or co-rotating interaction regions passed the comet. To advance our understanding of
the physics of these interactions, we need to go beyond what was possible with previous
missions, all of which could only measure at one location at one time, by measuring at
multiple positions simultaneously, allowing the separation of spatial and temporal changes.
Whether one, two, or all three of the Comet Interceptor platforms will enter the diamagnetic
cavity, will on its own give information on the cavity's size and shape, as well as indications
of its dynamics, and will be expanded upon by other measurements.

Solar wind charge exchange produces cometary ions as well as energetic neutral hydro-
gen and helium that can be observed remotely through ENA imaging (Simon Wedlund et al.
2016, 2017, 2019c,a,b): their presence testifies to the efficiency of energy and momentum
transfer between the solar wind and the cometary coma. Observations of ENAs are also usef-
ful to characterise plasma interactions in the region where the solar wind reaches deepest
into the neutral atmosphere (Ekenbäck et al. 2008), possibly partaking in the sputtering of
the nucleus's surface (Nilsson et al. 2015). Ion chemistry within the coma also depends on
neutral gas composition and, therefore, ion observations can shed light on the bulk compo-
sition of the nucleus, and the still poorly understood complex chemical and photo-ionisation
<figure><img src="image_16.png" /><figcaption>**Fig. 14** The highly-structured ion tail of Comet C/2016 R2 (Pan-STARRS). This particular comet was an atypical dust-poor LPC (e.g. Biver et al. 2018) which reached perihelion at 2.6 au from the Sun. Image: ESO, under license Attribution 4.0 Interactional (CC BY 4.0)</figcaption></figure>

reactions in the coma (Haeberli et al. 1995; Fuselier et al. 2016; Heritier et al. 2017; Beth et al. 2017).

Waves take on an important role in the cometary plasma environment, transferring energy across boundaries and heating particle populations through wave-particle interactions. The plasma environment of a comet is a complex mix of ions of different species and origin and relative velocities, electrons of different temperatures, neutral molecules and dust particles of different sizes and charge states. Solar wind interactions with the cometary plasma gives rise to instabilities that drive waves of various kinds, including ion-cyclotron and/or mirror-mode waves, e.g., Mazelle et al. (1991), harmonic waves created by the ion-Weibel instability (the “singing comet” waves found by Rosetta; Weibel (1959), Richter et al. (2015), Meier et al. (2016), Glassmeier (2017)), lower hybrid waves, e.g., Karlsson et al. (2017), and ion acoustic waves, e.g., Gunell et al. (2021). It is, however, not clear in which region of the coma these waves are present and how they depend on comet activity. Through multipoint measurements in the coma one can determine, in principle, the temporal and spatial development of the waves. Going from single spacecraft to multi-spacecraft observations thus enables new insights into both the physics of the waves themselves and how they affect boundaries and the surrounding plasma.

The comet-solar wind interaction region can also be observed remotely, due to resonance fluorescence processes occurring in common ions such as CO⁺ and H₂O⁺. The structures observed in the ion coma and tail reveal the spatial distribution and motion of cometary ions, e.g., Fig. 14. Coma and near-tail ions could be observable with Comet Interceptor's cameras, providing complementary observations of ion structures and their dynamics, especially during the approach to the comet. The rate of motion and relative densities of these structures can be compared to the *in situ* observations, providing ground truth data for remote plasma observations from Earth.
<figure><img src="image_17.png" /><figcaption>**Fig. 14** The highly-structured ion tail of Comet C/2016 R2 (Pan-STARRS). This particular comet was an atypical dust-poor LPC (e.g. Biver et al. 2018) which reached perihelion at 2.6 au from the Sun. Image: ESO, under license Attribution 4.0 Interactional (CC BY 4.0)</figcaption></figure>

reactions in the coma (Haeberli et al. 1995; Fuselier et al. 2016; Heritier et al. 2017; Beth et al. 2017).

Waves take on an important role in the cometary plasma environment, transferring energy across boundaries and heating particle populations through wave-particle interactions. The plasma environment of a comet is a complex mix of ions of different species and origin and relative velocities, electrons of different temperatures, neutral molecules and dust particles of different sizes and charge states. Solar wind interactions with the cometary plasma gives rise to instabilities that drive waves of various kinds, including ion-cyclotron and/or mirror-mode waves, e.g., Mazelle et al. (1991), harmonic waves created by the ion-Weibel instability (the “singing comet” waves found by Rosetta; Weibel (1959), Richter et al. (2015), Meier et al. (2016), Glassmeier (2017)), lower hybrid waves, e.g., Karlsson et al. (2017), and ion acoustic waves, e.g., Gunell et al. (2021). It is, however, not clear in which region of the coma these waves are present and how they depend on comet activity. Through multipoint measurements in the coma one can determine, in principle, the temporal and spatial development of the waves. Going from single spacecraft to multi-spacecraft observations thus enables new insights into both the physics of the waves themselves and how they affect boundaries and the surrounding plasma.

The comet-solar wind interaction region can also be observed remotely, due to resonance fluorescence processes occurring in common ions such as CO⁺ and H₂O⁺. The structures observed in the ion coma and tail reveal the spatial distribution and motion of cometary ions, e.g., Fig. 14. Coma and near-tail ions could be observable with Comet Interceptor's cameras, providing complementary observations of ion structures and their dynamics, especially during the approach to the comet. The rate of motion and relative densities of these structures can be compared to the *in situ* observations, providing ground truth data for remote plasma observations from Earth.
<figure><img src="image_17.png" /><figcaption>**Fig. 15** The remote sensing instruments aboard all three platforms will return complementary views of the nucleus from different directions. Inset images show representative complementary views of the single nucleus from two of the three spacecraft platforms</figcaption></figure>

# 3 Scientific Objectives and Requirements

## 3.1 Overview

In Sect. 2, we summarized the scientific context of the Comet Interceptor mission. The many unanswered questions regarding the potential targets provide the strong rationale for a multi-platform mission to an LPC, ideally a DNC, together with great advances in the understanding of solar wind interactions with comets in general. Here we identify the key objectives to be addressed by the mission, and the measurements that are needed to achieve them.

The multiple spacecraft approach of Comet Interceptor means that its target comet will be observed from different angles during the flyby, building up a 3D picture of the nucleus, coma, and interaction with the solar wind. This will allow the separation of differences in time (e.g., due to the changing activity of the comet) and in space (e.g., due to inhomogeneities in the outgassing pattern). Such a separation has not been possible in any previous flyby mission, or even with Rosetta, which could only sample one location in the coma at a time.

Concerning nucleus science, Comet Interceptor will measure the size, shape, and rotation rate of the target comet nucleus. It will return resolved images of the surface that reveal its morphology. The nucleus composition will be constrained directly via remote sensing observations (imaging, spectroscopy, and polarimetry). Furthermore, Comet Interceptor will measure directly, for the first time at a comet, nucleus thermal properties via thermal infrared imaging. Visible and near-infrared images of the nucleus will be returned from three different viewpoints, as illustrated in Fig. 15: from the main spacecraft (A) and both probes. The highest resolution images should have a resolution of ~10 m/pixel, comparable to previous comet flyby missions, allowing direct comparison of an LPC with the more evolved short period comet nuclei imaged previously.

Data will be captured through the flyby from a wide range of phase angles, further allowing the reflectance properties of the nucleus (and different resolved regions on it) to be assessed. The variety of viewpoints, and the possibility to constrain the unilluminated portions of the nucleus through a combination of thermal imaging and imaging of its silhouette against the background coma, will give good constraints on the size and shape of the entire nucleus, even if, due to the flyby nature of the mission, detailed imaging can only be returned
<figure><img src="image_18.png" /><figcaption>**Fig. 15** The remote sensing instruments aboard all three platforms will return complementary views of the nucleus from different directions. Inset images show representative complementary views of the single nucleus from two of the three spacecraft platforms</figcaption></figure>

# 3 Scientific Objectives and Requirements

## 3.1 Overview

In Sect. 2, we summarized the scientific context of the Comet Interceptor mission. The many unanswered questions regarding the potential targets provide the strong rationale for a multi-platform mission to an LPC, ideally a DNC, together with great advances in the understanding of solar wind interactions with comets in general. Here we identify the key objectives to be addressed by the mission, and the measurements that are needed to achieve them.

The multiple spacecraft approach of Comet Interceptor means that its target comet will be observed from different angles during the flyby, building up a 3D picture of the nucleus, coma, and interaction with the solar wind. This will allow the separation of differences in time (e.g., due to the changing activity of the comet) and in space (e.g., due to inhomogeneities in the outgassing pattern). Such a separation has not been possible in any previous flyby mission, or even with Rosetta, which could only sample one location in the coma at a time.

Concerning nucleus science, Comet Interceptor will measure the size, shape, and rotation rate of the target comet nucleus. It will return resolved images of the surface that reveal its morphology. The nucleus composition will be constrained directly via remote sensing observations (imaging, spectroscopy, and polarimetry). Furthermore, Comet Interceptor will measure directly, for the first time at a comet, nucleus thermal properties via thermal infrared imaging. Visible and near-infrared images of the nucleus will be returned from three different viewpoints, as illustrated in Fig. 15: from the main spacecraft (A) and both probes. The highest resolution images should have a resolution of ~10 m/pixel, comparable to previous comet flyby missions, allowing direct comparison of an LPC with the more evolved short period comet nuclei imaged previously.

Data will be captured through the flyby from a wide range of phase angles, further allowing the reflectance properties of the nucleus (and different resolved regions on it) to be assessed. The variety of viewpoints, and the possibility to constrain the unilluminated portions of the nucleus through a combination of thermal imaging and imaging of its silhouette against the background coma, will give good constraints on the size and shape of the entire nucleus, even if, due to the flyby nature of the mission, detailed imaging can only be returned
from one side. The nucleus rotation rate will be constrained by both resolved imaging and images of the unresolved nucleus in the days before and after the flyby.

A mission to a relatively pristine LPC will be an important advance in cometary science as a whole, as direct measurements of the coma composition can be related to nucleus ices with a very different processing history, and the distribution of active areas can be studied. Mapping the distribution of neutral gasses in the coma will give information on bulk composition and nucleus inhomogeneity and will probe coma chemistry. Measuring the composition of the coma at different distances from the nucleus will provide information for coma chemistry models. Remote sensing of the large-scale distribution of different species will be combined with *in situ* sampling to derive production rates of the individual volatile species for the time of the flyby (for comparison to ground-based observations). Identification of parent and daughter neutral and ion species will enable an assessment of their relationship in the coma. All of these individual elements will combine to give a comprehensive picture of the similarities and differences between comets with different evolutionary histories. The spacecraft measurements are critical: while we have ground-based observations of many comets of different classes, it is unclear what differences are evolutionary versus inherent. Comparison of the direct inner coma measurements at an LPC with previous JFC missions, in particular Rosetta, will help disentangle coma processes and long-term evolutionary trends.

Comet Interceptor will detect small-scale structures within the coma, <1 km in scale for a slow flyby, through *in situ* measurements of dust flux and gas density, along with remote sensing imaging from cameras on all platforms. Such an analysis, combined with nucleus shape observations, will supply information about the complex coma-nucleus relationship. For a bright LPC, measurements of even larger-scale spatial distribution of species in the coma will be possible from ground-based observatories and will provide, for the very first time, a clear link between the features observed from the ground and the nucleus of a comet.

Mapping of the dust and neutral gas jets both near to and far from the nucleus, especially on approach and post-encounter, where dust jet and shell structuring can constrain the time history of nucleus activity, will reveal more active locations and periodicities in the ejection rate of material. The jets should be mappable to their source locations, tying into high resolution images of the nucleus. The composition of the gas jets can also reveal a considerable amount about the active regions, such as determining whether all active regions have the same composition, and whether or not the relative abundances of different ices vary across the body. Observations in the IR will isolate jet emission by H₂O, CO₂, and CO, but also typically less abundant species such as methane, ethane, and methanol. Spectral information about the coma, and, better still, spectral imaging, will be highly beneficial. The spatial distribution of neutral hydrogen would provide a time history of activity in the comet; this could be achieved with imaging of the Lyman-α line in the UV range. The composition of the coma can be measured directly using *in situ* observations by a mass spectrometer. Coverage of masses up to a few hundred Daltons (amu) would be particularly beneficial. More direct inferences about surface activity can be made using thermal maps of the nucleus. For these, the surface would need to be resolved to better than ~300 m for a 3 km-wide body.

Measuring *in situ* absolute densities of major neutral gasses and comparing with ground-based observations will allow coma models used to deduce production rates from ground-based observations to be tested and constrained. This is crucial to interpret the wealth of ground-based data using a common baseline. These measurements will also allow Comet Interceptor to help investigate the contribution of distributed sources and the pathways leading to different gas species and their complex relation with the bulk composition. Mass spectroscopy will also reveal minor constituents of the gas coma, and isotopic ratios of D/H
from one side. The nucleus rotation rate will be constrained by both resolved imaging and images of the unresolved nucleus in the days before and after the flyby.

A mission to a relatively pristine LPC will be an important advance in cometary science as a whole, as direct measurements of the coma composition can be related to nucleus ices with a very different processing history, and the distribution of active areas can be studied. Mapping the distribution of neutral gasses in the coma will give information on bulk composition and nucleus inhomogeneity and will probe coma chemistry. Measuring the composition of the coma at different distances from the nucleus will provide information for coma chemistry models. Remote sensing of the large-scale distribution of different species will be combined with *in situ* sampling to derive production rates of the individual volatile species for the time of the flyby (for comparison to ground-based observations). Identification of parent and daughter neutral and ion species will enable an assessment of their relationship in the coma. All of these individual elements will combine to give a comprehensive picture of the similarities and differences between comets with different evolutionary histories. The spacecraft measurements are critical: while we have ground-based observations of many comets of different classes, it is unclear what differences are evolutionary versus inherent. Comparison of the direct inner coma measurements at an LPC with previous JFC missions, in particular Rosetta, will help disentangle coma processes and long-term evolutionary trends.

Comet Interceptor will detect small-scale structures within the coma, <1 km in scale for a slow flyby, through *in situ* measurements of dust flux and gas density, along with remote sensing imaging from cameras on all platforms. Such an analysis, combined with nucleus shape observations, will supply information about the complex coma-nucleus relationship. For a bright LPC, measurements of even larger-scale spatial distribution of species in the coma will be possible from ground-based observatories and will provide, for the very first time, a clear link between the features observed from the ground and the nucleus of a comet.

Mapping of the dust and neutral gas jets both near to and far from the nucleus, especially on approach and post-encounter, where dust jet and shell structuring can constrain the time history of nucleus activity, will reveal more active locations and periodicities in the ejection rate of material. The jets should be mappable to their source locations, tying into high resolution images of the nucleus. The composition of the gas jets can also reveal a considerable amount about the active regions, such as determining whether all active regions have the same composition, and whether or not the relative abundances of different ices vary across the body. Observations in the IR will isolate jet emission by H₂O, CO₂, and CO, but also typically less abundant species such as methane, ethane, and methanol. Spectral information about the coma, and, better still, spectral imaging, will be highly beneficial. The spatial distribution of neutral hydrogen would provide a time history of activity in the comet; this could be achieved with imaging of the Lyman-α line in the UV range. The composition of the coma can be measured directly using *in situ* observations by a mass spectrometer. Coverage of masses up to a few hundred Daltons (amu) would be particularly beneficial. More direct inferences about surface activity can be made using thermal maps of the nucleus. For these, the surface would need to be resolved to better than ~300 m for a 3 km-wide body.

Measuring *in situ* absolute densities of major neutral gasses and comparing with ground-based observations will allow coma models used to deduce production rates from ground-based observations to be tested and constrained. This is crucial to interpret the wealth of ground-based data using a common baseline. These measurements will also allow Comet Interceptor to help investigate the contribution of distributed sources and the pathways leading to different gas species and their complex relation with the bulk composition. Mass spectroscopy will also reveal minor constituents of the gas coma, and isotopic ratios of D/H
if the gas production rate is sufficiently high (and possibly the most abundant isotopes of O, C, and S), allowing detailed comparison of composition differences and/or similarities between a less evolved comet and results from Rosetta at 67P.

Observations of all-sky brightness within the coma, and polarimetric curves, will allow us to study the observed phase function curve and the degree of linear polarisation from the same data set, including the important forward- and back-scattering regimes. The combination of both datasets will yield key constraints for the physical properties of the dominant population of dust particles in the coma, alongside the dust mass distribution measured along two different trajectories. A detailed characterisation of the plasma environment of a comet requires measurements of the magnetic and electric field strength and direction, the electron and ion distribution functions (density, temperature, and flow velocity vector), and the ion composition, in the different comet-solar wind interaction regions (e.g., solar wind, diamagnetic cavity) and at their boundaries.

Comet Interceptor will assess, through sensors on spacecraft A and B2, and the B1 Plasma Suite, the energy, mass, and momentum transfer in the cometary environment, through the coma and across boundaries. Multi-point measurements of ions, magnetic fields, and nm- to mm-scale dust will elucidate the physics behind mass transfer and the consequences for both the coma and tail. Unprecedented ENA observations will help us to understand the role of charge exchange collisions in the transfer of energy and momentum from the solar wind to the coma. Solar wind and cometary ion and electron dynamics will enable the assessment of the amount of electrically-charged material impacting the (pristine) comet surface.

For the magnetometers on all three platforms on this mission, the bow shock (or bow wave), the field-line draping pattern around the inner coma, and the diamagnetic cavity, are of specific interest, amongst other features. Using the spatial separation of the three spacecraft and their magnetometer measurements, one can deduce the three-dimensional shape of the magnetic field, and study the differences in magnetic activity. The magnetic field measurements also play a defining role in analysing the internal structure and type of the plasma boundaries. Given the possibly high flyby speed, the planned high cadence measurements of the most prominent plasma properties (ion and electron density, electron temperature) are essential to capture as much as possible of the detailed spatial and temporal structure of the plasma. This will help in particular in studying the internal structure of the boundaries (shape, spatial extent, etc.). The ion coma and ion tail in its vicinity will be observed by the visible light cameras on all three platforms.

## 3.2 Science Objectives and Requirements

The above summary of the most desirable measurements possible during a fast flyby and modest spacecraft size are formalized as Science Objectives and Requirements, summarized here.

The overall goal of the Comet Interceptor mission is to provide the first investigation of an LPC, and to sample different regions of the coma simultaneously. The scientific requirements that need to be met to achieve this goal can be conveniently divided into the two Science Themes presented previously: First, measurement of the properties of the cometary nucleus that will allow the comparison of an LPC with short-period comet nuclei investigated by previous missions. Second, investigation of the coma, its connection to the nucleus (cometary activity) and its interaction with the solar wind, will take advantage of the multi-point perspective through three spacecraft.
if the gas production rate is sufficiently high (and possibly the most abundant isotopes of O, C, and S), allowing detailed comparison of composition differences and/or similarities between a less evolved comet and results from Rosetta at 67P.

Observations of all-sky brightness within the coma, and polarimetric curves, will allow us to study the observed phase function curve and the degree of linear polarisation from the same data set, including the important forward- and back-scattering regimes. The combination of both datasets will yield key constraints for the physical properties of the dominant population of dust particles in the coma, alongside the dust mass distribution measured along two different trajectories. A detailed characterisation of the plasma environment of a comet requires measurements of the magnetic and electric field strength and direction, the electron and ion distribution functions (density, temperature, and flow velocity vector), and the ion composition, in the different comet-solar wind interaction regions (e.g., solar wind, diamagnetic cavity) and at their boundaries.

Comet Interceptor will assess, through sensors on spacecraft A and B2, and the B1 Plasma Suite, the energy, mass, and momentum transfer in the cometary environment, through the coma and across boundaries. Multi-point measurements of ions, magnetic fields, and nm- to mm-scale dust will elucidate the physics behind mass transfer and the consequences for both the coma and tail. Unprecedented ENA observations will help us to understand the role of charge exchange collisions in the transfer of energy and momentum from the solar wind to the coma. Solar wind and cometary ion and electron dynamics will enable the assessment of the amount of electrically-charged material impacting the (pristine) comet surface.

For the magnetometers on all three platforms on this mission, the bow shock (or bow wave), the field-line draping pattern around the inner coma, and the diamagnetic cavity, are of specific interest, amongst other features. Using the spatial separation of the three spacecraft and their magnetometer measurements, one can deduce the three-dimensional shape of the magnetic field, and study the differences in magnetic activity. The magnetic field measurements also play a defining role in analysing the internal structure and type of the plasma boundaries. Given the possibly high flyby speed, the planned high cadence measurements of the most prominent plasma properties (ion and electron density, electron temperature) are essential to capture as much as possible of the detailed spatial and temporal structure of the plasma. This will help in particular in studying the internal structure of the boundaries (shape, spatial extent, etc.). The ion coma and ion tail in its vicinity will be observed by the visible light cameras on all three platforms.

## 3.2 Science Objectives and Requirements

The above summary of the most desirable measurements possible during a fast flyby and modest spacecraft size are formalized as Science Objectives and Requirements, summarized here.

The overall goal of the Comet Interceptor mission is to provide the first investigation of an LPC, and to sample different regions of the coma simultaneously. The scientific requirements that need to be met to achieve this goal can be conveniently divided into the two Science Themes presented previously: First, measurement of the properties of the cometary nucleus that will allow the comparison of an LPC with short-period comet nuclei investigated by previous missions. Second, investigation of the coma, its connection to the nucleus (cometary activity) and its interaction with the solar wind, will take advantage of the multi-point perspective through three spacecraft.