gradients between shades where a variety of approaches or targets give a wide
range of possibilities. Hatched squares indicate that such a combination
(e.g., nucleus sample return [SR] from an interstellar object) seems
unfeasible with current or near future technology, or within the cost caps of
the largest current interplanetary missions. Checks mark those combinations
already tried (various JFC and Halley flybys described in section 3, JFC coma
sample return by Stardust, and JFC rendezvous and landing by Rosetta and
Philae) and stars indicate those selected for future missions (flyby of a new
LPC and rendezvous with a MBC, by Comet Interceptor and ZhengHe,
respectively). Based on a figure from _Bockelée-Morvan et al._ (2021).

###  5.5 Target types

####  5.5.1 Jupiter Family Comets and Centaurs

Of the two major comet classes, JFCs with orbits near the ecliptic and a
source region in the Kuiper belt and scattered disc, and nearly-isotropic LPCs
or Halley-types (HTCs) from the Oort cloud222The distinction between LPCs and
HTCs is on orbital period (longer or shorter than 200 years, respectively),
but from a mission point of view they present similar challenges (potentially
high inclinations), so we group them together for the purposes of this
chapter. We note that HTCs may not have an Oort cloud source after all
(_Levison et al._ , 2006; _Fernández et al._ , 2016); see chapters by Kaib &
Volk and Fraser et al. in this volume., the former present much easier
spacecraft targets. This is due to their typically low inclinations and short
periods, with aphelia typically within the orbit of Jupiter. As demonstrated
by Rosetta, it is feasible with current launchers to match these orbits and,
with modern solar panels such as those used by Juno or Lucy (_Matousek_ 2007;
_Olkin et al._ 2021; respectively), perform scientific operations all the way
to aphelion. JFCs have well characterized orbits and all classes of mission
can be planned well in advance. This is reflected in the dominance of these
comets in the list of previous missions. However, there remain interesting
cases for future missions to JFCs: different types of mission (see section
5.6), or exploring the long-term evolution of these bodies by visiting split
or fragmented comets or related populations, e.g., Centaurs or extinct comets.

Centaurs are thought to be precursors to JFCs as objects transitioning from
outer Solar System orbits. They have a wide range of orbits and sizes (see
chapter by Fraser et al. in this volume.), and consequently vary considerably
in how feasible a mission to one would be. Feasible medium-sized missions have
been proposed (e.g., _Harris et al._ , 2019) for flyby or rendezvous with
29P/Schwassmann-Wachmann 1, which has a relatively circular orbit just beyond
that of Jupiter, and is particularly interesting as it is likely in the
process of transitioning from a Centaur to a JFC (_Sarid et al._ , 2019). More
distant Centaurs, e.g., the large ($\sim$200 km diameter) ringed worlds
Chariklo or Chiron, with orbits between 8 and 18 au from the Sun, have been
proposed as mission targets as more accessible Kuiper Belt Objects, rather
than due to their relationship with comets as such, but would be more costly:
e.g., NASA Frontiers class for a Deep Impact-style flyby and impactor
combination (_Howell et al._ , 2018) or multi-Centaur tour (_Singer et al._ ,
2021).

Comets at the other end of their active lifetimes can present more accessible
targets. A large number of asteroids in cometary orbits (ACOs) have been
identified, including many in near-Earth space, some of which may have low
$\Delta v$ transfers for flyby, rendezvous, or even sample return (see chapter
by Jewitt & Hsieh in this volume). As these objects are thought to be dormant
or extinct comets, with their near-surface ices either exhausted or buried at
sufficient depth to extinguish activity, they offer an opportunity to conduct
a _relatively_ easy comet mission, without the concerns for spacecraft safety
caused by activity, but a scientifically compelling one, to understand what it
is that causes comet activity to cease. However, the most significant
uncertainty with such a proposal is in being sure of the identification of any
individual ACO as definitely a previously active comet, rather than a body
with an asteroidal origin but somehow scattered onto a comet-like orbit.

####  5.5.2 Long Period Comets

As noted in section 5.3, LPCs or HTCs are much more challenging targets to
reach, as they can have high inclinations (including retrograde orbits) and
relatively high speeds relative to Earth and any spacecraft as they pass
through the inner Solar System. For this reason, only flyby missions to LPCs
seem reasonably feasible in the short term. The Halley missions took advantage
of a long-predicted return of a rare well known comet in this class, while
Comet Interceptor has the chance to visit a new comet by implementing the use
of a waypoint. Further sampling the diversity of LPCs may be rewarding,
especially given the recent discoveries of activity at large heliocentric
distances well beyond the water snow line (e.g., C/2017 K2; _Meech et al._
(2017)) and low activity or inactive ‘Manx’ or ‘Damocloid’ objects in LPC
orbits (see chapter by Jewitt & Hsieh in this volume). Discussions of a return
to 1P at its 2061 perihelion, to rendezvous or even return a sample, highlight
the current need for significant technology development to achieve this step
(_Ozaki et al._ , 2021). While another mission at the next return of this most
famous comet has strong cultural and historic appeal, and its predictability
decades in advance inspires some of the long-term planning needed to develop
the necessary technology, there are perhaps easier LPCs/HTCs to consider.
Although it comes close to Earth, 1P’s highly retrograde orbit ($i=162\degr$)
will always mean very high energy requirements to match its velocity. A
concept was investigated to address this issue for 1P’s 1986 apparition
through the use of a solar sail (_Friedman_ , 1976); this technology deserves
to be considered further.

####  5.5.3 Main Belt Comets

In the decades since the last comet mission was launched, it has become
apparent that there may be a third class of comet orbit, and source reservoir,
within the Solar System: the Main Belt Comets (MBCs), which suggest an
unexpected significant population of ice-rich bodies in the outer asteroid
belt (see chapter by Jewitt & Hsieh in this volume). With very low activity
levels and relatively circular orbits, MBCs are reasonably accessible for
rendezvous missions, particularly for missions propelled by ion engines or
other low thrust technology, as employed by the Dawn mission to Vesta and
Ceres (_Thomas et al._ , 2011). As described in section 5.2, an upcoming
Chinese mission may make the first visit to a MBC as a secondary target, but
may not encounter an active body. A number of other missions have been
proposed to other agencies, but not yet selected. As a potential guide to
future proposals in this area, we identify some of the approaches and
technologies proposed in those with publicly available information. These
include, for example, the NASA Discovery class Proteus rendezvous mission
concept (_Meech and Castillo-Rogez_ , 2015); the Castalia rendezvous concept
proposed as an ESA medium-sized (M-class) mission and described in detail by
_Snodgrass et al._ (2018); and a Stardust-like coma sample return mission,
Caroline, which was also previously proposed as an ESA M-class (_Jones et al._
, 2018). The main science drivers for most MBC missions rely on in situ
sampling of their volatiles, to confirm that MBC activity is indeed driven by
sublimating water ice, which has not yet been possible via remote observation
(_Snodgrass et al._ , 2017a), and to reveal their composition and thus test
Solar System formation and evolution models, and whether or not the asteroid
belt could have been a significant source of Earth’s water. The very low
activity levels of MBCs make this challenging – to build up the necessary
signal-to-noise to make a sensitive detection via mass spectroscopy, Castalia
proposed extended periods of automated close proximity station keeping. This
would have been a significant technological advance from Rosetta, where
navigation with ground control in the loop meant reaction times of a few days,
but automated on-board navigation is now a mature spacecraft technology, to
the point of enabling precision landings on asteroids or Mars (_Ge et al._ ,
2019). MBCs also differ from other comets in that their ices may be buried
deeper under a refractory crust, or exposed in only a small and recently
uncovered active area, and locating volatiles must constitute an important
part of the mission. Surface (or sub-surface) exploration with landers,
drills, or penetrators would also be very interesting, if they can be targeted
to volatile-rich areas, although estimates of the depth of buried ice vary
from cm to tens of metres, depending on location and the individual MBC’s age,
orbit, and pole orientation (_Snodgrass et al._ , 2017a).

####  5.5.4 Interstellar Objects

Finally, recent years have seen the discovery of the first interstellar
comets, which would naturally be very high priority spacecraft targets, as
they represent an opportunity to sample extra-solar material without the
massive technological challenges or centuries-timescales of an interstellar
probe. Our knowledge of interstellar objects (ISOs) and the motivations and
concepts for a mission to one are discussed in the chapter by Fitzsimmons et
al. in this volume, and were explored by a recent NASA Planetary Science
Summer Seminar cohort (_Moore et al._ , 2021), but we briefly discuss the
technology requirements for such a probe. In terms of mission design, ISOs are
similar to new LPCs, with high velocities and potentially high inclinations,
and consequently present similar challenges. Fast flyby missions, perhaps
combined with a Deep Impact-style impactor (_Moore et al._ , 2021), appear to
be the limit of what is feasible. A waiting spacecraft, similar to Comet
Interceptor in concept, seems the most likely way to reach an ISO, but the
current poor constraints on the total population size mean that it is hard to
judge the necessary waiting times or $\Delta v$ requirements. Given the
limited heliocentric distance range in which Comet Interceptor will be able to
operate, it seems that it will have to be exceedingly lucky to be able to
encounter an ISO within its planned 6-year lifetime and $\Delta v<$ 1 km s-1.
A more capable version of a similar concept could be imagined for a larger-
budget mission class (e.g. ESA M-class or NASA Discovery), as the true size of
the ISO population becomes clearer following the expected decade of discovery
with the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST)
(_Seligman and Laughlin_ , 2018). Alternatively, ‘launch on discovery’
concepts, rather than waiting in space, have been proposed to encounter ISOs
(_Seligman and Laughlin_ , 2018; _Moore et al._ , 2021), although these also
require quite rapid reaction (see section 5.6.5). Clearly cometary ISOs like
2I/Borisov, if discovered inbound, present the best chances of success for
interception, while the available warning times for another small
1I/‘Oumuamua-like object, with no visible coma, remain impractically short
even with LSST (one or two months at best). There are suggestions to launch
missions to chase after departing ISOs, catching up to them at very large
distance from the Sun (100 au or more) decades after their perihelion, but
these are limited by both available spacecraft technology (propulsion, power,
communications) and the navigation and operational practicalities of even
locating such a small object so far from the Sun while approaching at tens of
km s-1 (_Hein et al._ , 2019, 2022). Such a mission does not appear feasible
in the near term.

###  5.6 Technology for Future Comet Missions

There still remains much to be learned about the more accessible JFCs, and new
missions to these, or even returns to previously visited comets, are well
justified. In this section we review current and near-future technology that
could enable the next comet missions to JFCs and beyond (see also _A’Hearn_
(2017) and _Thomas et al._ (2019)).

####  5.6.1 Landing on the Nucleus

As (cryogenic) sample return remains the ultimate goal of comet exploration, a
key technology requirement is the ability to land on the nucleus safely:
either in a brief ‘touch and go’ maneuver, as used by the asteroid missions
Hayabusa, Hayabusa 2 and OSIRIS-REx, and proposed at a comet by the Triple-F
mission (_Küppers et al._ , 2009), or for a longer period of
measurement/drilling. Landers also have a lot of potential to advance our
knowledge of cometary activity, independent of any sample return attempt,
through local measurements of the surface (micro-)physics, as was planned for
Philae. Sampling multiple locations on the nucleus would be particularly
valuable for this, whether using one mobile lander or a network of static ones
(the latter also having the advantage of redundancy and being able to perform
seismology or CONSERT-like measurements between probes; _Thomas et al._ 2019).
Future mission architectures to consider include very low speed landers,
rovers, and multi-point landers or hoppers, like the proposed implementation
of NASA’s Phase A Comet Hopper mission concept (_Clark et al._ , 2008), and a
great deal of autonomous operations. Taking into account the micro-gravity
environment at the surface of a comet and balancing it with the desire to
minimize surface alteration upon arrival, various spacecraft designs coupled
with landing and holding strategies have been evaluated in concept studies
(_Clark et al._ , 2008; _Ulamec et al._ , 2011) or executed without success to
date (section 4.4). With our knowledge of the surface strength and regolith
properties still ranging an order of magnitude or more after Philae’s
inconclusive results (_Groussin et al._ , 2019), there remains a need for
replicating and modeling porous regolith in micro-gravity conditions on Earth
for landing tests, and to assess the particle movement near the output of
spacecraft thrusters, and the thermal and electrostatic response of the
regolith to an approaching spacecraft. Low speed landers will reduce the
potential for bouncing on the surface and numerous small thrusters oriented at
an angle from the sub-spacecraft point will undoubtedly prove to be
beneficial.

Once in the vicinity of the comet nucleus or safely landed, interacting with
the comet’s surface is also technologically challenging, whether to rove or
hop around to explore, to gather a surface sample, or to penetrate and sample
the sub-surface. There are still many unknowns in each individual comet’s
properties and orders of magnitude in possible strength regimes and surface
topographies to design for. A significant difference between classes of comet
(e.g., possibly asteroid-like MBCs vs ‘pristine’ LPCs) could also be expected.
Mission designs employing smaller articulating solar panels or nuclear power
sources will enable agile traversal of the diverse terrains observed on the
comets visited thus far. Additionally, missions powered by nuclear sources
will enable operation at larger heliocentric distances with fewer limitations
on shared spacecraft resources as well as in non-illuminated regions of a
comet, the most likely location to find surface ice (_Sunshine et al._ , 2006;
_De Sanctis et al._ , 2015; _Filacchione et al._ , 2016).

####  5.6.2 Sample return

The Stardust mission showed the immense value of laboratory analysis of comet
samples. The next step in this field should be to collect a sample from the
nucleus, although a more advanced coma sample return mission could also be
imagined. Stardust and Rosetta showed that coma dust includes an easily
fragmented and fragile component, that would have to be collected very gently
(e.g., at near zero relative velocity by a rendezvous mission) to obtain a
sample that retains structural information. Measuring the refractory-to-ice
abundance in the coma would also be an important, but challenging, goal for
such a sample.

For nucleus sample return, there is a need for community guidelines and
agreement on the depth to which a sample should be collected to have the
greatest science return, from the exposed regolith (_Nakamura-Messenger et
al._ , 2021) to reaching the least-altered high-volatility sub-surface ice
reservoirs. As the surface composition of comets remains uncertain even after
Rosetta, there is value in the former approach, which formed the basis of the
CAESAR proposal that was the runner up in the 2019 NASA New Frontiers call
(_Squyres et al._ , 2018). A surface regolith sample, which could be returned
to Earth in non-cryogenic conditions, would allow for detailed investigation
of thermally processed material and surface chemistry at a level beyond what
Philae could have hoped to achieve, even if it had operated perfectly, and
could be compared to similar asteroid samples from Hayabusa 2 and OSIRIS-REx.
However, a surface sample return mission that scrapes, sucks, or brushes up
regolith makes detailed chemistry measurements at the cost of knowledge of the
physical structure of the surface material. A more ambitious sample return
approach would be to collect a core that preserves both composition and its
stratigraphy; information on the size and shape of grains, the bulk strength,
porosity, and relative locations of different components (see _Thomas et al._
, 2019, for a more detailed discussion). The depth to which a core must reach
to sample various ices is not well determined, but is expected to be of order
meters, or possibly considerably more to reach ‘pristine’ layers (see
discussion in chapter by Guilbert-Lepoutre et al. in this volume). Comet
missions can look to technological advances from ground penetrating radar
designed to characterize the subsurface structure 3-10 m deep at Mars
(_Ciarletti et al._ , 2017) to first locate the true ice depths. This would be
a useful measurement for future rendezvous missions whether or not they also
attempt to land on or sample the nucleus (e.g., the CORE mission concept, an
orbiter whose primary payload is a radar mapping experiment to study nucleus
interior structure; _Asphaug_ 2015). Drilling into the ice, and extracting an
unaltered core, would be technically challenging. The SD2 drill on Philae was
designed to reach a depth of 23 cm and retrieve material (but not an intact
core) for composition analysis (_Finzi et al._ , 2007), while Triple-F
proposed to extract 50 cm cores in touch-and-go landings (_Küppers et al._ ,
2009). Neither of these has been successfully demonstrated at a comet;
obtaining meters-long ice cores will require significant advances of these
technologies, but may benefit from current developments for ocean world
applications (_Dachwald et al._ , 2020). The largest drill being developed for
a near-future flight opportunity is the 1 m long TRIDENT drill (_Zacny et al._
, 2021), expected to launch to the Moon in 2022, on a large lander or rover
(450 - 2000 kg; a very different class of mission than the 100 kg Philae).

Cryogenic sample return missions face the challenge not only of acquiring a
sample with context and retaining stratigraphy, but also preserving it during
the return flight, Earth atmospheric entry, descent, landing, recovery, and
curation. The latter is a significant challenge in its own right; we do not
currently have suitable facilities, but progress is being made (_McCubbin et
al._ , 2021). The sample would need to be protected to minimize and ideally
prevent alteration that would affect interpretation (this includes shock,
vibration, and heat). Space-based retrieval (i.e., docking with a crewed space
station, and subsequent ‘gentle’ return of the sample with the astronauts) has
even been considered to minimize some of these effects. There have been recent
technological advances in cryogenic flight systems and instrumentation, as
well as in cryogenic sample curation, handling, and analysis (_Westphal et
al._ , 2021). _Küppers et al._ (2009) proposed a passive system that would
keep the sample below 133 K during cruise and 163 K during reentry, which
would return a valuable sample at relatively low cost, even if some ices were
lost. _Bockelée-Morvan et al._ (2021) suggest that temperatures must be kept
below 90 K, and pressure at 1 bar, to preserve most volatile ices (H2O, CO2,
HCN). Even lower temperatures or higher pressures would be necessary to keep
CO stable. _Westphal et al._ (2021) argue that sample collection and transport
at 60 K is necessary to prevent an amorphous-amorphous phase transition in
water ice that occurs at 80 K, and that this is considered achievable with
current flight system technologies. While challenging, it appears that
cryogenic sample preservation is technically possible in the near future. In
the end, a small advantage that comet missions have once a sample is collected
and contained, is that operating in low gravity environments will allow for
easier departure.

####  5.6.3 Sub-surface penetrators

Penetrator probes, which reach sub-surface layers of a planetary body by
burying themselves in a high speed impact, rather than a gentle landing and
then drilling, have been proposed for some time (_Lorenz_ , 2011). Although
typically suggested as a solution for larger bodies such as the Moon or icy
moons of the outer planets (e.g., _Gowen et al._ , 2011), where gravity makes
soft-landing far more challenging, they were studied in detail for comets as
part of the NASA-led Comet Rendezvous Asteroid Flyby mission, which was
cancelled in the early 1990s (_Swenson et al._ , 1987; _Boynton and Reinert_ ,
1995). Penetrator probes rely on sufficiently robust instrumentation that can
survive the sudden deceleration as the probe hits the ground, typically at
$\sim$100 m s-1, but can reveal physical structure of the surface by measuring
this deceleration. They can measure sub-surface composition, obtain
(microscopic) images of the surrounding material, and measure physical
properties such as temperatures, conductivity, etc., in situ. Although a
penetrator has yet to be successfully flown on a planetary mission, they have
been studied in detail, including high speed impact tests of instrumentation
on Earth, and similar technology has been widely developed for military
purposes; penetrators are therefore a realistic and mature technology that
would be of value on a suitable rendezvous mission. One may be included on the
ZhengHe mission to 311P (_Tang et al._ , 2021), and, if it is, probably
represents the best chance for this mission to detect any sub-surface ice.

####  5.6.4 Small satellites and multi-point measurements

A relatively new mission architecture is proving to be beneficial in planetary
sciences and has application for comets: the development of small,
standardized spacecraft (e.g., CubeSats). The advantage of small, fast, and
relatively cheap missions should not be overlooked. Small-sats can play a role
in reconnaissance, rapid response, and could be employed as a fleet. They can
also be used to address focused questions and may be the only way in the near
future to explore an ISO or LPC up close, either as released probes as part of
a larger mission (e.g., Comet Interceptor) or even as independent spacecraft
(_Clark et al._ , 2018). A CONTOUR-like tour (_Birch et al._ , 2020) and Comet
Interceptor-like ‘waiting in space’ LPC mission (_Hewagama et al._ , 2018)
have been proposed within the NASA ‘SIMPLEx’ call for small missions.

The idea of having a large number of independent spacecraft at a comet has
many advantages, particularly for coma sampling and measurements of the plasma
environment. Comet Interceptor will perform the first multi-point sampling,
but the only instrument common to all three spacecraft is a magnetometer, and
it is a fast flyby mission. A plasma-science dedicated rendezvous mission with
at least four identical spacecraft measuring the interaction between a comet
and the solar wind has been proposed as the next big step forward for this
field (_Goetz et al._ , 2021). While this concept assumes four small-to-medium
sized independent spacecraft, it is also interesting to consider the
possibilities presented by advances in technology for very small platforms.
The Comet Interceptor released probes, while small, will still be bespoke
spacecraft with rigorous design and build standards. The use of real CubeSats
(i.e., standardized units built with relatively cheap off-the-shelf parts) in
deep space has been demonstrated to be feasible by the MarCO mission to Mars
(_Baker et al._ , 2019). Even smaller ‘ChipSats’ have been demonstrated to be
capable of making simple single measurements at many separate locations in
Earth orbit, and have been proposed for small body missions (_Ledbetter et
al._ , 2018). Many ($N>>1$) point measurements with a ‘coma swarm’ would
provide fascinating insights into the temporal and spatial variations within a
coma, whether simply by tracing dynamics and variations in coma structure
(dust or gas density, temperature, or magnetic fields), or even making
multiple in situ composition measurements (e.g., with CubeSat-sized mass
spectrometers based on Philae Ptolemy technology; _Wright et al._ 2007).
Having a large number of small probes also enables a paradigm shift in build
and operation costs that is useful in a cometary environment – with
sufficiently large $N$, failure _is_ an option, and results are assured even
if some of the probes fail or are lost soon after deployment.

####  5.6.5 Rapid reaction

One mission type that is at a true disadvantage in the current program
architecture for most, if not all, space agencies is a rapid response mission
(_Moore et al._ , 2021). These missions would undoubtedly result in high
reward science and could be in response to any number of phenomena or target
type, e.g., a major splitting event of an JFC on its inbound perihelion leg, a
newly discovered inbound LPC, or an ISO quickly passing through our Solar
System. Implementations of rapid response missions may have inherent schedule
risk to mission success, or risk failing to achieve their full science
potential due to launch vehicle, trajectory, and orbital constraints. Rapid
response missions could be executed via a very short and rushed design, build,
test, and launch phase, with the major disadvantages of high likelihood of
schedule slip, ballooning costs, and potential lack of dedicated launch
vehicle or timely ride-share launch. In contrast, rapid response missions
could take advantage of ground- or space-based storage and thus allow for a
more typical and lengthy design and build phase. Having a pre-built spacecraft
in storage or in a parking orbit in space waiting for a target has drawbacks,
manifesting as having been designed with a target agnostic approach, requiring
a minimum intact skeleton team for a long period of low to no mission
activity, utilizing technology that is not state-of-the-art by the time of
target discovery, degrading parts in orbit, and having the potential need for
a dedicated launch vehicle. The key to justifying such a mission will be some
reasonably reliable statistical knowledge of how long the wait may have to be,
and that the (presumably rare) opportunity requiring rapid reaction is worth
the associated cost.

##  6 Summary

There has been fantastic progress in spacecraft exploration of comets since
the beginning of this field around the time of comet Halley’s last return. As
this comet approaches aphelion in 2024, it is instructive to look at what we
have achieved, and what we may look forward to in the next half of its orbit,
before its 2061 return. Reviewing the three major themes we described in the
introduction (formation, composition, and activity), we see significant
progress with each past mission and clear priorities for future missions and
technology. Our understanding of nucleus formation and structure has primarily
advanced by the increasing resolution of surface imaging with each mission to
date, revealing an apparent preference for bilobed shapes and a wide variety
of surface morphologies. The next step needs to be probing microphysical
details of the nucleus and its interior structure. Composition measurements in
situ have examined gas, dust, and ions, including isotopic abundances, and
Stardust enabled incredibly detailed laboratory analysis of solid grains – but
increasing the diversity of comets visited and sample return from the nucleus
are promising next steps to take, especially to investigate how the volatile,
organic, and refractory components are associated. Finally, processes linked
to activity have been localized but fundamentally remain a puzzle that will
likely require lander missions, capable of temporally studying the physical,
thermal, and chemical properties of the nucleus (sub-)surface at microscopic
scales, to solve. There are still many knowledge gaps that require in situ
measurement to explore: There are strong cases for missions to comets to
better understand nucleus surfaces at small scales, their activity, and the
diversity between comets from different source regions or at different
evolutionary stages. The return of frozen ices from a comet nucleus remains
the highest priority mission objective, yet experiences from flyby, impactor,
coma sample return, rendezvous, and landed missions in the past decades have
shown that this will be highly challenging. Returning to 1P, and/or cryogenic
sample return, in the distant future necessitates long-term planning and
investment in technology now. In the meantime, there are still many questions
that can, and should, be addressed with a range of smaller missions, taking
advantage of current advances in small, fast, and cheap spacecraft. No matter
what the mission design and target, the science objectives should be
compelling and concise, and drive the spacecraft design and technological
advances needed for future cometary exploration.

Acknowledgments.

The authors thank the referees, Jessica Sunshine and Horst Uwe Keller, for
their helpful and constructive reviews of this chapter. We thank Abbie
Donaldson for providing Fig. 11. C.S. and G.H.J. acknowledge support from the
UK Science and Technology Research Council and UK Space Agency. For the
purpose of open access, the author has applied a Creative Commons Attribution
(CC BY) licence to any Author Accepted Manuscript version arising from this
submission.

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