Cometary Science with the James Webb Space Telescope

Abstract

The James Webb Space Telescope (JWST), as the largest space-based
astronomical observatory with near- and mid-infrared instrumentation, will
elucidate many mysterious aspects of comets. We summarize four cometary science
themes especially suited for this telescope and its instrumentation: the
drivers of cometary activity, comet nucleus heterogeneity, water ice in comae
and on surfaces, and activity in faint comets and main-belt asteroids. With
JWST, we can expect the most distant detections of gas, especially CO2, in what
we now consider to be only moderately bright comets. For nearby comets, coma
dust properties can be studied with their driving gases, measured
simultaneously with the same instrument or contemporaneously with another.
Studies of water ice and gas in the distant Solar System will help us test our
understanding of cometary interiors and coma evolution. The question of
cometary activity in main-belt comets will be further explored with the
possibility of a direct detection of coma gas. We explore the technical
approaches to these science cases and provide simple tools for estimating comet
dust and gas brightness. Finally, we consider the effects of the observatory's
non-sidereal tracking limits, and provide a list of potential comet targets
during the first 5 years of the mission.

Full text

Accepted for publication in the Publications of the Astronomical
Society of the Pacific, 2015 Oct 15
Cometary Science with the James Webb Space Telescope
Michael S. P. Kelley1, Charles E. Woodward2, Dennis Bodewits1, Tony L. Farnham1, Murthy S.
Gudipati3,4, David E. Harker5, Dean C. Hines6, Matthew M. Knight7, Ludmilla Kolokolova1,
Aigen Li8, Imke de Pater9, Silvia Protopapa1, Ray W. Russell10 , Michael L. Sitko11, Diane H.
Wooden12
ABSTRACT
The James Webb Space Telescope (JWST ), as the largest space-based astronomical
observatory with near- and mid-infrared instrumentation, will elucidate many mysteri-
ous aspects of comets. We summarize four cometary science themes especially suited
for this telescope and its instrumentation: the drivers of cometary activity, comet nu-
cleus heterogeneity, water ice in comae and on surfaces, and activity in faint comets
and main-belt asteroids. With JWST , we can expect the most distant detections of
gas, especially CO2, in what we now consider to be only moderately bright comets. For
nearby comets, coma dust properties can be simultaneously studied with their driving
gases, measured simultaneously with the same instrument or contemporaneously with
another. Studies of water ice and gas in the distant Solar System will help us test our
understanding of cometary interiors and coma evolution. The question of cometary
1Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
2Minnesota Institute for Astrophysics, 116 Church Street S. E., University of Minnesota, Minneapolis, MN 55455,
USA
3Science Division, Jet Propulsion Laboratory, California Institute of Technology, Mail Stop 183-301, 4800 Oak
Grove Drive, Pasadena, CA 91109, USA
4Institute for Physical Sciences and Technology, University of Maryland, College Park, MD 20742, USA
5Center for Astrophysics and Space Sciences, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA
92093-0424, USA
6Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA
7Lowell Observatory, 1400 W. Mars Hill Rd, Flagstaff, AZ 86001, USA
8Department of Physics and Astronomy, University of Missouri, Columbia, MO 65211, USA
9Department of Astronomy, 501 Campbell Hall, University of California, Berkeley, CA, 94720, USA
10The Aerospace Corporation, Los Angeles, CA 90009, USA
11Space Science Institute, Boulder, CO 80301, USA
12NASA Ames Research Center, Moffett Field, CA 94035-0001 USA
arXiv:1510.05878v1 [astro-ph.EP] 20 Oct 2015

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activity in main-belt comets will be further explored with the possibility of a direct
detection of coma gas. We explore the technical approaches to these science cases and
provide simple tools for estimating comet dust and gas brightness. Finally, we con-
sider the effects of the observatory’s non-sidereal tracking limits, and provide a list of
potential comet targets during the first 5 years of the mission.
Subject headings: Solar System
1. INTRODUCTION
The large aperture of the James Webb Space Telescope (JWST ) in combination with its sensi-
tive near-infrared (near-IR) and mid-infrared (mid-IR) instruments will provide new opportunities
to study comet dust, gas, and nuclei at moderate and large heliocentric distances (defined here as
beyond 3 AU), and at excellent spatial resolutions for closer objects. Moreover, JWST ’s operational
strategies, scheduling, and instrument capabilities allow us to follow comets over a wide range of
times and heliocentric distances. Nominal performance expectations for JWST hardware and flight
software will enable the telescope to follow moving targets with apparent rates up to '0.
00030 s−1
with small pointing errors (currently specified to be 17 mas RMS at 0.
00030 s−1). JWST will use
the JPL HORIZONS system ephemerides for pointing and tracking of comets with known orbital
elements, although an observer may provide ephemerides for targets not in the JPL database.
Accurate ephemerides and/or peak-up operations are essential to place comets and other Solar
System small bodies in the narrow (.100 ) spectrometer slits. Observing visits are limited to the
dwell time of the guide star on the fine-guidance detectors, which have a field-of-view of 2.2×2.2
arcmin. Non-sidereal guiding strategies are beginning to be developed and refined by the JWST
team and will be verified during the initial observatory commissioning period. Release of moving
target capabilities is anticipated to be at the start of routine science operations. See Norwood et al.
(submitted) and Milam et al. (this issue) for summaries of Solar System science with JWST .
The study of comets with JWST will address current key decadal questions in planetary
science, as well as astronomy and astrophysics, that are challenging to address with ground-based
observations alone. These questions pertain to the initial stages of solar system formation and its
subsequent evolution, and include: “What were the initial stages, conditions, and processes of Solar
System formation and the nature of the interstellar matter that was incorporated?” “What were
the primordial sources of organic matter, and where does organic synthesis continue today?” “How
have the myriad chemical and physical processes that shaped the solar system operated, interacted,
and evolved over time?” (National Academy Space Studies Board 2013).
Here, we highlight some of the observational challenges, experimental techniques, and repre-
sentative science opportunities that JWST offers as a tool to improve our knowledge of comets.
Such a space-based platform has easy access to key cometary emission bands from H2O, CO2, CO,
and CH4that would otherwise be hindered by severe telluric absorption. Except for CO2, studies

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of these latter molecules are possible from ground-based telescopes, e.g., through photodissocia-
tion products (e.g., OH and O from H2O, O from CO2), through non-resonance emissions (e.g.,
water hot-bands) that are not blocked by the Earth’s atmosphere, or individual rovibrational lines
Doppler-shifted from their terrestrial counterparts (for a review, see Bockel´ee-Morvan et al. 2004).
However, having direct access to the fundamental bands, and without Doppler shift restrictions,
can enable new and exciting results. Above the atmosphere, background emission is greatly re-
duced, allowing for greater sensitivity to the 3-µm region and beyond, enabling survey studies of
dust emission, water ice absorption, or (refractory) organic features that might otherwise take years
to achieve from ground-based telescopes. Time-domain JWST observational campaigns covering a
wide range of heliocentric distances become possible for many more targets, rather than just the
occasional Hale-Bopp-class comet.
In Section 2, we present our methods to estimate the continuum arising from dust (reflected and
thermal emission), and to estimate the strengths of emission bands from molecules. These methods
are used to determine integration times and signal-to-noise ratios throughout our paper. We then
propose four science examples that JWST will be especially suited to address. In Section 3.1, we
discuss the telescope’s ability to measure the primary drivers of comet activity: H2O, CO2, and
CO. In Section 3.2, we summarize simultaneous observations of water gas and dust comae and how
they can be used to assess nucleus heterogeneity. In Section 3.3, we propose observations that can
address the nature of ice in cometary comae and nuclei. Finally, in Section 3.4, we demonstrate
how JWST can be used to detect gas in main belt and other faint comets. The effects of the
observatory’s non-sidereal tracking limit on observations of comets, and specific comet observing
opportunities are summarized in Section 4.
2. COMET BRIGHTNESS ESTIMATION TOOLS
2.1. Continuum
At JWST wavelengths (0.6µm<λ<28.5µm), comet spectra are generally dominated by
sunlight scattered by and thermal emission from coma dust. It is a challenge for observers to
accurately predict the surface brightness of a given target, due to each comet’s inherent variability
and coma physical properties (dust and gas ejection speeds, grain parameters, presence of ice, etc.).
For comets in the inner Solar System, the observed energy from dust thermal emission and that
from light scattered by dust are approximately equal in the 2–4 µm range, thus a single Planck
function or stellar template are poor approximations. Whether the goal is to observe the continuum
from dust, absorption or emissivity features, or gas emission, having a simple model to estimate
continuum brightness is beneficial to observers interested in this wavelength regime.
Light scattered by comet dust can be estimated using the Afρ parameter of A’Hearn et al.
(1984). Afρ is the product of the albedo, A, the dust filling factor, f, and the radius of the circular
aperture in consideration, ρ. Under certain assumptions and conditions, it is proportional to the

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dust production rate (Fink & Rubin 2012). It carries the units of ρ, and is typically expressed in
cm,
A(θ)fρ =4∆2r2
h
ρ
Fλ,c
Fλ,
(cm), (1)
where ∆ is the observer-comet distance (cm), rhis the heliocentric distance of the comet (AU),
Fλ,c is the observed flux density of the comet within a circular aperture (W m−2µm−1), ρis the
radius of the aperture projected to the distance of the comet (cm), and Fλ,is the flux density of
sunlight (W m−2µm−1AU−2). We have written the albedo as A(θ) to emphasize that the observed
albedo is a function of phase angle (Sun-comet-observer angle, θ). For observations corrected to
a phase angle of 0◦, we will simply write Afρ. The albedo of A’Hearn et al. (1984) is a factor of
4 larger than the geometric albedo, Ap, of Hanner et al. (1981): A(θ)=4Ap(θ). Throughout the
paper, we use the combined Halley-Marcus phase function from D. Schleicher to scale A(θ)f ρ to
Afρ (Schleicher et al. 1998; Marcus 2007; Schleicher & Bair 2011).
For quantifying the thermal emission, Kelley et al. (2013) introduced the parameter f ρ, which
is the thermal emission equivalent to Af ρ:
fρ =∆2
πρ
Fλ,c
Bλ(T)(cm), (2)
where is the apparent emissivity, Fλ,c is now the observed thermal emission from dust (W m−2µm−1),
and Bλ(T) is the Planck function evaluated at temperature T(W m−2µm−1, and K, respectively).
Similar to Afρ, this quantity carries units of length, determined by the units of ρ. Kelley et al.
(2013) recommend using the effective (color) temperature of the continuum, with a default tempera-
ture of T= 306 r−1/2
hK, 10% warmer than an isothermal blackbody sphere in local thermodynamic
equilibrium with insolation (TBB = 278 r−1/2
hK).
For exposure time calculations, we propose using a combination of the two empirical quantities
above:
Fλ,c =Afρ Φ(θ)ρFλ,
4∆2r2
h
+fρ πρBλ(T)
∆2,(W m−2µm−1) (3)
where Φ(θ) is the phase function of the dust evaluated at the phase angle θ. The filling factors
for Afρ and fρ are not necessarily the same, as different populations of dust could dominate
the measured scattered and thermal emission. In Table 1 we present seven observations of five
comets with contemporaneous Afρ and fρ estimates. The ratios fρ/Afρ =fem/Afsca aspan
the range 2.3 to 4.2. When fρ is not known, we suggest scaling Afρ by the ratio fem/Afsca = 3.5,
based on the observed range in our small sample, and the assumptions: (1) ≈0.85 −0.95, (2)
A≈0.25, and (3) the emission and scattering filling factors are similar, fem ≈fsca. Because the
fρ parameter depends on effective temperature, the ratio fem /Afsca may also be correlated with
T/TBB , although this is not evident in the limited dataset in Table 1. A caveat to our approach
arises if T/TBB and f ρ rely on data measured over a limited wavelength range. Any derived values
may not be valid for other wavelengths, or even heliocentric distances. Specifically, short ward of

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∼7.5 µm silicates have limited absorptivity so the 3–7 µm continuum is dominated by a warmer
continuum from more absorbing carbonaceous grains.
In Fig. 1, we plot a near-IR spectrum of comet 73P-C/Schwassmann-Wachmann 3 (Sitko
et al. 2011), and our proposed model with the parameters T/TBB = 1.12, Af ρ=1400 cm, and
f/Af = 3.0. The agreement is acceptable for first-order integration time estimations. Further
refinements, e.g., wavelength dependent Af ρ to account for coma color, or wavelength dependent
fρ to account for grain composition and size distribution, are not necessary.
2.2. Gas Flux
In addition to the dust continuum, gas emission bands are commonly observed. Here, we sum-
marize our method to estimate total band flux. Using fluorescence g-factors (photons s−1molecule−1
at 1 AU) from Crovisier & Encrenaz (1983), we generate total band fluxes in a 0.
004 radius aperture,
for either the NIRSpec integral field unit or micro-shutter assembly, assuming an optically thin
coma (Table 5),
F=Qρhcg
8λv∆2r2
h
(W m−2),(4)
where Qis the production rate of the molecule (molecules s−1), ρis is the radius of the aperture
projected to the distance of the comet, his the Planck constant, cis the speed of light, λis the band
central wavelength, v= 800 r−1/2
hm s−1is the expansion speed of the gas, ∆ is the observer-comet
distance, and rhis the heliocentric distance of the comet (AU). All parameters have MKS units,
unless otherwise noted.
Although we assume an optically thin coma, optical depth effects must be carefully considered
when interpreting real spectra. For comet 9P/Tempel 1, Feaga et al. (2007) showed that optical
depth effects are significant inside of ∼10 km for a water production rate of 5 ×1027 molecules s−1
and a CO2production rate of 5 ×1026 molecules s−1at 1.5 AU from the Sun. For integration time
estimation purposes, we neglect line opacity effects, which should only affect total band fluxes at
the 10% level or less (e.g., Feaga et al. 2014).
2.3. Comet Comae and Exposure Time Calculators
AJWST exposure time calculator could provide a two-component model that would be adapt-
able to a variety of Solar System sources. Continuum spectra of comet comae, comet nuclei, and
asteroids can all be approximated by: (1) Fλ,refl , based on a solar-type spectrum, potentially
reddened, representing the reflected or scattered light; and (2) Fλ,therm, a blackbody spectrum of

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arbitrary temperature, representing the thermal emission,
Fλ,ref l =Cref l
Fλ,(λ)
Fλ,(λref l,0)(5)
Fλ,therm =Ctherm
Bλ(T, λ)
Bλ(T, λtherm,0)(6)
where Cref l(λrefl,0) and Ctherm(λtherm,0) are the reflected (or scattered) and thermal emission
normalization factors; Fλ,is the solar flux density (see, e.g., ASTM International 2006); and Bλ
is the Planck function. The template spectra are normalized to 1.0 at reference wavelengths λrefl,0
and λtherm,0. Because Af ρ and f ρ can change with wavelength, their values and the normalization
wavelengths should be carefully chosen for the observation in consideration. For example, one might
choose λref l,0= 1.0µm and λtherm,0= 4.0µm for the spectrum in Fig. 1. The normalization factors
for a comet coma model would be based the Af ρ and f ρ components in Eq. 3,
Cref l =Afρ Φ(θ)ρ
4∆2r2
h
(7)
Ctherm =f ρ πρ
∆2.(8)
Thus, comet-specific parameters would not need to be incorporated into an observing tool, but
arbitrary scale factors would be a necessity. Observers will need to estimate Afρ and f ρ based on
the literature, their own experience, or recent observations.
3. COMET SCIENCE
3.1. Comet Gas Coma Orbital Evolution
Three gas species, H2O, CO2, and CO, are primarily responsible for driving cometary activity.
These molecules have very different levels of volatility (e.g., Langer et al. 2000; Meech & Svore˘n
2004; Huebner et al. 2006), suggesting that each species should dominate the comet’s activity at
different parts of the orbit, delineated by the abundance of each ice, and the amount of solar energy
available for sublimating those ices. CO may be released at heliocentric distances out to tens of
AU, while CO2sublimation dramatically increases around 6–8 AU and H2O sublimation near 2–3
AU. However, the limited observations available for these species indicate that cometary behavior is
more complex than would otherwise be suggested by a simple energy balance model. The relative
abundances not only differ from comet to comet, but also in unexpected ways with respect to
heliocentric distance and sometimes before and after perihelion in a single comet (Bockel´ee-Morvan
et al. 2004; Mumma & Charnley 2011; A’Hearn et al. 2012; Ootsubo et al. 2012; Feaga et al. 2014;
Bodewits et al. 2014). These variations are due presumably to inherent compositional differences
between objects reflecting their origins in the early Solar System, heterogeneities in individual
nuclei, and evolutionary processing from insolation.

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Comprehensive data sets, i.e., those covering the production of all three molecules (H2O, CO2,
and CO) over a wide range of times or heliocentric distances, typically depend on observations from
multiple instruments and techniques. How systematic uncertainties (instrumental, observational,
or theoretical) affect the relative abundances is therefore of concern. In addition, CO2cannot be
observed from the ground, thus little is known regarding its abundance in comets or its role in comet
activity outside of recent snapshot surveys (Crovisier et al. 2000; Ootsubo et al. 2012; Reach et al.
2013) and missions to comets (A’Hearn et al. 2005; Feaga et al. 2007; A’Hearn et al. 2011; H¨assig
et al. 2015). The wavelength coverage and sensitivities of NIRSpec, NIRCam, and NIRISS will
allow JWST to simultaneously or contemporaneously measure all three of the primary species over
a range of heliocentric distances. Such measurements enable studies of activity and composition
at various distances and points around a comet’s orbit, providing a uniform dataset that cannot
be achieved by other means. The high-spatial-resolution imaging available with NIRCam and
NIRISS will also allow the investigation of heterogeneities in these species, revealed in the spatial
distribution of the gases around the nucleus and their variations with time.
Here, we demonstrate the capabilities of time-domain spectroscopy with NIRSpec. Our goal
is to observe H2O, CO2, and CO in a single comet over a wide range of heliocentric distances.
We take the orbit of C/2013 A1 (Siding Spring) as a baseline (perihelion distance q= 1.4 AU)
to provide a rough idea for potential observing windows and comet brightnesses during an Oort
cloud comet’s journey from 10 AU pre-perihelion to 10 AU post-perihelion. We assume a coma
mixing ratio of 100/10/10 for H2O/CO2/CO at rh= 1.5 AU, approximately equal to the average
observation listed in A’Hearn et al. (2012). Using the sublimation model of Cowan & A’Hearn
(1979), this ratio corresponds to equivalent active areas of 6 km2for H2O, 0.26 km2for CO2, and
0.080 km2for CO. Other model quantities are summarized in Table 2. For the sublimation model,
we assume a rapidly rotating nucleus with a visual albedo of 0.05, an infrared emissivity of 1.0,
and the pole direction perpendicular to the orbit. Gas production rates are the product of the
sublimation rate and the active area.
Water molecules have the shortest photodissociation length scale of the molecules in our study,
but our apertures are no larger than 13% of this scale, so for simplicity, we neglect photodissociation.
The continuum arising from dust, a source of noise, is modeled using Eq. 3, with the following
parameters: Afρ = 2000 cm at 1 AU, scaling with r−2
h,fem/Afsca = 3.5.
Production rates and Af ρ values derived from the gas and dust models are listed at approx-
imately 1 AU steps in Table 5. We have selected epochs that would be within JWST ’s solar
elongation constraints (85◦to 135◦). The water production rate (9 ×1027 molecules s−1) and Afρ
(925 cm) at 1.5 AU are within a factor of two of actual measurements of the comet near perihelion
(Schleicher et al. 2014; Bodewits et al. 2015). Comet Siding Spring’s Afρ at 4 AU (pre-perihelion)
observed by Li et al. (2014) is higher than our model by an order of magnitude (120 cm versus
2000 cm). Dynamically new comets tend to have shallow lightcurves upon approach to perihelion
(Oort & Schmidt 1951; Whipple 1978), and we do not incorporate this aspect into our model.

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Using the prototype NIRSpec exposure time calculator, we compute the integration time re-
quired to achieve a peak signal-to-noise ratio (SNR) of 10 for each band (Table 5) and plot it in
Fig. 2. For our low-resolution spectroscopy observations, we assumed each band may be approxi-
mated by a single line with a full-width at half-maximum of 0.2 µm. Both H2O and CO are detected
in a few thousand seconds at production rates Q&5×1025 molecules s−1out to ∼4.5 AU; CO2
is detected in a few thousand seconds at Q&1×1025 molecules s−1out to ∼7 AU. Thus, JWST
will easily be the best telescope for spectroscopic studies of comet activity at moderate and large
heliocentric distance.
3.2. Comet Dust Heterogeneity With MIRI
Comet nuclei are aggregates of the planetesimals present in the outer Solar System during the
epoch of planet formation (Weissman et al. 2004; Belton et al. 2007). There is an open question
concerning the homogeneity of accreted planetesimals for particular comets, i.e., are individual
comets aggregates of a heterogeneous population of planetesimals formed over a range of times
and distances (e.g., Weidenschilling 1997)? Are comets collisional fragments (e.g., Stern & Weiss-
man 2001; Morbidelli & Rickman 2015) and less likely to be heterogeneous despite their origins?
High spatial resolution images and spectral data cubes from flyby spacecraft have suggested comet
volatiles are not uniformly distributed about nuclei (e.g., Feaga et al. 2007; A’Hearn et al. 2011), but
the evidence for regions heterogeneous in dust properties relies on poorer resolution telescopic data,
usually through multiple observations of a rotating comet (e.g., Ryan & Campins 1991; Wooden
et al. 2004). A close approach (here, ∆ <1 AU) between a comet and JWST would allow us
to observe coma features on .200 km spatial scales, potentially revealing distinct active regions
before they spatially mix in the coma. Such a study can test the heterogeneity of comet nuclei by
observing and comparing multiple spatially resolved coma jet features.
JWST ’s MIRI instrument will be able to simultaneously observe both coma dust and water
gas. At λ > 5µm, mid-infrared spectroscopy reflects a coma’s dust composition, grain sizes, and
porosities through solid-state emission features and thermal emission pseudo-continuum. We use
the terms grain and aggregate interchangeably. They represent an entire particle of dust, which
may be composed of smaller sub-units (the latter is implied for aggregate). Typical dust features
include a broad plateau from 8 to 12 µm, primarily arising from anhydrous amorphous silicates
(non-stochiometric olivine and pyroxene), and narrow emission peaks (λ/∆λ∼10 −20) from
crystalline silicates, dominated by Mg-rich olivine (Wooden 2002; Hanner & Bradley 2004). MIRI
also covers the ν2water gas emission band at 6 µm, observed and studied in several comets with
the Infrared Space Observatory and Spitzer Space Telescope (Crovisier et al. 1997; Woodward et al.
2007; Bockel´ee-Morvan et al. 2009). In addition, the NIRSpec instrument holds the potential to
simultaneously detect emission from water gas and hydrocarbons, the latter via the aromatic C–H
stretch at ∼3.3 µm and the aliphatic C–H stretch at ∼3.4 µm (see Li 2009).
We consider comet 46P/Wirtanen as an example exercise demonstrating the technical feasibil-

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ity of an observing program designed to test coma heterogeneity. This comet has a close approach
to JWST in December 2018. In November 2018 the comet is within the solar elongation constraints
(85◦and 135◦) and quite bright, with integrated flux densities of order 1 Jy at 13 µm (0.
005 radius
aperture). But rather than consider an observing window so soon after JWST ’s launch (October
2018), we instead consider the post-closest approach window starting 2019 March 13 (rh= 1.56 AU,
∆=0.69 AU) with non-sidereal rates of .5300 hr−1, well under the JWST limit of 10800 hr−1.
Spatial-spectral maps of a comet’s dust coma can be performed with either the Low Resolution
Spectrometer (LRS, λ/∆λ∼100) or Medium Resolution Spectrometer (MRS, λ/∆λ∼3000) modes
of MIRI. Typically, comet dust studies would use the LRS, because the spectral resolving power of
the MRS is much greater than necessary to resolve solid-state emission features. However, we will
design an observation for the MRS integral field units (IFU), which can efficiently map a 300 ×400
field of view, and have a spectral resolving power appropriate for measuring individual lines in the
ν2water band.
For our model, we adopt an effective nucleus radius of 0.6 km (Lamy et al. 1998), a maximum
Afρ of 380 cm (Farnham & Schleicher 1998), and a maximum water production rate Q(H2O) =
1.6×1028 molecules s−1at perihelion q= 1.05 AU (Bertaux et al. 1999). We also assume that the
dust production rate scales with water production as r−4.9
h(Bertaux et al. 1999).
The IFUs can obtain complete spectra from 5 to 28.5 µm in 3 exposures over a 3.
000×3.
009 field
of view, corresponding to 1500 km ×1900 km at the comet, with a spatial resolution of about
100 km per resolution element. For comparison, the current best spatial resolution of this comet
obtained with mid-infrared spectroscopy is 1300 km with Spitzer/IRS.
Comet Wirtanen’s peak surface brightnesses estimated for the MRS IFUs are listed in Table 4.
The surface brightness is an average within a 0.
004 radius aperture (corresponding to approximately
10 resolution elements at 6 µm and a few resolution elements at 23 µm). Using the estimated MRS
line sensitivities (in units of W m−2pixel−1) and dividing by the unresolved line width for each
module yields continuum sensitivity thresholds in units of flux density. These values are scaled by
t−1/2to an integration time of 100 s and listed in Table 4. We expect high signal-to-noise ratios in
the central aperture, easily enabling mapping of the inner coma.
For the H2O band at 6 µm we assume an effective g-factor of 2.4×10−5photons s−1molecule−1
at 1 AU for the brightest lines, based on modeled Spitzer spectra of C/2003 K4 (LINEAR),
71P/Clark, and C/2004 B1 (LINEAR) (Woodward et al. 2007; Bockel´ee-Morvan et al. 2009). These
lines will be affected by optical depth effects, but introducing opacity is not necessary for our first-
order feasibility exercise. Following Eq. 4, we predict a total line flux ∼2×10−18 W m−2in a 0.
004
radius aperture, or about 2 ×10−19 W m−2pixel−1. The lines will be easily detected above the
continuum (∼10−20 W m−2pixel−1) with the MRS IFUs. The morphology of the water gas would
be used to locate active areas on the nucleus, and to identify dust that may originate from those
areas. Additional context with CO2and CO may be obtained with immediate follow-up observa-
tions with the near-IR spectrometer. Comparisons of a particular active area’s dust composition to

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that of the ambient coma or of other regions can be used to test the heterogeneity of the nucleus.
3.3. Water Ice in Comae and on Nuclei
As preserved leftovers from the formation of the Solar System, comets serve as one of the best
probes for studying primitive water ice in the form it may have existed in the early outer Solar
System. Thus observations of water ice in comets may allow us to test the accretion processes
that led to the formation of comet nuclei and therefore of planets (e.g., Greenberg & Li 1999).
Contrasting coma ice properties to observations of ice on comet surfaces will allow us to study
coma-nucleus interactions, and nucleus surface evolution.
Water ice has absorption bands in the near-infrared at 1.5, 2.0, and 3.0 µm. The relative
strength and shape of these features provides information on aggregate porosities, presence of dust
within the ice, total abundance, and the size of the scattering units (following Section 3.2, we use the
term grain and aggregate interchangeably). Laboratory measurements show that infrared water-ice
absorption bands change position and shape as a function of phase (crystalline or amorphous) and
temperature: e.g., crystalline water ice differs from its amorphous counterpart by the presence
of a sharp and narrow feature at 1.65 µm apparent at T.200 K (Grundy & Schmitt 1998).
The best approach to investigate water-ice physical properties is through near-infrared (1–5 µm)
spectroscopy.
Water ice has been observed in comae using ground- and space-based telescopes and flyby
spacecraft (e.g., Davies et al. 1997; Kawakita et al. 2004; Yang et al. 2009; Protopapa et al. 2014).
The small sample of comae detections presently available has revealed a range of water-ice charac-
teristics: µm and sub-µm water-ice particles have been identified, as well as one case of crystalline
water ice (e.g., Yang et al. 2009; Yang & Sarid 2010; Yang et al. 2014; Protopapa et al. 2014). How-
ever, the limited number of detections prevents development of a classification scheme or taxonomy
based on water-ice properties. The sample population is too small to apply statistical analyses with
confidence, thereby limiting our understanding of the origins of this diversity. It may relate to the
different mechanisms responsible for delivering water ice from the interior into the coma (sublima-
tion of more volatile ices e.g., CO2, CO, or through large outbursts in activity). Alternatively, it
may be the outcome of a preserved comet-to-comet heterogeneity. Establishing these connections
is crucial if water-ice properties are to be used as observationally imposed boundary conditions on
comet nucleus formation models.
On nucleus surfaces, however, water ice has only been observed with flyby and orbiting space-
craft. The observations of ice on the surfaces of comets 9P/Tempel 1 and 103P/Hartley 2 point
towards sub-units with sizes of order 10 to 100 µm (Sunshine et al. 2006, 2012). This is much
larger than the coma ice summarized above, and the ice excavated from the interior of Tempel 1
(Sunshine et al. 2007). Thus, Sunshine et al. (2006) hypothesize that this ice is more likely due
to re-condensation of water gas, rather than recently uncovered interior ice. Observations of bare

– 11 –
nuclei can help test when this re-condensation occurs: does it require constant replenishment from
an active source, or is it deposited by low activity levels as a comet recedes from the Sun? Com-
plicating matters, Capaccioni et al. (2015) report the detection of a broad absorption band at 2.9
to 3.6 µm in Rosetta observations of the surface of comet 67P/Churyumov-Gerasimenko. The ab-
sorption band has a peak depth of 20% located at 3.2 to 3.3 µm. They suggest this feature is due
to the presence of aromatic and aliphatic C-H bonds, carboxylic groups, and/or alcoholic groups.
The absorption feature has not been reported on the surfaces of 9P/Tempel 1 or 103P/Hartley 2
(Sunshine et al. 2006; A’Hearn et al. 2011). Whether absorption features are due to water ice or
organic molecules, the study of comet surfaces at 3–4 µm is best addressed through space-based
missions or observatories.
NIRSpec is ideal for the study of water ice in comets. The prism mode covers 0.7 to 5 µm
at a spectral resolving power of R∼100, ideal not only to detect the broad absorption features
of surface and coma ice but also to determine its chemical phase (amorphous or crystalline). The
instrument’s higher spectral resolving powers (R∼1000) may be sensitive to trapped CO2or
other volatiles. Below we summarize observations of a distant comet nucleus and coma with this
instrument to illustrate future science opportunities.
3.3.1. Surface ice
At 5 AU and beyond, water ice deposited on the cold surface of a comet could persist until
vigorous sublimation returns when surface temperatures rise to 160 K or more. Where that ice
resides would depend strongly on deposition and insolation history, including latitude and surface
roughness considerations. Cooler nuclei in the outer Solar System, e.g., cometary Centaurs, may
have surface ice present throughout their orbit. As an example of NIRSpec’s capabilities with
respect to spectroscopy of distant nuclei, we consider comet 172P/Yeung. This comet was observed
to be a point source by Spitzer (Kelley et al. 2013) at rh= 4.25 AU (pre-aphelion), with an effective
nucleus radius of 5.7 km (Fern´andez et al. 2013). We model the comet nucleus thermal emission
with the near-Earth asteroid thermal model of Harris (1998) and the parameters of Fern´andez et al.
(2013). The scattered light contribution is modeled assuming a geometric albedo of Ap= 0.04
(Lamy et al. 2004). Our goal is to detect the presence of water ice on a surface. We choose the
3-µm water ice because of its high relative absorption compared to the other water ice bands in the
near-infrared. The same methodology could be applied to the detection of surface organics, similar
to those seen at comet 67P/Churyumov-Gerasimenko (Capaccioni et al. 2015). There is, however,
an important caveat: observations of a point source are not necessarily those of a bare nucleus, as
a faint and/or unresolved coma may be present. Supporting observations may be needed to rule
out coma contamination in any particular data set.
We target a water-ice band with a band depth of 10% or more at a confidence level of 5-σ.
Averaging over a 0.3-µm-wide band (2.9 to 3.2 µm), and taking a spectral resolving power of 100, we
set our signal-to-noise ratio goal to 20 per resolution element. JWST can observe comet Yeung on

– 12 –
2019 June 30 (rh= 4.5 AU, ∆ = 4.4 AU, phase angle = 13◦), when the comet’s 3-µm flux density
would be approximately 5.7 µJy (the thermal contribution is <5% at λ <4µm). We meet our
SNR goal with NIRSpec’s low-resolution, fixed-slit mode in 300 seconds of integration time. Since
the integration time is short compared to all known comet rotation periods (>5 hr), longitudinally
resolved observations would be possible, providing important motivation to measure this comet’s
rotation period by 2019.
The Centaur 95P/Chiron (2060) is known to have water ice and activity (Hartmann et al.
1990; Luu & Jewitt 1990; Foster et al. 1999; Luu et al. 2000; Duffard et al. 2002; Ruprecht et al.
2015), and a spectrum taken in Jul 2019 (rh= 19 AU, ∆ = 18 AU, Fν= 21 µJy for Ap= 0.15 and
80 km radius, Campins & Fern´andez 2000) could easily observe a 3-µm feature as weak as a few
percent. In 1.0 hr, an R∼1000 spectrum from 2.9 to 5.0 µm can be obtained with a 3 µm SNR of
60 per resolution element. This higher resolution spectrum, and complementary spectra at shorter
wavelengths, could be used to investigate the structure and composition of the ice (crystallinity,
dirt fraction, trapped volatiles), as well as to look for signatures of organics in the 3- to 4-µm
region.
In an hour of integration time, we can achieve a SNR of ∼20 on Centaurs as small as ∼12 km
radius at the distance of Chiron, sufficient for detecting a 3-µm absorption feature at the 10%
level. Observations of a representative sample of Centaur surfaces within JWST ’s lifetime will be
possible. These observations of Centaurs, as transitional objects between the trans-Neptunian and
Jupiter-family comet regions, will yield new insight into the evolutionary processes currently at
work in our Solar System.
3.3.2. Water Ice in Comet Comae
For our comet coma case, we move beyond ice detection and instead focus on physical charac-
terization. A signal-to-noise ratio of >50 is necessary to ensure that our observations are sensitive
to water-ice fractions of a few percent by area, to constrain the size of the scattering units, and to
test the presence of the potentially weak crystalline water-ice feature at 1.65 µm.
As an example case we consider the observations of comet C/2012 S1 (ISON) by Li et al. (2013)
with the Hubble Space Telescope when the comet was at 4.15 AU from the Sun and 4.24 AU from
Earth. The authors report an A(θ)f ρ of 1300 cm at 0.6 µm within a distance of 5000 km from the
nucleus, which corresponds to a flux density of 50 and ∼20 µJy at 0.6 and 3 µm, respectively, within
an angular diameter of 0.
004. We conclude that in 900 s we can obtain a spectrum of comet ISON
with a SNR of 100 using the current NIRSpec prototype exposure time calculator. Li et al. (2013)
report reddening of the dust coma between 5000 km and 10000 km from the nucleus, compatible
with the presence of icy grains close to the nucleus. It would be interesting to test this hypothesis
by measuring the coma ice distribution with NIRSpec. The flux density at 10000 km from the
nucleus at 3 µm is estimated to be 0.4 µJy. An exposure time of 3 h yields a SNR of 30, high

– 13 –
enough quality to test for the presence and size of water ice grains, and to study its evolution from
the inner-coma out to 10000 km. This exercise demonstrates that JWST will be able to measure
ice properties throughout the coma of moderately bright comets at rh.4 AU.
3.4. Activity in Faint Comets and Main-Belt Asteroids
JWST ’s capabilities will transform our understanding of faint and/or weakly active comets by
enabling observations that were heretofore at the limit of experimental techniques. Such comets
comprise several categories:
1. Main-belt comets (MBCs). Activity recurring from orbit to orbit has been identified in a
handful of MBCs (e.g., Hsieh & Jewitt 2006; Hsieh et al. 2011, 2015), but the mechanism for
this activity is unknown. To date, there have been no successful searches for signatures of
volatiles on MBCs.
2. Nearly inactive and dormant comets. A large number of suspected dead comets (inactive
objects in comet-like orbits) have been identified in the asteroid population (e.g., Jewitt
2005; Jenniskens 2008), and occasionally asteroids show cometary activity. For example,
Spitzer/IRAC observations of “asteroid” (3552) Don Quixote exhibited signs of CO2out-
gassing (Mommert et al. 2014). In addition, there are known comets with extremely weak or
only sporadic activity, e.g., 209P/LINEAR 41 (Schleicher 2014) or 107P/Wilson-Harrington
(Fern´andez et al. 2007). High-sensitivity observations of volatiles can help confirm or better
characterize their cometary natures.
3. Very small comets. The population of small near-Sun comets are generally thought to be
<50 m in radius (Knight et al. 2010), too small or weakly active to be observed beyond
the solar coronagraphs on SOHO or STEREO. Little is known about these small bodies,
including what drives their activity: normal cometary activity via sublimation of ices or the
sublimation of dust and refractory grains due to the extreme temperatures experienced near
perihelion (equilibrium temperatures exceeding 1000 K; Mann et al. 2004).
4. Distant comets. Many comets show continuous activity outside the so-called water-ice line.
Their activities may be driven by the more volatile ices CO2or CO (Meech & Svore˘n 2004;
Meech et al. 2013; Stevenson et al. 2015), or by the exothermic annealing of water ice from
amorphous to crystalline states Meech et al. (2009); Jewitt (2009). Owing to their large
distances, studies of activity are generally limited to observations of dust comae and tails.
A common theme in the above cases is faint activity. NIRCam imaging would be capable of
searching for signs of very weak activity (coma and/or tail, both dust and gas) and of efficiently
characterizing nucleus properties such as size, elongation, albedo, and beaming parameter. NIR-
Spec might be able to spectroscopically detect the driving volatiles at production rates lower than

– 14 –
previously achievable. The comet-asteroid transition is currently poorly understood, and such
results, whether of severely processed sun-grazing/sun-skirting comets, dormant comets, distant
comets, or of activated asteroids, would yield insight into the ongoing evolutionary processes of our
solar system.
To demonstrate JWST ’s capabilities for characterizing activity in faint comets, we consider
observations of main belt comet P/2010 R2 (La Sagra) in late Aug 2021 (rh= 2.7 AU, ∆ = 1.9 AU).
This observing window is post-perihelion, during a period when comet La Sagra has been observed
to be active with an Afρ near 50 cm (Hsieh et al. 2012). Estimates of the dust production rate by
Moreno et al. (2011) and Hsieh et al. (2012) span from 0.1 to 4 kg s−1. Assuming a dust to gas mass
production ratio of 1:1, and assuming water sublimation is driving the activity, this corresponds to
a water production rate with an order of magnitude of 1025 molecules s−1.
For this exercise, we use NIRSpec to detect the ν3water band at 2.7 µm, one of the strongest
water bands in comets (Crovisier & Encrenaz 1983) and one that cannot be observed from the
ground (Bockel´ee-Morvan et al. 2004). In a small aperture of radius 0.
002, the estimated continuum
flux density at 2.7 µm is 1.7×10−18 W m−2µm−1, and the water band flux is 2.3×10−20 W m−2.
With the NIRSpec prototype exposure time calculator and the R∼100 prism, we model the water
band as a single 0.2-µm-wide line. In a 1 hr exposure, the peak of the line is detected above the
continuum with a signal-to-noise ratio of 4. Thus, we can expect JWST to provide one of the best
tests for the drivers of main-belt comet activity.
4. NON-SIDEREAL RATES AND POTENTIAL TARGETS
JWST is expected to have the ability to track moving targets with proper motions ≤10800 hr−1
(30 mas s−1) (Norwood et al. submitted). To estimate the impact this limit may have on cometary
science, we searched the NASA Jet Propulsion Laboratory Solar System Dynamics Group database
for all comets that reached perihelion during the five-year period 2010 Jan 1 to 2015 Jan 1. This list
includes 393 comets: 221 short-period comets (with numbered or provisional “P/” designations),
and 172 Oort cloud and long-period comets (with provisional “C/” designations). For each target,
we used the IAU Minor Planet Center’s ephemeris service to generate ephemerides with 3-day
intervals, using the center of the Earth as the observer. The differences in ephemerides between the
Earth and JWST at the L2 Lagrange point (1.5×106km or 0.01 AU away) are not significant for this
exercise. The proper motions of all targets within JWST ’s solar elongation constraints (85–135◦)
are shown in Fig. 3. Not all of the observing windows make sense in a real world situation, e.g.,
the plot includes epochs before some comets are discovered, and all epochs are shown independent
of the comet’s apparent brightness. However, we neglect these and other considerations and focus
on the orbital characteristics of the known comet population.
In Fig. 4, we show histograms of our comet ephemerides binned by heliocentric distance. Oort
cloud and long-period comets have higher proper motions in the inner Solar System than short-

– 15 –
period comets, due to their higher eccentricities and the greater potential for high inclinations,
including retrograde orbits. These faster moving comets are, therefore, more likely to exceed a
proper motion of 10800 hr−1. Less than 50% of the opportunities to observe Oort cloud or long-
period comets inside of 1.7 AU are possible, given the current anticipated tracking limit. Inside of
1.3 AU from the Sun, there is a .25% probability that any particular comet could be observed.
This limitation can significantly affect science programs requiring the maximum possible spatial
resolution in studies of nearby comets. Any observations dedicated to the near-nucleus environment
will have a limited set of objects to consider, if any. If we artificially increase the non-sidereal
tracking rate by a factor of 2 to 21600 hr−1, the situation is substantially improved: the 50% limit
for Oort cloud/long-period comets is reduced to rh≈1.3 AU and most comets can be observed
down to 1.1 AU from the Sun.
In Table 5 we present a select list of targets observable by JWST, based on: (1) comets with
perihelion dates during the first 5 years of the observatory’s mission, (2) Centaurs with known or
suspected cometary activity, (3) prior and potential spacecraft targets, and (4) known main-belt
comets and activated asteroids. Our choice of Jupiter-family comets is biased towards previous
or potential spacecraft targets, and targets with favorable observing circumstances (small comet-
observer distance, or otherwise easily observable at perihelion). This list of comets, as well as
undiscovered comets (especially Oort cloud and long-period comets), comets near aphelion, and
unanticipated outbursts and fragmentation events will provide many targets of opportunity for
JWST observers.
5. SUMMARY
We considered simple tools for estimating comet dust and gas brightness based on known
comet properties. We then explored four science themes that JWST will be especially suited to
address: (1) For a moderately bright comet, we can expect studies of the main drivers of cometary
activity, H2O, CO2, and CO, out to a heliocentric distance of at least 4 AU; CO2can be observed
even farther, out to at least 7 AU. (2) We assessed the observatory’s potential to detect ice and
organics in the comet population. A survey of surface ice in Centaurs appears feasible for objects
as small as 12 km out to 19 AU from the Sun. Spectroscopy of Jupiter-family comets, targeted near
aphelion during periods of inactivity, can be used to determine ice deposition rates (if occurring),
and further understand the uniqueness of the recent discovery of organics on the surface of comet
67P/Churyumov-Gerasimenko. The properties of water ice grains can be measured, and spatially
resolved throughout the coma of a moderately bright comet at 4 AU. Such a study would provide
observational constraints on the lifetime of coma water ice, which can in turn be compared to
their physical properties derived from light scattering. (3) Through the fundamental emission
band of water at 2.7 µm, JWST will be able to provide direct tests of the cometary nature of
main-belt comets. (4) Spatially resolved dust studies in the mid-IR can be used to examine the
physical properties of the grains, and to test the heterogeneity of nuclei through their coma dust

– 16 –
properties. Finally, there will be many opportunities to observe comets during the first five-years of
operations, but the proper motion limit significantly affects target availability within 1.7 AU of the
Sun. Improved tracking performance is highly desired to enhance the ability of the community to
further studies of the near-nucleus environment of comets at the best-spatial resolutions possible.
The authors thank an anonymous referee for their insightful critique that improved the manuscript.
MSPK acknowledges support for this work from NASA (USA) grant NNX13AH67G, and CEW ac-
knowledges partial support from grant NNX13AJ11G. This work is supported at The Aerospace
Corporation by the Independent Research and Development program.
This research made use of Astropy, a community-developed core Python package for Astronomy
(Astropy Collaboration et al. 2013).
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Table 1. Observed Afρ and fρ values, and their ratio.
Comet Afρ λAfρ λT/TBB fem/Afsca Referencea
(cm) (µm) (cm) (µm)
1P/Halley 1205 1.25 3450 10.1 1.13 2.9 1985-08-25.6, T86, T88
1P/Halley 5710 1.25 15400 10.1 1.13 2.7 1985-12-13.3, T86, T88
73P-C/S-W 3 243 0.7 1000 13 1.07 <4.1bS06, S11
73P-C/S-W 3c1400 1.0 4150 4.0 1.12 3.0 S11 and this work
103P/Hartley 2 233 0.445 970 13 1.08 4.2 S10, M11
C/1986 P1 (Wilson)d4230 1.25 9660 10.1 1.11 2.3 1987-06-1.2, H89
C/2012 K1 (Pan STARRS) 5731 0.64 14900 19.7 1.02 2.6 W15
aH89 = Hanner & Newburn (1989), M11 = Meech et al. (2011) S06 = Sitko et al. (2006), S10 = Sitko et al. (2010),
S11 = Sitko et al. (2011), S13 = Sitko et al. (2013), T86 = Tokunaga et al. (1986), T88 = Tokunaga et al. (1988),
W15 = Woodward et al. (2015).
bThe observations of 73P-C with the BASS (Broadband Array Spectrograph System) indicate the comet’s IR
brightness was increasing on hour timescales, and the Afρ and f ρ values given are based on observations separated
by about 40 min. Therefore, f /Af is potentially an upper-limit.
cValues in this row are based on the fit to the near-IR spectrum presented in Fig. 1. The continuum temperature
scale factor was fixed at T/TBB = 1.12.
dOld provisional designation: 1986l.
Table 2. NIRSpec case study: adopted volatile parameters.
Species Band λcArea log hZ(1.5AU)ilog hZ(10AU)i
(µm) (km2) (molec. s−1cm−2) (molec. s−1cm−2)
H2Oν32.7 6.0 17.15 8.30
CO2ν24.3 0.26 17.51 14.84
CO ν(1 −0) 4.7 0.080 18.03 16.36
Note. — λCis the approximate band center, (Crovisier & Encrenaz 1983),
Area is effective active area, and hZiis the mean sublimation rate per unit area,
given at two heliocentric distances.

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Table 3. NIRSpec case study: Production rates, total band fluxes, and integration times.
H2O CO2CO
rh∆Eρ Af ρ log Qlog F t10 log Qlog F t10 log Qlog F t10
(AU) (AU) (◦) (km) (cm) (s−1) (W m−2) (s) (s−1) (W m−2) (s) (s−1) (W m−2) (s)
9.90 9.20 133 2668 20 19.18 -26.64 · · · 24.29 -20.74 · · · 25.27 -20.86 · · ·
8.04 8.04 87 2330 31 21.32 -24.31 · · · 24.90 -19.93 12000 25.45 -20.48 · · ·
7.05 6.55 116 1901 40 22.54 -22.91 · · · 25.16 -19.50 2110 25.57 -20.19 59300
5.00 4.90 90 1421 80 25.26 -19.84 28200 25.69 -18.62 127 25.87 -19.54 4100
4.01 3.63 106 1052 124 26.23 -18.60 306 25.95 -18.08 32.7 26.06 -19.08 841
1.92 1.68 87 488 540 27.62 -16.39 0.731 26.69 -16.53 0.942 26.70 -17.62 52.4
1.47 1.08 90 313 925 27.94 -15.71 0.148 26.93 -15.92 0.288 26.94 -17.02 36.8
2.39 2.16 91 627 350 27.33 -16.94 2.79 26.48 -16.99 2.56 26.51 -18.06 81.2
3.03 2.65 102 769 218 26.93 -17.59 14.9 26.25 -17.46 7.61 26.31 -18.51 186
5.19 5.10 89 1480 74 25.00 -20.14 98400 25.64 -18.71 163 25.84 -19.61 5430
6.05 5.41 126 1569 55 23.86 -21.41 · · · 25.42 -19.05 449 25.70 -19.88 15400
8.09 8.03 90 2329 31 21.26 -24.37 · · · 24.88 -19.95 13200 25.45 -20.49 · · ·
9.04 8.50 120 2464 24 20.12 -25.61 · · · 24.56 -20.38 84600 25.35 -20.69 · · ·
Note. — As an approximation, an Earth-based observer viewing comet C/2013 A1 (Siding Spring) was used to generate observer-
comet distances, ∆, and solar elongations, E.ρis the radius of a 0.
004 synthetic aperture projected at the distance of the comet,
Afρ is the parameter of A’Hearn et al. (1984), Qis the production rate, Fis the total band flux, t10 is the integration time to
achieve a signal-to-noise ratio of 10.

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Table 4. Comet 46P/Wirtanen surface brightness and MIRI MRS IFU sensitivities.
λFOV IλσSNR
(µm) (00) (10−5W m−2µm−1sr−1) (10−5W m−2µm−1sr−1)
6.4 0.18 ×0.19 1.1 0.11 10
9.2 0.28 ×0.19 2.9 0.05 60
14.5 0.39 ×0.24 3.2 0.01 320
22.5 0.64 ×0.27 1.6 0.02 80
Note. — Computed for 2019 March 13 (rh= 1.56 AU, ∆ = 0.69 AU) in a 0.
004 radius
aperture and 100 s of integration time per module. λis the central wavelength of the
estimate, FOV is the field of view of the instrument’s native resolution element, σis the
estimated 1-σcontinuum sensitivity, Iis the modeled continuum surface brightness, and
SNR is the signal-to-noise ratio per resolution element.

– 24 –
Table 5. Potential target comets during the first 5 years of JWST.
Comet TPNotes
2P/Encke 2020-06-26 Potential spacecraft target
2023-10-23
6P/d’Arrest 2021-09-18 Potential spacecraft target
21P/Giacobini-Zinner 2018-09-10 Spacecraft target
29P/Schwassmann-Wachmann 1 2019-03-07 Centaur, frequent strong outbursts
38P/Stephan-Oterma 2018-11-10 Halley-type
46P/Wirtanen 2018-12-12 Potential spacecraft target; historic close approach to
Earth (0.078 AU)
64P/Swift-Gehrels 2018-11-03 Excellent apparition
67P/Churyumov-Gerasimenko 2021-11-02 Spacecraft target
95P/Chiron (2060) 2046-08-03 Centaur with jets/arcs/cometary activity (Bus et al.
1991; Elliot et al. 1995; Ruprecht et al. 2015)
99P/Kowal 1 2022-04-12 Large q(4.7 AU)
103P/Hartley 2 2023-10-12 Spacecraft target
104P/Kowal 2 2022-01-08 Excellent apparition
107P/Wilson-Harrington 2022-08-25 Active asteroid with small q(0.97 AU)
117P/Helin-Roman-Alu 1 2022-07-08 Large q(3.0 AU)
126P/IRAS 2023-07-05 Halley-type orbit
176P/LINEAR 52 2022-11-21 MBC active by sublimation
133P/Elst-Pizarro 2018-09-21 MBC active by sublimation
289P/Blanpain 2019-12-20 Nearly extinct comet (Jewitt 2006); historic close ap-
proach to Earth (0.086 AU)
311P/PanSTARRS 23 2020-10-07 MBC active by rotational breakup (Moreno et al.
2014)
P/2010 R2 (La Sagra) 2021-05-09 MBC active by sublimation (Hsieh et al. 2012)
P/2013 R3 (Catalina-PANSTARRS 1) 2018-12-06 MBC active by rotational breakup (Jewitt et al. 2013)
C/2010 U3 (Boattini) 2019-02-26 Oort cloud comet with very large q(8.5 AU)
C/2014 F3 (Sheppard-Trujillo) 2021-05-23 Unusual orbit: low inclination and long (60-year) pe-
riod
(596) Scheila 2022-05-26 MBC active by impact (Jewitt et al. 2011; Bodewits
et al. 2011)
(3200) Phaethon 2019-07-03 Active asteroid with small q(0.14 AU) (Li & Jewitt
2013)
(10199) Chariklo 2066-06-26 Centaur with rings (Braga-Ribas et al. 2014)
Note. — TPis the comet’s perihelion date, MBC = main belt comet, qis perihelion distance.

– 25 –
Fig. 1.— Spectrum of the coma of comet 73P-C/Schwassmann-Wachmann 3 from Sitko et al. (2011)
and a model fit using Eq. 3 (solid line). The dotted and dashed lines are the model scattered and
thermal components, respectively.

– 26 –
Fig. 2.— Estimated NIRSpec integration times (low-resolution mode) to achieve a signal-to-noise
ratio of 10 at the band peak for each of H2O, CO2, and CO, given our example model described by
Section 3.1 and Table 5. Thick lines mark the fictitious JWST observing windows (solar elongation
between 85◦and 135◦). On the left are pre-perihelion epochs, on the right are post-perihelion
epochs.

– 27 –
Fig. 3.— Comet proper motion (µ) versus heliocentric distance (rh) for all comets in our non-
sidereal rate study. Only epochs within JWST ’s solar elongation constraints (85–135◦) are shown.
A dashed horizontal line marks the observatory’s current non-sidereal rate limit of 10800 hr−1. The
isolated comet with high proper motion at 5 AU is P/2011 P1 (McNaught). This object was
potentially in a large orbit before passing Jupiter from a distance of 0.0008 AU in December 2010,
and is currently in a Jupiter-family comet orbit.

– 28 –
Fig. 4.— Top: A histogram of the number of epochs within JWST ’s elongation constraints,
n, versus heliocentric distance, rh, for our non-sidereal rate study. The solid line includes all
comets, the dotted line only considers short-period comets, and the dashed line is limited to long-
period and Oort cloud comets. Bottom: The fraction of epochs with non-sidereal rates within
JWST ’s observing limits, f, versus heliocentric distance. In general, short period comets are more
accessible to JWST , and the observatory’s tracking limit significantly impacts observations within
rh.2.0 AU.