# Near-Infrared Spectroscopy Of The Nucleus Of Low-Activity
Comet P/2016 BA14 During Its 2016 Close Approach

Theodore Kareta Lowell Observatory
Flagstaff, AZ, USA  Lunar and Planetary Laboratory
University of Arizona
Tucson, AZ, USA  Vishnu Reddy  Lunar and Planetary Laboratory
University of Arizona
Tucson, AZ, USA  Juan A. Sanchez  Planetary Science Institute
Tucson, AZ, USA  Walter M. Harris  Lunar and Planetary Laboratory
University of Arizona
Tucson, AZ, USA

(Received February 2, 2022; Revised March 15, 2022; Accepted April 1, 2022)

###### Abstract

The Near-Earth Comet P/2016 BA14 (PanSTARRS) is a slow-rotatating nearly-
dormant object, a likely dynamical twin of 252P/LINEAR, and was recently shown
to have a mid-infrared spectrum very dissimilar to other comets. BA14 also
recently selected one of the back-up targets for the ESA’s Comet Interceptor,
so a clearer understanding of BA14’s modern properties would not just improve
our understanding of how comets go dormant, but could also aid planning for a
potential spacecraft visit. We present observations of BA14 taken on two dates
during its 2016 Earth close approach with the NASA Infrared Telescope
Facility, both of which are consistent with direct observations of its
nucleus. The reflectance spectrum of BA14 is similar to 67P/Churyumov-
Gerasimenko, albeit highly phase-reddened. Thermal emission contaminates the
reflectance spectrum at longer wavelengths, which we correct with a new Markov
Chain Monte Carlo thermal modeling code. The models suggest $BA_{14}$’s
visible geometric albedo is $p_{V}=0.01-0.03$, consistent with radar
observations, its beaming parameter is typical for NEOs observed in its
geometry, and its reflectrance spectrum is red and linear throughout H and K
band. It appears very much like a “normal” comet nucleus, despite its mid-
infrared oddities. A slow loss of fine grains as the object’s activity
diminished might help to reconcile some of the lines of evidence, and we
discuss other possibilities. A spacecraft flyby past BA14 could get closer to
the nucleus than with a more active target, and we highlight some science
questions that could be addressed with a visit to a (nearly-)dormant comet.

comets, near-earth objects, spectroscopy, spacecraft targets

††facilities: IRTF(SpeX)††software: NumPy (Harris et al., 2020), SciPy
(Virtanen et al., 2020), AstroPy (Astropy Collaboration et al., 2013, 2018),
spextool (Cushing et al., 2004), emcee Foreman-Mackey et al. (2013)

##  1 Introduction

The study of low activity or fully dormant comets is critical to understanding
how comets age and deplete their volatiles as well as learning how to identify
them among other low-albedo small bodies in the inner Solar System. The
problem is identifying them. While some orbits might allow for more definitive
conclusions about whether or not a particular object is a dormant comet (e.g.
lower Tisserand parameters), many objects cannot be so conclusively identified
as such without additional information. One scenario where a cometary origin
can be more clearly identified is if a candidate dormant comet is dynamically
linked to a traditionally active comet, such as with the dormant comet 2003
EH1 and active comet 96P/Machholz (Jenniskens, 2004; Wiegert & Brown, 2005).
(EH1 is also the parent of the Quadrantids meteor shower, so their dynamical
relationship wasn’t the only clue.) Dynamically similar or linked objects have
likely experienced similar thermal conditions over the past few thousand years
due to their similar orbits, and if they are genetically related (e.g.
descended from a common progenitor object), one can assume that their bulk
compositions are quite similar as well. The problem is then to constrain which
scenarios can explain their modern differences so as to better understand how
comets age and turn off more directly than would otherwise be possible. When
did they split, and why are they so different now? Studies of dormant-and-not-
dormant comet pairs or groups are thus of great interest to those studying the
life cycles of comets in particular and the divide between comets and
asteroids in general.

The close approach of comet 252P/LINEAR (hereafter just ’252P’) to the Earth
on March 21st, 2016 was followed closely $\sim$ 26 hours later by the approach
of P/2016 BA14 (PANSTARRS, hereafter just ’BA14’) on the 22nd. BA14 had only
been discovered in late January of that year (Linder et al., 2016) as an
apparently inactive Near-Earth Object (NEO) on the orbit of a Jupiter Family
Comet extremely similar to that of 252P. Observations of BA14 prior to the
close approach revealed that it too was undergoing cometary mass-loss (first
reported by Knight et al. 2016), and it was subsequently given the cometary P/
designation completing its modern name (Naves et al., 2016). Post-encounter,
252P has a perihelion distance of $q=0.996$ AU and an eccentricity of
$e=0.673$, while BA14 has $q=1.008$ AU and $e=0.666$.

The two comets, despite their similar orbits, appear to be quite different. Li
et al. (2017) characterized 252P with the Hubble Space Telescope and both 252P
and BA14 with the Lowell Discovery Telescope (LDT, then named the Discovery
Channel Telescope) during and around their close approach, and found that
while 252P is small ($D\sim 0.6$km) and was very active for its size (a water-
derived active fraction of 40 to more than 100%), BA14’s active fraction was
on the order of $\sim 0.01\%$ or so, or a factor of $\sim 10^{4}$ less,
despite its larger nucleus. Li et al. (2017) estimated BA14’s nucleus to be
approximately $D\sim 1$km in size based on its JPL Horizon’s reported absolute
magnitude of $H_{V}=19.2$ and an assumed albedo of $4\%$. Radar observations
presented by Naidu et al. (2016) found the size of the object to likely be
more than a kilometer in diameter (implying an optical albedo of $\sim 0.03$
or less) and an extremely slow rotation period of $\sim 40$ hours with hints
of an angular and blocky surface. While Hyland et al. (2019) did not detect
gas emission from BA14, Li et al. (2017) did detect CN emission on April 17th,
2016 consistent with a production rate of $Q(CN)=(1.4\pm 0.1)\times
10^{22}mol/s$. The visible reflectivity of BA14’s nucleus or coma is minimally
constrained, with Li et al. (2017) only observing BA14 in a single non-gas
filter (r’) and Hyland et al. (2019) obtaining a red slope beyond $\sim
0.58\mu{m}$. Their results hinted at a possible blue slope at shorter
wavelengths ($<0.54\mu{m}$) but calibration issues hampered their ability to
ascertain how reliable that measurement was.

Perhaps most relevant to the current study is the recent work of Ootsubo et
al. (2021) which presented and analyzed mid-infrared observations of BA14 just
prior to its close approach. Those authors argued that their observations were
dominated by emission from BA14’s nucleus as opposed to from grains in its
coma. The direct detection of cometary nuclei during close approaches is
possible due to the larger apparent area of the sky that the coma is spread
over, which in this case would be assisted by the overall minimal activity of
the object. A similar scenario of somewhat-unplanned direct mid-infrared
observations of the nucleus of a comet during close approach is described in
the seminal Hanner et al. 1985. Ootsubo et al. (2021) present mid-infrared
photometry and low-resolution spectra of BA14 that do not look similar to
typical mid-infrared observatons of cometary comae (see, e.g., Kelley & Wooden
2009) or the nuclei of more active comets observed at larger heliocentric
distances (see, e.g., Kelley et al. 2017). Instead of an emission spectrum
dominated by emission features from silicates (perhaps most notably the
10-$\mu{m}$ excess), the retrieved spectrum appears dominated instead by
absorption features due to phyllosilicates and organics. While Lisse et al.
(2006) did find evidence for phyllosilicates in the ejecta of the Deep Impact
experiment (A’Hearn et al., 2005), the dominant materials (by areal fraction
in the retrieved spectral modeling, at least) were that of amorphous and
crystalline silicates. Ootsubo et al. (2021) argue that if they did indeed
observe the nucleus of BA14, then a phyllosilicate-rich surface might be
common surface composition for comets, or at least those on the verge of going
dormant. Considering the lack of information about the surface of BA14 at the
wavelengths where comet nuclei are increasingly able to be studied (visible
and near-infared), it is challenging to ascertain which of these scenarios is
more likely, or if BA14 is an outlier for a reason yet to be considered.

We thus have one primary question we aim to address in this study and a second
that will require additional work at a latter date. Primarily, what is the
actual nature of the surface of BA14 and how could it inform our understanding
of the surfaces of low-activity comets and how comets go dormant in general?
Secondarily, what is the relationship between BA14 and 252P, and what would
explain their differences if they are genetically related? We do not present
new observations of 252P or address this question in great detail in this
work, but highlight part of the path forward at the end of the Section 6. It
is particularly critical to address these questions now, as BA14 has recently
been selected as a potential backup target for the ESA’s Comet Interceptor
mission (Snodgrass & Jones, 2019; Schwamb et al., 2020). It is critical to
understand these two objects in as much detail now such that the best-informed
choice can be made nearer to the end of the decade when the Comet Interceptor
team is choosing a target.

In this work, we present low-resolution near-infared ($0.7-2.5\mu{m}$) spectra
of BA14 taken with SpeX (Rayner et al., 2003) on the NASA Infrared Telescope
Facility (IRTF) from approximately 11 and 14 days prior to the observations of
Ootsubo et al. (2021) to attempt to diagnose the surface reflectivity and
albedo of this object and place it in context among the broader population of
comets whose nuclei have been studied directly. In Section 2, we present a
journal of observations and describe our data reduction procedure. In Section
3, we present and describe the reflectance spectra we obtained of BA14 on
March 7th and 10th UTC and make comparisons to other comets observed at the
same wavelengths. We also present spatial profiles of BA14 at visible and
near-infrared wavelengths for context about any possible coma contamination.
In Section 4, we describe and utilize our efforts to remove the thermal excess
from the longer wavelength portions of our spectrum to analyze the underlying
reflectance there and to constrain its albedo. Finally, in Section 5, we
synthesize all of these results and those in the literature to better
understand how to interpret the properties of P/2016 BA14 (PanSTARRS) and
address what potential spacecraft exploration could do for the target.

##  2 Observations

P/2016 BA14 (PanSTARRS) was observed on March 7 and March 10 2016 UTC with the
SpeX instrument (Rayner et al., 2003) on the NASA Infrared Telescope Facility
(IRTF) as part of an observational program centered on time-sensitive
observations of small and/or close-approaching NEOs. The observational details
are summarized in Table 2.1. All observations were obtained with the low-
resolution ’Prism’ mode on SpeX, providing simultaneous coverage from
$0.7-2.5\mu{m}$ at an effective resolution of $R\sim 100$ with a 0.8”-wide
slit. Observations of the target were ’bookended’ by observations of a G-type
star nearby on the sky at similar airmass for proper telluric correction, and
observations of a proper solar analog star (SAO 93936 on both nights) was
later observed near 1.0 airmass for further slope correction of the final
output spectrum. Observations of the target and calibration stars were all
obtained at the local parallactic angle to mitigate wavelength-dependent slit
losses. The observational circumstances were such that all observations of
BA14 were between airmasses of $1.55-1.70$. The reduction, extraction, and
telluric correction of the spectra were accomplished using the IDL-based
’spextool’ package (Cushing et al., 2004) using the default (optimal) settings
for an unresolved point-source, while the final solar analog correction was
accomplished using a custom-written script in Python.

###  2.1 Comparison of Visible and Near-IR PSFs

While the reduction was completed shortly after observations, the SpeX
observations are (thankfully) available on the IRTF Legacy
Archive111http://irtfdata.ifa.hawaii.edu/search/, and we present a comparison
of the PSFs of local telluric star observed before and after observations of
the target were taken to assess whether or not any dust coma might be visible
in Figure 1. The data shown are from 2016 March 7, where seeing was much
better ($\sim 0.51\arcsec$ at visible wavelengths as measured by the visible
wavelength guide camera). The shown spatial profiles were derived by combining
the ”A” and ”B” profiles after doing an A-B subtraction on paired sequetial
frames (labeled as a ”folded spatial profile” in Figure 1 x-axis as it
effectively only shows half the slit due to their combination), such that the
shown BA14 profile is the aligned-and-stacked average of all 12 200-second
exposures on that night. The spatial profiles of the star are from single
combined A-B pairs due to the much higher SNR on the brighter calibration
star. The profiles were extracted at wavelengths corresponding to the
J-filter, or between 1.17 and 1.33 $\mu{m}$. The Full-Width Half-Max (FWHM) of
a best-fit Gaussian profile to the stars retrieved FWHM$=0.28\arcsec\pm
0.01\arcsec$ before and FWHM$=0.28\arcsec\pm 0.01\arcsec$ after observations
of the BA14, which was measured to have FWHM$=0.39\arcsec\pm 0.01\arcsec$.
Performing the same analyses at H or K band retrieved the same FWHMs for the
stars ($0.26-0.28\arcsec$) and comet ($0.38-0.39\arcsec$) as for J to within
$\sim 1.0\sigma$.

The extended PSF of BA14 in our IRTF observations could be due to an actual
detection of its coma in the near-infrared (despite the generally much lower
visibility of typically sized cometary dust at those wavelengths) or could be
due to slight guiding imperfections that might not be noticed under ordinary
seeing conditions. An object being tracked at non-sidereal rates
($1.4\arcsec/min$ on the 7th) with long exposure times will inevitably lead to
some drift within the slit over the course of the exposures; both of these
issues do not apply to the sidereally tracked short exposures on the nearby
standard stars. If the detection is indicative of the coma’s size, given the
geocentric distance of $\Delta=0.125AU$, we estimate the dust coma BA14 to
have a FWHM of $\sim 35-36$ kilometers, and a maximum detectable extent of
$\sim 240$ kilometers at J-band on March 7th UTC. If the coma was the same
size on March 10, we estimate it’s FWHM to have been $\sim 0.49\arcsec$,
though we note that this is considerably smaller than the seeing limit on that
date.

To assess the appearance of the coma at optical wavelengths around the same
time, we queried the Mission Accessible Near-Earth Object Survey (MANOS)’s
database and found visible-wavelength imaging observations from the SOAR
telescope and Lowell Discovery Telescope (LDT) on 2016 February 21 and 22,
respectively, when the heliocentric and geocentric distances of BA14 were
$R_{H}=1.055-1.059AU$ and $\Delta=0.229-0.237AU$. The SOAR observations
utilized the Goodman High-Throughput Spectrograph (Clemens et al., 2004) in
imaging mode with the Sloan r filter, while the LDT observations utilized the
Large Monolithic Imager (LMI, Massey et al. 2013) with a ”VR” filter. The SOAR
observations consisted of one 10 second exposure and five 60 second exposures,
while the LDT observations were six 60 second exposures. We applied standard
corrections to each set of images (debiasing, flattening) and then calibrated
them astrometrically and photometrically using the Photometry Pipeline
(Mommert, 2017), with the photmetric calibration being derived exclusively
from Sun-like stars within the fields of view. (The ”VR” filter was calibrated
as if it was a Sloan r filter due to their similar central wavelengths.)

In the LDT observations, the FWHM of BA14 was $0.95\pm 0.02\arcsec$ along the
direction perpendicular to its apparent motion and $1.07\pm 0.02$ along the
direction of its motion compared to a uniform $0.91\pm 0.01$ for nearby field
stars. For the SOAR observations, the cross track profile has $0.51\pm
0.01\arcsec$, the along-track profile has $0.59\pm 0.02$, and nearby stars
have $0.49\pm 0.01\arcsec$. The images were all taken with exposure times
short enough to prevent the along-track extension being due to trailing. These
quoted numbers are measured from the composite images composed of the
individual frames stacked at the object’s non-sidereal rates, but the same
results were found through inspection of individual frames as well. Through
these analyses and visual inspection of all frames individually and stacked,
it seems rather firm that we detect BA14’s small coma in these images. The
magnitude of BA14 was $m_{r}=18.71\pm 0.02$ on Feb. 21 and $m_{VR-r}=18.64\pm
0.02$.

The visible coma of BA14 was two-to-three times the size of the NIR spatial
profile at twice the geocentric distance and when the comet was slightly
further from the Sun and thus presumably slightly less active. If any of the
slightly-larger-than-stellar spatial profile in our IRTF observations is from
reflected or thermally emitted light from the coma of BA14, it is sampling a
small areal fraction of an already small coma at wavelengths where dust should
be less prominent. In the Section 3, we discuss the very limited possible coma
contamination of our NIR spectra.

Table 1: Summary of Infrared Observational Data

UT Dateaa UTC at start of sequence. | UTC Timeaa UTC at start of sequence. | $R_{H}$ | $\Delta$ | $\alpha$ | Seeing | RHbbfootnotemark:  | Total Time
---|---|---|---|---|---|---|---
(YYYY-mm-dd) | (d) | (au) | (au) | $(^{\circ})$ | (arcsec) | % | seconds
2016-03-07 | 05:26 | 1.015 | 0.125 | 77.0 | $\sim 0.51\arcsec$ |  $\sim$ 22 | 2400.0
2016-03-10 | 05:12 | 1.011 | 0.102 | 76.1 | ∼1.5″ |  $\sim$ 6 | 2400.0

bbfootnotemark:

Relative Humidity as measured at the IRTF at the start of sequence.

Figure 1: The along-slit spatial profiles are shown for P/2016 BA14
(PanSTARRS) as well as the telluric star observed immediately before and
afterwards as obtained on 2016 March 7. The shown profiles are medians over
the wavelengths corresponding to J-band ($1.17-1.33\mu{m}$). The visible
seeing was quite good ($\sim 0.51\arcsec$) and the resulting profiles are
quite narrow, but the profile of P/2016 BA14 is still significantly wider than
the star was before or after. Worse seeing (such as that obtained on 2016
March 10) would not have shown this clear hint for activity, however weak it
might be. See text for more details.

##  3 BA14 in the NIR

###  3.1 Retrieved Reflectance Spectra

The retrieved reflectance spectra of P/2016 BA14 (PanSTARRS) are shown in
Figure 2. The spectra obtained are similar on both dates, with an overall red-
slope becoming more neutral with wavelength and an upturn in reflectivity at
wavelengths longer than $\sim 2.25\mu{m}$ that is likely excess thermal
emission ”on top” of the reflected light from the solid coma and nucelus.
However, the spectrum obtained on March 10 is noticably redder at shorter
wavelengths becoming similar to the March 7 observations by $\sim$H-band, the
possible slope-break near $1.4-1.5\mu{m}$ appears somewhat sharper, and the
region $\lesssim 1.35$ appears slightly more linear. Both stars were slope-
calibrated to the same well-studied solar analog (SAO 93936) and inspection of
the ratios of the spectra of each star’s local G-type telluric would not have
introduced the change either.We also note that any wavelength-dependent slit
losses would be smooth functions of wavelength, unlike the difference in the
retrieved spectra, so we think this an unlikely cause of the discrepancy as
well – especially considering that our observations were all at the local
parallactic angle to mitigate just this effect. Considering that conditions on
both nights were rather good (low and stable humidity), the phase angle did
not change appreciably, and the target was observed at similar airmass on both
nights, it seems like the difference is real, or at least mostly so.

Figure 2: The retrieved reflectance spectra of P/2016 BA14 (PanSTARRS) as
obtained on 2016 March 7 and 10 from the NASA IRTF, normalized such that
$R(\lambda=1.5\mu{m})=1.0$. The spectra on both nights are similarly red-
sloped with a thermal excess visible at longer wavelengths ($>2.25\mu{m}$, and
the spectrum obtained on March 10th is slightly redder at all wavelengths with
a slight slope-break near $1.4-1.5\mu{m}$ and a more linear spectrum at
shorter wavelengths. See text for more details.

To quantify the slopes of the two spectra, we use the $S^{\prime}$ framework
of Luu & Jewitt (1990) and report slopes for the J, H, and K spectral ranges
in units of $\%/0.1\mu m$ for both nights. For the March 7 data, we find
$S_{J}^{\prime}=4.6\pm 0.8$, $S_{H}^{\prime}=3.4\pm 0.7$, and
$S_{K*}^{\prime}=4.3\pm 1.5$. For March 10, we find $S_{J}^{\prime}=7.7\pm
0.3$, $S_{H}^{\prime}=3.0\pm 0.3$, and $S_{K*}^{\prime}=5.9\pm 0.6$. The
K-slopes are contaminated from the thermal emission, and we report slopes
measured over the range ($1.95-2.20\mu{m}$) to attempt to mitigate this. At
wavelengths away from the thermal excess, the slopes of the two spectra agree
at the $1-\sigma$ level at H-band, but March 10 is significantly redder at
J-band as expected.

Furthermore, it is worth stating explicitly that our spectra are almost
certainly quite phase-reddened (Sanchez et al., 2012), meaning that the
retrieved reflectance spectra appear redder than they would if observed at a
lower phase angle. However, estimating the magnitude of this effect is
challenging as we don’t know the phase function of the nucleus or the dust
coma, the relative contribution of either to the reflectance spectrum, or how
that would vary with phase angle itself.

###  3.2 Did we observe the nucleus?

Perhaps the most obvious factor to consider, given the claimed direct
observation of the nucleus by Ootsubo et al. (2021), would be the $\sim 20\%$
drop in geocentric distance between the two dates from $\Delta=0.125AU$ on
March 7 to $\Delta=0.102AU$ on March 10th. One would expect, a priori, that if
the coma was more spread out on the sky then the brightness of the nucleus
might dominate more in the later, closer observations. Making a quantitative
prediction of how much fainter a dust coma might be more straightforward at
larger geocentric distances, but for the present case is likely to remain
fairly speculative. Given the rather long rotation period ($P\sim 40$ hours,
Naidu et al. 2016) and the small scales probed, it seems reasonable to assume
that the innermost coma of BA14 is highly anisotropic unless the nucleus’s
surface is uniformly active, which seems rather unlikely given its extremely
low active fraction (Li et al., 2017). The visible light curve of the object
presented in (Li et al., 2017) shows that the object’s overall brightness
trend is dominated by the nuclear signal, though this does not necessarily
preclude some contamination.

The slight change in spectral behavior between the two observations also
provides insight into the issue. The spectrum obtained on 2016 March 10, when
coma contamination should have (in principle) be lower and the nucleus more
prominent, shows a slope-break near $1.4-1.5\mu{m}$. A population of small
grains comparable to the wavelengths of note (as expected for cometary comae)
reflects light according to Mie theory, which should produce reflectance
spectra that do not have discontinuities or sudden slope changes with
wavelength but instead more gradual continuous ones. By contrast, light
reflected by solid surfaces or by particles much larger than the wavelength of
light in question, which are out of the Mie regime and better modeled with
Hapke photometric theory (Hapke, 1993), can produce such sudden slope breaks.
In fact, Sharkey et al. (2019) found a slope-break at a nearly-identical
wavelength on the Jupiter Trojan and Lucy (Levison & Lucy Science Team, 2016)
mission target (21900) Orus. A review of the limited number of nuclear spectra
of confirmed low-activity or dormant comets in DeMeo & Binzel (2008) and
Kareta et al. (2021) show that some but not all comets in those studies have
slope breaks in the same wavelength region. Furthermore, the change in slope
is very apparent on comet 67P/Churyumov-Gerasimenko (Filacchione et al.,
2019). As to whether or not the slope-break derives from large grains or the
surface of the nucleus itself, a nucleus with as low an active fraction as
BA14 ($\sim 0.01\%$, Li et al. 2017) would make ejection of large grains in
large number physically challenging. Furthermore, if the shorter wavelength
section of March 7 dataset is more curved and slightly less red than the March
10 spectrum but the two spectra converge at longer wavelengths, one possible
explanation is that the shorter wavelengths of the March 7 reflectance
spectrum contain some light from grains similar in size to the wavelengths
being studied here – e.g., $\mu{m}$-sized, within the Mie regime and outside
of the Hapke regime. We thus conclude that the most likely scenario to explain
our observations is that the nucleus is a majority of the reflected light
contained in the spectra collected on March 7 and March 10, and that the March
10 spectrum is likely even more dominated by the nucleus itself.

##  4 Thermal Excess Modeling

###  4.1 Modeling Description and Initial Results

Understanding, modeling, and correcting for the exponential-like upturn at
wavelengths longer than $\sim 2.25\mu{m}$ in spectra of low-albedo NEOs is
critical to both understanding their reflectivities at those wavelengths as
well as understand the thermal properties of their surface and near-surface.
The go-to method for these kinds of studies is the Near-Earth Asteroid Thermal
Model (NEATM, Harris 1998), a modification of the earlier Standard Thermal
Model (Lebofsky et al., 1986) to better suit observations of small NEOs. The
specifics of applying NEATM to thermally-contaminated reflectance spectra at
these and similar wavelengths is outlined in Rivkin et al. (2005) and Reddy et
al. (2009). NEATM (and STM, for that matter) assume that all thermal emission
from the object in question is blackbody emission from the dayside of the
object. The surface temperature at a particular point is calculated as:

$T(\theta,\phi)=[\frac{(1-A)\times S_{\odot}}{\eta\epsilon\sigma\times
R_{H}^{2}}\times cos(\theta)\times cos(\phi)]^{1/4}$ (1)

where $A$ is the Bond Albedo of the object (which in turn is a function of
$p_{V}$), $S_{\odot}$ is the Solar Constant (1370 $Wm^{-2}$ at $R_{H}=1$ AU),
$\eta$ is the infrared beaming parameter, $\epsilon$ is the (bulk) emissivity
of the object (assumed to be the canonical $0.9$), $\sigma$ is the Stefan-
Boltzmann Constant, and $R_{H}$ is the heliocentric distance of the object at
the time of observation in AU. $\theta$ and $\phi$ are the angular coordinates
on the body itself moving away from the subsolar point, such that the
temperature of the object is modeled as $T=0$ at the terminator and on the
whole of the nightside. The infrared beaming parameter $\eta$ is essentially
the ’fudge factor’ that allows the model to overcome its rather large
assumptions. A body with ’normal’ (e.g. regolith covered and moderate-to-slow
rotation speed around its prinicpal axis) thermal parameters which observed at
rather small phase angle should have a value of $\eta\sim 0.9-1.1$ – only a
small deviation from a traditional greybody, but at larger phase angles or
with less common thermal states (less regolith, non-principal axis rotation,
etc.), the value of $\eta$ should get increasingly large, up to a
(theoretical) maximum near $\pi$. While a more detailed description of the
model and its serious limitations in this capacity is included in Kareta et
al. (2018), we note that as the model is really constraining $T_{SS}$, the fit
parameters $p_{V}$ and $\eta$ are expected to be somewhat degenerate.

In order to account for this degeneracy and better understand the errors on
our fit parameters, we update the model of Kareta et al. (2018) by changing it
from a least-squares fitting technique into a Markov-Chain Monte Carlo
technique using the package emcee (Foreman-Mackey et al., 2013). The best-fit
from the least-squares technique is used as an initial guess for the
distribution of walkers in emcee. We utilized 32 walkers and 2000 iterations
as an initial attempt due to the low amount of parameters and length it took
for each iteration. The results are presented in Figure 3.

Figure 3: Left: The Maximum Likelihood Thermal Model, with $p_{V}=0.014$ and
$\eta=1.627$, or $p_{V}=0.023$ and $\eta=1.622$ after a correction for phase
reddening), is shown as a dark red line, while the uncorrected data is shown
as black unfilled circles. The thermal-model-corrected all-reflectance
spectrum is shown as blue unfilled circles. Right: A corner plot shows the
distribution of fit parameter values for $p_{V}$ and $\eta$ for both models.
The albedo constraint from radar observations (Naidu et al., 2016) is plotted
as a vertical line. As expected, the albedo and beaming parameters show some
degeneracy, as the model constrains the peak subsolar temperature which is a
function of both parameters. (See text for more details.)

The fit parameters are constrained to be in the range of
$p_{V}=0.016_{-0.011}^{+0.049}$ and $\eta=1.560_{-0.441}^{+0.424}$ at the
99.7$\%$ confidence level, with the Maximum-Likelihood values being
$p_{V}=0.0135$ and $\eta=1.627$. The shapes of the distributions of the
parameters are as expected, with the bottom left panel of the corner plot (the
right side of Figure 2) showing a curve. Essentially, this is the curve of
constant peak subsolar temperature. While the marginalized distribution of
$\eta$ values is symmetric about its approximate maximum likelihood value, the
distrubtion of $p_{V}$ values is highly skewed, with the maximum likelihood
value being on the absolute lowest end of the confidence interval.

These values for the infared beaming parameter $\eta$ are quite reasonable for
a typical NEO, in fact almost exactly what would be estimated from the Delbó
et al. (2003) empirical relationship between phase angle and beaming
parameter. The range of allowed visible albedos are all really quite low – if
taken at face value, it would make BA14 among the darkest objects in the Solar
System. The radar data do mandate that the object’s albedo has to be below
3$\%$, so our range of values do satisfy the one pre-existing constraint. The
distribution for NEOs as found by the all-cryogenic phase of the WISE mission
(Wright et al., 2016) finds that approximately $5\%$ of objects have geometric
albedos of $0.02$ or less, so our nominal range of values place the albedo of
$BA_{14}$ in the ’quite rare but not implausible’ range. The radar albedo
upper limit and an absolute magnitude of $H_{V}=19.2$ result in a diameter
estimate of $D\sim 1.1$ km, and our maximum-likelihood albedo of
$p_{V}=0.0135$ results in a diameter of $D\sim 1.6$ km. In the next two sub-
sections, we discuss complications to this albedo estimate that would likely
result in it being a slight under-estimate, and thus our estimated diameters
being a slight over-estimate.

###  4.2 Phase-Reddening Complications and Correction

In order to estimate the ratio $f_{IR}=p_{IR}/p_{V}$, we extrapolated
downwards to $0.55\mu{m}$ by way of a linear fit to all datapoints short of
$0.9\mu{m}$. This is the data-driven way to estimate $f_{IR}$, but is
susceptible to over-or-under-estimating the actual ratio should there be
slope-changes in the visible. A linear extrapolation of the March 10 data
results in $f_{IR}=3.24$. In many cases, one can check this estimate by
comparing the retrieved value against typical values for the taxonomic class
of the object, most commonly the Bus-Demeo System (DeMeo et al., 2009), but
our spectrum is even redder than the very-red A and D types (before phase
reddening is accounted for, at the very least.) For context, the low-activity
Halley-Type Comet P/2006 HR30 (Siding Spring) had $f_{IR}\sim 1.92$ (Kareta et
al., 2021) and the more active Jupiter Family Comet 67P/Churyumov-Gerasimenko
had $\sim 2.0$ depending on which area of the surface was studied (Quirico et
al., 2016). The retrieved spectrum of $BA_{14}$ itself is almost certainly
significantly phase-reddened, which we first highlighted in Section 3. The
reflectance spectra of Solar System objects appear redder at larger phase
angles than they would at zero-phase (e.g. opposition). While the magnitude of
this change per degree can be quite small (often smaller than the other
measurement uncertainties at hand), at the large phase angles sometimes
occupied by NEOs, it can be critical to account for it to estimate what the
’real’ spectrum looks like.

In the case of phase-reddened and hot NEOs, the thermal emission at longer
wavelengths is being added onto a reddened underlying continuum, making
guesses at the underlying reflectivity at longer wavelengths challenging.
Phase reddening is not constant with wavelength, so there is some question of
how much of the thermal upturn is actual thermal emission and how much is the
phase-induced redness of the object underneath the thermal photons. There has
been some work done to correct NEATM’s imperfections at larger phase angles
(e.g. Mommert et al. 2018 calculates corrections to NEATM for high phase
angles, but their work assumes all light being collected is flux-calibrated
observations at longer wavelengths than studied here), but we are not aware of
a pre-existing prescription in the literature for dealing with this particular
issue.

Given the lack of additional information, we attempt to ”undo” the phase
reddening’s impact on our thermal modeling by re-running our MCMC modeling
routine with a lower $f_{IR}=1.92$, implicitly assuming that BA14’s surface is
similarly red (measured in bulk from the visible to K-band) at zero phase
angle to P/2006 HR30 (Siding Spring)’s. These results are plotted in red on
Figure 3. The distribution of $\eta$ is essentially unchanged
($\eta=1.553_{-0.330}^{+0.409}$), but the distribution of visible geometric
albedo $p_{V}$ is shifted higher ($p_{V}=0.028_{-0.018}^{+0.046}$) by
approximately the ratio of $f_{IR}$ values ($3.24/1.92\sim 1.7\sim
0.28/0.016$). This is the expected behavior for small enough changes in
$f_{IR}$: the actual thermal model can fit the data just as well (in this
case, it overlaps nearly exactly), it is the distribution of plausible visible
albedos that is modified.

So, which distribution of albedos is more ”likely” for this object? There is
no obvious answer without a complementary visible spectrum or broadband color
measurement or phase reddening study of this object. It seems likely that
BA14’s visible colors are similar to that of HR30 or 67P, and it seems
inevitable that BA14’s spectrum is at least somewhat phase reddened, so some
correction is definitely in order, but the ”correct” magnitude of such a
correction is unknowable at present. In any case, the quality of the data
doesn’t allow an obvious discrimination between such small changes in albedo
anyways. The albedo of BA14 is almost certainly between $0.01-0.03$, but
precision beyond that would require higher signal to noise thermal
observations at longer wavelengths – or, of course, a visit by Comet
Interceptor. More constraints from other techniques would not allow us to
constrain the albedo better, but it would allow us to model the thermal
emission more accurately and better understand the underlying reflectance
spectrum with more confidence.

In summary, our thermal models of BA14’s thermal emission naturally agree with
the $p_{V}\leq 0.03$ of Naidu et al. (2016) regardless of a phase-reddening
correction, the distributions of beaming parameters $\eta$ seem entirely
typical for a NEO observed in this geometry, and the underlying spectrum
appears to be featureless and linear from $1.4\mu{m}$ onwards like many other
comets, including 67P.

###  4.3 Possible Thermal Contamination By Dust

Despite BA14’s thin-and-small coma, the dust grains that are there would still
be quite warm at $R_{H}=1.01$ AU, suggesting that some of the thermal emission
that we model as coming from the surface of the object could be coming from
the coma instead. If the coma of BA14 is composed at least partially of rather
small grains only a few $\mu{m}$ in size – which seems natural for an object
with such weak activity – the grains in the coma might appears ”hotter” than
the nucleus as they cannot effectively emit at wavelengths larger than their
physical size (see, e.g., Sitko et al. 2004). This process, often called
”superheating”, would further bias the retrieved thermal fits towards higher
temperatures and thus lower albedos and beaming parameters. However, BA14’s
activity is quite weak, the quality of the fits are rather good, and the
retrieved range of values for the infrared beaming parameter $\eta$ are quite
reasonable for a small NEO. If there were significant ’extra’ thermal
emission, then the shape of the blackbody would change and become steeper –
the fact that we were able to fit it reasonably implies that the subsolar
temperature our reflected-light-and-thermal models used was reasonable to
correct the data assuming that the thermal emission and reflected light were
coming from the same source. In principle, we might be unable to differentiate
our all-nucleus model from a stranger situation, such as one with a higher
albedo nucleus and a darker albedo dust that balance such that their composite
thermal emission looks like the curve we’ve modeled here. However, we would
expect the dust coming off the nucleus’s surface to look similar to the
surface itself, and thus we argue that, again, this is likely not a huge
factor for this dataset.222We remind the reader here that there are
shortcomings of NEATM in general, summarized in Kareta et al. (2018) and
elsewhere. We also note that if the contamination by dust was a large factor,
it seems likely that our model would not be able to satisfy the albedo
constraint imposed by the radar albedo. As with phase reddening, this might
result in a slight under-estimate of the true albedo of BA14 if it were
significant.

##  5 Discussion

The first reason that we were interested in re-examining our spectra of P/2016
BA14 (PanSTARRS) was to compare with the recent work of Ootsubo et al. (2021).
If BA14 looked strange or anomalous at mid-infrared wavelengths, what would it
look like at other wavelengths? Do the reported mid-IR phyllosilicate and
organic absorption features reported for BA14’s nucleus manifest with any
signatures in the near-IR? If BA14 looks like a ’typical’ comet using these
other approaches, how does one interpret the general applicability of the
Ootsubo et al. (2021) result? The second reason was the selection of BA14 as a
back-up target for Comet Interceptor (Schwamb et al., 2020) should a suitable
Long Period Comet (or Interstellar Object!) not be found in time. In this
section, we first compare and contextualize our results with those of Ootsubo
et al. (2021) and then discuss its viability as a spacecraft mission target
afterwards.

###  5.1 Comparison to the Mid-Infrared

The reflectance spectra we obtained of BA14 in this work on both nights appear
very much ’typical’ for comets observed in the near-infrared when phase-
reddening is considered. We interpreted our observations in Section 3 as both
being primarily dominated by light from the nucleus itself, with the March 10
spectrum being even more so. The different spectral behaviors on the two
nights are consistent with a thin dust coma around the object object composed
of $\mu{m}$-sized grains, which is what would be expected for such a weakly
active object (Li et al., 2017). Our thermal model estimates the visible-
wavelength albedo of BA14 to be quite low at $p_{V}=0.016_{-0.011}^{+0.049}$
at the $99.7\%$ formal confidence interval, though the large phase angle and
continuing low-level dust contamination both likely result in this being a
slight under-estimate of the albedo of the nucleus. A simple phase-reddening
correction results in a slightly higher modeled albedo distribution of
$p_{V}=0.028_{-0.018}^{+0.046}$ ($99.7\%$ confidence interval). Radar data
from that apparition (Naidu et al., 2016) suggest that the albedo has to be
below $0.03$, and there are cometary nuclei whose albedos are within this
range, like 19P/Borrelly (Soderblom et al., 2002) or 103P/Hartley 2 (Lisse et
al., 2009). If our albedo estimates are accurate, then we estimate a diameter
between $1.1-1.6$ km, again consistent with the current understanding of the
radar data, and very much a typical size for a Jupiter Family Comet nucleus
(Bauer et al., 2017). In the unlikely case that the object’s albedo really is
on the low end of our modeled albedo distributions, the diameter of $BA_{14}$
could be up to $\sim 2$ km across. The infrared beaming parameter modeled here
($\eta=1.560_{-0.441}^{+0.424}$) is a typical range of values for an NEO
observed at similarly high phase angles as is the case in this study (Delbó et
al., 2003). In summary, BA14 very much appears to have the properties expected
for a normal comet at these wavelengths, though it might be of slightly lower
albedo than many cometary nuclei (pending the previous discussions of phase
reddening and dust, of course). The only truly rare traits it seems to have
are its low activity levels, its slow rotation state, and its anomalous
appearance at mid-infrared wavelengths.

While the mid-infrared spectrum of BA14 suggests a surface incorporating more
processed substances like phyllosilicates, the near-infrared nuclear spectrum
does not appear different than other comets expected to have a more typical
composition. Ootsubo et al. (2021) attribute several features that they cannot
identify to being from a matrix of organics, which is thought to be a dominant
surface material in comets like 67P/C-G and others (Filacchione et al., 2019).
One possibility is simply that the matrix of complex organics is the dominant
factor in determining the visible and near-infrared spectral properties of the
nucleus (considering that cometary nuclei are dark and red at those
wavelengths, this seems plausible) but the balance changes as one observes at
longer and longer wavelengths. Could BA14 have a different surface composition
compared to other objects, even if NIR spectroscopy cannot discern it? Ootsubo
et al. (2021) argue based on the apparent similarity of their spectrum to
heated micrometeorites that the difference is evolutionary and likely driven
by previous high-temperature conditions brought on by a past orbit with a
lower perihelion. However, Kareta et al. (2021) found some evidence that
cometary objects should have spectral changes in the visible/near-infrared
range if they had been heated significantly, so the fact that BA14 looks like
a ’typical’ comet in the NIR does not necessarily support a heating origin,
but more data are needed on these kinds of objects to say with any more
certainty. A future detailed study of BA14’s and 252P’s dynamical evolution is
certainly warranted, both to understand their mutual dynamical relationship
should it be robust and to better understand BA14’s past thermal state to
assess the viability of the hypothesis that its modern surface is thermally
altered.

Comets with decreasing activity levels (e.g. lowering gas production rates)
can only lift smaller and smaller grains with time, in principle leading to a
preferential loss of small grains from the surface. If this is a reasonable
description for the recent evolution of BA14’s surface, then it might be the
loss of these small and porous grains which might normally produce the
10$\mu{m}$ excess is what allowed the observers to see ’through’ to the
underlying material which is usually buried in the mid-IR data. The study of
Kareta et al. (2021) found that the nearly-dormant Halley-Type Comet P/2006
HR30 (Siding Spring) had retained a classic comet/D-type slope despite
whatever processes are ongoing as it became less active. That object and BA14
both have perihelia similar to or greater than 1 AU, suggesting that they’re
also in similar thermal regimes, at least in their current and recent orbits.

One key problem going forward is the lack of clear mid-IR observations of many
cometary nuclei and associated objects to compare and contextualize BA14 and
related objects in. Even low-level comae might be able to hide the kinds of
surface features that Ootsubo et al. (2021) observed, assuming that they (and
we!) were successful in actually observing the nucleus. One path forward is to
obtain more $10\mu{m}$ observations of dormant comet candidates and low-
activity comets in general to compare to and better estimate which attributes
of objects like BA14 are evolutionary and which parts are simply challenging
to observe because of coma interference (a preliminary study reported in
McAdam et al. (2018) suggests this might be quite fruitful.). The recent
(successful) launch of JWST will facilitate a generational breakthrough in our
ability to measure near- and mid-infrared reflectivities and emissivities of
the surfaces of Solar System Small Bodies. This is doubly true for cometary
surfaces, for which observations at larger heliocentric distances are usually
required to mitigate the confusing effects of dust and gas emission and
discern the properties of the surface more directly. The NIRSPEC (up to
$5\mu{m}$) and MIRI ($5.0-28.5\mu{m}$) instruments are likely to be the
instruments of choice, though the infrared imager NIRCAM might be better
suited to fainter targets. At present, we are not aware of any currently-
approved JWST programs to use these mid-infrared wavelengths to better
understand the composition of low-activity/dormant comet nuclei, but it seems
like a highly fruitful avenue for future research. Approved studies of the
Trojan asteroids and the Centaurs, both populations thought to share
significant similarities with the objects discussed here, should be highly
useful for planning future low-activity comet studies in addition to their
obvious scientific benefits. Another hopeful note is the L’TES instrument on
the Lucy mission will be able to measure the surface properties of more trojan
asteroids in more detail, which have previously been shown to have mid-
infrared spectra like that of ’typical’ (non-BA14-like) comets. Lastly, we
also note that many datasets obtained of BA14 during its 2016 close approach
have yet to be fully analyzed and could provide key insights to help better
understand how this object fits into the population of the comets in general.

###  5.2 Comet Interceptor

Among the back-up targets for Comet Interceptor listed in Schwamb et al.
(2020), BA14 is one of two low-activity targets along with 289P/Blanpain. Low
activity targets naturally allow for safer flybys with the possibility of
closer approaches to the nucleus of the chosen comet, but also would result in
less material being encountered and identified by the remote sensing
instruments as well as (presumably) a less complex plasma environment. BA14
does put off some gas (Li et al., 2017), so there is no reason to think this
object would be strictly a bare nucleus, but the focus of a spacecraft visit
would simply have to be more focused on studies of the surface of the comet.
The slow rotation of the object as determined by radar when combined with the
lower activity state could allow for higher resolution imaging and mapping of
the nucleus than many of the other targets. There are a variety of interesting
questions that could be addressed through detailed imaging and spectroscopy of
a nearly dormant comet, such as ”how does the bulk topographic roughness
evolve as comets age?” or ”can the activity history of a nearly/dormant comet
be determined through detailed mapping of its surface?” Discriminating between
surface properties (reflectance spectra, topography, thermal characteristics)
between the small fraction of the surface that is active at present and the
inactive fraction can only be accomplished through spatially resolved studies.
If an understanding can be developed of why certain areas are still active but
others aren’t (and how that might affect other observable features), then a
more general purpose understanding of how comets age can be developed in a
much more nuanced way than just assuming that their gas production rates trend
towards zero with time. The dormant comets constitute at least a few percent
of the NEO population, and understanding their material and structural
properties would thus benefit studies of similar objects from a planetary
defense point of view in addition to developing a broader understanding of
cometary life cycles. The flyby of BA14, a ’traditional’ comet that has gone
dormant, would provide a fascinating counterpoint to the upcoming flyby of
(3200) Phaethon, an extremely low activity object that is quite unlikely to be
a ’traditional’ comet as opposed to a former more active asteroid, by JAXA’s
DESTINY+ (Arai et al., 2018).

The MIRMIS instrument (Bowles et al., 2020) onboard Spacecraft A of Comet
Interceptor could produce exceptionally interesting results at characterizing
BA14. MIRMIS is a near-and-mid-IR spectrograph that would facilitate a
detailed mapping of the cometary surface and would be able to near-
simulatenously observe at the wavelengths of the data presented here and the
mid-IR observations of Ootsubo et al. (2021) in a spatially resolved way
across much of the nucleus. This could more clearly reveal what compounds are
most dominant in determining the typically featureless red spectra of cometary
nuclei at visible and near-infrared wavelengths through detecting their
absorption features directly throughout the mid-IR as in Ootsubo et al.
(2021). The sensitivity of the instrument to multiple absorption features of
water ice would also allow a more direct and sensitive measurement of its
modern near-surface volatile content than would be capable from the ground. A
comparison between the thermal stability of the surface materials, their
distribution across the surface, and the orbital history of the object could
then also directly assess how each of those materials responds to heating and
thermal degradation once the cooling effect of sublimation is minimized. This
would be a more direct test of Ootsubo et al. (2021)’s suggestion that BA14’s
nucleus shows signs of past more intense heating than we were able to achieve
with our observations.

##  6 Summary

We observed the low-activity Jupiter Family Comet P/2016 BA14 (PanSTARRS) on
March 7 and March 10, 2016 using SpeX on the NASA IRTF some days before its
closest approach to the Earth later that month as part of an ongoing program
to obtain reflectance spectra of close-approaching and notable NEOs. We were
prompted to re-reduce and re-inspect the dataset after the recent publication
of Ootsubo et al. (2021), which found BA14 to have an atypical spectrum at
mid-infrared wavelengths compared to other comets. In particular, they
interpret their observations as being directly of the nucleus of the object
and for it to be covered primarily by phyllosilicates, as opposed to anhydrous
materials more commonly found in cometary dust comae. The selection of BA14 as
a backup target for the ESA’s Comet Interceptor (Schwamb et al., 2020)
provides another pressing reason to reinvestigate this intriguing object now.
In particular, we sought to infer whether or not our NIR observations also
probed the properties of the nucleus and how it compared to other comets
observed in the NIR in general. We found:

  * •

The reflectance spectrum of BA14 on both nights is steeply red and shows a
significant thermal upturn at longer wavelengths. The March 7 spectrum,
observed at a geocentric distance of $\Delta=0.125$ AU, has a more slowly
varying spectrum, while the March 10 spectrum ($\Delta=0.102$ AU) has a
sharper change in slope near $\sim 1.4-1.5\mu{m}$. We interpret both spectra
as being nucleus dominated through studies of their spatial profiles compared
to visible observations from the same apparition. The latter spectrum is
likely even further dominated by the nucleus due to the lower surface
brightness of the coma being spread out over a larger area of the sky and the
challenges with producing such a spectrum from a population of grains as
opposed to a solid surface. The slope break at $1.4-1.5\mu{m}$ is spectrally
similar to 67P (Quirico et al., 2016), but at a longer wavelength to the slope
break on comets like P/2006 HR30 (Siding Spring) (Kareta et al., 2021).

  * •

We employ the Near-Earth Asteroid Thermal Model (NEATM) using a new MCMC-based
implementation to our March 10 spectrum to constrain the visible wavelength
albedo ($p_{V}$) and the near-infrared beaming parameter ($\eta$). At the
$99.7\%$ confidence level and assuming typical values for emissivity, the
slope parameter G, etc., we retrieve $p_{V}=0.016_{-0.011}^{+0.049}$ and
$\eta=1.560_{-0.441}^{+0.424}$. This visible wavelength albedo is low, but
within line for some comets visited by spacecraft and a $\sim$ few $\%$ of
NEOs. The beaming parameter values are what would be expected for an object
observed at these phase angles. Thermal contamination by dust and the large
phase-reddening expected for this object make these albedo estimates slightly
too low. As a result, we re-ran our thermal models accounting for phase
reddening by assuming it had a typical cometary surface and retrieved
$p_{V}=0.028_{-0.018}^{+0.046}$ also at a $99.7\%$ confidence interval. Radar
observations (Naidu et al., 2016) constrain the albedo to be $p_{V}\leq 0.03$,
which our models naturally reproduce.

  * •

Assuming our nominal albedo range is accurate, we estimate the diameter of
BA14 to be $D\sim 1.1-1.6$ km, which would be a typical size for a Jupiter
Family Comet. The reflectance properties at near-infrared wavelengths of BA14
(e.g., similar to many previously observed cometary nuclei) are seemingly at
odds with a rare surface composition suggested by the mid-infrared
observations of Ootsubo et al. (2021). The two could be reconciled if VIS/NIR
spectral response of BA14 is minimally effected by the phyllosilicates that
appear more prominent in the mid-infrared and more dominated by the low-albedo
featureless matrix of organics that appears to be a common surface material on
other comets.

  * •

While it is tempting to infer that BA14’s mid-IR derived surface composition
is evolutionary, we note that the observational circumstances comparable to
those for BA14’s close approach are rare and high SNR observations of cometary
nuclei even more so, so it cannot be ruled out that more ’typical’ comets
might look as BA14 did in the near- and mid-IR should a similar circumstance
present itself. If its properties are evolutionary, we argue for a slow loss
of smaller grains with diminishing gas production as a more plausible
mechanism to explain its surface composition as opposed to thermal alteration
from a past hotter orbit, but more observations are needed.

We encourage observers who obtained data on BA14 and its purported sibling
252P/LINEAR during their 2016 close approach but have yet to present it
formally somewhere to re/analyze their observations to help understand these
compelling objects and where they fit within the continuum of low activity and
nearly dormant comets and their higher activity (possible) siblings. We also
support continued observation of low activity and dormant comet candidates at
near- and mid-infrared wavelengths to add to the limited number of well-
studied objects. Finally, a full dynamical study of the orbital evolution of
P/2016 BA14 (PanSTARRS) and 252P would be critical to understanding the mutual
relationship between the two bodies and understanding the role that orbital
evolution might have played in BA14’s modern near-dormancy. All of these
observations would also help the Comet Interceptor team make an informed
decision if their ideal comet is not found in time.

The MCMC thermal model described and implemented here is available upon
reasonable request to the authors, and is being developed for public release
in 2022. The authors wish to recognize and acknowledge the very significant
cultural role and reverence that the summit of Mauna Kea has always had within
the indigenous Hawaiian community. We are most fortunate to have the
opportunity to conduct observations from this mountain. This work made use of
observations obtained by the Mission Accessible Near Earth Object Survey
(MANOS) which is funded through the NASA NEOO program grants NNX14AN82G and
NNX17AH06G.

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