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On the Proposed Interstellar Origin of the USG 20140108 Fireball

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A critical review of the evidence for the interstellar origin for the USG 20140108 fireball is presented. Examining USG fireball velocities where independent data are available shows the former to have significant (10-15 km/s) uncertainties at large speeds and highly variable radiant accuracy, with average errors in excess of ten degrees. Ablation model fits to the observed lightcurve are possible for normal chondritic impactors only assuming low speeds. To match the high speed and low fragmentation height of the USG 20140108 fireball would require a high density/strength object with low drag and highly aerodynamic shape not made of iron. We suggest the simpliest explanation for the unusual characteristics of USG 20140108 is that the speed, in particular, is substantially overestimated.
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On the Proposed Interstellar Origin of the USG 20140108 Fireball
Peter G. Brown 1, 2 and Jiˇ
r
´
ı Boroviˇ
cka 3
1Department of Physics and Astronomy
University of Western Ontario
London, Ontario, N6A 3K7, Canada
2Institute for Earth and Space Exploration
University of Western Ontario
London, Ontario, N6A 3K7, Canada
3Astronomical Institute of the Czech Academy of Sciences, Friˇcova 298, CZ-25165 Ondˇrejov, Czech Republic
ABSTRACT
A critical review of the evidence for the interstellar origin for the USG 20140108 fireball is presented.
Examining USG fireball velocities where independent data are available shows the former to have
significant (10-15 km/s) uncertainties at large speeds and highly variable radiant accuracy, with average
errors in excess of ten degrees. Ablation model fits to the observed lightcurve are possible for normal
chondritic impactors only assuming low speeds. To match the high speed and low fragmentation
height of the USG 20140108 fireball would require a high density/strength object with low drag and
highly aerodynamic shape not made of iron. We suggest the simpliest explanation for the unusual
characteristics of USG 20140108 is that the speed, in particular, is substantially overestimated.
Keywords: interstellar meteors, meteoroids methods: numerical
1. INTRODUCTION
In the early 20th century the field of me-
teor science was driven largely by the question
of the interstellar vs. interplanetary nature of
most meteors (Hughes 1982;Gunn 2005). This
controversy developed due to the large fraction
of apparently hyperbolic meteors found from
visual observations (Opik 1950;Almond et al.
1950;Williams 2004). Optical and radar tech-
niques of higher precision than visual measure-
ments conclusively showed by the mid-20th
century that the vast majority of apparently
interstellar velocities were the result of mea-
surement errors (Lovell 1954;Stohl 1970;Ha-
jdukov´a Jr et al. 2019).
More recent ground-based optical surveys
focused on interstellar meteor detection (e.g.
Hajdukov´a 1994;Hawkes & Woodworth 1997;
Musci et al. 2012) have not found convinc-
ing evidence (>3σ) of any clear hyperbolic
population at mm-cm sizes. Hajdukova et al.
(2020); Hajdukova & Kornoˇs (2020) have anal-
ysed optical meteor datasets and find a clear
correlation between orbit determination qual-
ity and the fraction of meteors appearing to be
hyperbolic.
At smaller meteoroid sizes, radar-based mea-
surements have proven more equivocal. Tay-
lor et al. (1994); Baggaley (2000) have claimed
detection of interstellar meteoroids at tens of
micron sizes using the Advanced Meteor Or-
bit Radar (which ceased operation in 2000).
Their most notable result was the claim of an
interstellar meteoroid stream emanating from
arXiv:2306.14267v1 [astro-ph.EP] 25 Jun 2023
2Brown and Boroviˇ
cka
the direction of the debris-sisk star β-Pictoris
(though the interpretation is questioned by
Murray et al. (2004)). In contrast, Weryk &
Brown (2004); Froncisz et al. (2020) used the
Canadian Meteor Orbit Radar to search for
hyperbolic meteoroids at larger sizes (of or-
der several hundred microns) and found only a
handful of potential detections out of millions
of orbits, which they ascribed to measurement
error.
While ground-based techniques using the
Earth’s atmosphere as a detector have failed to
provide clear evidence for an interstellar mete-
oroid population at tens of microns - decime-
ter sizes, spacecraft measurements at small
sizes are abundant. The in-situ dust detec-
tors on the Galileo, Cassini and Ulysses space-
craft have unambiguously detected the inflow
of interstellar dust at sub-micron sizes in the
middle-outer solar system (Gr¨un & and 22
Other authors 1993;Kruger et al. 2001;Al-
tobelli 2003).
The recent telescopic detection of two in-
terstellar objects in the inner solar system,
namely 1I/’Ouamumua and 2I/Borisov (Je-
witt & Seligman 2022), have reinvigorated
the search for interstellar meteoroids in the
Earth’s neighborhood. Siraj & Loeb (2022)
have recently claimed detection of a meter-
sized interstellar meteoroid detected on Jan 8,
2014 based on data from US Government sen-
sors. This is a surprising result given that nu-
merous surveys at smaller meteoroid sizes have
searched for and as yet not found convincing
evidence for true interstellar meteoroids from
ground-based detectors (Hajdukov´a Jr et al.
2019;Froncisz et al. 2020;Musci et al. 2012).
The claim is impossible to verify conclusively
as independent analysis of the original data
is not possible since the data originates from
classified US Government sensors (Brown et al.
2016).
However, while specific data for this event
are not open for analysis, the general accu-
Table 1. Published data for USG 20140108. The
time, location, altitude and speed refer to the
point of peak brightness. The three velocity com-
ponents are given in Earth Centered Fixed coordi-
nates. The total radiated energy represents the es-
timated electromagnetic radiation emitted during
the entire fireball in the silicon bandpass assuming
a 6000 K blackbody (Tagliaferri et al. 1994).
Parameter Value
Date/Time of Peak Brightness 2014-01-08 17:05:34
Latitude 1.3S
Longitude 147.6E
Altitude (km) 18.7
Speed (km/s) 44.8
Vx(km/s) -3.4
Vy(km/s) -43.5
Vz(km/s) -10.3
Total Radiated Energy (J) 3.1×1010
racy of US Government sensor observations are
better known through comparison with com-
mon fireballs detected using other instruments
(Devillepoix et al. 2019). The crux of the in-
terstellar origin claim for USG 20140108 is the
measured velocity vector.
Here we examine the metric data and other
information related to this fireball in an ef-
fort to better interpret its history and physical
properties.
2. AVAILABLE DATA FOR USG 20140108
2.1. US Government Sensor Data
The USG website 1is the source of the met-
ric and lightcurve information related to USG
20140108. Table 1summarizes the primary
data from this source.
There are no documented uncertainties for
these data - the precision as published is the
only constraint on these values.
1https://cneos.jpl.nasa.gov/fireballs/
critical review - interstellar 3
Compared to all other events in the USG
dataset, USG 20140108 stands out in sev-
eral respects. Of the 280 fireballs which have
speeds and velocity components, it is the 2nd
highest reported speed and the 6th lowest al-
titude, an extremely unusual combination. Of
the fireballs in the USG dataset with altitudes
of peak brightness below 20 km, all others
have speeds half that (or lower) than USG
20140108. Similarly, of all events with speeds
above 40 km/s, the next lowest altitude of peak
brightness is 46.3 km. Clearly, USG 20140108
is an extreme outlier.
Complementing these data is the recently
released lightcurve, reporting brightness as a
function of time for USG 20140108. This
is shown in Figure 1. The main feature of
the lightcurve is the presence of three equally
spaced flares separated by only a few tenths of
a second. Using the radiated energy to total
energy conversion from Brown et al. (2002),
the estimated total energy for the fireball is
0.11 kT TNT (1 kT = 4.185×1012J).
2.2. Infrasound
In addition to the optical detection by USG
sensors, a search for infrasound (low frequency
sound waves with f<20 Hz) signals from the
fireball was performed across the International
Monitoring System (Le pichon et al. 2019) op-
erated as part of the Comprehensive Test Ban
Treaty Organization (CTBTO).
Acoustic signals from CTBTO infrasound
stations within 5000km of the impactor loca-
tion were examined. Of these, three stations
showed signals which could be confidently as-
sociated with the fireball based on arrival time
and backazimuth association. Table 2summa-
rizes the properties of these signals. Analy-
sis was conducted according to the techniques
described in Ens et al. (2012); Gi & Brown
(2017). All signals showed celerities consistent
with stratospheric ducting. These detections
are summarized in detail in Appendix A.
The period and amplitude (with suitable
corrections for winds) of these signals can be
used to estimate the source energy following
the methodology described in Ens et al. (2012);
Gi & Brown (2017). As has been noted previ-
ously, period estimates are usually more robust
than those derived from amplitude due to the
large influence winds can have on the latter
(Silber & Brown 2019).
We find a best estimate for the source energy
of 0.010 ±0.001 kT TNT equivalent using the
relationship between multi-station period av-
erage and fireball energy as given in Ens et al.
(2012). The Air Force Technical Applications
Center (AFTAC) period-yield relation derived
from nuclear tests (ReVelle 1997) produces an
identical result. Applying doppler wind cor-
rections to these periods lowers the yield ap-
proximately 20%.
The average yield using wind-corrected in-
frasound maximum amplitude produces a
source energy estimate of 0.04 ±0.03 kT TNT.
While the yield energies from amplitudes
and periods show large differences (as ex-
pected), taken together these data suggest a
source energy a factor of several to as large
as an order of magnitude smaller than the
USG estimate. Such large differences betweeen
USG energy and infrasound energy are atypi-
cal, with normal agreement being within a fac-
tor of two for events where multi-station in-
frasound data can be averaged (see Fig 16 in
Ens et al. (2012)). The infrasound source en-
ergy is comparable to the nominal optical en-
ergy, implying an unphysically high luminous
efficiency approaching 70%. Such a large ap-
parent luminous efficiency has only been docu-
mented for one other fireball, namely the April
23, 2001 Pacific bolide (Brown 2002).
This is yet another peculiar aspect of USG
20140108.
4Brown and Boroviˇ
cka
Figure 1. Light curve for USG 20140108. The top plot shows the radiant intensity as a function of
time while the lower plot shows the equivalent bolometric magnitude assuming a 6000K blackbody. The
intensity appears to drop below zero in the upper plot due to background subtraction; here the magnitude
approaches the limit near -16 to -17.
3. ESTABLISHING UNCERTAINTIES OF
USG THROUGH COMPARISON WITH
GROUND-BASED MEASUREMENTS
The available metric data for USG 20140108
does not permit a detailed analysis of uncer-
tainties for this particular case. We can, how-
ever, gain insight into the general size of USG
uncertainties by comparing fireballs simulta-
neously detected by USG sensors and ground-
based instruments.
Devillepoix et al. (2019) performed such an
analysis for USG data available through late
2018. They found that the time of the fire-
ball, the altitude of peak brightness, fireball
energy and location were most consistent with
ground-based records (within the reported pre-
cision of the USG data). The radiant and par-
ticularly the speed were least reliable. How-
ever, their analysis was largely qualitative and
limited to only 10 events.
In Table 3we summarize all 17 USG
events where ground-based or other indepen-
dent measurements are also available as of
mid-2023.
As the main focus of the interstellar nature
of USG 20140108 depends on its measured ve-
locity vector we focus on the accuracy of USG
measurements of speed and radiant direction.
Using the values from Table 3we show in
Figure 2the apparent speed difference be-
tween USG speeds and ground-based speeds as
a function of speed. We expect a priori that
any technique used by USG sensors to sample
position as a function of time will necessarily
be limited by sample cadence and, all other
things being equal, will result in higher uncer-
tainties at higher speeds. Precisely this trend
is also seen with ground-based cameras (e.g.
Vida et al. 2020).
critical review - interstellar 5
Table 2. Infrasound signal characteristics for USG 20140108. Here station is the International Monitoring
System designation for each infrasound array, range is the great circle distance from the USG fireball
location to the array (in kilometers) and Az is the difference between the theoretical and observed airwave
backazimuth. The delay is the total time from fireball occurrence to signal detection (given as arrival time in
UT) at a given array. Cel is the celerity (mean signal travel speed in km/s) while max amp is the maximum
amplitude of the Hilbert transformed signal (in Pa) and Max P-P is the maximum Hilbert transformed peak-
to-peak amplitude (in Pa). The Period is the period at maximum amplitude using the zero crossing method
described in ReVelle (1997); Edwards et al. (2006) while TP S D is the period in seconds corresponding to the
peak spectral energy from the power spectral density function of the bolide signal following the approach
of Ens et al. (2012).
Station Range Az Delay (sec) Cel (km/s) Arrival Max Amp (Pa) Max P-P (Pa) Period(sec) TP S D
I39PW 1749.7 0.95726.0 0.306 18:41 0.016 ±0.005 0.029 ±0.001 1.24 ±0.15 1.2
I07AU 2525.1 -2.58366.0 0.302 19:25 0.04 ±0.04 0.06 ±0.08 1.31 ±0.17 1.2
I04AU 4819.0 -116466.0 0.293 21:40 0.02 ±0.01 0.03 ±0.02 1.15 ±0.3 1.3
While the number statistics are low, we do
see in Figure 2a trend that the highest speed
USG fireballs show the largest difference com-
pared to ground-based estimates. Moreover,
USG tends to overestimate speeds as the event
speed increases.
A similar comparison of the radiant differ-
ence between USG and ground based data is
shown in Figure 3. Here no clear trend with
speed is apparent, though the uncertainty dis-
tribution is bimodal: a significant minority
have differences of order a couple of degrees
or less (low uncertainty population) while the
remainder (high uncertainty population) have
differences more than the median which is 10
degrees. The low uncertainty events have aver-
age radiant errors of less than 2 degrees, while
the high uncertainty population have a mean
and median error of just over 30 degrees.
Finally, we note that the absolute height dif-
ference between the USG peak brightness loca-
tion and that from ground-based instruments
is only 3 ±2.6 km and shows no obvious trend
with speed.
One other USG event provides insight into
uncertainties, namely the Bering Sea bolide of
Dec 18, 2018 . The radiant provided by USG
was found by Borovicka et al. (2020a) to be
13±9different than that found from triangu-
lation using several satellite views of the dust
cloud. While no independent speed estimate
was available, we note that the original speed
of 32 km/s posted on the CNEOS site was later
amended to 13.6 km/s, suggesting that factor
of two uncertainties in speeds are possible, par-
ticularly at higher apparent speeds.
These results agree qualitatively with those
of Devillepoix et al. (2019), with radiant and
speed being much less secure than the height
of peak brightness. What does this relation-
ship suggest about the likely uncertainty in the
speed and radiant for USG 20140108?
As the uncertainty in the speed correlates
positively with speed, a USG speed measure-
ment of 45 km/s could be expected to be 10-15
km/s above the true speed through simple lin-
ear regression extrapolation of Figure 2. The
radiant difference is harder to establish, but
at such high speeds it is difficult to imagine
the direction vector would be in the low un-
certainty group. Assuming association in the
high uncertainty group, a plausible radiant er-
ror would be of order 30 degrees based on Fig-
ure 2.
Thus, examination of USG metric data
where ground-truth measurements from other
6Brown and Boroviˇ
cka
instruments are available suggests plausible er-
rors for USG 20140108 are at least 10-15 km/s
in speed and 30in radiant direction.
4. ABLATION AND FRAGMENTATION
MODELLING
As another independent check on the prop-
erties of USG 20140108 we have tried to re-
produce the USG reported light curve with
the semi-empirical fragmentation model of
Boroviˇcka et al. (2020). The detected part
of the light curve consists of one faint flare
followed after 0.25 s by the series of three
bright flares separated mutually by about 0.1 s
(see Figure 1). The last (fourth) flare was
the brightest. According to USG, the maxi-
mum brightness point occurred at a height of
18.7 km. From the previous section, we expect
this height to be accurate to of order 3 km.
A series of models was created by varying
the initial meteoroid speed. The height of the
fourth flare was set to 18.7 km in all models
and the trajectory slope to the vertical line (i.e.
the zenith distance of the radiant) was kept at
the USG reported value of 63.2. The assumed
mass- and speed-dependence of the luminous
efficiency (τ) was similar to that Boroviˇcka et
al. (2020) but was scaled down two times since
the USG absolute magnitudes are computed
under the assumption that 3000 W source is
needed to produce a zero-magnitude meteor
while the model uses 1500 W. Moreover, τof
small fragments was increased, which helped
to reproduce the high amplitude of the flares.
While Boroviˇcka et al. (2020) used τ= 5% for
large (1 kg) and 2.5% for small (1 kg)
meteoroids at the speed of 15 km s1, we used
2.5% and 2% (instead of 1.25%), respectively.
The solid lines in Fig. 4shows the computed
light curves for five nominal models with initial
speeds between 12 and 30 km s1. These are
compared with the USG measurements. The
three main flares could be fitted similarly well
in all cases. They were modelled by sudden re-
lease of small fragments in the mass range 0.1
10 kg from the main meteoroid. Nevertheless,
there are significant differences between the
models in the earlier parts of the light curve,
namely between the first and the second flare
and before the first flare. The models with low
initial speeds, 12 and 16 km s1, are consistent
with the scattered data points but for higher
speeds the expected light curve becomes too
bright. At 30 km s1, the steady brightness
before the second flare is already much higher
than the actual brightness reached in the first
flare.
The parameters of the models are given in
Table 4. The meteoroid density was assumed
to correspond to typical stony meteorites. The
product of the drag and shape coefficients, ΓA,
corresponds approximately to spherical mete-
oroids. The ablation coefficient 0.005 s2km2,
which worked well in fitting fireball data for
which meteorites have also been recovered in
many instances (Boroviˇcka et al. 2020), was
used in all cases. The initial meteoroid mass
was adjusted so that all flares could be pro-
duced. As the light curve is a proxy for energy
deposition, assuming a constant initial kinetic
energy, the lower the speed the higher is the
mass needed to produce a fireball of the de-
sired brightness.
critical review - interstellar 7
Table 3. Comparison of speed, radiants and height of peak brightness for fireballs recorded in common
by USG sensors and other techniques. For fireballs producing recovered meteorites, the meteorite name is
given; for fireballs not producing meteorites, the name used in the associated reference is used. The first
line contains the ground-based data and the second line USG data. The date is given as year-month-day.
The columns ϕ,θgive the azimuth and zenith angle of the apparent local radiant while Rad is the angular
difference between radiants. The speed is given at the height of peak brightness, and V(km/s) is the
difference between speeds. Similarly, height (km) here refers to the height of peak brightness and ∆H (km)
is the difference in peak brightness height between USG and the other techniques. Also shown are the type
of recovered meteorites (eg How - Howardites, Euc - Eucrites ) as well as the reference where independent
metric data used for comparison to USG estimates were extracted. For Almahata Sitta, 2019MO and 2022
EB5 the meteoroid was detected prior to impact and the orbit used to compute the radiant of the fireball.
References are: 1. Jenniskens et al. (2009), 2. Milley (2010), 3. Borovicka et al. (2013b), 4. Boroviˇcka et
al. (2013), 5. Devillepoix et al. (2019), 6. Borovicka et al. (2017), 7. Unsalan et al. (2019), 8. Hildebrand
et al. (2018), 9. Jenniskens et al. (2021), 10. Kartashova et al. (2020), 11. Ceballos Izquierdo et al. (2021),
12. JPL-Horizons (2023a), 13. Borovicˇcka et al. (2021), 14. NorskMeteorNetwork (2023), 15. Vida et al.
(2021), 16. JPL-Horizons (2023b)
Fireball/
Meteorite
name
Date ϕθRad V
(km/s)
V
(km/s)
Ht
(km)
H
(km)
Met
Type
Ref
Almahata
Sitta
20081008 281.7 70 34.2 12.4 37 Euc 1
249.5 83.8 13.3 0.9 38.9 1.9
Buzzard
Coulee
20081121 348.6 22.8 31.9 18 31 H4 2
330.6 53 12.9 -5.1 28.2 -2.8
Kosice 20100228 297.1 56.3 38.1 15 36 H5 3
257.6 27.1 15.1 0.1 37 1
Chelyabinsk 20130215 103.5 71.5 4.3 19 30 LL5 4
99.9 74.1 18.6 -0.4 23 -7
Kalabity 20150102 346.4 70 14 13.4 40.2 5
331.7 71.6 18.1 4.7 38.1 -2.1
Romania 20150107 232.2 47 6.8 27.8 42.8 6
222.9 46.4 35.7 7.9 45.5 2.7
Sari¸ci¸cek 20150922 319.6 46.2 70.4 17.1 36.5 How 7
48.8 61.8 24.1 7 39.8 3.3
Baird Bay 20170630 6.7 17.2 2 9.6 25.7 5
11.6 18.6 15.2 5.6 20 -5.7
Crawford Bay 20170905 177.6 53.8 92 16.5 27 8
280.1 76.7 14.7 -1.8 36 9
Motopi Pan 20180602 95.2 64.9 1.9 17 27.8 How 9
95.9 66.7 16.9 -0.1 28.7 0.9
Ozerki 20180621 238 12.4 2.4 14.9 27.2 L6 10
237.5 10 14.4 -0.5 27.2 0.0
Vinales 20190201 171.8 39.5 15.6 16.6 22 L6 11
176.1 54.8 16.3 -0.3 23.7 1.7
2019MO 20190622 109.7 62.1 15.2 16.5 12
117.9 48.4 14.9 -1.6 25
Flensburg 20190912 188.1 65.3 1.2 19.4 40.7 C1-
ung
13
188.5 66.4 18.5 -0.9 42 1.3
Froslunda 20201107 243.6 17.9 1.2 17.4 Iron 14
242.2 16.8 16.7 -0.7 22.3
Novo Mesto 20200228 156.5 42.1 1.1 22.1 35 L5 15
155 42.7 21.5 -0.6 34.5 -0.5
2022 EB5 20220311 129.3 33.4 1.2 18.6 16
129.8 32.2 17.2 -1.4 33.3
8Brown and Boroviˇ
cka
Figure 2. Speed difference (USG-ground-based) as a function of USG reported speed among USG fireballs
for which independent speed estimates are available. Data are from Table 3.
Table 4. Parameters of light curve models
Model Initial Density ΓALumin. Initial First Flare Last Flare
Speed Efficien. Mass Height Speed Press. Height Speed Press.
km s1kg m3kg km km s1MPa km km s1MPa
12 12 3400 0.6 normal 45000 20.9 11.0 10 18.7 10.5 13
16 16 3400 0.6 normal 15000 21.5 14.2 15 18.7 13.2 21
20 20 3400 0.6 normal 6800 22.0 17.4 20 18.7 15.4 28
24 24 3400 0.6 normal 4600 22.6 20.7 26 18.7 17.6 37
30 30 3400 0.6 normal 3400 23.4 25.8 35 18.7 20.6 50
30-lt 30 3400 0.6 constant 4100 23.5 26.1 35 18.7 21.0 53
45-lt 45 3400 0.6 constant 5700 18.7 29.5 100
45-lt-hd 45 8000 0.6 constant 2100 18.7 33 130
45-exotic 45 8000 0.2 constant 800 27.2 44 54 18.7 40 190
critical review - interstellar 9
Figure 3. Radiant difference between USG and ground-based data as a function of USG reported speed.
Note that the ordinate is a log scale. Data are from Table 3.
10 Brown and Boroviˇ
cka
Figure 4. Models of the light curve for various speeds (colored curves) compared with the USG data (black
dots). See text for details.
critical review - interstellar 11
We have tried to change some of the param-
eters to be able to reproduce the amplitude of
the main flares in the light curve at differing
speeds. We note that the luminous efficiency
at higher speeds is uncertain. Boroviˇcka et al.
(2020), following earlier work of Ceplecha (e.g.
Ceplecha and ReVelle 2005), assumed that τ
is increasing linearly with speed above 25 km
s1. We have relaxed that assumption and
set τto 2.5% for all speeds and masses. The
corresponding light curve for 30 km s1, desig-
nated 30-lt, was then closer to the observations
but still too bright. The discrepancy was much
higher for 45 km s1, where the brightness was
well above the first two flares (see model 45-lt
in Fig. 4). The fragment masses in that model
needed to be set to 1 10 kg to fit the last
flare.
Next we investigated the case where the me-
teoroid density was increased to 8000 kg m3,
corresponding approximately to metallic me-
teorites. Still, the early part of the light curve
was too bright for 45 km s1(model 45-lt-
hd). To be able to produce the first flare
with such a high initial speed, we had to de-
crease ΓAto 0.2. This would correspond to
a very aerodynamic body with low drag and
low cross-section relative to the mass. We will
call this model ‘exotic’ henceforth. But even
the exotic model had problems in reproduc-
ing the light curve well. The brightness was
too high between the first and the second flare
(see Fig. 4). It was possible to remove these
discrepancies only by additional decreasing of
the ablation coefficient to 0.002 s2km2. The
fragment masses in this model were 0.1 kg.
Table 4also contains the height, speed, and
dynamic pressure at the time of the first and
the last flare. Dynamic pressure, computed
as p=ρv2, where ρis atmospheric density
and vis speed, is an approximate measure of
meteoroid strength. It is commonly assumed
that fragmentation occurs when dynamic pres-
sure exceeds the tensile or shear strength of the
meteoroid (Popova et al. 2011;Robertson and
Mathias 2017). In ordinary fireballs studied
by Boroviˇcka et al. (2020), pdid not exceed
5 MPa. The large Chelyabinsk impactor frag-
mented heavily under 1 5 MPa and only the
strongest daughter fragment reached 18 MPa
(Boroviˇcka et al. 2013).
The data compilation of Pohl and Britt
(2020) showed that strengths of meteorites
vary widely. Tensile strengths of stony non-
carbonaceous meteorites were measured in the
range 2 80 MPa, with the most typical value
about 40 MPa. For iron meteorites it was 40
500 MPa with the mean value being 340 MPa.
The mean values of compressive strengths are
about 250 MPa, and 400 MPa for stones and
irons, respectively. Data for shear strengths
are not available. It is evident, that atmo-
spheric fragmentation occurs at lower pres-
sures than the meteorite tensile strength, at
least for stones (atmospheric fragmentation of
large iron meteoroids has not been studied as
yet). This behaviour is usually explained as
being due to the presence of cracks in incom-
ing meteoroids (Boroviˇcka et al. 2020).
Dynamic pressures for the USG fireball were
large in all cases. Only for the lowest speeds
below 16 km s1, are they comparable or lower
than the maximum dynamic pressure reached
in Chelyabinsk. Of course, we cannot exclude
that the meteoroid did not contain any cracks
and the fragmentation occurred when the ma-
terial strength was reached, a rare but not un-
heard of situation as exemplified by the forma-
tion of the Carancas crater (Brown et al. 2008;
Borovicka & Spurn´y 2008). If the material was
similar to iron, the dynamic pressure even for
the highest speed (190 MPa) is consistent with
the material properties. However, it would be
strange in that case that the meteoroid frag-
mented repeatedly and into large fragments
and not into constituent grains (which would
lead to a single terminal flare).
12 Brown and Boroviˇ
cka
Moreover, the exotic model is not appropri-
ate for iron since it uses a low ablation coef-
ficient 0.005 s2km2. Since the melting tem-
perature of iron is lower than that of stone,
the ablation coefficient should be at least sev-
eral times higher. Under that condition, a me-
teoroid of speed of 45 km s1would not be
able to penetrate to the height of 18.7 km un-
less much larger than modelled (and orders of
magntitude more energy than indicated by the
infrasound records). The fireball would also
be much brighter than observed. The exotic
model can therefore be applied only to a hypo-
thetical material of high density and low ab-
lation ability (and body with highly aerody-
namic shape).
In summary, lightcurve modelling using the
measured speed of USG 20140108 can only re-
produce the observed lightcurve/flares for an
extremely unusual (high density, low ablation
coefficient) object with extremely low drag. In
contrast, assuming a lower speed allows rea-
sonable matches to the observed lightcurve
for non-exotic entry model parameters, though
with a relatively high strength compared to
other fireballs, but within the range of stony
meteorite tensile strengths.
5. DISCUSSION
In addition to USG 20140108, the dataset of
all USG fireballs shows five other fireballs with
hyperbolic orbits. The properties of all USG
hyperbolic fireballs are summarized in Table 5.
To illustrate the speed and radiant un-
certainties which can potentially move these
events from unbound to bound orbits we plot
each event in Figure 6, following the approach
of Kres´ak & Kres´akoa (1976); Hajdukov´a Jr
et al. (2019). Here the elongation of the appar-
ent radiant from the apex of the Earth’s way
(the direction vector of the Earth’s velocity) is
shown as a function of the entry speed. The
blue line demarcates bound orbits (to the left
of the line) from unbound orbits (to the right
of the line).
As shown by the figure, most of the USG
unbound orbits become bound presuming an
overestimate in speed of order a few km/s even
if the radiants are perfectly known. This is
well within expected uncertainties shown ear-
lier. Two of the events (at 30 and 35 km/s)
are closer to 10 km/s over the unbound limit
for fixed radiants. However, for these events,
adopting an uncertainty in the radiant of only
10-20 degrees result in bound orbits for much
smaller velocity overestimates, which is well
within expected USG radiant accuracy estab-
lished earlier.
The USG 20140108 event (represented by
the red cross) is again the most extreme of the
unbounded group. It would require an over-
estimate of more than 20 km/s presuming an
accurate radiant to move from bound to un-
bound. This is not much higher than our ex-
trapolated speed error estimate at such high
speeds. However, when allowing for radiant
uncertainty as well, an error of 20-30 degrees
would translate into a required speed reduc-
tion of only 10-15 km/s which together could
push the orbit to be bound. These values are
compatible with errors of similar magnitude
found in our earlier comparison between USG
fireballs also recorded by other instruments.
Given the high speed of USG 20140108 com-
pared to our calibration dataset, the required
uncertainties to bring the event to a bound or-
bit are, in our view, very reasonable.
Our lightcurve modelling assuming a chon-
dritic object agrees with the observed
lightcurve for cases where speeds are much
lower than the nominally measured value of 45
km/s. In particular, we cannot reconcile the
apparent lack of light production before the
observed flares for an object travelling at even
half the observed speed. If the speed is much
lower, there is no need to invoke exotic mate-
rials or shapes, only the requirement that the
critical review - interstellar 13
meteoroid be stronger compared to the back-
ground population of fireballs (Borovicka et al.
2020b), but still within the range of tensile
strength of stony meteorites.
The factor of several difference between the
infrasound source energy and USG lightcurve-
derived energy may be due to a smaller lumi-
nous efficiency used by the USG website com-
pared to the actual value of τ. The USG effi-
ciency was derived empirically from a subsam-
ple of USG fireballs where infrasound signals or
meteorites were available to calibrate impactor
size/energy (Brown et al. 2002) at character-
istic speeds of 15-18 km/s and peak bright-
ness heights of 30km. For the lower altitude
and probable higher speed of USG 2010108, a
larger τis likely, potentially explaining part of
the difference (Nemtchinov et al. 1997). A sim-
ilar difference may arise if the effective black-
body temperature of the fireball was different
than the USG assumed value of 6000K (Ce-
plecha et al. 1998).
Additionally, the low fragmentation height
of the fireball could also lead to smaller fun-
damental acoustic periods than the average
USG events used to produce period-yield re-
lations for bolides (Gi & Brown 2017). Since
the blast radius for fragmentation events scales
with ambient pressure as P1/3(McFadden
et al. 2021), energy release at lower heights
will produce smaller (apparent) source ener-
gies using the average relations as periods will
be smaller.
One interpretation of all the foregoing data
could be that USG 20140108 was produced
by an iron meteoroid. Irons constitute about
3.6% of all cm-sized fireballs observed by the
European Network (Voj´cek et al. 2020) and
just over 4% of all observed meteorite falls
(Greenwood et al. 2020). However, the frac-
tion of iron meteorite falls increases sharply
with mass (Bland & Artemieva 2006), with
25% of all meteorite falls of between 200kg and
1000 kg being irons. A clear strength selec-
tion effect is evident; the proportional increase
in irons as a function of mass underscores the
higher strength of the latter compared to stony
meteorites and their resistance to fragmenta-
tion.
Assuming 5% of all USG events are iron
we would expect of order 15 to have been ob-
served by USG and had speeds measured. Fig-
ure 5shows the height of peak brightness for
all USG fireballs as a function of speed over-
laid on lines of equal dynamic pressure. The
vast majority of all bolides detected by USG
have their peak brightness at locations where
the dynamic pressure is between 0.5 - 10 MPa.
While modest differences in height of peak
brightness are expected for objects of simi-
lar composition given the random nature of
cracks, populations separated by large peak
height differences may reflect more significant
physical property differences. For example, a
population which reach peak brightness below
0.2-0.3 MPa dynamic pressure is noticeable in
the top left of Figure 5and indicative of unusu-
ally weak objects separated from the remain-
ing population.
Similarly, there are a dozen events where
peak brightness is reached when dynamic pres-
sure exceeds 25 MPa. These dozen events
could be interpreted as being stronger on av-
erage than the majority of the USG popula-
tion (if the speeds are correct) or perhaps as
having internal components that are strong.
This includes USG 20140108 (extreme point
in the lower right hand corner) and also an-
other fireball which occurred on 20170309 with
a reported speed of 36.5 km/s and height of
peak brightness of 23 km. This also hap-
pens to be the 2nd most unbound orbit in the
USG dataset. The remaining four unbound
USG fireballs (see Table 5) are indistinguish-
able from the bound USG population in terms
of their height of peak brightness, speed.
While an iron interpretation is attractive,
the modelling in section 4strongly argues
14 Brown and Boroviˇ
cka
against an iron object. This is because an
iron would have much higher ablation coeffi-
cients than used in our modelling (Ceplecha
et al. 1998) and even at low speeds such high
mass loss rates would cause the fireball to be
visible much earlier than is observed. The
object would also have to be much larger
than modelled, further increasing the difficulty
in matching the lightcurve. Moreover, irons
do not typically show flares but rather have
smooth light curves (Voj´cek et al. 2020) as
would be physically expected. So, we can fit
the light curve either with a low speed (<20
km/s) and a reasonable set of parameters or
with a high speed combined with low ΓA, low
σand very high density/ strength. Such ma-
terial cannot be iron but some dense material
with high melting temperature.
Finally, the one fireball observed by USG
for which iron meteorites were recovered
(Froslunda - see Table 3) shows no distinct
flares and at 17 km/s entry speed reached peak
brightness at 23 km altitude. While this at the
bottom of the dense USG population in Figure
5it is still well above the deepest penetrators.
6. CONCLUSION
Our modelling of the USG 20140108 fireball
shows that we can fit the observed lightcurve
and heights with a low speed (<20 km/s)
using physical parameters consistent with a
stony impactors as measured for other simi-
lar fireballs (Borovicka et al. 2020b). The re-
ported high speed can only be matched with an
unusually low luminous efficiency, extremely
aerodynamic (low drag) shape and high den-
sity/strength. We are able to rule out an iron
composition for the latter as iron ablation pro-
ceeds via melting (rather than vaporization as
is the case for stony meteoroids) which leads
to high mass loss and a brightness too high to
match the measured lightcurve and deep pen-
etration of the fireball.
The source energy derived from infrasound
records is much smaller than the nominal USG
energy. We suggest the most likely explanation
is the lower than average fragmentaton height
produces an artifically small blast cavity which
translates into smaller periods and hence ap-
parent source energies than the typical USG
fireball population.
Examination of the 17 USG fireballs having
independent metric measurements show that
the speed and radiant errors are large in many
cases and vary widely in a non systematic man-
ner. Higher speed events tend to be overesti-
mated in speed, while radiant errors of order
tens of degrees are common in the dataset. To
resolve these contradictions access to the orig-
inal metric data and an understanding of the
measurement process is required. Ultimately
this is the only means to conclude with confi-
dence the hyperbolic or bound nature of USG
20140108.
Appealing to Occam’s razor we suggest that
a significant error in the velocity vector is the
most probable explanation for the apparent in-
terstellar nature of USG 20140108 (i.e. it did
not have an unbound orbit prior to Earth im-
pact). This follows from our determination of
the apparent USG uncertainties, and the fact
that for any fixed time sampling observing sys-
tem the relative meteor velocity error will in-
crease with apparent speed (Vida et al. 2020).
Based on the foregoing analysis as USG
20140108 is at the very upper limit of speeds
reported by USG systems, the required error
in velocity is compatible with moving the or-
bit from an unbound to bound state. More-
over, the extraordinary physical properties of
the associated meteoroid required to model the
height and brightness behaviour of the event if
it is high speed can instead be matched using
properties similar to other chondritic fireballs
presuming the measured speed and/or radiant
are in error.
critical review - interstellar 15
Figure 5. All USG fireballs where speeds and heights of peak brightness have been measured. The labelled
curves are lines of equal dynamic pressure.
A similar conclusion is reached by both
Vaubaillon (2022) and Hajdukova (2023).
7. ACKNOWLEDGEMENTS
PGBs contribution was made possible
by funding under NASA Meteoroid En-
vironment Office Cooperative agreement
80NSSC21M0073 and by the Natural Sciences
and Engineering Research Council of Canada
(Grants no. RGPIN-2018-05659)as well as by
the Canada Research Chairs Program. JB was
supported by grant no. 19-26232X from the
Czech Science Foundation.
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APPENDIX
A. INFRASOUND SIGNALS
Figure 7shows an example of the beamformed signal as detected at the closest station, I39PW.
Figure 7. Beamformed array arrivals of the infrasound from USG 20140108 as detected at the I39PW
array in Palau some 1750 km from the fireball. Window lengths of two minutes across the seven element
array were stepped with 80% overlap to find the best beam direction per window. The top plot shows the
signal correlation in each window - here the coherent airwave is visible for values of the coefficient above
0.2. The second plot shows the apparent speed of the signal across the array while the third shows the
backazmiuth of the signal in each time window. The bottom plot is the pressure signal at array element 1
bandpassed from 0.7-3 Hz. The signal is highlighted in green.
22 Brown and Boroviˇ
cka
Table 5. All observed USG fireballs with hyperbolic orbits. All angular
elements are in degrees and J2000.0. Here Ht refers to height at peak
brightness and Vel is apparent in atmosphere speed in km/s. The total
radiated energy (J) as given on the USG website is also shown, together
with each fireball’s orbital elements. ϵis the elongation of the apparent
radiant from the apex of the Earth’s way, Vhis the heliocentric velocity
of the bolide and αgand δgare the geocentric radiant coordinates.
Time (UT) Lat Long Ht (km) Vel Rad Energy (J) q(AU) a (AU) e inc ωϵVh αgδg
2014-01-08 17:05 1.3S 147.6E 18.7 44.8 3.1×1010 0.7 -0.5 2.4 9.7 57.7 108.2 108.1 61.0 88.5 13.3
2017-03-09 4:16 40.5N 18.0W 23.0 36.5 4.0×1011 0.7 -1.3 1.5 23.9 239.8 348.5 99.4 49.8 170.2 34.3
2022-07-28 1:36 6.0S 86.9W 37.5 29.9 2.5×1011 0.9 -1.9 1.5 23.4 221.0 124.7 119.4 47.0 276.1 14.8
2009-04-10 18:42 44.7S 25.7E 32.4 19.1 2.7×1011 1.0 -3.6 1.3 6.2 357.7 200.9 157.2 44.9 108.0 4.2
2021-05-06 5:54 34.7S 141.0E 31.0 26.6 2.1×1010 0.7 -6.6 1.1 5.7 298.2 225.6 108.8 43.5 61.9 12.1
2015-02-17 13:19 8.0S 11.2W 39.0 28.8 3.3×1010 0.6 -10.5 1.1 0.4 286.9 148.5 100.5 43.4 339.3 -9.3
critical review - interstellar 23
Figure 8. Beamformed array arrivals of the infrasound from USG 20140108 as detected at the I07AU
array in Australia at a range of 2525 km. Window lengths of two minutes across the seven element array
were stepped with 90% overlap to find the best beam direction per window. The top plot shows the signal
correlation in each window - here the coherent airwave is visible for values of the coefficient above 0.1. The
second plot shows the apparent speed of the signal across the array while the third shows the backazmiuth
of the signal in each time window. The bottom plot is the pressure signal at array element 1 bandpassed
from 0.5-2 Hz. The signal is highlighted in green.
24 Brown and Boroviˇ
cka
Figure 9. Beamformed array arrivals of the infrasound from USG 20140108 as detected at the I04AU
array in Australia at a range of 4820 km. Window lengths of two minutes across the seven element array
were stepped with 90% overlap to find the best beam direction per window. The top plot shows the signal
correlation in each window - here the coherent airwave is visible for values of the coefficient above 0.15. The
second plot shows the apparent speed of the signal across the array while the third shows the backazmiuth
of the signal in each time window. The bottom plot is the pressure signal at array element 1 bandpassed
from 0.9-1.6 Hz. The signal is highlighted in green.
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