# The Winchcombe Fireball—that Lucky Survivor

Sarah McMullan Impact and Astromaterials Research Centre, Dept. Earth Science
and Eng., Imperial College London, SW7 2BP, UK  UK Fireball Network (UKFN)  UK
Fireball Alliance (UKFAll)  Global Fireball Observatory (GFO)  Denis Vida
Department of Physics and Astronomy, University of Western Ontario, London,
Ontario, N6A 3K7, Canada  Global Meteor Network (GMN)  Global Fireball
Observatory (GFO)  Hadrien A. R. Devillepoix School of Earth and Planetary
Sciences, Curtin University, Perth WA 6845, Australia  Global Fireball
Observatory (GFO)  Jim Rowe  UK Fireball Alliance (UKFAll)  System for the
Capture of Asteroid and Meteorite Paths (SCAMP)  Luke Daly School of
Geographical and Earth Sciences, University of Glasgow, Glasgow, UK
Department of Materials, University of Oxford, Oxford, UK  UK Fireball Network
(UKFN)  UK Fireball Alliance (UKFAll)  Global Fireball Observatory (GFO)
Ashley J. King Planetary Materials Group, Department of Earth Sciences,
Natural History Museum, Cromwell Road, London SW7 5BD, UK  UK Fireball
Alliance (UKFAll)  Martin Cupák School of Earth and Planetary Sciences, Curtin
University, Perth WA 6845, Australia  Global Fireball Observatory (GFO)
Robert M. Howie School of Earth and Planetary Sciences, Curtin University,
Perth WA 6845, Australia  Global Fireball Observatory (GFO)  Eleanor K. Sansom
School of Earth and Planetary Sciences, Curtin University, Perth WA 6845,
Australia  Global Fireball Observatory (GFO)  Patrick Shober IMCCE,
Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités,
UPMC Univ. Paris 06, Univ. Lille, 77 Av. Denfert-Rochereau, 75014, Paris,
France  Global Fireball Observatory (GFO)  Martin C. Towner School of Earth
and Planetary Sciences, Curtin University, Perth WA 6845, Australia  Global
Fireball Observatory (GFO)  Seamus Anderson School of Earth and Planetary
Sciences, Curtin University, Perth WA 6845, Australia  Global Fireball
Observatory (GFO)  Luke McFadden  Department of Physics and Astronomy,
University of Western Ontario, London, Ontario, N6A 3K7, Canada  Jana Horák
National Museum Cardiff, Cardiff, C10 3NP, UK  System for the Capture of
Asteroid and Meteorite Paths (SCAMP)  UK Fireball Alliance (UKFAll)  Andrew R.
D. Smedley Department for Earth and Environmental Sciences, The University of
Manchester, Manchester, UK  System for the Capture of Asteroid and Meteorite
Paths (SCAMP)  UK Fireball Alliance (UKFAll)  Katherine H. Joy Department for
Earth and Environmental Sciences, The University of Manchester, Manchester, UK
System for the Capture of Asteroid and Meteorite Paths (SCAMP)  UK Fireball
Alliance (UKFAll)  Alan Shuttleworth  System for the Capture of Asteroid and
Meteorite Paths (SCAMP)  Francois Colas IMCCE, Observatoire de Paris, PSL
Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ.
Lille, 77 Av. Denfert-Rochereau, 75014, Paris, France  Brigitte Zanda  Áine C.
O’Brien School of Geographical and Earth Sciences, University of Glasgow,
Glasgow, UK  SUREC, University of Glasgow, Glasgow, UK  UK Fireball Network
(UKFN)  UK Fireball Alliance (UKFAll)  Ian McMullan  UK Fireball Network
(UKFN)  Clive Shaw  UK Fireball Network (UKFN)  University of Cambridge,
Cavendish Laboratory, Cambridge, UK  Adam Suttle  UK Fireball Network (UKFN)
Martin D. Suttle UK Fireball Network (UKFN)  School of Physical Sciences, The
Open University, Walton Hall, Milton Keynes, MK7 6AA, UK  John S. Young  UK
Fireball Network (UKFN)  University of Cambridge, Cavendish Laboratory,
Cambridge, UK  Peter Campbell-Burns  UK Meteor Observation Network (UKMON); UK
UK Fireball Alliance (UKFAll)  Richard Kacerek  UK Meteor Observation Network
(UKMON); UK  UK Fireball Alliance (UKFAll)  Richard Bassom  UK Meteor
Observation Network (UKMON); UK  Global Meteor Network (GMN)  Steve Bosley  UK
Meteor Observation Network (UKMON); UK  Richard Fleet  UK Meteor Observation
Network (UKMON); UK  Dave Jones  UK Meteor Observation Network (UKMON); UK
Mark McIntyre UK Meteor Observation Network (UKMON); UK  Global Meteor Network
(GMN)  Nick James  NEMETODE Network; UK  Derek Robson  UK Meteor Observation
Network (UKMON); UK  NEMETODE Network; UK  Paul Dickinson  Global Meteor
Network (GMN)  Philip A. Bland School of Earth and Planetary Sciences, Curtin
University, Perth WA 6845, Australia  Global Fireball Observatory (GFO)
Gareth S. Collins Impact and Astromaterials Research Centre, Dept. Earth
Science and Eng., Imperial College London, SW7 2BP, UK  UK Fireball Network
(UKFN)  Global Fireball Observatory (GFO)

###### Abstract

On February 28, 2021, a fireball dropped $\sim 0.6$ kg of recovered CM2
carbonaceous chondrite meteorites in South-West England near the town of
Winchcombe. We reconstruct the fireball’s atmospheric trajectory, light curve,
fragmentation behaviour, and pre-atmospheric orbit from optical records
contributed by five networks. The progenitor meteoroid was three orders of
magnitude less massive ($\sim 13$ kg) than any previously observed
carbonaceous fall. The Winchcombe meteorite survived entry because it was
exposed to a very low peak atmospheric dynamic pressure ($\sim 0.6$ MPa) due
to a fortuitous combination of entry parameters, notably low velocity (13.9 km
s-1). A near-catastrophic fragmentation at $\sim 0.07$ MPa points to the
body’s fragility. Low entry speeds which cause low peak dynamic pressures are
likely necessary conditions for a small carbonaceous meteoroid to survive
atmospheric entry, strongly constraining the radiant direction to the general
antapex direction. Orbital integrations show that the meteoroid was injected
into the near-Earth region $\sim 0.08$ Myr ago and it never had a perihelion
distance smaller than $\sim 0.7$ AU, while other CM2 meteorites with known
orbits approached the Sun closer ($\sim 0.5$ AU) and were heated to at least
100 K higher temperatures.

††journal: MAPS

##  1 Introduction

On the 28th February 2021, at 21:54:16 UTC, a bright fireball lasting 8
seconds was observed above southern Wales, ending around Gloucester, UK (King
et al., 2022). It was witnessed by over 1000 people and captured by many
doorbell and dashboard cameras. It was also captured by 16 dedicated
meteor/fireball cameras of the UK Fireball Alliance (UKFAll), making it a
meteorite fall with one of the highest number of instrumental records to date.

The UKFAll consortium was established in 2018 as a collaboration between the
five meteor camera networks in the UK, with an aim to streamline data sharing
and meteorite recovery efforts (Daly et al., 2020). The precursory work that
UKFAll had done prior to this event enabled the team to share data, establish
an initial strewn field, and handle press inquiries, all within 12 hours of
the fall. This streamlined process enabled the recovery of a portion of the
339 g main mass the morning following the fall. The meteorite was discovered
as a rubble pile on a driveway in the town of Winchcombe, about 60 km south of
Birmingham (King et al., 2022). The rest of the main mass was collected from
this same site the next day, with another 283 g of fragments recovered from
the surrounding area over the next week (Gattacceca et al., 2022). The
meteorite was identified as a CM2 carbonaceous chondrite (Krot et al., 2014;
Suttle et al., 2021); a rare type as only $\sim 4$% of meteorite falls
globally are carbonaceous chondrites (Scott & Krot, 2014).

Instrumentally observing a meteorite fall enables the computation of its pre-
atmospheric orbit (Ceplecha, 1961; Devillepoix et al., 2020). This can link
the meteorite sample to a particular source region in the solar system
(Granvik & Brown, 2018). Pairing an orbit with meteorite laboratory analyses
enlightens our understanding of the composition of that particular region. Due
to the presence of aqueous alteration (Bischoff, 1998), carbonaceous
chondrites are known to have formed close to the snow line in the outer solar
system (Krot et al., 2015). The exact mechanism of delivery of carbonaceous
material into the asteroid belt is still a matter of discussion, but a
migration event of the giant planets appears to be a necessary condition
(Meech & Raymond, 2020; Vida et al., 2022).

Carbonaceous chondrites contain abundant water and organic matter (Trigo-
Rodríguez et al., 2019; Matlovič et al., 2022). They are proposed as a
significant source of Earth’s water and organic material so may be key to
understanding the origins of life on Earth (Pizzarello et al., 2006; Marty,
2012; Alexander et al., 2012). Prior to the Winchcombe meteorite, only four
carbonaceous chondrites have been linked to their pre-atmospheric orbit—Tagish
Lake (Brown et al., 2000), Maribo (Haack et al., 2012), Sutter’s Mill
(Jenniskens et al., 2012), and Flensburg (Borovička et al., 2021). However,
none of them were well observed instrumentally from multiple stations and for
most, trajectories were reconstructed from either casual images and videos or
non-optical recordings.

Each of these events had a limited number of high-precision observations,
especially from nearby dedicated cameras. Additionally, three out of four were
daylight fireballs which makes calibration of video records more difficult,
requiring the use of proxy objects as calibration points instead of stars
(Borovička, 2014).

Winchcombe is the first carbonaceous chondrite fall which was recorded by
multiple dedicated meteor/fireball cameras within 150 km of the fireball, and
thus, observed with unprecedented detail. It was an evening event, providing
both an opportunity for capturing high-precision optical recordings, as well
as being widely witnessed—over 1000 eyewitness reports—resulting in much
public interest.

In this paper, we perform a complete analysis of the fireball from the
available optical records. In Section 2 we describe the observations and
camera networks that observed the fireball. In Section 3 we discuss the
trajectory and fragmentation modelling, continuing with the orbital analysis
in Section 4. We compare the strewn field calculation with the locations of
the meteorites found in Section 5. Finally, in Section 6 we put Winchcombe in
context with previous orbital carbonaceous chondrites and discuss the
relevance of this unique fall.

##  2 Data & Methods

Table 1: List of coordinates of UKFAll cameras that observed the Winchcombe fireball. The coordinates have been truncated to three decimal places due to privacy reasons, but the full trajectory solutions were produced using at least 5 decimal places ($\sim 1$ m accuracy). Astrometry/Photometry indicates whether the data from the camera was used for astrometric picks (A) or for photometry (P). The altitude is given in the mean sea level convention, not WGS84. F is the plate scale, and A err is the astrometric fit error.  Location | Network | Latitude (°) | Longitude (°) | Alt. (m) | Instrument | Used for | F (arcmin/px) | A err (arcmin)
---|---|---|---|---|---|---|---|---
Cardiff | SCAMP | 51.486 | -3.178 | 33 | all-sky video | A + P | 10.1 | 2.92
Cambridge | UKFN | 52.165 | 0.039 | 8 | all-sky photo | - | - | -
Chard | UKMON | 50.878 | -2.950 | 100 | video | A | 5.8 | 1.25
Chelmsford | NEMETODE | 51.745 | 0.494 | 45 | video | - | - | -
Clanfield | UKMON | 50.939 | -1.020 | 158 | video | - | - | -
Honiton | SCAMP | 50.802 | -3.184 | 119 | all-sky video | A + P | 10.1 | 3.42
Hullavington | GMN | 51.535 | -2.149 | 103 | video | A | 3.8 | 0.83
Lincoln | UKFN | 53.222 | -0.464 | 16 | all-sky photo | A | 2.0 | 0.64
Loughborough | NEMETODE | 52.751 | -1.213 | 73 | video | - | - | -
Manchester | SCAMP | 53.474 | -2.234 | 69 | all-sky video | P | - | -
Ringwood | GMN | 50.858 | -1.778 | 24 | video | A | 3.8 | 1.32
Tackley | GMN | 51.883 | -1.306 | 80 | video | - | - | -
Welwyn | UKFN | 51.268 | -0.394 | 78 | all-sky photo | A | 2.0 | 1.04
Wilcot | UKMON | 51.352 | -1.802 | 133 | video | P | - | -

The details of cameras that observed the fireball are given in Table 1. All
networks which contributed optical observations are part of the UK Fireball
Alliance111https://www.ukfall.org.uk/ (Daly et al., 2020). Figure 1 shows the
image of the fireball from several cameras.

Figure 1: The Winchcombe fireball as seen from Welwyn (upper left),
Hullavington (upper right), Cardiff (bottom left), and Wilcot (bottom right).
The Hullavington and Wilcot images show a reflection parallel to the fireball.
The fireball is moving from left to right in all images.

After the fall, all astrometry and photometry measurements produced by
individual networks were mutually exchanged through the Global Fireball
Exchange format (Rowe et al., 2020). Standard specifications are documented at
https://github.com/UKFAll/standard, and the full final measurements are given
in Supplementary Materials in this format.

Although each network employs its own reduction software suite for day-to-day
operations, all astrometric and photometric data used for the analysis
presented in this work have been re-measured following the methods of Vida et
al. (2021b). In this section, we provide a brief description of each camera
network.

###  2.1 SCAMP/FRIPON

SCAMP, the System for the Capture of Asteroid and Meteorite Paths, is the UK
arm of the French Fireball Recovery and InterPlanetary Observation Network
(FRIPON)222https://www.fripon.org that extends over Europe (Colas et al.,
2020). This network uses cameras with an all-sky lens to capture high-
resolution video recordings (30 frames per second) of fireballs (Colas et al.,
2015). There are currently 7 cameras in the UK network, with the aim to have
72 cameras in total to provide full coverage of the UK and Ireland.

###  2.2 UKFN/GFO

UKFN, the UK Fireball Network is the UK arm of the Global Fireball Observatory
(GFO) collaboration333https://gfo.rocks (Devillepoix et al., 2020). This
network uses all-sky cameras based on the DSLR system developed by the Desert
Fireball Network (DFN) in Australia (Howie et al., 2017a). These cameras
capture a 27 s long-exposure photograph every 30 seconds. Absolute timing
along the fireball track is encoded using a liquid crystal shutter and the de
Bruijn method of Howie et al. (2017b). The cameras require a spacing of $<$200
km for accurate fireball detection and observation (Devillepoix et al., 2019);
there are currently 6 cameras deployed in the UK, with plans to expand the
network to a total of 11 cameras in the British Isles.

###  2.3 GMN

The Global Meteor Network (GMN)444https://globalmeteornetwork.org operates
over 700 video meteor stations in 38 countries (Vida et al., 2021b). The
stations use low-cost consumer-grade IMX291 and IMX307 CMOS sensors paired
with wide-field lenses (most commonly $88^{\circ}\times 48^{\circ}$). All
cameras are operated at 25 frames per second. 3.6 mm and 6 mm f/0.95 lenses
are most commonly used, giving a similar field of view and sensitivity to a
human observer (limiting magnitude $+6.0^{\mathrm{M}}\pm 0.5^{\mathrm{M}}$.
The cameras are connected to Raspberry Pi single-board computers which run
open-source software (Vida et al., 2016, 2018). Currently (circa. mid-2022),
the GMN operates around 220 cameras in the UK.

###  2.4 UKMON

UKMON555https://ukmeteornetwork.co.uk, the UK Meteor Network, is a group of
amateur astronomers who use commercial CCTV video cameras for meteor
monitoring (Campbell-Burns & Kacerek, 2014). The network’s main focus are
fainter meteors and meteor showers, however the cameras also detect fireballs.
The UKMON mainly uses GMN camera systems but also operates several older
analog Watec cameras. UKMON currently has over 200 cameras in the UK, but at
the time of the Winchcombe fall only around 30 cameras were installed.

###  2.5 NEMETODE

NEMETODE666http://www.nemetode.org, the Network for Meteor Triangulation and
Orbit Determination, is an amateur group with a network of analog and digital
cameras to monitor the night sky for meteors and meteor showers, mostly based
on UFOCapture software (Stewart et al., 2013). Currently, NEMETODE has over 40
stations, generally with multiple cameras at each station, with significant
coverage of much of Northern England and Ireland.

###  2.6 Other data

Other observational data of the Winchcombe fall were collected along with the
optical data described above. However, as they did not inform the astrometric
and entry modelling of the Winchcombe fall we only briefly summarise them
below:

  * •

A low-resolution visible spectrum was captured by two cameras (NEMETODE and
UKMON).

  * •

Some infrasound signals were detected by sensors from the Raspberry Shake &
Boom network 777https://raspberryshake.org, however no useful measurements
could be made due to data timestamping issues.

  * •

No seismic signals were detected by any seismographs within 200 km.

##  3 Trajectory

In this section, we describe the details of the trajectory, discuss the data
reduction procedure and calibration quality, and present the results of
fireball ablation modelling.

###  3.1 Astrometry and photometry

All optical data sets were manually calibrated and reduced using the SkyFit2
software (Vida et al., 2021b)888The code is available in the RMS repository:
https://github.com/CroatianMeteorNetwork/RMS. Both the all-sky and narrow-
field data were calibrated using the radial distortion model with odd terms up
to the seventh order ($r,r^{3},r^{5},r^{7}$), taking atmospheric refraction
and lens anisotropy into account. All calibrations showed only random errors
with no systematic trends. Table 1 summarises the cameras used in the
solution, together with the plate scales and the average astrometric fit
errors, which were on the order of a few arc minutes for all systems. Of the
total 16 cameras which observed the fireball, only seven were used in the
final trajectory solution. Others were less optimal due to large distance, bad
geometry, CCD blooming, or frame drops. We note that these data would also be
useful if the seven picked stations did not offer the best view of the
fireball.

The unsaturated light curve was exclusively measured on SCAMP video data for
magnitudes fainter than $-8^{\mathrm{M}}$, at which point the cameras
saturated (Cardiff, Honiton, and Manchester stations). All other cameras were
either already saturated or observed the fireball through thin clouds which
would degrade the quality of photometry measurements. Fortuitously, during the
time of SCAMP camera saturation, the analogue CCD camera video from Wilcot
showed an unsaturated lens reflection which was used to measure the brightest
portion of the fireball. Independent absolute calibration of the reflection
could not be done, the measurements yielding only an instrumental magnitude
estimate. However, there were several common points with the unsaturated SCAMP
portion of the light curve which were used to scale the instrumental magnitude
of the reflection, allowing the full light curve to be reconstructed to a high
degree of accuracy. Fig. 6 shows the measured light curve.

###  3.2 Atmospheric trajectory

The nominal trajectory and orbital solution were calculated using the method
of Vida et al. (2020)999The code is available in the WesternMeteorPyLib
repository: https://github.com/wmpg/WesternMeteorPyLib. The uncertainties were
computed by adding Gaussian noise that is two times larger than the measured
random fit errors, as per Vida et al. (2020), and re-fitting the trajectory
solution. The trajectory details are given in Table 2. Figure 2 shows the
fireball trajectory in relation to the seven stations. All selected stations
are within 200 km of the fireball, and the Hullavington station was only 50 km
away, allowing it to capture the details of fragmentation and track the final
fragments just before the dark flight began. Despite most stations being south
of the fireball, the observation geometry was favourable — the maximum
convergence angle of 89∘ was between the Cardiff and Welwyn stations.

Table 2: Fireball trajectory parameters with associated $1\sigma$ uncertainties. All values are given in the Earth-fixed system. The values of initial and final azimuth and altitude differ because the trajectory of the fireball is considered to be a straight line in the Earth-Centred Inertial (ECI) coordinate system, and are recomputed to ground-fixed values to be compatible with previous publications. The final azimuth and altitude do not take the deviating fragment into account. | Beginning | End
---|---|---
Time | 21:54:15.88 | 21:54:24.12
Latitude (+N) | 51.870970∘ | 51.940114∘
|  $\pm$15.3 m |  $\pm$16.2 m
Longitude (+E) | -3.109378∘ | -2.096335∘
|  $\pm$8.8 m |  $\pm$5.6 m
Height (km) | 90.599 | 27.554
|  $\pm$0.020 |  $\pm$0.015
Velocity (km s-1) | 13.86 | $\sim 3$
| $\pm 0.01$ | -
Azimuth | 263.342∘ | 263.906∘
| 0.046∘ | -
Altitude | 41.919∘ | 41.530∘
| 0.029∘ | -
Figure 2: The Winchcombe fireball trajectory (red line) and locations of
cameras used for astrometric measurements.

Figure 3 shows the trajectory fit residuals from a straight line. The
trajectory fit residuals were all below 100 m across the observed span of
almost 100 km. Following Vida et al. (2020), the initial velocity was computed
as the average velocity up to the time when deceleration became statistically
significant. This is achieved by progressively including more points from the
beginning of the trajectory until the end in a linear time vs. distance fit,
and choosing the solution with the smallest standard deviation. The fireball
was first observed at the height of 90.6 km moving at a velocity of 13.86 km
s-1, and it was last observed at 27.6 km decelerating below the ablation limit
at 3 km s-1.

Figure 3: Total trajectory fit residuals versus height (the sign of the
deviation is taken from the vertical direction). RMSD stands for root-mean-
square deviation. The measurements from Hullavington were only done at the
beginning and the end, as the fireball was too bright in the middle of the
flight for accurate astrometric picks.

The Winchcombe meteoroid experienced several major fragmentation, dramatically
increasing the observed fireball brightness and deceleration. The trajectory
followed a straight line up until the final fragmentation at a height of 35
km. A sudden change in the direction of fragments was observed afterwards. The
final portion of the trajectory which showed the deviation was only observed
from the Hullavington station due to its closeness to the fireball and higher
sensitivity. This final portion was either outside of the fields of view of
other cameras, or they were not sensitive enough to observe it. The observed
deviation was not due to calibration issues—the total observed deviation from
a straight-line trajectory was 12 arc minutes, and the astrometry fit accuracy
around the end of the fireball was 0.83 arc minutes. The apparent cross-track
velocity of the fragment relative to a straight line was $\sim 100$ m s-1;
however, this represents a lower limit as the orientation of the plane of
fragmentation cannot be measured from a single-station observation. Only the
part of the trajectory above 35 km was used for orbit estimation, to avoid any
influence of the deviating fragment on the radiant.

###  3.3 Final mass estimation

This final fragmentation produced four discrete fragments (Figure 4) that
could be individually tracked until they dimmed below magnitude
$+2^{\mathrm{M}}$. These measurements allowed an accurate estimation of the
dynamic mass of the largest fragment (Figure 5). Following a classical
approach (McCrosky et al., 1971), a line was fit on time vs. velocity
measurements near the fireball’s end to obtain an estimate of the velocity and
the deceleration. The dynamic mass $m_{dyn}$ is computed as:

$m_{dyn}=\frac{1}{\rho_{m}^{2}}\left(\frac{\Gamma
A\rho_{a}v^{2}}{\dot{v}}\right)^{3}\,,$ (1)

where $\rho_{m}$ is the meteoroid bulk density, $\Gamma$ is the drag
factor101010$\Gamma$ is referred to as the drag factor in many meteoroid
trajectory works, including (Ceplecha & Revelle, 2005). The aerodynamic drag
coefficient, $c_{d}$ = 2$\Gamma$ (Bronshten, 1983; Borovička et al., 2015).,
$A$ is the shape factor, and $\rho_{a}$ is the atmospheric mass density at the
point where the velocity $v$ and deceleration $\dot{v}$ are measured.

In this method, the underlying assumption is that the mass loss is no longer
the dominant driver of energy loss and that the fit can be fully described by
the single-body drag equation. A bulk density of $\rho_{m}=$2100 kg m-3 was
used, informed by the atmospheric ablation characteristics which indicate a
carbonaceous body. This is consistent with the density of the recovered
meteorites from micro-X-ray computed tomography ($\sim$2090 kg m-3 King et al.
2022). The product $\Gamma A$ can be treated as a free parameter, and has
previously been found empirically to fall in the 0.5–1.0 range near the end of
fireballs, representing spherical to cylindrical shapes (Boroviĉka & Kalenda,
2003; Borovička et al., 2015; Gritsevich et al., 2014).

Figure 4: Colour-inverted mosaic of the last moments (every second frame)
observed from Hullavington. Four discrete fragments can be seen at the end.
Figure 5: Left: Observed point-to-point velocities, and the dynamic mass fit
(red line) done using final velocity measurements below the dashed line.
Right: A section of velocity measurements at the end of the fireball (blue
dots) used for the dynamic mass fit (red line). The mass was estimated in the
middle (green dot). Black lines indicate the nominal simulation (solid) and
the $2\sigma$ variations (dashed and dotted).

To constrain the mass by adjusting the $\Gamma A$ factor, and to compute the
final location of the fragments prior to dark flight, we introduce a novel
method. We perform a forward integration of single-body ablation equations
(Ceplecha et al., 1998) starting at the point where the dynamic mass is
estimated. The equations are integrated until the velocity falls below 3 km
s-1, i.e. the ablation limit. An intrinsic ablation coefficient of
$\sigma=0.005$ kg MJ-1 is used (Ceplecha et al., 1998). $\Gamma A$ factor is
adjusted until the simulated velocity matches the observed velocity at the
end. The final geographical coordinates and height above ground are computed
at the cessation of ablation and are used for dark flight modelling. The
azimuth and elevation of the trajectory with respect to the ground at this
endpoint are computed taking the drop due to gravity and the curvature of the
Earth into account.

$\Gamma A=0.55$ was found to best fit the observations. A range of masses
between 0.225 and 0.588 kg was measured (95% confidence interval) for the main
fragment, with 0.350 kg being the nominal value. The dynamic mass was
estimated at the height of 29.69 km and at the velocity of 5.68 km s-1, at
which a deceleration of $3.75\pm 0.30$ km s-2 was measured. The NRLMSISE-00
model (Picone et al., 2002) was used to obtain the atmospheric mass density.
After integrating the ablation equations, only a further 5% of mass was lost
(the final nominal value for the main fragment was 0.330 kg). This nominal
estimate for the lead fragment is consistent with the largest sample recovered
which has a mass of 0.319 kg. The dynamic mass estimates for the smaller
fragments are between 50 and 100 g, which is also consistent with the other
recovered samples (King et al., 2022).

###  3.4 Dynamics and Fragmentation Modelling

The dynamics, light curve, and fragmentation behaviour were modelled using the
Borovička et al. (2013) semi-empirical ablation model. The manual modelling
procedure described in detail in Borovička et al. (2020) was followed. The
luminous efficiency model used was from the same paper. Of the many semi-
analytical approaches developed to model meteoroid trajectories and
fragmentation processes in our atmosphere (e.g. Johnston et al., 2018; Wheeler
et al., 2017), this method gives us the highest fidelity and enables direct
modelling of each observed feature and fragment individually.

In summary, the initial meteoroid is modelled as a classical single body that
fragments at manually determined points. Physical parameters of the initial
meteoroid and individual fragments are also manually estimated. All generated
fragments are considered single bodies. The fragmentation points are informed
by brightness increases in the light curve (i.e. flares) and increases in the
observed deceleration. Most fragmentations were modelled as a release of an
eroding fragment (see Borovička et al., 2015, for more details) which is a
body that rapidly erodes by the release of mm-sized grains. The grain masses
are distributed according to a power law (differential mass index of $s=2.0$
is assumed) within a given range of masses. The amount of erosion is regulated
by the erosion coefficient $\eta$, which determines how much mass is eroded
from the fragment per unit of kinetic energy loss.

An excellent fit to the data was obtained, within observational uncertainty
(Figure 6). The only significant discrepancy between the model and
observations is at the beginning of the fireball above $\sim 60$ km. For this
portion, the fit of the model to the light curve could not be improved, even
with an unphysical and extremely low ablation coefficient of $\sigma=0.0001$
kg MJ-1 ($50\times$ lower than the intrinsic $\sigma$ of ordinary chondrites).
This suggests a significant influence of preheating of carbonaceous material,
as was similarly noticed for ordinary chondrites (Spurnỳ et al., 2020). The
model indicates that the mass loss was not important in this initial stage,
meaning the light production was purely caused by the drag component. This
discrepancy calls for a revision of luminous efficiency models or a separation
of models into mass loss and drag components with different values. For
example, Borovička et al. (2011) have measured a luminous efficiency of the
drag component of the Hayabusa capsule re-entry, as its ablation shield
prevented any significant mass loss. The capsule entered the atmosphere at 12
km s-1, a similar speed to Winchcombe, and its observed luminous efficiency
was 1.3%. The luminous efficiency in our model was $\sim 5\%$, which can
explain the observed discrepancy. Nevertheless, the initial portion of the
light curve does not play a significant role in deriving physical properties
nor fragmentation behaviour and does not affect the conclusions below which
stand for the given model assumptions.

The inverted physical properties of the fireball are given in Table 3 and the
fragmentation behaviour is given in Table 4. We note that error estimation
remains a challenge with this empirical method, and this might not be a unique
solution, thus concrete error estimates are hard to give (Vida et al., 2022).
Nevertheless, the data is accurate enough to constrain the model bulk density
to $\pm 10$ kg m-3 while keeping all other parameters fixed. Progressive
fragmentation was not modelled, so it was assumed that the released fragments
do not have an intrinsic ablation coefficient $\sigma=0.005$ kg MJ-1 of
ordinary chondrites, but an apparent ablation coefficient appropriate for
C-type meteoroids $\sigma=0.04$ kg MJ-1 (Ceplecha et al., 1998).

Table 3: Modelled physical properties of the fireball. Description |  | Value
---|---|---
Initial mass (kg) | $m_{0}$ | 12.5
Initial speed at 180 km (km s-1) | $v_{0}$ | 13.86
Zenith angle | $Z_{c}$ | $48.08^{\circ}$
Bulk density (kg m-3) | $\rho$ | 2100
Grain density (kg m-3) | $\rho_{g}$ | 3000
Ablation coefficient (kg MJ-1) | $\sigma$ | 0.005
(above 66 km) | $\sigma$ |  $<$0.0001
Shape factor (sphere) | $A$ | 1.21
Drag factor | $\Gamma$ | 0.8
(above 66 km) | $\Gamma$ | 1.0
(below 35.1 km) | $\Gamma$ | 0.55
Table 4: Modelled fragmentation behaviour. The fragment mass percentage in the table is reference to the mass of the main fragment at the moment of ejection. The mass distribution index for all grains was $s=2.0$. The values of the dynamic pressure are computed using the drag factor $\Gamma=1.0$. Timea | Height | Velocity | Dyn pres | Main $m$ | Fragment | $m$ | $m$ | Erosion coeff | Grain $m$ | Meteorites
---|---|---|---|---|---|---|---|---|---|---
(s) | (km) | (km s-1) | (MPa) | (kg) |  | (%) | (kg) | (kg MJ-1) | range (kg) | Mass (kg)
2.71 | 65.30 | 13.80 | 0.025 | 12.50 | EF | 0.2 | 0.025 | 1.50 | $10^{-9}-10^{-8}$ | -
3.06 | 62.00 | 13.78 | 0.040 | 12.45 | EF | 1.0 | 0.124 | 1.00 | $10^{-9}-10^{-8}$ | -
3.39 | 59.00 | 13.73 | 0.059 | 12.29 | EF | 1.5 | 0.184 | 1.00 | $10^{-9}-10^{-8}$ | -
3.68 | 56.25 | 13.68 | 0.083 | 12.06 | EF | 36.0 | 4.341 | 0.10 | $10^{-4}-10^{-3}$ | -
3.69 | 56.20 | 13.68 | 0.084 | 7.72 | EF | 46.0 | 3.550 | 0.80 | $10^{-5}-10^{-3}$ | -
4.58 | 48.00 | 13.22 | 0.214 | 4.04 | EF | 20.0 | 0.808 | 0.05 | $10^{-4}-10^{-3}$ | -
4.81 | 46.00 | 13.00 | 0.265 | 3.19 | EF | 30.0 | 0.956 | 0.05 | $10^{-4}-10^{-3}$ | -
4.98 | 44.50 | 12.78 | 0.307 | 2.20 | EF | 30.0 | 0.660 | 0.10 | $10^{-6}-10^{-4}$ | -
5.42 | 40.80 | 11.94 | 0.428 | 1.46 | EF | 37.0 | 0.541 | 0.05 | $10^{-4}-10^{-3}$ | $10^{-3}$
6.19 | 35.10 | 9.45 | 0.584 | 0.81 | D | 28.0 | 0.226 | - | $10^{-5}-10^{-3}$ |
6.23 | 34.90 | 9.35 | 0.591 | 0.58 |  $3\times$ F | 25.0 | 0.144 | - | - | $3\times 0.12$
Endb | 25.31 | - | - | - | - | - | - | - | - | 0.35
a Seconds after 2021-02-28 21:54:15.9 UTC. |
b Final mass of the main fragment at the end of ablation. |
EF = New eroding fragment; D = Dust ejection; F = Single-body fragment. |
Figure 6: Fit of the ablation model (black line) to the observed light curve
(left) and the lag (right) for the Winchcombe fireball. The lag is the
distance a decelerating meteoroid falls behind a hypothetical non-decelerating
meteoroid moving at the initially observed velocity (Vida et al., 2021a).
Inferred contributions to the total light production of individual fragments
are given in dashed lines; black for the magnitude of the main body, green for
eroding fragments, purple for grains released by erosion, blue for single-body
fragments, and orange for dust released directly from the main body. Figure 7:
Details of individual fragmentation events. The fragmentation points, the type
of fragmentation, total fragment mass, fragment ablation and erosion
coefficients are shown.

Figure 7 shows the details of the modelled fragmentation events. Starting with
an initial mass of 12.53 kg, the meteoroid only experienced minor
fragmentation (a loss of a few percent of mass) above 60 km of height, at
dynamic pressures $P_{\mathrm{dyn}}\lesssim 0.06$ MPa (Figure 8;
$P_{\mathrm{dyn}}=\Gamma\rho_{a}v^{2}$, where $\rho_{a}$ is the atmospheric
mass density at a given height and $v$ is the meteoroid speed at that height).
However, the rapid mass loss occurred between $0.08$–$0.1$ MPa when over 80%
of mass was lost into two eroding fragments. The fireball continued to lose
20-30% of instantaneous mass ($\sim 0.5$ kg each) in each of the five
subsequent fragmentations between $0.1$–$0.5$ MPa. Due to rapid deceleration,
the rise in the dynamic pressure slowed and the fireball only reached a peak
pressure of $0.6$ MPa at the height of 35 km. At this point of peak pressure,
a final fragmentation with a sharp flare was observed which could only be
explained by a sudden release of mm-sized dust.

Direct observations of the fragment train from the Hullavington station
(Figure 4) showed that the final fragmentation event at the height of 35 km
produced four distinct fragments. We reproduce the observed fragment mass
distribution by assuming the final fragmentation produces three $\sim 100$ g
fragments. Even smaller fragments have also been observed in the video, but
there is no practical way to directly measure their size. We note that 10
fragments total have been recovered (Russell et al., 2022), which is at odds
with the four final fragments observed in the video. However, further
fragmentation during the dark flight is a known phenomenon that might have
been significant for this fragile body (Boroviĉka & Kalenda, 2003; Spurnỳ et
al., 2020), and some may even have been released higher up and obscured by the
wake.

The fragmentation of Winchcombe occurred at dynamic pressures consistent with
previously observed carbonaceous chondrite falls (Figure 8), which have also
all shown fragmentation in the $0.1$–$0.5$ MPa range (Borovička et al., 2019,
2021). This is in stark contrast to ordinary chondrites which have two
distinct phases of fragmentation, the first between 0.04–0.12 MPa and the
second between 0.5–5 MPa Borovička et al. (2020).

Figure 8: Mass of the main fragment (solid line) and the total mass (dashed
line) versus the dynamic pressure. Individual fragmentation points and the
total fragment masses are marked in red circles. The first fragmentation
($m=0.02$ kg) is outside the plot for presentation purposes.

##  4 Orbital analysis

###  4.1 Radiant and pre-encounter orbit

The pre-encounter orbit was calculated from a trajectory that excluded all
measurements below 35 km (Sec. 3), due to the fragment deviation below that
altitude. Orbital elements are given in Table 5. Winchcombe’s orbit is well
within the main belt (not evolved). The semi-major axis (2.5855 AU) places it
between the 3:1 (2.5 AU) and the 5:2 (2.82 AU) mean-motion resonances with
Jupiter, and points to these as probable mechanisms for delivering Winchcombe
to near-Earth space. Although Winchcombe shares a mid-belt semi-major axis
with two other CM2 meteorites (Sutter’s Mill (Jenniskens et al., 2012) and
Maribo (Borovička et al., 2019)), its Tisserand parameter with respect to
Jupiter (TJ = 3.12) places it on the asteroid side (T${}_{J}>$3), contrary to
the other two. The dynamical evolution of these objects can sometimes
obfuscate their true origins Shober et al. (2021).

Table 5: Pre-encounter orbital parameters expressed in the heliocentric ecliptic frame (J2000) and associated $1\sigma$ formal uncertainties.  | Unit | Value | $1\sigma$ | 95% Confidence Interval
---|---|---|---|---
|  |  |  | Lower | Upper
Semi-major axis | AU | 2.5855 | $\pm 0.0077$ | 2.5686 | 2.5980
Eccentricity |  | 0.6183 | $\pm 0.0011$ | 0.6158 | 0.6201
Inclination | ° | 0.460 | $\pm 0.014$ | 0.440 | 0.490
Argument of periapsis | ° | 351.798 | $\pm 0.018$ | 351.759 | 351.824
Longitude ascending node | ° | 160.1955 | $\pm 0.0014$ | 160.1933 | 160.1985
Perihelion | AU | 0.986839 | $\pm 0.000012$ | 0.986814 | 0.986861
Aphelion | AU | 4.184 | $\pm 0.015$ | 4.150 | 4.209
Tisserand’s parameter |  | 3.1207 | $\pm 0.0056$ | 3.1117 | 3.1331
Last perihelion | days | 2021 Feb 22.446 | $\pm 0.015$ | 22.413 | 22.469
Geocentric Right Ascension | ° | 56.638 | $\pm 0.017$ | 56.604 | 56.671
Geocentric Declination | ° | 17.713 | $\pm 0.069$ | 17.555 | 17.816
Geocentric velocity | m s-1 | 8123 | $\pm 13$ | 8093 | 8143
Figure 9: Ecliptic projection representing the pre-encounter orbit of the
Winchcombe meteoroid, in context with other carbonaceous orbital meteorites.
Inclination to the ecliptic for Winchcombe is 0.46°.

###  4.2 Orbital history

To gain insight into the recent dynamical past of the meteoroid before it
crossed the Earth’s path, we use backward integrations of the orbit following
the method used by Shober et al. (2021). 1000 orbital clones of the meteoroid
are created based on the uncertainties (Table 2) and then integrated backwards
using the Rebound IAS15 adaptive time step integrator (Rein & Spiegel, 2015)
with the Sun, 8 planets, and the Moon as active bodies. The state vector of
the test particles is recorded every 1000 years, both in barycentric
coordinates and as osculating ecliptic orbital elements. Backward integrations
ended at 3 million years in the past, way past the time for which meaningful
dynamical insights into the meteoroid’s history can be gained. Because the
meteoroid was affected by Earth, Mars, and Jupiter in the recent past, the
dynamical system is very rapidly chaotic. In post-analysis, we record at what
time each meteoroid clone entered near-Earth space (perihelion distance $<1.3$
AU). This gives us a median near-Earth entry of $\sim$0.08 Myr in the past,
with 50% of particles entering between 0.035 and 0.24 Myr ago. In comparison,
measured cosmic-ray exposure ages for Winchcombe are $\sim$0.3 Myr for 21Ne
and 0.27 $\pm$ 0.08 Myr for 26Al (King et al., 2022). This indicates that the
ejection of the Winchcombe meteoroid from a larger parent asteroid (start of
exposure to cosmic rays), and its orbital migration from the main belt to
near-Earth space were either contemporaneous events or the ejection happened
while the parent body was already in near-Earth space.

We also track the median perihelion distance of the particles: although the
meteoroid has likely spent time closer to the sun than its impact perihelion
distance suggests ($\sim$0.9868 AU), it most likely has remained higher
(Figure 10) than that of both Sutter’s Mill and Maribo ($\sim 0.5$ AU; Toliou
et al., 2021). This suggests that in its recent NEO history, Winchcombe
underwent less radiant heating ($\lesssim$400 K using Marchi et al. (2009))
than its orbital CM counterparts, which were heated to at least 100 K higher
temperatures.

Figure 10: Orbital elements for the Winchcombe meteoroid over the previous ten
thousand years, from the integration of 1000 particles using the IAS15
integrator (Rein & Spiegel, 2015). The black line is indicative of the median
value, meanwhile, the red lines show the 1-sigma variation.

##  5 Strewn field

###  5.1 Atmospheric model

The atmospheric conditions and winds for the dark flight were modelled
numerically using the Weather Research and Forecasting (WRF) model version 4.0
with dynamic solver ARW (Advanced Research WRF) (Skamarock et al., 2019). The
weather models include wind speed, wind direction, pressure, temperature, and
relative humidity at heights ranging up to 30 km. Three runs were processed,
starting the weather simulation at different times before the meteorite fall,
on 2021-02-28 at 6:00, 12:00, and 18:00 UTC. The 12:00 UTC profile is shown in
Figure 11. Fortuitously, the atmospheric conditions were stable and all three
times gave relatively similar profiles, which is not always the case for other
falls (e.g. Devillepoix et al., 2018). The atmospheric models are available in
Supplementary Materials.

Figure 11: Wind model (speed and direction for a given altitude), extracted as
a vertical profile at the location and time of the lowest visible bright
flight measurement. Model integration started at 2021-02-12T12:00. The winds
affecting this fall were very low, as the maximum wind encountered by the
meteoroid during the dark flight was $\sim$10 m s-1 coming from the East, at
around 0.4 km altitude.

###  5.2 Dark flight

The 12:00 UTC wind model was used to predict where meteorites would land,
based on the last observed bright flight state vector (Sec. 3), using the
method of Towner et al. (2022) 111111Code openly available at
https://github.com/desertfireballnetwork/DFN_darkflight. We create Monte Carlo
clones by varying the final observed state vector within uncertainty (Sec. 3),
as well as meteorite physical parameters such as mass, shape and density
(fixed at 2090 kg m-3 based on recovered meteorite properties King et al.
(2022)). The azimuth and altitude used for dark flight followed the original
straight-line trajectory, prior to the deviation observed at 35 km, as the
absolute amount and direction of the deviation could not be determined from
single-station observations. The masses are randomly sampled logarithmically
from $\sim$5 g to $\sim$0.8 kg, and shape coefficients drawn from a normal
distribution that covers predominantly spherical and cylindrical shapes:
$A=\mathcal{N}(\mu=1.4,\,\sigma^{2}=0.33)$ (Zhdan et al., 2007; Sansom et al.,
2017). The drag is calculated dynamically as a function of the atmosphere and
flight conditions via the Reynolds number (see Towner et al. (2022)). For this
case, a model was also run for extreme non-aerodynamic shapes (A=2.7; tile-
like Zhdan et al. (2007)) to extend the fall line to all recovered fragment
locations (see discussion in section 5.2.1).

Figure 12: Fall line for the Winchcombe meteorite fall. Red points correspond
to the location of the meteorites found with their estimated masses (Russell
et al., 2022). Monte Carlo points from the darkflight simulation are blue
shaded based on the assumed shape. In this case, the shape is the dominant
parameter shifting the points along the fall line: dark blue are more brick-
like and less aerodynamic in shape, while light blue is more sphere-like and
aerodynamic shapes. Grey points to the west are from a custom model using an
extreme, tile-like shape for reference.

The predicted strewn field is in reasonable agreement with the positions of
the recovered meteorites (Fig. 12). Nonetheless, our modelling is unable to
predict the lateral extent of the fall area. Below, we treat along and across
the fall line shifts separately.

####  5.2.1 Along the fall line

The position of masses along the fall line is primarily controlled by the
shape and the mass of the fragments (East-West direction in Fig. 12). When
performing Monte Carlo dark flight simulations, these parameters are assumed
_a priori_ in order to predict where certain masses would fall (points in Fig.
12). Knowing the location and masses of recovered fragments with respect to
these simulated stones can inform us of the approximate shape, impact speed,
and flight time for each sample.

Cross-matching samples with simulations, the main mass found (Site 1 in
Russell et al. (2022)) would have had a moderate shape coefficient of 1.4 to
1.5 (similar drag profile to a cylinder) and hit the ground at 46 to 49 m s-1
after a dark flight free fall time of $\sim$4.5 minutes. The other large mass
(160 g at site 5) likely had a shape coefficient of 1.5 to 1.7 and hit the
ground at 35 to 40 m s-1 after a flight time of $\sim$6 minutes. Smaller
fragments would require extreme shapes to match the fall line (grey points in
Fig. 12). In particular, the 16.5 g mass at Site 6 requires a shape
coefficient of $\sim 2.7$ (as found by Čapek et al., 2021) and would have
impacted at 17 to 19 m s-1, after a $\sim$16 minute flight. An alternative
explanation is that this fragment entered the dark flight phase at a higher
altitude (consistent with 2-3 km before the end), as was in fact observed in
the Hullavington video. The lateral deviation of this fragment from the line
also supports a longer dark flight phase (see also section 5.2.2). The spread
of recovered meteorites along the modelled fall line is consistent with
differences in their physical characteristics, as well as the late
fragmentation of the smaller stones.

####  5.2.2 Across the fall line

The position of masses across the fall line is primarily controlled by the
state vector at the end of the bright flight (North-South direction in Fig.
12). The lateral spread of the Monte Carlo simulations due to uncertainties in
this final state vector is $\sim$200 m (90 m s-1). Some of the Winchcombe
fragments were recovered outside of this area by up to $\sim$300 m on either
side. This creates a $\sim$600 m corridor formed by the found masses. It
should be noted that our $\sim$200 m wide predicted fall line is in the middle
of this $\sim$600 m corridor. To account for the fragments lying outside the
lateral spread of the Monte Carlo simulations, the various meteoritic
fragments must have had differences in their final bright flight velocity
vector that model uncertainties alone cannot account for. Strong evidence of
this direction change process is evident in Fig. 3, in which the Hullavington
viewpoint displays a projected deviation of $\sim$200 m. With a single
viewpoint on the end of the bright flight trajectory, it is difficult to
estimate the true spread of the fragments in 3D space. Nonetheless, an
observed 200 m projected deviation could well explain a 500 m cross-line
positional difference between different fragments if their flight was
completely tracked to the ground.

Passey & Melosh (1980) have also proposed significant lateral velocities in
fragments just after a breakup from studying strewn fields on Earth. These
could be due to lift effects, bow shock interactions, centripetal separation
of a rotating meteoroid, or transverse separation from reaching meteoroid
crushing strengths (Passey & Melosh, 1980). These authors propose a spreading
velocity proportional to the velocity at breakup such that
$V_{spreading}=V_{breakup}\times\sqrt{C\times\rho_{a}/\rho_{m}}$, where here
the atmospheric density and meteoroid density refer to those at breakup
altitude, and $C$ is an empirical constant that ranges from 0.02 to 1.5 (based
on strewn field analyses on Earth, Venus, and Mars; Passey & Melosh, 1980;
Herrick & Phillips, 1994; Popova et al., 2007; Collins et al., 2022). For the
final fragmentation of the Winchcombe meteoroid at 34.9 km, this relation
predicts spreading velocities in the range of 2.5 - 22.5 m/s. This would
account for a maximum deviation over the final 2.01 s of $\sim$45 m. The
observed 200 m deviation and a minimum lateral velocity of 90 m/s would
require $C$ values of over 30. Such high lateral spreading velocities are not
however unreasonable. Both Boroviĉka & Kalenda (2003) for the Morávka
fireball, and Docobo & Ceplecha (1999) for another fragmenting event, show
spreading velocities of up to 300 m/s for observed fragments. These
significantly higher values than represented in strewn field data show the
energetic nature of the fragmentation processes that simple models of mere
atmospheric loading cannot account for. The addition of any volatile materials
in a carbonaceous-type meteoroid would be expected to exaggerate these
effects. We note that the deviation occurred during a fragmentation event
which released 28% of the remaining mass ($\sim 150$ g) into dust. We
postulate that a preferential direction for the dust release might also
explain the gain in transverse momentum.

A common assumption for fall area estimation is that the more precise the
overall bright flight trajectory is, the smaller the search area will be.
Although this is likely true, there is a catch: the individual fragments must
be well observed from multiple sites at the end of the bright flight.
Therefore, unless multi-station viewpoints are available at high-resolution
(arc minute) and high sensitivity (down to magnitude 0M), it is not possible
to predict the width of meteorite fall line within less than a couple of
hundreds of meters, no matter how precisely the upper trajectory is
determined.

##  6 Discussion

The limited number of carbonaceous chondrites with known pre-atmospheric
orbits is largely due to their poor survivability (Ceplecha & McCrosky, 1976).
Carbonaceous chondrites are weak, so they fragment and ablate quickly in the
Earth’s atmosphere (Ceplecha et al., 1998). To survive as a meteorite, their
atmospheric trajectories require one or more specific properties: low entry
speeds (approach from the antapex, i.e. opposite to the direction of Earth’s
motion around the Sun), shallow entry angles, and large initial masses.

The first instrumentally observed carbonaceous chondrite meteorite fall,
Tagish Lake, fell on 18 January 2000 at 16:43 UTC in a remote region of
northern British Columbia, Canada (Brown et al., 2000). The meteorite was
categorised as a C2-ungrouped carbonaceous chondrite with an initial mass of
up to 200,000 kg. There were over 70 eyewitness reports, with 24 photographs
and five videos of the dust cloud taken by observers within 1-2 mins after the
event. It was also detected by infrared and optical sensors aboard US
Department of Defense Satellites and the corresponding shock wave was detected
by local seismic and infrasound stations (Brown et al., 2002). More than 500
meteorites were recovered, with the largest fragment 2.3 kg, and a total mass
of 16.3 kg (Popova et al., 2011). The lack of direct ground-based optical
recordings and reliance on classified satellite data mean that the orbit might
contain systematic uncertainties that are not well understood (Devillepoix et
al., 2019).

The Maribo meteorite fell on 17 January 2009, at 18:08:28 UTC in Denmark
(Haack et al., 2012), and was identified as a CM2 meteorite with a calculated
initial meteoroid mass of 2000$\pm$1000 kg (Borovička et al., 2019). There
were 550 eyewitness reports and it was captured by a surveillance camera in
southern Sweden, a photo from an all-sky fireball camera in the Netherlands,
and by three all-sky meteor radars in Germany. The sonic boom was recorded by
11 seismometers and an infrasound station. Seven radiometers in the Czech
Republic were able to measure the radiometric light curve, allowing for the
first detailed ablation and fragmentation modelling of a carbonaceous
meteorite fall.

The Sutter’s Mill meteorite fell on 22 April 2012, at 14:51:12 UTC in
California and was identified as a CM2 carbonaceous chondrite with a
calculated initial meteoroid mass of 20,000–80,000 kg (Jenniskens et al.,
2012). It was observed by 3 Doppler weather radars, 2 infrasound stations, and
8 seismic stations, along with a set of three photographs from Nevada, and a
few videos.

The most recent recorded carbonaceous meteorite fall, Flensburg, occurred on
12 September 2019, 12:50 UTC over northern Germany. It was identified as a
C1-ungrouped carbonaceous chondrite with a calculated initial meteoroid mass
of 10,000–20,000 kg (Borovička et al., 2021). There were 584 eyewitness
reports and it was recorded by one AllSky6 camera and three dash cameras. A
single meteorite of 24.5 g was recovered, and there was insufficient data to
estimate the total mass or number of fragments.

Each of the four previous carbonaceous falls had high entry velocities ($>15$
km s-1) and consequently experienced high dynamic pressures ($>1$ MPa) which
caused mechanical disintegration of the weak bodies. The reason why any
meteorites survived at all is that their pre-atmospheric masses were large
($>$1 t) and they had shallow entry angles ($<30^{\circ}$) (Table 6), allowing
few meteorites to survive by chance while $>>99$% of the initial mass was
destroyed. In contrast, Winchcombe experienced the most favourable entry
conditions possible which enabled a significant amount of material from the
smallest ever observed carbonaceous meteoroid to survive to the ground.

Name | Classification | Max | Initial Mass | Initial | Entry | Semi-major | $T_{J}$ | Ref.
---|---|---|---|---|---|---|---|---
|  | Pressure | Range | Velocity | Angle | Axis |  |
|  | (MPa) | (kg) | (km s-1) | (∘) | (AU) |  |
Tagish Lake | C2(ungrouped) | 2.2 | 50,000–200,000 | 15.8 | 16.5 | $2.1\pm 0.2$ | 3.66 | 1
Maribo | CM2 | 5 | 1,000–3,000 | 28.3$\pm$0.3 | 31 | $2.43\pm 0.12$ | $2.95\pm 0.11$ | 2, 3
Sutter’s Mill | CM2 | $>1$ | 20,000–80,000 | 28.6$\pm$0.6 | 26.3$\pm$0.5 | $2.59\pm 0.35$ | $2.81\pm 0.32$ | 4
Flensburg | C1(ungrouped) | 2 | 10,000–20,000 | 19.43$\pm$0.05 | 24.4 | $2.82\pm 0.03$ | $2.89\pm 0.02$ | 5
Winchcombe | CM2 | 0.6 | 9–15 | 13.547$\pm$0.008 | 41.92$\pm$0.03 | $2.586\pm 0.008$ | $3.121\pm 0.006$ | 6, 7
Table 6: Physical and orbital properties of previous carbonaceous chondrite
falls, in comparison to Winchcombe. The entry angle is reference to the
horizontal. See references for details on falls and their orbits: (1) Brown et
al. (2000), (2) Haack et al. (2012), (3) Borovička et al. (2019), (4)
Jenniskens et al. (2012), (5) Borovička et al. (2021), (6) King et al. (2022),
(7) this work.

##  7 Conclusions

Winchcombe is the first meteorite recovered in the UK in 30 years, the first
carbonaceous chondrite recovered in the UK, and the first instrumentally
observed meteorite fall in the UK.

The main scientific takeaways of this work are:

  * •

The Winchcombe meteoroid entered with favourable entry parameters (low
velocity and entry angle) to avoid $>$1 MPa dynamic pressures; an uncommon
range of conditions that were necessary for the survival of weak carbonaceous
material. So far, amongst meteorites with measured orbits, only multi-ton
objects have been proven to drop carbonaceous chondrites. In the more
traditional meteorite dropping size (decimetre), a strong velocity bias exists
against the survival of carbonaceous material. Winchcombe is the first
evidence that survival of smaller cabronaceous bodies is possible, but only
for objects approaching from the antapex which by rule have the slowest entry
velocities.

  * •

The surviving fragments experienced a significant flight vector change before
entering dark flight, obtaining a velocity kick perpendicular to a straight-
line trajectory of at least 90 m s-1. The physical phenomenon that caused this
could not be definitely determined, but we investigate several possibilities
that require further study. The deviation resulted in fragments being
scattered beyond the nominal error boundaries of the fall area. Without
detailed observations at both high-resolution and high sensitivity of the very
end of the bright flight, it may not be possible to predict meteorite fall
positions to better than a few hundred metres.

  * •

The recent ejection of Winchcombe from a parent asteroid as measured by the
cosmic-ray exposure ages ($\sim 0.3$ Myr) is longer or contemporary with the
time spent as a Near-Earth Object inferred from its orbit (0.035—0.24 Myr,
0.08 Myr nominal). This indicates a minimal time delay between ejection from
its parent body and the insertion into near-Earth space.

  * •

The analysis of the Winchcombe fireball involved five independent optical
observation networks. This validates the need for standard data exchange
procedures, as proposed by Rowe et al. (2020), in order to enable a quick
turnaround time from the time the fireball happens to when a fall area is
calculated.

##  8 Supplementary materials

Supplementary materials have been uploaded as a Zenodo record at
http://doi.org/10.5281/zenodo.6685719. It contains fireball images (including
calibration data), astrometry tables in Global Fireball Exchange standard, the
trajectory report file, and the wind profiles.

This publication is part of the Winchcombe science team consortium, organised
by the UK Fireball Alliance and conducted by the UK Cosmochemistry Network.
The authors of this paper would like to thank the UK Fireball Alliance, its
constituent networks (UK Fireball Network, SCAMP, UKMON, NEMETODE, GMN),
international collaborators (FRIPON, Global Fireball Observatory, Desert
Fireball Network, University of Western Ontario, and University of Helsinki)
and the meteor observation camera owners who participate in the UK Fireball
Alliance network for their aid in observing the fireball and helping to
predict its fall position. We would also like to thank the scientists and
volunteers that participated in the UK Fireball Alliance led search and
recovery of the Winchcombe meteorite, and the local community, who generously
reported and donated meteorite finds and enabled the team to search the strewn
field. STFC is acknowledged for supporting the “Curation and Preliminary
Examination of the Winchcombe Carbonaceous Chondrite Fall” project
(ST/V000799/1), and Natural History Museum staff for curatorial support. The
Curtin University authors acknowledge their contribution was made possible by
the Australian Research Council as part of the Australian Discovery Project
scheme (DP170102529, DP200102073), the Linkage Infrastructure, Equipment and
Facilities scheme (LE170100106), and through institutional support from Curtin
University. The Global Fireball Observatory data reduction is supported by
resources provided by the Pawsey Supercomputing Centre with funding from the
Australian Government and the Government of Western Australia, and makes
intensive use of Astropy, a community-developed core Python package for
Astronomy (Astropy Collaboration et al., 2013). The DFN gets software support
resources awarded under the Astronomy Data and Computing Services (ADACS)
Merit Allocation Program. ADACS is funded from the Astronomy National
Collaborative Research Infrastructure Strategy (NCRIS) allocation provided by
the Australian Government and managed by Astronomy Australia Limited (AAL).
This project has received funding from the European Union’s Horizon 2020
research and innovation programme under the Marie Sklodowska-Curie grant
agreement No 945298. Several authors acknowledge funding from UK Science and
Technology Facilities Council; SM and GSC ST/S0000615/1, LD ST/T002328/1, and
KJ ST/R000751/1. AK is funded by UK Research and Innovation Grant
MR/T020261/1. LD thanks a University of Glasgow COVID-19-Research Support
Scheme grant.

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