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# The main asteroid belt: the primary source of debris on comet-like orbits

P.M. Shober1∗, E.K. Sansom1, P.A. Bland1, H.A.R. Devillepoix1 M.C. Towner1, M.
Cupák1, R.M. Howie1, B.A.D. Hartig1, S.L. Anderson1

1Space Science & Technology Centre, School of Earth and Planetary Sciences,
Curtin University,
GPO Box U1987, Perth, Western Australia 6845, Australia
∗ E-mail<EMAIL_ADDRESS>
###### Abstract

Jupiter family comets contribute a significant amount of debris to near-Earth
space. However, telescopic observations of these objects seem to suggest they
have short physical lifetimes. If this is true, the material generated will
also be short-lived, but fireball observation networks still detect material
on cometary orbits. This study examines centimeter-meter scale sporadic
meteoroids detected by the Desert Fireball Network from 2014-2020 originating
from Jupiter family comet-like orbits. Analyzing each event’s dynamic history
and physical characteristics, we confidently determined whether they
originated from the main asteroid belt or the trans-Neptunian region. Our
results indicate that $<4\%$ of sporadic meteoroids on JFC-like orbits are
genetically cometary. This observation is statistically significant and shows
that cometary material is too friable to survive in near-Earth space. Even
when considering shower contributions, meteoroids on JFC-like orbits are
primarily from the main-belt. Thus, the presence of genuine cometary
meteorites in terrestrial collections is highly unlikely.

##  1 Introduction

Jupiter family comets (JFCs) are short-period comets near the ecliptic plane
that are strongly influenced by Jupiter. They originate from the outer solar
system in the scattered disk (Duncan & Levison, 1997) and are comprised of
primitive volatile-rich material. They are conventionally defined by their
Tisserand’s parameter ($2<T_{J}<3$), which is an approximately conserved value
in the circular restricted three-body problem (object - Jupiter - Sun). This
is a better way to classify the orbits of small planetary bodies compared to
the traditionally used orbital periods (Levison & Duncan, 1994). The physical
properties of JFCs have been explored in numerous studies based on
meteor/fireball analysis (Jenniskens, 2006; Borovička et al., 2007; Madiedo et
al., 2014), telescopic observations (Fernández et al., 2005, 2013), dynamical
modeling (Duncan et al., 2004; Di Sisto et al., 2009; Nesvorný et al., 2010;
Tancredi, 2014), and by space missions (Brownlee et al., 2004; A’Hearn et al.,
2005; Sunshine et al., 2006; Fornasier et al., 2015). Each method provides
information about different size-ranges of objects on JFC-like orbits.

When observing meteors streak across the night sky, we are typically
witnessing the size-range of the smallest objects in the near-Earth space
(dust-sized; $\ll 10^{-3}$ kg). Conversely, telescopic observations are
instrumental in characterizing the most massive objects (hundreds of meters to
kilometers in diameter). This gap between the meteor and telescopic
observations is the primary range for the progenitors of meteorites. Usually
spanning centimeters to meters in scale, this size-range has been interpreted
from fireballs and the meteorites they produce on the ground (Borovička et
al., 2015; Granvik & Brown, 2018).

Based on telescopic observations and dynamical studies, many researchers
believe there is a paucity of JFCs at sub-kilometer scales (Meech et al.,
2004; Fernández & Morbidelli, 2006; Nesvorný et al., 2017). This lack of
objects at smaller sizes is thought to be related to the inferred brief
physical lifetimes ($10^{3}-10^{4}$ yrs) of JFCs in the inner solar system
(Levison & Duncan, 1997; Fernández et al., 2002; Di Sisto et al., 2009). Due
to their low-bulk density and high volatile content, JFCs should fade away
relatively quickly when within $\sim$3 au.

Nevertheless, despite the predicted sub-kilometer paucity, fireball
observation networks have many accounts of durable centimeter to meter-sized
meteoroids originating from JFC-like orbits (Brown et al., 2000; Borovička et
al., 2013; Spurny et al., 2013; Spurný et al., 2017). By examining meter-size
terrestrial impactors, it was found that about $5-10\%$ had JFC-like orbits,
relatively consistent with the flux estimates based on larger kilometer-scale
near-Earth objects (NEOs) (Brown et al., 2016). Yet, they only found half of
these displayed weaker than average structures based on their atmospheric
ablation characteristics. Two of the five JFC-like events discussed were
calibrated meteorite falls (Maribo and Sutter’s Mill) likely to be from the
main asteroid belt. Meanwhile, the remaining three events all came from US
Government sensor (USG) data, two with a semi-major axis of $\sim 2.9$ au,
i.e., well within the bounds of the outer main-belt. Also, it has since been
demonstrated that the orbital information collected by USG events is generally
unreliable (Devillepoix et al., 2019).

Additionally, another study recalculated the orbits for 25 meteorite falls and
identified their likely source regions using an advanced NEO model (Granvik &
Brown, 2018; Granvik et al., 2018). The only meteorite that had a mean JFC
probability $\geq 50\%$ was an H5/6 ordinary chondrite, Ejby. Furthermore, the
fall with the second-highest JFC source region probability was another H5,
Košice. However, both of these falls have relatively significant initial
velocity uncertainties, increasing the uncertainty for the source region
analysis. Carbonaceous-chondrites Maribo and Sutter‘s Mill (both CM2), also
had probabilities of coming from the JFCs above $20\%$. Nevertheless, no
precisely observed meteorite falls have yet come from an unambiguously
cometary orbit.

The Taurid complex has also been discussed as being capable of larger,
possibly meteorite-dropping, meteoroids (Brown et al., 2013). These meteoroids
can be hundreds of kilograms in size, an order of magnitude higher than other
showers. Although other studies have found that while the Taurids can produce
very large meteoroids, the Taurids are still very weak and were unlikely to
produce meteorites (Borovička et al., 2017). Given the highly evolved orbit of
comet 2P/Encke and the other non-cometary possible parent bodies in similar
orbits, this subset of Taurids observed also does not likely reflect the vast
majority of JFC meteoroids (Asher et al., 1993).

Based on debiased NEO orbital and absolute-magnitude distributions, it is
expected that the contribution from the JFC region is a few percent on average
(Bottke Jr et al., 2002; Granvik et al., 2018). The contribution seems to
increase at smaller sizes, reaching $\sim 10\%$ of the NEO population below
diameters of 100 m (Granvik et al., 2018). However, this estimate assumes that
JFCs are more likely to become dormant than disintegrate as JFC disruptions
are less commonly observed compared to long-period comets. If this assumption
breaks down at smaller sizes, then the JFC contribution to the NEO meteoroid
flux may be negligible.

If the hypothesis that the physical lifetimes of sub-kilometer comets are very
short is accurate, why do fireball networks still observe centimeter-meter
debris originating from comet-like orbits? Does the larger cometary debris we
observe impacting the Earth originate from comets?

The massive orbital dataset collected by the Desert Fireball Network (DFN) was
utilized to answer these questions. The DFN is the largest single photographic
fireball network in the world, covering over $2.5$ million km2 of Australia.
This massive project is part of a worldwide collaboration, the Global Fireball
Observatory (https://gfo.rocks/) (GFO), currently consisting of 18 partner
institutions (Devillepoix et al., 2020). The GFO has a semi-automated data
processing pipeline which sorts images, detects fireball events, and
triangulates these events (Howie et al., 2017; Jansen-Sturgeon et al., 2019;
Towner et al., 2020; Sansom et al., 2019a, b).

##  2 Materials and Methods

###  2.1 Experimental Design

The dataset used for this study was collected by the Desert Fireball Network
(DFN). Covering over one third of Australian outback, the DFN is the largest
photographic fireball network globally. The observations made by the DFN are
invaluable, and provide key insights into the debris impacting the Earth
daily.

When parsing through the dataset for this study, all events that originated
from an orbit with a JFC-like Tisserand’s parameter ($2<T_{J}<3$) and had a
significant initial mass ($\geq 0.01$ kg) were gathered. The Tisserand’s
parameter is an approximately conserved value in the three-body problem and is
regularly used to distinguish between different kinds of small planetary
bodies (Levison & Duncan, 1994). Typically this three body problem includes
the sun, Jupiter, and the asteroid or comet; since Jupiter is seen as the
primary perturber the parameter is written as $T_{J}$.

####  2.1.1 Trajectory Analysis & Orbit Determination

The fireball events detected by the DFN have atmospheric trajectories fitted
using a modified straight-line least-squares (SLLS) and an extended Kalman
smoother for the velocity profile (Sansom et al., 2015). The uncertainties
associated with the fitted fireball trajectories are propagated from the
residuals of the fit itself along with the timing and positional uncertainties
of the events‘ observations. The observational uncertainties are handled and
incorporated by the Kalman filter. The initial masses and corresponding
uncertainties were also determined using a dynamic model in a reverse extended
Kalman filter (Sansom et al., 2015). The pre-atmospheric orbits are then
determined numerically including any relevant perturbations by integrating the
meteoroid‘s state until it was outside the Earth‘s sphere of influence
(Jansen-Sturgeon et al., 2019). The orbital uncertainties are then obtained
with a Monte Carlo approach, by numerically integrating samples randomly drawn
from within the initial state uncertainties at the top of the atmosphere.

####  2.1.2 Addressing Observational Biases

The DFN uses photographic observations of fireballs to help better understand
the debris in the inner solar system. However, to obtain meaningful results
from this dataset, we must first address any observational biases.

The first bias we address is that the DFN is optimized for meteorite-dropping
fireball events ($\sim 0.5$ limiting magnitude) (Howie et al., 2017). In this
study, we limit the meteoroids considered to a minimum initial mass of 10
grams. This bias does not affect the results as both asteroidal and cometary
populations are known to produce material within this size-range (Boehnhardt,
2004; Fernández, 2009).

Secondly, we must ensure that the DFN observations do not bias against either
observing asteroidal or cometary debris due to the orbits they exist. The
primary bias to be concerned with is due gravitational focusing. Since
meteoroids with a lower relative velocity are more gravitationally focused,
the terrestrial impact population contains a larger proportion of asteroidal
impactors from orbits with smaller semi-major axis values. However, this study
is concerned with meteoroids all originating from similar orbits, thus they
are all weighted nearly equally. Other observational biases are associated
with photographic fireball observations, however, none of these would have any
affect on the results of this study. For a more exhaustive list and discussion
of DFN observational biases please reference Shober et al. (2020a).

####  2.1.3 Data Selection

A manual check of all the fireball observations and triangulations, any events
with poor data were removed (e.g. fireball was very far from cameras,
convergence angles for triangulations were too small, etc.). The orbital
uncertainties of the meteoroids must be sufficiently low to obtain meaningful
statistics from the Monte Carlo numerical integrations. If the uncertainties
are too large, the number of samples required to get statistically reasonable
results may be computationally unfeasible. Additionally, meteoroids with
$T_{J}$ values within three standard deviations of $T_{J}=3.0$ or $T_{J}=2.0$
were excluded to remove ambiguous events.

In this study, only fireballs with no shower-associations were considered,
i.e., sporadic. Therefore, any events with distinctly cometary physical and
dynamical characteristics could indicate a process to preserve cometary
material in this range for longer periods. Using primarily the $D_{J}$
similarity criterion, we sorted through associations with a limiting value of
$D_{J}<0.15$ (Jopek, 1993). A more inclusive limiting value was used to
eliminate any potential shower events. We verified associations from this
subset by comparing orbital elements, shower dates, right ascensions and
declinations, initial velocities, nodal directions, arguments of perihelion,
along with other similarity criterions (Southworth & Hawkins, 1963; Drummond,
1981). The meteor shower data used was taken from all showers listed by the
IAU Meteor Data Center 111https://www.ta3.sk/IAUC22DB/MDC2007/, with a higher
weight placed on established showers.

The fireball data used within this study, including uncertainties, are
available in the Appendix.

We characterized the physical and dynamical characteristics of the sporadic
meteoroids on JFC-like orbits using fireball data from the DFN with the
following goals:

  1. 1.

Determine proportion of genetically cometary material in excess of 10 g on
sporadic JFC-like orbits

  2. 2.

Assess whether or not the physical lifetimes of JFC meteoroids in the
meteorite-dropping size range are shorter than the disassociation lifetimes of
cometary streams

  3. 3.

Find out if it is possible to get a meteorite from a JFC-orbit, and what it
would look like.

###  2.2 Dynamic Analysis

As shown in previous studies, genuine JFCs originating from the outer solar
system tend to move on chaotic orbits over thousands of years, encountering
Jupiter frequently over their lifetime (Tancredi, 1995; Levison & Duncan,
1997). In this work, we have adopted a similar search strategy to past studies
by numerically integrating the fireball trajectories backward 10 kyrs,
identifying fireballs originating from ‘stable’ and ‘unstable’ orbits
(Tancredi, 2014; Fernández et al., 2014; Fernández & Sosa, 2015). For each
event, a Monte Carlo simulation was undertaken where ten simulations with 100
particles were integrated -10 kyrs. These simulations were conducted using the
IAS15 integrator, taking into account all planetary perturbations and close
encounters considering all of the planets and the Moon (Rein & Liu, 2012; Rein
& Spiegel, 2015). The particles were initialized within the formal
triangulation uncertainties assuming a Gaussian distribution. The particles’
histories were then each assessed and labeled as either ‘stable’ or ‘unstable’
over the previous 10 kyrs. Those with frequent unpredictable jumps in their
orbital elements were considered unstable (Fig. 11(a)), whereas those in
resonances and followed smooth trajectories with little changes in perihelion
distance were considered stable (Fig. 11(b)). The final instability
probabilities and associated uncertainties were then obtained by taking the
mean and standard deviation of the sample means.

(a) Unstable

(b) Stable

Figure 1: Backward integrations -10 kyrs of two particles generated within the
triangulated trajectory uncertainties of event DN200104_01 where q is
perihelion, a is semi-major axis, $\iota$ is inclination, $\omega$ is argument
of perihelion, and $\Omega$ is longitude of ascending node. On the left, a
particle on a chaotic unstable orbit typical of a comet with numerous Jupiter
close encounters. On the right, another particle provides an example of a
stable orbit following a predictable path with no close encounters with
Jupiter. This specific fireball event lasted over 5 sec and was recorded by 4
DFN stations, providing an excellent trajectory fit with very minimal
uncertainties. Thus, this demonstrates the difficulty associated with backward
propagation of meteoroids coming from orbits on the edge of two source regions
(JFC and main-belt in this case).

###  2.3 Physical Analysis

In order to characterize the physical properties, we determined the PE value
for each fireball. The PE criterion was first introduced as a way to
discriminate between different meteoroid compositions based on the ability to
penetrate the atmosphere (Ceplecha & McCrosky, 1976). It has since been used
in numerous studies to assess the physical nature of impacting meteoroids
(Ceplecha, 1994; Brown et al., 2013; Borovička et al., 2015; Brown et al.,
2016). The PE value is given by:

$PE=\log(\rho_{E})-0.42\log(m_{\infty})+1.49\log(V_{\infty})-1.29\log(\cos
Z_{R})$ (1)

where $\rho_{E}$ is the density of the atmosphere at the end the luminous
trajectory (and the reason for the acronym ‘PE’), $m_{\infty}$ is the initial
mass of the meteoroid in grams, $V_{\infty}$ is the initial velocity of the
meteoroid in $km/s$, and $Z_{R}$ is the local entry angle measured from the
zenith.

The PE values are divided into four types: ordinary chondrite-like (type I),
carbonaceous (type II), short-period cometary (type IIIa), and weak cometary
(type IIIb) (Ceplecha et al., 1998). The boundaries for these groups are
denoted in Table 1. The PE criterion is an extremely useful metric to evaluate
the physical properties of meteoroids, however, the types are not strictly
correct. While providing generally accurate results of relative strength, the
physical composition of any individual meteoroid is uncertain due to the other
factors that affect the durability while transiting the atmosphere (e.g.
macro-scale fractures) (Popova et al., 2011; Borovička et al., 2020).

We additionally computed the ballistic coefficient ($\alpha$) and the mass-
loss parameter ($\beta$) based on the fireball observations (Gritsevich &
Stulov, 2006; Gritsevich, 2007; Lyytinen & Gritsevich, 2016). Using these
parameters one can quickly identify potential meteorite-dropping events
(Sansom et al., 2019b). They can be calculated for any event showing some
level of deceleration by using the velocity and height data
(https://github.com/desertfireballnetwork/alpha_beta_modules).

In Fig. 4, we determined potential meteorite dropping fireballs using the
$\alpha$-$\beta$ values. Assuming a 50 g final mass is the minimum meteorite
dropping mass, the meteorite dropping lines were calculated with the following
equations:

$\ln\beta=\ln[13.2-3\ln(\alpha\sin\gamma)],\quad\mu=0$ (2)
$\ln\beta=\ln[4.4-\ln(\alpha\sin\gamma)],\quad\mu=\frac{2}{3}$ (3)

when assuming $\rho_{m}$ = 3500 kg m-3 (meteoroid density) and cd A = 1.5
(product of drag coefficient and initial shape coefficient). Considering that
the contribution from the outer main-belt may be significant, where many
objects have low-albedos (Takir et al., 2015; DeMeo & Carry, 2014; DeMeo et
al., 2015) (e.g., C- and D-types), we also plotted the equivalent lines for a
CM-like density (Consolmagno et al., 2008) ($\rho_{m}$ = 2240 kg m-3) using
the following equations:

$\ln\beta=\ln[14.09-3\ln(\alpha\sin\gamma)],\quad\mu=0$ (4)
$\ln\beta=\ln[4.7-\ln(\alpha\sin\gamma)],\quad\mu=\frac{2}{3}$ (5)

Together, the dynamic simulations and analysis of the material properties can
better elucidate what types of material regularly impact the Earth from JFC-
like orbits in the considered size-range.

###  2.4 Statistical Analysis

Based on the dynamical and physical analysis, the source populations can be
inferred for each meteoroid in this study. However, given the sample size,
what does this tell us about the entire population of small JFC-like
meteoroids?

In order to answer this question, we utilize a Markov Chain Monte Carlo (MCMC)
to sample from the posterior probability distribution. The Bayesian model was
created with the Python PyMC3 probabilistic programming package using the No-
UTurn Sampler (Salvatier et al., 2016). A beta distribution was used as a
prior with the hyperparameters ($\alpha$) based on a study that examined the
dynamical characteristics of 58 near-Earth JFCs ($q<1.3$ au) (Fernández &
Sosa, 2015). This corresponded to hyperparameters $\alpha=[1.38,8.62]$ and an
observation vector $c=[48,2]$; the hyperparameters were weighted less as they
were based on kilometer-scale objects. Additionally, as a comparison, an
uninformed uniform prior of $\alpha=[1.0,1.0]$ was included. In total, fifteen
thousand samples from the posterior were taken to estimate the posterior. To
be counted as genetically JFC in origin in the model, the meteoroids would
need to have a $>50\%$ probability of originating from an unstable orbit and
be of PE types II or III. We considered a PE type II as plausibly cometary due
to previous arguments that meteorites from JFCs could possibly be represented
by primitive carbonaceous chondrites (Gounelle et al., 2008).

##  3 Results

In this study, we examined fireballs not associated with meteor showers
detected by the DFN during four years of observations. All the fireballs
considered were generated by meteoroids that originated from JFC-like orbits
($2<T_{J}<3$). The source region of these meteoroids, main-belt or JFC, was
determined by analyzing the orbital stability and physical durability during
atmospheric transit. It takes only $\sim 1000$ yrs to become orbitally
disassociated from a JFC shower. Therefore, the identification of sporadic JFC
material would indicate a minimum physical lifetime equal to the
disassociation time (Tancredi, 1995). However, a lack of sporadic cometary
debris in fireball observations would be clear evidence that the physical
lifetimes are less than one thousand years.

###  3.1 Orbital characteristics of sporadic JFC-like meteoroids

JFCs move on chaotic orbits compared to main-belt asteroids; on the order of
thousands of years (Tancredi, 1995). During their lifetimes, their orbital
evolution is controlled by the numerous close encounters they have with
Jupiter. Of the 50 sporadic JFC-like fireball events analyzed in this study,
nearly all the meteoroids come from stable orbits over the previous 10 kyrs
(see Appendix). This is inconsistent with the dynamics diagnostic of JFCs.

Only three events have a high probabilities of originating from an unstable
orbits. However, one of these comes from within 3 au and crosses one of the
major mean-motion resonances (MMRs). Events DN150817_01 and DN161028_02 have
the highest probability of originating from unstable cometary orbits, with
$86.1\pm 5.5\%$ and $90.8\pm 3.0\%$ of the particles integrated being chaotic
respectively. As seen in Fig. 2, event DN150816_04 overlaps the 7:3 MMR when
considering triangulation uncertainties, whereas, events DN150817_01 and
DN161028_02 both lie outside the normal range of the main-belt with a semi-
major axis of $\sim 3.6$ au. Several meteoroids have inconclusive dynamical
histories; these include six events where several of the particles integrated
evolved chaotically over thousands of years. On the other hand, nearly all of
these inconclusive events come from orbits near MMRs, thus indicating that
orbital uncertainty is very likely the cause of the inconclusive results.
Otherwise, $>80\%$ of the events originate from extremely stable orbits nearby
one of the primary MMRs (Fig. 2).

Figure 2: Triangulated semi-major axis vs. eccentricity for 50 non-shower JFC-
like fireballs observed by the DFN that have predicted initial masses greater
than 10 g. Error bars denote the formal $1\sigma$ orbital uncertainties
associated with each meteoroid, and coloration is indicative of the orbital
stability determined through numerical integrations -10 kyrs. The vertical red
lines indicate the major MMRs with the dashed gray lines marking the maximum
libration in semi-major axis (Tancredi, 2014). Most of the events originate
from stable orbits near the MMRs. The densogram
(http://www.star.bris.ac.uk/~mbt/stilts/sun256/layer-densogram.html) at the
bottom of the plot represents the semi-major axis density for near-Earth
($q<1.3$ au) JFCs within according to the NASA HORIZONS system
(http://ssd.jpl.nasa.gov/horizons.cgi).

A large majority of the fireballs examined in this study do not fall within
the range consistent with observed kilometer-scale JFCs. As seen in Fig. 2, a
majority of the near-Earth JFCs exist near $\sim 3.1$ au, where only a handful
of events originate. However, only one of the four events in this range comes
from an unstable orbit. This is in line with a previous model, where they
found that main-belt material can contaminate the JFC population but would
primarily be contained to the original bounds of the main-belt (Hsieh &
Haghighipour, 2016). Albeit, four of the 50 events examined fall beyond the
2:1 MMR, and of these, only two have higher probabilities of originating from
unstable orbits (Fig. 2). Additionally, the inclinations of the fireballs
examined tend to be relatively diffuse, but are most concentrated towards low-
values within the $0-25^{\circ}$ range. This is more reconcilable with the
main-belt inclination distribution than the JFC distribution, which is more
highly concentrated around $\sim 12^{\circ}$ (see Appendix).

###  3.2 Physical characteristics of sporadic JFC-like meteoroids

The PE criterion characterizes the composition of meteoroids based on their
ability to penetrate the atmosphere (Ceplecha & McCrosky, 1976). This metric
is empirically determined based on the terminal height of the fireball, the
entry mass, the velocity at the top of the atmosphere, and the impact angle.
The PE criterion has been used for decades to understand the physical
properties of meteoroids based on their atmospheric ablation characteristics
(Borovička et al., 2015; Brown et al., 2016). The values are traditionally
split accordingly (Ceplecha et al., 1998):

type I | $PE>-4.60$ | Ordinary chondrite-like
---|---|---
type II | $-5.25<PE\leq-4.60$ | Carbonaceous chondrite
type IIIa | $-5.70<PE\leq-5.25$ | Short-period cometary
type IIIb | $PE\leq-5.70$ | Weak cometary material
Table 1: Traditional PE classifications based on the atmospheric density at
terminal height (Ceplecha & McCrosky, 1976).

The use of the PE criterion can be quite valuable when analyzing many
fireballs; however, the types are not strictly exclusive and should not be
taken as absolute truth. Other properties beyond physical composition can blur
the lines between the types, such as macro-scale cracks (Popova et al., 2011;
Borovička et al., 2020). As expected, considering the dynamic analysis, many
of the JFC-like fireballs in this study are quite durable and strong (Fig.3).
We only see meteoroids of types I or II, i.e., none fall into the standard
categories associated with cometary material. Out of the 50 fireballs, 28
would be classified as type I and 22 as type II. Some short-period showers
actually can produce more durable debris with similar strengths (Brown et al.,
2013). Therefore, the PE criterion alone is not a sure way to distinguish
between different materials for an individual fireball. In general, the PE
types can give a sense of how strong the initial meteoroid was, but when
combined with the dynamic analysis, the source population of a meteoroid can
be more confidently obtained (main-belt or JFC).

Figure 3: Sporadic JFC-like fireballs observed by the DFN with a minimum
initial mass of 10 g. The end heights of meteors can be used to ascertain the
physical-nature of meteoroids using the PE criterion (Ceplecha & McCrosky,
1976). The PE values for the 50 fireballs examined are overwhelmingly stronger
than the traditional comet types (Types IIIa and IIIb). The error bars
associated with each meteoroid‘s semi-major axis and PE value are due to
triangulation uncertainties. Figure 4: Distribution of non-shower JFC
fireballs with enough deceleration to determine $\alpha-\beta$ values (at
least $\sim 20\%$ deceleration). $\gamma$ is the trajectory slope relative to
the horizontal. If a macroscopic event is considered to have a final mass of
$\geq 50$ g, assuming $\rho_{m}$ = [2240, 3500] kg m-3 (carbonaceous and
ordinary chondrite respectively) and cdA = 1.5, meteorite dropping events can
easily be identified given the range of possible shape change coefficients
($\mu$).

####  3.2.1 Meteorite production from JFC-like meteoroids

Traditionally, the empirical criteria to predict whether a fireball has
produced a meteorite on the ground (“a fall”) is height and velocity below 35
km and 10 kms-1 respectively at the end of the luminous portion of the
atmospheric trajectory (Wetherill & ReVelle, 1981). This gives us an easy way
to identify possible recoverable falls quickly. A more rigorous way to
determine which fireballs result in meteorites has since been demonstrated,
where the ballistic coefficient ($\alpha$) and the mass-loss parameter
($\beta$) are fit to the fireball observations (Sansom et al., 2019b).

In Fig. 4, we see that several of the JFC-like fireballs examined in this
study are quite durable. At least some events are likely to have meteorites on
the ground $>50$ g (DN150905_01 and DN191020_02). However, these events are
likely asteroidal in origin based on their dynamic stability over the previous
10 kyrs and their PE classification. No events with high instabilities over
the previous 10 kyrs or of type II drop meteorites in this study. Thus,
meteorites from JFC-like orbits do occur, but they are all representative of
main-belt material.

##  4 Discussion

Based on dynamical and physical analysis of sporadic JFC-like ($2<T_{J}<3$)
fireballs with initial masses $>$10 g, we find a clear abundance of stronger
dynamically stable meteoroids (Fig. 3). As seen in Fig. 2, most meteoroids
evolve in a deterministic way over the previous 10 kyrs, and have proximity to
major MMRs. Dynamic instability is a diagnostic feature of JFCs originating
from the outer solar system, as they suffer frequent close encounters with
Jupiter (Tancredi, 1995, 2014). The predictable evolution of nearly all the
meteoroids in this study is therefore indicative of their likely origin in the
main belt. Additionally, events with indeterminate histories, where a non-
negligible subset of particles evolved chaotically, also mostly lie near the
primary MMRs. As does one of the three events with $>80\%$ instability (Fig.
3). This proximity to MMRs could indicate possible main belt origins.
Moreover, JFCs’ arguments of perihelia tend to concentrate near $0^{\circ}$
and $180^{\circ}$ (Quinn et al., 1990), and there is no such correlation for
the sporadic JFC-like fireballs in this study (see supplementary material).
This further supports a dominantly main belt, asteroidal source.

Based on the calculated PE values, most of the events are also very durable
and are classified as type I chondritic material. In fact, all of the
fireballs in this sample are either classified as chondritic or carbonaceous
chondritic material (type I or II; Fig. 3). No events have PE types
traditionally associated with cometary material (type III). However, it has
been previously argued that primitive carbonaceous chondrites may have come
from cometary sources (Gounelle et al., 2008). Thus, any DFN events that
likely evolved chaotically over the previous 10 kyrs ($>50\%$) and are of type
II were also considered as cometary. At most only 2 meteoroids studied here
are likely to originate from a short-period comet (DN150817_01 and
DN161028_02). However, this would require centimeter to meter cometary debris
to be stronger than expected. The subset of weaker type II meteoroids in our
data are also generally smaller ($<50\,g$), perhaps debris released by dark
carbonaceous asteroids in the outer main-belt (Takir et al., 2015; DeMeo &
Carry, 2014; DeMeo et al., 2015). A novel process has been recently observed
to release centimeter-sized particles on carbonaceous asteroid Bennu during
the OSIRIS-REx mission (Lauretta et al., 2019).

Albeit, caution should be used with such a criterion to characterize
meteoroids; a wide range of meteorite types have been recovered from type
II/III events. For example, Almahatta Sitta (2008 TC3), Košice, and Sutter‘s
Mill had PE values near the II/IIIa boundary, despite all being of different
meteorite types on asteroidal orbits (polymict breccia, H5, and CM2
respectively) (Borovička et al., 2015; Brown et al., 2016). This is indicative
that other physical factors, such as macroscopic cracks or porosity, can
significantly affect the apparent strengths of impactors, and that there is
still much unknown about the physical nature of meter-scale NEOs (Borovička et
al., 2017, 2020). The PE criterion is useful to characterize the relative
strength of meteoroids, but how much the meteorite type influences the
strength is debatable.

###  4.1 Meteoroids beyond the 2:1 resonance

Previous studies have shown that a minor component of main-belt material can
be transferred to JFC-like orbits, even in some circumstances, onto
dynamically unstable orbits via close encounters with the terrestrial planets
(Fernández et al., 2002, 2014; Hsieh & Haghighipour, 2016; Shober et al.,
2020b, a). Nevertheless, these objects’ semi-major axes are predicted to be
primarily constrained to the original range of the main-belt (Hsieh &
Haghighipour, 2016). Here we see that of the four events beyond 3.27 au, two
are likely asteroidal in origin based on their PE classifications and
stability (Fig. 3). The two most probable events to originate from unstable
comet-like orbits are DN150817_01 and DN161028_02, with $86.1\pm 5.5\%$ and
$90.8\pm 3.0\%$ probabilities of having chaotic histories based on Monte Carlo
simulations along with orbits beyond the 2:1 MMR (each around $a\sim 3.6$ au).
The meteoroids from events DN150817_01 and DN161028_02 both fall into the type
II, nominally carbonaceous and equally or more durable than many of the main-
belt debris (Fig. 3). Even if both events are genuinely cometary in origin,
they only comprise $4\%$ of the sample.

###  4.2 Statistical Significance

Let’s consider the null hypothesis to be that the source regions of sporadic
meteoroids on JFC orbits match those of kilometer-scale JFCs. A P-test was
used to determine if our results were significant enough to reject the null
hypothesis. The P-value reflects the probability of observing an as or more
extreme result. We considered a P-value of 0.01 to be significant enough to
reject the null hypothesis. At most, two events are genetically cometary
within the dataset (DN150817_01 and DN161028_02). Both of these events meet
the criteria of being relatively weaker (type II) and likely originating from
unstable orbits over the previous 10 kyrs ($>50\%$). We assume the probability
of observing a meteoroid from the main-belt within our sample matches the
maximum value found from a dynamical study based on telescopic observations
(19 out of 58 JFCs from the main-belt) (Fernández & Sosa, 2015). From these
results, we find the P-value to be equal to $3.1\times 10^{-21}$. This
extraordinarily low probability clearly indicates we can reject the null
hypothesis. Thus, the source region for centimeter-scale meteoroids on JFC-
like orbits does not match the cometary kilometer-sized bodies.

We additionally utilized Bayesian inference to estimate the proportion of
main-belt material on JFC-like orbits (Salvatier et al., 2016). The informed
prior used is a beta distribution with hyperparameters $\alpha=[1.38,8.62]$
and an observation vector $c=[48,2]$. The hyperparameters are derived from
observational data, but the weight of these prior observations on our expected
values was slightly reduced as they corresponded to kilometer-scale objects
(Fernández & Sosa, 2015). Using a Markov-Chain Monte Carlo to draw samples
from the posterior distribution, we found that sporadic JFC-like meteoroids in
NEO space are $82.1\pm 4.9\%$ from the main-belt and $17.9\pm 4.9\%$ from the
trans-Neptunian region with our reasonably informed prior. Whereas when an
uninformed prior is used (i.e., the prior probability of observing a JFC or
main-belt meteoroid is equal), the relative components are $94.2\pm 3.2\%$
from the main-belt and $5.8\pm 3.2\%$ from JFCs.

Furthermore, while processing the data in this study, we found that of the
meteoroids originating from JFC-like orbits with initial masses greater than
10 g, about $78\pm 3\%$ were sporadic and $22\pm 3\%$ were associated with
showers. Thus, even when considering showers along with the sporadic
population, it is likely that main-belt material makes up the majority of
near-Earth material on orbits with $2<T_{J}<3$ within this size range.
Evidently the Tisserand’s parameter is not a very informative metric when
analyzing meteoroids in the inner solar system.

###  4.3 Why are there no sporadic cometary meteoroids?

####  4.3.1 Alternative explanations

A possible explanation for the lack of dynamically unstable meteoroids in our
sample could be that non-gravitational forces are decoupling the material from
Jupiter. Previous work has found that sporadic micron-sized meteoroids of the
zodiacal cloud are not able to decouple from Jupiter via planetary
perturbations along with solar radiation pressure before having another close
encounter removing them from Earth-crossing orbits (Nesvorný et al., 2010).
However, the meteoroids studied by Nesvorný et al. (2010) are in a completely
different size range compared to those discussed here, and the effects on
these meteoroids are very different.

Nevertheless, non-gravitational forces are highly unlikely to be responsible
for the meteoroids with asteroidal dynamics in our data set. Non-gravitational
forces, such as the Yarkovsky effect, would not be efficient enough to
decouple these meteoroids before having another close encounter with Jupiter
($<10^{3}$ yrs).

Alternatively, one could argue that we may not expect to see many JFC
meteoroids on these low-perihelion orbits or within the examined size range
(10 grams to a few kilograms). Firstly, there have been multiple studies on
Earth-crossing JFCs with the current number known to be $\sim 140$ objects
(http://ssd.jpl.nasa.gov/horizons.cgi). A previous study investigated the
dynamic stability of 58 JFCs in orbits with $q<1.3$ au, and they found that
most of the objects (over two thirds) move on unstable orbits over $\pm 10$
kyrs (Fernández & Sosa, 2015). Thus, showing at the kilometer scale objects on
these orbits are dominated by genetically cometary material. Secondly, larger
meteoroids (centimeter or larger) are capable of being produced by comets
through a few mechanisms such as cometary splitting or gas drag (Boehnhardt,
2004; Fernández, 2009). Thus, we should expect to observe cometary meteoroids
within the range detected by the DFN.

####  4.3.2 Short physical-lifetimes

Given the numerous JFCs on Earth-crossing orbits and their ability to produce
meteoroids within the range considered in this study, it is surprising that we
observe nearly no meteoroids clearly sourced from the JFC population
(Fernández & Morbidelli, 2006). According to debiased NEO models, as much as
$10\%$ of the NEO population at the smallest sizes considered ($\sim 100$ m),
should be sourced from the JFC population (Granvik et al., 2018). Yet, in this
sample, we see no distinctly cometary meteoroids.

This lack of cometary debris within the sporadic population strongly supports
the hypothesis that sub-kilometer JFCs have limited physical lifetimes in the
inner solar system (Meech et al., 2004; Fernández et al., 2002; Fernández &
Morbidelli, 2006; Nesvorný et al., 2017). The physical lifetimes of
centimeter-meter debris must be less than $\sim 1000$ years (the decoherence
time). This physical breakdown is consistent with the observations that there
are less high-volatile asteroids at low-perihelion distances (Granvik et al.,
2016). Planetary debris seems to break down in the inner solar system closer
to the sun, depending on the object’s size and volatile content. At large
sizes ($>10^{3-4}$ m), material from the trans-Neptunian region is dominant on
JFC-like orbits. However, as the diameters decrease to typical meteorite-
producing objects, the physical lifetimes are likely so short that main-belt
material becomes a more viable source. Conversely, according to Nesvorný et
al. (2010), micron–sized meteoroid impacts on Earth are dominated by JFC
particles ($\sim 85\%$ of the mass influx). If accurate, these results suggest
that dust-sized and kilometer-sized objects on JFC-like orbits are genetically
cometary, however, intermediate sizes are sourced from the main asteroid belt.
This is consistent with the prediction by Nesvorný et al. (2010) that cm-sized
particles ejected from comets likely quickly disrupt ($<10$ kyrs) and form the
robust micron-sized population observed within the zodiacal cloud.

###  4.4 Meteorites from Jupiter-family comets

A couple of the fireballs likely resulted in surviving meteorites $>50$ g
(Fig. 4), however, all of these are stable dynamically over the previous 10
kyrs and only have PE values of type I. Thus, even in the occasion where we do
have some genetically cometary material impact the Earth, according to our
sample, it is unlikely to survive. It has been argued that primitive
carbonaceous chondrites may be the best candidates to originate from cometary
bodies, however, given our results, this may be only possible if the meteoroid
was recently released from the parent body (within a thousand years) (Gounelle
et al., 2008). While the cosmic ray exposure ages are significantly shorter
for some carbonaceous chondrites ($<1$ Myr), they are still usually much
longer than the $\sim 10^{3}$ yrs expected from our results (Eugster et al.,
2006). Therefore, meteorites from $2<T_{J}<3$ orbits are expected, but they
will not be cometary.

###  4.5 Limitations

This study is constrained by the amount of data collected by the DFN. However,
given that the DFN is the largest fireball network in the world, this cannot
be improved upon. Additionally, the PE value methodology is a very limited
metric of meteoroid strength as macroscopic features of the meteoroids can
significantly affect the strengths. However, the PE value was utilized
liberally in this study to determine potentially cometary meteoroids. Any
meteoroid with a slightly weaker strength was considered as potentially
cometary. Even with this very inclusive criterion, we found nearly no
meteoroids that fulfilled our physical and orbital requirements to be
considered cometary. The results clearly demonstrate that cometary debris
larger than dust-sized is too friable to survive in near-Earth space for
longer periods.

##  5 Summary

Sporadic fireball data collected by the DFN demonstrates there is a lack of
cometary material on JFC-like orbits for meteoroids in the gram to kilogram
size range. This supports the short physical lifetime hypothesis for JFC
material (Meech et al., 2004; Fernández et al., 2002; Fernández & Morbidelli,
2006; Nesvorný et al., 2017). Additionally, it indicates that the primary
source region for Earth-crossing material on JFC-like orbits ($2<T_{J}<3$) is
size-dependent. A majority of the objects at dust-sizes and on kilometer-
scales are likely sourced from the trans-Neptunian region of the outer solar
system (Nesvorný et al., 2010; Fernández & Sosa, 2015). However, objects
centimeters to meters in size are dominated by asteroidal material diffused
out from the main-belt. This diffusion process likely occurs via some
combination of orbital resonances, Kozai resonances, non-gravitational forces,
and close encounters with terrestrial planets (Bottke Jr et al., 2002;
Fernández et al., 2014; Hsieh & Haghighipour, 2016; Shober et al., 2020b, a).
In this study, we found a minor fraction of unstable possibly cometary
objects, but the number of fireballs observed to originate from stable orbits
from the main-belt over the previous 10 kyrs is statistically significant. We
estimate that as much as $94.2\pm 3.2\%$ of Earth-crossing centimeter-sized
debris on these orbits is sourced from the main-belt. Furthermore, considering
that $22\pm 3\%$ of the JFC-like fireballs observed by the DFN over the
previous five years are associated with showers, our results suggest that the
majority of centimeter material on JFC-like orbits in near-Earth space is
main-belt in origin. Thus, we should expect to see ordinary types of meteorite
falls coming from Jupiter-crossing orbits.

##  6 Acknowledgments

This work was funded by the Australian Research Council as part of the
Australian Discovery Project scheme (DP170102529). SSTC authors acknowledge
institutional support from Curtin University. This work was also supported by
resources provided by the Pawsey Supercomputing Centre with funding from the
Australian Government and the Government of Western Australia. This research
made use of TOPCAT for visualization and figures (Taylor, 2005). This research
also made use of Astropy, a community-developed core Python package for
Astronomy (Robitaille et al., 2013). Simulations in this paper made use of the
REBOUND code which can be downloaded freely at
http://github.com/hannorein/REBOUND (Rein & Liu, 2012). The authors declare no
competing interests. All data needed to evaluate the conclusions in the paper
are present in the paper and/or at https://doi.org/10.5281/zenodo.4710556.

## References

  * A’Hearn et al. (2005) A’Hearn, M. F., Belton, M., Delamere, W., et al. 2005, science, 310, 258
  * Asher et al. (1993) Asher, D., Clube, S., & Steel, D. 1993, Monthly Notices of the Royal Astronomical Society, 264, 93
  * Boehnhardt (2004) Boehnhardt, H. 2004, Comets II, 745, 301
  * Borovička et al. (2015) Borovička, J., Spurný, P., & Brown, P. 2015, Asteroids IV, 257
  * Borovička et al. (2017) Borovička, J., Spurný, P., Grigore, V. I., & Svoreň, J. 2017, Planetary and Space Science, 143, 147
  * Borovička et al. (2007) Borovička, J., Spurný, P., & Koten, P. 2007, Astronomy & Astrophysics, 473, 661
  * Borovička et al. (2020) Borovička, J., Spurný, P., & Shrbený, L. 2020, arXiv preprint arXiv:2006.07080
  * Borovička et al. (2013) Borovička, J., Tóth, J., Igaz, A., et al. 2013, Meteoritics & Planetary Science, 48, 1757
  * Bottke Jr et al. (2002) Bottke Jr, W. F., Morbidelli, A., Jedicke, R., et al. 2002, Icarus, 156, 399
  * Brown et al. (2013) Brown, P., Marchenko, V., Moser, D. E., Weryk, R., & Cooke, W. 2013, Meteoritics & Planetary Science, 48, 270
  * Brown et al. (2016) Brown, P., Wiegert, P., Clark, D., & Tagliaferri, E. 2016, Icarus, 266, 96
  * Brown et al. (2000) Brown, P. G., Hildebrand, A. R., Zolensky, M. E., et al. 2000, Science, 290, 320
  * Brownlee et al. (2004) Brownlee, D. E., Horz, F., Newburn, R. L., et al. 2004, Science, 304, 1764
  * Ceplecha (1994) Ceplecha, Z. 1994, Astronomy and Astrophysics, 286, 967
  * Ceplecha et al. (1998) Ceplecha, Z., Borovička, J., Elford, W. G., et al. 1998, Space Science Reviews, 84, 327
  * Ceplecha & McCrosky (1976) Ceplecha, Z., & McCrosky, R. 1976, Journal of Geophysical Research, 81, 6257
  * Consolmagno et al. (2008) Consolmagno, G., Britt, D., & Macke, R. 2008, Geochemistry, 68, 1
  * DeMeo et al. (2015) DeMeo, F., Alexander, C., Walsh, K., Chapman, C., & Binzel, R. 2015, Asteroids IV, 1, 13
  * DeMeo & Carry (2014) DeMeo, F. E., & Carry, B. 2014, Nature, 505, 629
  * Devillepoix et al. (2019) Devillepoix, H. A., Bland, P. A., Sansom, E. K., et al. 2019, Monthly Notices of the Royal Astronomical Society, 483, 5166
  * Devillepoix et al. (2020) Devillepoix, H. A. R., Cupák, M., Bland, P. A., et al. 2020, A Global Fireball Observatory.  https://arxiv.org/abs/2004.01069
  * Di Sisto et al. (2009) Di Sisto, R. P., Fernández, J. A., & Brunini, A. 2009, Icarus, 203, 140
  * Drummond (1981) Drummond, J. D. 1981, Icarus, 45, 545
  * Duncan et al. (2004) Duncan, M., Levison, H., & Dones, L. 2004, Comets II, 193, 204
  * Duncan & Levison (1997) Duncan, M. J., & Levison, H. F. 1997, Science, 276, 1670
  * Eugster et al. (2006) Eugster, O., Herzog, G., Marti, K., & Caffee, M. 2006, Meteorites and the early solar system II, 829
  * Fernández et al. (2002) Fernández, J. A., Gallardo, T., & Brunini, A. 2002, Icarus, 159, 358
  * Fernández & Morbidelli (2006) Fernández, J. A., & Morbidelli, A. 2006, Icarus, 185, 211
  * Fernández & Sosa (2015) Fernández, J. A., & Sosa, A. 2015, Planetary and Space Science, 118, 14
  * Fernández et al. (2014) Fernández, J. A., Sosa, A., Gallardo, T., & Gutiérrez, J. N. 2014, Icarus, 238, 1
  * Fernández et al. (2013) Fernández, Y., Kelley, M., Lamy, P., et al. 2013, Icarus, 226, 1138
  * Fernández (2009) Fernández, Y. R. 2009, Planetary and Space Science, 57, 1218
  * Fernández et al. (2005) Fernández, Y. R., Jewitt, D. C., & Sheppard, S. S. 2005, The Astronomical Journal, 130, 308
  * Fornasier et al. (2015) Fornasier, S., Hasselmann, P., Barucci, M., et al. 2015, Astronomy & Astrophysics, 583, A30
  * Gounelle et al. (2008) Gounelle, M., Morbidelli, A., Bland, P. A., et al. 2008, The solar system beyond Neptune, 525
  * Granvik & Brown (2018) Granvik, M., & Brown, P. 2018, Icarus, 311, 271
  * Granvik et al. (2016) Granvik, M., Morbidelli, A., Jedicke, R., et al. 2016, Nature, 530, 303
  * Granvik et al. (2018) —. 2018, Icarus, 312, 181
  * Gritsevich & Stulov (2006) Gritsevich, M., & Stulov, V. 2006, Solar System Research, 40, 477
  * Gritsevich (2007) Gritsevich, M. I. 2007, Solar System Research, 41, 509, doi: 10.1134/S003809460706007X
  * Howie et al. (2017) Howie, R. M., Paxman, J., Bland, P. A., et al. 2017, Experimental Astronomy, 43, 237
  * Hsieh & Haghighipour (2016) Hsieh, H. H., & Haghighipour, N. 2016, Icarus, 277, 19
  * Jansen-Sturgeon et al. (2019) Jansen-Sturgeon, T., Sansom, E. K., & Bland, P. A. 2019, Meteoritics & Planetary Science, 54, 2149
  * Jenniskens (2006) Jenniskens, P. 2006, Meteor showers and their parent comets (Cambridge University Press)
  * Jopek (1993) Jopek, T. J. 1993, Icarus, 106, 603
  * Lauretta et al. (2019) Lauretta, D., Hergenrother, C., Chesley, S., et al. 2019, Science, 366
  * Levison & Duncan (1994) Levison, H. F., & Duncan, M. J. 1994, Icarus, 108, 18
  * Levison & Duncan (1997) —. 1997, Icarus, 127, 13
  * Lyytinen & Gritsevich (2016) Lyytinen, E., & Gritsevich, M. 2016, Planetary and Space Science, 120, 35
  * Madiedo et al. (2014) Madiedo, J. M., Trigo-Rodríguez, J. M., Zamorano, J., et al. 2014, Astronomy & Astrophysics, 569, A104
  * Meech et al. (2004) Meech, K., Hainaut, O., & Marsden, B. 2004, Icarus, 170, 463
  * Nesvorný et al. (2010) Nesvorný, D., Jenniskens, P., Levison, H. F., et al. 2010, The Astrophysical Journal, 713, 816
  * Nesvorný et al. (2017) Nesvorný, D., Vokrouhlický, D., Dones, L., et al. 2017, The Astrophysical Journal, 845, 27
  * Popova et al. (2011) Popova, O., Borovička, J., Hartmann, W. K., et al. 2011, Meteoritics & Planetary Science, 46, 1525
  * Quinn et al. (1990) Quinn, T., Tremaine, S., & Duncan, M. 1990, The Astrophysical Journal, 355, 667
  * Rein & Liu (2012) Rein, H., & Liu, S. F. 2012, Astronomy & Astrophysics, 537, A128, doi: 10.1051/0004-6361/201118085
  * Rein & Spiegel (2015) Rein, H., & Spiegel, D. S. 2015, Monthly Notices of the Royal Astronomical Society, 446, 1424, doi: 10.1093/mnras/stu2164
  * Robitaille et al. (2013) Robitaille, T. P., Tollerud, E. J., Greenfield, P., et al. 2013, Astronomy & Astrophysics, 558, A33
  * Salvatier et al. (2016) Salvatier, J., Wiecki, T. V., & Fonnesbeck, C. 2016, PeerJ Computer Science, 2, e55
  * Sansom et al. (2015) Sansom, E. K., Bland, P., Paxman, J., & Towner, M. 2015, Meteoritics & Planetary Science, 50, 1423
  * Sansom et al. (2019a) Sansom, E. K., Jansen-Sturgeon, T., Rutten, M. G., et al. 2019a, Icarus, 321, 388
  * Sansom et al. (2019b) Sansom, E. K., Gritsevich, M., Devillepoix, H. A., et al. 2019b, The Astrophysical Journal, 885, 115
  * Shober et al. (2020a) Shober, P. M., Jansen-Sturgeon, T., Bland, P., et al. 2020a, Monthly Notices of the Royal Astronomical Society, 498, 5240
  * Shober et al. (2020b) Shober, P. M., Jansen-Sturgeon, T., Sansom, E. K., et al. 2020b, The Astronomical Journal, 159, 191
  * Southworth & Hawkins (1963) Southworth, R., & Hawkins, G. 1963, Smithsonian Contributions to Astrophysics, 7, 261
  * Spurný et al. (2017) Spurný, P., Borovička, J., Baumgarten, G., et al. 2017, Planetary and Space Science, 143, 192
  * Spurny et al. (2013) Spurny, P., Borovicka, J., Haack, H., et al. 2013, in Meteoroids 2013 conference, Poznan, Poland
  * Sunshine et al. (2006) Sunshine, J., A’hearn, M., Groussin, O., et al. 2006, Science, 311, 1453
  * Takir et al. (2015) Takir, D., Emery, J. P., & McSween Jr, H. Y. 2015, Icarus, 257, 185
  * Tancredi (1995) Tancredi, G. 1995, Astronomy and Astrophysics, 299, 288
  * Tancredi (2014) —. 2014, Icarus, 234, 66
  * Taylor (2005) Taylor, M. B. 2005, in Astronomical Data Analysis Software and Systems XIV, Vol. 347, 29
  * Towner et al. (2020) Towner, M. C., Cupák, M., Deshayes, J., et al. 2020, Publications of the Astronomical Society of Australia, 37, e008, doi: 10.1017/pasa.2019.48
  * Wetherill & ReVelle (1981) Wetherill, G., & ReVelle, D. 1981, Icarus, 48, 308