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Accepted for publication in Monthly Notices of the Royal Astronomical Society
(MNRAS) on 2019 March 29
MULTIWAVELENGTH OBSERVATIONS OF A
BRIGHT IMPACT FLASH DURING THE JANUARY
2019 TOTAL LUNAR ECLIPSE
José M. Madiedo
1
, José L. Ortiz
2
, Nicolás Morales
2
, Pablo Santos-Sanz
2
1
Facultad de Ciencias Experimentales, Universidad de Huelva. 21071
Huelva (Spain).
2
Instituto de Astrofísica de Andalucía, CSIC, Apt. 3004, Camino Bajo de
Huetor 50, 18080 Granada, Spain.
ABSTRACT
We discuss here a lunar impact flash recorded during the total lunar eclipse
that occurred on 2019 January 21, at 4h 41m 38.09 ± 0.01 s UT. This is the
first time ever that an impact flash is unambiguously recorded during a lunar
eclipse and discussed in the scientific literature, and the first time that lunar
impact flash observations in more than two wavelengths are reported. The
impact event was observed by different instruments in the framework of the
MIDAS survey. It was also spotted by casual observers that were taking
images of the eclipse. The flash lasted 0.28 seconds and its peak luminosity
in visible band was equivalent to the brightness of a mag. 4.2 star. The
projectile hit the Moon at the coordinates 29.2 ± 0.3 ºS, 67.5 ± 0.4 ºW. In
this work we have investigated the most likely source of the projectile, and
the diameter of the new crater generated by the collision has been
calculated. In addition, the temperature of the lunar impact flash is derived
Accepted for publication in Monthly Notices of the Royal Astronomical Society
(MNRAS) on 2019 March 29
from the multiwavelength observations. These indicate that the blackbody
temperature of this flash was of about 5700 K.
KEYWORDS: Meteorites, meteors, meteoroids, Moon.
1. INTRODUCTION
The Earth and the Moon continuously experience the impact of meteoroids
that intercept the path of both celestial bodies. The analysis of these
collisions provides very valuable data that allows us to better understand the
Earth-Moon meteoroid environment. The study of meteoroid impacts on the
Moon from the analysis of the brief flashes of light that are generated when
these particles hit the lunar ground at high speeds has proven to be very
useful to investigate this environment. For instance, the analysis of the
frequency of these events can provide information about the impact flux on
Earth (see e.g. Ortiz et al. 2006; Suggs et al. 2014; Madiedo et al. 2014a,
2014b). Also the initial kinetic energy of the projectile, its mass, and the
size of the resulting crater can be obtained. For events produced by large
(cm-sized or larger) particles, one of the main benefits of this technique over
the systems that analyze meteors produced by the interaction of meteoroids
with the atmosphere of our planet is that a single instrument covers a much
larger area on the lunar surface (typically of an order of magnitude of 10
6
km
2
) than that monitored in the atmosphere of the Earth by a meteor-
observing station.
Accepted for publication in Monthly Notices of the Royal Astronomical Society
(MNRAS) on 2019 March 29
The monitoring of lunar impact flashes by means of telescopes and high-
sensibility cameras dates back to the 1990s. Since the first systematic
observations performed by Ortiz et al. (1999) in this field, different authors
have obtained information about the collision with the lunar surface of
meteoroids from several sources. Thus, flashes associated with impactors
belonging to the sporadic meteoroid background and to different meteoroid
streams have been recorded and described (see for instance Madiedo et al.
2019 for a comprehensive review about this topic). Some synergies have
been found when this method is employed in conjunction with the technique
based on the monitoring and analysis of meteors produced by meteoroids
entering the atmosphere (Madiedo et al. 2015a,b). Even fresh impact craters
associated to observed lunar impact flashes have been also observed by
means of the Lunar Reconnaissance Orbiter (LRO) probe, which is in orbit
around the Moon since 2009 (Robinson et al. 2015, Madiedo and Ortiz
2018, Madiedo et al. 2019). More recently, since 2015, lunar impact flashes
observations simultaneously performed in several spectral bands allowed us
to estimate the temperature of impact plumes (Madiedo and Ortiz 2016;
Madiedo et al. 2018; Bonanos et al. 2018).
Despite its multiple advantages, this technique has also some important
drawbacks, since the results are strongly dependent on the value given to the
luminous efficiency. This parameter is the fraction of the kinetic energy of
the projectile emitted as visible light as a consequence of the collision. The
value of the luminous efficiency is not known with enough accuracy. The
Accepted for publication in Monthly Notices of the Royal Astronomical Society
(MNRAS) on 2019 March 29
comparison between the calculated size of fresh craters associated to
observed impact flashes and the experimental size measured by probes
orbiting the Moon can play a fundamental role to better constrain the value
of this efficiency (Ortiz et al. 2015).
Another drawback of this technique is related to the fact that, since most of
these flashes are very dim, they must be recorded against a dark
background. For this reason, the method is based on the monitoring of the
nocturnal region of the Moon. The area directly illuminated by the Sun must
be avoided in order to prevent the negative effects of the excess of scattered
light entering the telescopes. This implies that, weather permitting, the
monitoring by means of telescopes of these flashes is limited to those
periods where the illuminated fraction of the lunar disk ranges between
about 5% and 50-60%, i.e., about 10 days per month during the waxing and
waning phases (Ortiz et al. 2006, Madiedo et al. 2019). Lunar eclipses
provide another opportunity to monitor lunar impact flashes out of this
standard observing period, since during these the Moon gets dark. However,
because of the typical duration of lunar eclipses, this extra observational
window is relatively short when compared to a standard observing session.
Besides, the possibility to detect dimmer impact flashes, which are more
frequent than brighter ones, depend on the intrinsic brightness of the eclipse,
which in turn depend on the aerosol content at stratospheric levels. In
general, the lunar ground is brighter in visible light during a lunar eclipse
than the lunar ground in standard observing periods during the waning and
Accepted for publication in Monthly Notices of the Royal Astronomical Society
(MNRAS) on 2019 March 29
waxing phases. These factors, which pose some difficulties to the detection
of lunar impact flashes, might have contributed to the fact that, despite
several researchers have conducted impact flashes monitoring campaigns
during lunar eclipses, no team succeeded until now. The first lunar impact
flash monitoring campaign performed by our team during a total lunar
eclipse was conducted by the second author of this work in October 2004. In
2009, the pioneer survey developed by Ortiz et al. (1999) was renewed and
named Moon Impacts Detection and Analysis System (MIDAS) (Madiedo
et al. 2010; Madiedo et al. 2015a, 2015b). This project is conducted from
three astronomical observatories located in the south of Spain: Sevilla, La
Sagra and La Hita (Madiedo and Ortiz 2018, Madiedo et al. 2019). In this
context, our survey observed a flash on the Moon during the total lunar
eclipse that took place on 2019 January 21. This flash was also spotted by
casual observers that were taking images of this eclipse, or streaming it live
on the Internet
(https://www.reddit.com/r/space/comments/ai79zy/possible_meteor_impact
_on_moon_during_the_eclipse/). The MIDAS survey was the first to
confirm that this flash was generated as a consequence of the collision of a
meteoroid with the lunar soil at high speed, so that this is the first lunar
impact flash ever recorded during a lunar eclipse and discussed in the
scientific literature. The news was covered by communication media all
around the world. From a scientific point of view, it offered the opportunity
to monitor the Moon with an angular orientation very different to that of the
regular campaigns at waxing and waning phases and it was a good
Accepted for publication in Monthly Notices of the Royal Astronomical Society
(MNRAS) on 2019 March 29
opportunity to test new equipment for the monitoring of lunar impact
flashes, and provided valuable data in relation to the study of impact
processes on the Moon. We focus here on the analysis of this impact event.
2. OBSERVATIONAL TECHNIQUE
The impact flash discussed in this work was observed from Sevilla on 2019
January 21. Our systems at the observatories of La Sagra and La Hita could
not operate because of adverse weather conditions. In Sevilla, five f/10
Schmidt-Cassegrain telescopes were used. Two of these instruments had an
aperture of 0.36 m, and the other three telescopes had a diameter of 0.28 cm.
These telescopes employed a Watec 902H Ultimate video camera connected
to a GPS-based time inserter to stamp time information on each vide frame.
The configuration of these cameras, which are sensitive in the wavelength
range between, approximately, 400 and 900 nm, is explained in full detail in
Madiedo et al. (2018). The observational setup consisted also of two 0.10 m
f/10 refractors endowed with Sony A7S digital cameras, which provided
colour imagery and employ the IMX235 CMOS sensor. One of these was
configured to take still images each 10 s with a resolution of 4240x2832
pixels, while the other recorded a continuous video sequence of the eclipse
at 50 fps with a resolution of 1920x1080 pixels. A third Sony A7S camera
working in video mode was attached to a Schmidt-Cassegrain telescope
with an aperture of 0.24 m working at f/3.3. However, because of a
technical issue that occurred during the eclipse, this telescope could not be
finally operated. The Sony A7S cameras are sensitive within the wavelength
Accepted for publication in Monthly Notices of the Royal Astronomical Society
(MNRAS) on 2019 March 29
range between, approximately, 400 and 700 nm. These have been used in
the framework of our survey for the first time during this monitoring
campaign to take advantage of the colour information they could provide.
Also, the larger field of view of these instruments allowed for a full
coverage of the lunar surface during the totality phase of the eclipse, in
contrast with the Schmidt-Cassegrain telescopes with the Watec cameras,
which can monitor only an area of the Moon of around 4· 10
6
to 8·10
6
km
2
(see for instance Madiedo et al. 2015a,b and Ortiz et al. 2015).
No photometric filter was attached to the cameras employed with the 0.36 m
and two of the 0.28 m Schmidt-Cassegrain telescopes. These provided
images in the wavelength range between, approximately, 400 and 900 nm.
The third 0.28 m SC telescope employed a Johnson-Cousin I filter.
Observations performed with the two refractors were also unfiltered.
We did not focus on the monitoring of any particular region on the lunar
disk. Instead, our telescopes were aimed so that the whole lunar disk was
monitored during the totality phase of the eclipse, with each instrument
covering a specific area of the lunar surface, and with at least two
instruments monitoring a common area. Before and after the totality, the
region of the Moon not occulted by the Earth's shadow was avoided. The
MIDAS software (Madiedo et al. 2010, 2015a) was employed to
automatically detect lunar impact flashes in the images obtained with the
above-mentioned instrumentation.
Accepted for publication in Monthly Notices of the Royal Astronomical Society
(MNRAS) on 2019 March 29
3. OBSERVATIONS
Our lunar monitoring campaign took place on 2019 January 21 from 3h 33m
UT to 6h 50m UT. These times correspond to the first and last contact with
the Earth's umbra, respectively. Excellent weather conditions allowed us to
monitor the Moon during the whole time interval, so the effective observing
time of was of 3.2 hours. This resulted in the detection of a flash at 4h 41m
38.09 ± 0.01 s UT (Figure 1), about 21 seconds after the totality phase of the
eclipse began. This event, which lasted 0.28 s, was simultaneously recorded
by two of our instruments: one of the 0.36 m Schmidt-Cassegrain
telescopes, and the 0.1 m refractor with the Sony A7S camera that recorded
the continuous video sequence of the eclipse. This flash was also reported in
social networks by several observers at different locations in Europe,
America and Africa
(https://www.reddit.com/r/space/comments/ai79zy/possible_meteor_impact
_on_moon_during_the_eclipse/). The MIDAS team confirmed that it was
associated with an impact event on the Moon. Table 1 contains the main
parameters derived for this impact flash. By means of the MIDAS software
(Madiedo et al. 2015a, 2015b) we determined that the impactor hit the
Moon at the selenographic coordinates 29.2 ± 0.3 ºS, 67.5 ± 0.4 ºW, a
position close to crater Lagrange H. This is located next to the west-south-
west portion of the lunar limb.
Accepted for publication in Monthly Notices of the Royal Astronomical Society
(MNRAS) on 2019 March 29
It is worth mentioning that astronomers at the Royal Observatory in
Greenwich reported a second flash at 4:43:44 UT (Emily Drabek-Maunder,
personal communication). We tried to locate this flash in our recordings by
checking them automatically with our MIDAS software. We also checked
them manually, by performing a visual inspection of the videos frame by
frame. We allowed for a timing uncertainty of around 1 minute, which is
well above the 5 seconds time difference between the time reported by this
observatory for the first flash (4:41:43 UT) and the time specified by our
GPS time inserters. However, this event was not present in any of the
images recorded by our systems and, to our knowledge, no other casual
observer spotted it. This means that it should have been produced by a
different phenomenon, and not by a meteoroid hitting the lunar ground. The
MIDAS survey uses at least two instruments monitoring the same lunar area
in order to have redundant detection to discard false positive impact flashes
due to cosmic ray hits, satellite glints and other possible phenomena that
may mimic the impact flashes.
4. RESULTS AND DISCUSSION
4.1. Impactor source
Since the technique employed to detect lunar impact flashes cannot
unambiguously provide the source of the impactors that produce these
events (Madiedo et al. 2015a, 2015b, 2019), we have followed the approach
described in (Madiedo et al. 2015a, 2015b) to determine the most likely
source of the meteoroid that generated the flash discussed here.
Accepted for publication in Monthly Notices of the Royal Astronomical Society
(MNRAS) on 2019 March 29
The observing date did not coincide with the activity period of any major
meteor shower on our planet and so the impactor should be associated either
with a minor meteoroid stream or with the sporadic meteoroid component.
Our meteor stations, which operate in the framework of the SMART project
(Madiedo 2014, 2017), recorded that night meteors from the January Comae
Berenicids (JCO), the δ-Cancrids (DCA), and the ρ-Geminids (RGE), but
the activity of all of these corresponded to a zenithal hourly rate (ZHR) < 1
meteor/h. Besides, the geometry for the impact of the DCA and RGE
streams did not fit that of the lunar impact flash: these meteoroids could not
hit the lunar region where the flash was recorded. So, we considered the
sporadic background and the JCO meteoroid stream as potential sources of
the event. The association probabilities corresponding to these sources,
labelled as p
SPO
and p
JCO
, respectively, were obtained by following the
technique developed by Madiedo et al. (2015a, 2015b). Thus we have
calculated p
JCO
with our software MIDAS, which obtains this probability
from Equation (15) in the paper by Madiedo et al. (2015b). In this
calculation the zenithal hourly rate and the population index of the January
Comae Berenicids have been set to 1 meteor/h and 3, respectively, and
HR=10 meteors/h was set for the activity of the sporadic component (see for
instance Dubietis and Arlt, 2010). From this analysis p
JCO
yields 0.01, with
p
SPO
= 1 - p
JCO
= 0.99. According to this, the probability that the impactor is
linked to the sporadic meteoroid component is of about 99%. In these
calculations an average impact velocity and an impact angle of sporadics on
Accepted for publication in Monthly Notices of the Royal Astronomical Society
(MNRAS) on 2019 March 29
the Moon of 17 km s
-1
and 45º, respectively, have been assumed (Ortiz et al.
1999). For impactors associated with the JCO meteoroid stream this velocity
was set to 65 km s
-1
(see e.g. Jenniskens 2006) and, according to the impact
geometry, the angle of impact would be of around 54º in this case.
4.2. Impactor kinetic energy and mass
We recorded the impact flash with the Watec camera in white light only.
Since no observations with different photometric filters were available for
this CCD device, we could not employ color terms for the photometric
analysis of the event. As explained in the next section, color terms could be
employed in the case of the Sony A7S camera. So, as in previous works
(see, e.g., Ortiz et al. 2000, Yanagisawa et al. 2006, Madiedo et al. 2014),
the brightness of the flash as recorded with the Watec camera was estimated
by comparing the luminosity of this event with the known V magnitude of
reference stars observed with the same instrumentation at equal airmass. In
this way we could determine that the peak magnitude of the impact flash
was 4.2 ± 0.2. Figure 2 shows the lightcurve of the flash as recorded by
means of the 0.36 m telescope that spotted the event. Using t=0.28 in the
empiric equation
t=2.10exp(-0.46±0.10m) (1)
that links impact flash duration t and magnitude m (Bouley et al. 2012), we
come up with a 4.1 mag for the flash, which is close to the derived 4.2 mag.
Accepted for publication in Monthly Notices of the Royal Astronomical Society
(MNRAS) on 2019 March 29
The calculations in this section are performed from the data collected by this
instrument, since its larger aperture and the higher sensitivity of its CCD
camera allowed us to record the evolution of the impact flash in much more
detail than with the 0.1 m refractor. This refractor telescope just registered
the peak luminosity of the flash and so the lightcurve of the event cannot be
constructed from its recordings.
As explained in detail in Madiedo et al. (2018), the energy radiated on the
Moon by the flash can be obtained from the integration of the power
radiated by the event:
2)5.2/m(8
Rf10·10·75.3P λ∆π=
−−
(2)
Here the magnitude of the flash varies with time according to the lightcurve
of the event, and f quantifies the degree of isotropy of the emission of light.
Since we have considered that light was isotropically emitted from the lunar
ground, we have set f = 2 (Madiedo et al. 2018). The distance between our
observatory on Earth and the impact location on the Moon at the instant
when the event took place was R= 364831.2 km. For the wavelength range
∆λ corresponding to the luminous range we have set ∆λ = 0.5 µm (see for
instance Ortiz et al. 2000 and Madiedo et al. 2019).. By entering these
parameters in Eq. (2) the energy radiated on the Moon yields E =
(1.96±0.39)·10
7
J.
Accepted for publication in Monthly Notices of the Royal Astronomical Society
(MNRAS) on 2019 March 29
This radiated energy is a fraction of the kinetic energy E
k
of the meteoroid.
That fraction is called the luminous efficiency, which is wavelength-
dependent and is usually denoted by η (Bellot Rubio et al. 2000a, 2000b;
Ortiz et al. 2000; Madiedo et al. 2018, 2019):
E = η E
k
(3)
Since the value of the radiated energy derived from Eq. (2) depends on the
wavelength range considered, the luminous efficiency for that same spectral
range defined ∆λ by must be employed. On the contrary, we would arrive to
the non-sense conclusion that the kinetic energy of the projectile would be
also a function of the spectral range, instead of depending only on the mass
and velocity of the projectile. The concept "luminous" refers to the above-
mentioned luminous range, and it was defined to correspond to the range of
sensitivity of typical CCD detectors (i.e., from around 400 to about 900 nm)
used in the first works on lunar impact flashes and luminous efficiencies
(see e.g. Bellot-Rubio et al. 2000a, 2000b; Ortiz et al. 2000; Yanagisawa et
al. 2006). Other wavelength ranges can be of course defined and employed,
but this consistency between ∆λ, E and η must be maintained. For other
spectral ranges the fraction of the kinetic energy of the impacting meteoroid
converted into radiation in the corresponding photometric bands should be
denoted by using subscripts, such as η
R
, for the R-band, η
I
for the I-band,
etc., to avoid confusing it with η (Madiedo et al. 2018, 2019). In previous
works the value employed for the luminous efficiency was η=2·10
-3
(Ortiz
Accepted for publication in Monthly Notices of the Royal Astronomical Society
(MNRAS) on 2019 March 29
et al. 2006, 2015). However, this value was derived by assuming f=3 for the
degree of isotropy factor (see, for instance, Ortiz et al. 2006). Since in this
work we have considered f=2, we have to multiply this value of the
efficiency by 3/2, as explained in Madiedo et al. (2018). As a consequence
of this, the value considered for η in the luminous range for the flash yields
η = 3·10
-3
. In this way, the kinetic energy E
k
of the impactor is E
k
=
(6.55±0.63)·10
9
J. The impactor mass M derived from this kinetic energy is
M = 45 ± 8 kg for a sporadic meteoroid impacting at velocity of 17 km s
-1
.
Its size is readily obtained from the bulk density of the particle. The average
value of this bulk density for projectiles associated with the sporadic
meteoroid background is ρ
P
=1.8 g cm
-3
according to Babadzhanov and
Kokhirova (2009). This density yields a diameter for the impactor D
P
= 36 ±
2 cm. However, if the projectile consisted of soft cometary materials, with a
bulk density of 0.3 g cm
-3
, or ordinary chondritic materials, with ρ
P
= 3.7 g
cm
-3
(Babadzhanov and Kokhirova 2009), the size of the projectile would
yield D
P
= 66 ± 4 cm and D
P
= 29 ± 2 cm, respectively.
4.3. Temperature of the impact plume
Unfortunately, the impact flash was not recorded by the 0.28 m telescope
with the Johnson I filter, since the event took place outside the field of view
of this instrument. So, we could not derive the temperature of the impact
flash by comparing the energy flux density measured in the luminous and
the I ranges (Madiedo et al. 2018). Instead, we followed here a different
approach on the basis of the colour images recorded by the 0.1 m refractor
Accepted for publication in Monthly Notices of the Royal Astronomical Society
(MNRAS) on 2019 March 29
and the Sony A7S camera. The decomposition of these colour images into
its individual R, G and B channels (Figure 3) provides a multiwavelength
observation of the impact flash, which can be employed, for instance to
derive the flash temperature, assuming blackbody emission. To do so, we
have performed a photometric calibration of the Sony A7S camera to derive
the flash magnitude in the Johnson-Cousins R, V and B bands from its
measured luminosity in R, G and B channels of the video stream. For this
conversion color term corrections are necessary. It is worth mentioning that
the Sony A7S camera has a built-in NIR blocking filter, but in the spectral
response of the device, no leakage in the NIR was observed. The calibration
procedure has been performed as follows.
The magnitudes m
R
, m
V
and m
B
in the Johnson-Cousins photometric system
are given by the following standard relationships:
m
R
= r + ZP
R
+ (m
V
-m
R
) C
R
- K
R
A (4)
m
V
= v + ZP
V
+ (m
V
-m
R
) C
V
- K
V
A (5)
m
B
= b + ZP
B
+ (m
B
-m
V
) C
B
- K
B
A (6)
In these equations ZP
R
, ZP
V
, and ZP
B
are the corresponding zero points for
each photometric band, K
R
, K
V
, and K
B
are the extinction coefficients, and
A is the airmass; r, v, and b are the instrumental magnitudes in R, V and B
band, and are defined by
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(MNRAS) on 2019 March 29
r = -2.5log(S
R
) (7)
v = -2.5log(S
G
) (8)
b = -2.5log(S
B
) (9)
where S
R
, S
G
, and S
B
are the measured signals. We employed 30 calibration
stars within the Messier 67 open cluster, with known m
R
, m
V
and m
B
, to
obtain the value of the color terms C
R
, C
V
, and C
B
and the coefficients ZP
R
,
ZP
V,
ZP
B
, K
R
A, K
V
A, and K
B
A by performing a least-squares fit (Figures 4
to 6). These stars were observed with the same refractor telescope and Sony
A7S camera employed to record the flash. Their signals S
R
, S
G
and S
B
were
measured by performing an aperture photometry. Since the calibration stars
and the impact flash were observed at the same airmass, the least-squares fit
provided the sum of ZP and KA in a single constant for each band R, V and
B. The values resulting from this fit are shown in Table 2. By inserting in
Eqs (4-6) the measured flash signals in R, G and B channels, the peak
magnitude of the flash in R, V and B bands yield, respectively, m
R
= 3.53 ±
0.19, m
V
= 4.08 ± 0.10 and m
B
= 4.75 ± 0.09. The value calculated for m
V
fits
fairly well the 4.2 ± 0.2 magnitude in V band derived from the images
obtained with the Watec camera.
From these magnitudes, the energy flux densities observed on our planet for
the above-mentioned bands (labelled as F
R
, F
V
, and F
B
) have been estimated
by employing the following equations:
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(MNRAS) on 2019 March 29
)5.2/m(
8
R
R
10·10·80.1F
−
−
= (10)
)5.2/m(
8
V
V
10·10·75.3F
−
−
= (11)
)5.2/m(
8
B
B
10·10·70.6F
−
−
= (12)
where the multiplicative constants 1.80·10
-8
, 3.75·10
-8
and 6.70· 10
-8
correspond to the irradiances, in Wm
-2
µm
-1
, for a mag. 0 star in the
corresponding photometric band. The effective wavelengths for these bands
are λ
R
= 0.70 µm, λ
V
= 0.55 µm, and λ
B
= 0.43 µm, respectively. These
parameters have been provided by the magnitude to flux converter tool of
the Spitzer Science Center
(http://ssc.spitzer.caltech.edu/warmmission/propkit/pet/magtojy/). The flux
densities given by Eqs (10-12) are plotted in Figure 7. By assuming that the
flash behaves as a blackbody, these flux densities have been fitted to
Planck's radiation law. The best fit is obtained for T = 5700 ± 300 K. This
temperature agrees with the statistics of flash temperatures derived with 2-
color measurements from the Neliota survey, for which blackbody
temperatures ranging between 1300 and 5800 K have been estimated
(Avdellidou and Vaubaillon 2019). Our result is in the high-end tail of the
blackbody temperature flash distribution shown in Avdellidou and
Vaubaillon (2019) from a sample of 55 impact flashes with magnitudes in R
band ranging between 6.67 to 11.80. Lower temperatures can be fit to our
data by assuming optically thin emission modulated by the optical depth,
but we cannot determine the optical depth of the emitting hot cloud at
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(MNRAS) on 2019 March 29
different wavelengths without making too many assumptions. When
observations at 4 or more wavelengths become available we will be able to
shed more light on this.
4.4. Crater size and potential observability by lunar spacecraft
The estimation of the size of fresh craters associated with observed lunar
impact flashes is fundamental to allow for a better constraint of the
luminous efficiency, a key parameter which is not yet known with enough
accuracy. Thus, if these craters are later on observed and measured by
probes in orbit around the Moon, the comparison between predicted and
experimental sizes is of a paramount importance to test the validity of the
parameters and theoretical models employed to analyze these impacts.
Different models, which are also called crater-scaling equations, can be
employed to estimate the size of these fresh craters, and most studies in
these field employ either the Gault model or the Holsapple model. The
Gault equation is given by the following relationship (Gault, 1974):
(
)
3/129.0
k
5.0
t
6/1
p
sinE25.0D θρρ=
−
(13)
D is the rim-to-rim diameter, ρ
p
and ρ
t
are the projectile and target bulk
densities, respectively, and the angle of impact θ is measured with respect to
the local horizontal (Melosh, 1989). We have employed θ=45º for sporadic
meteoroids, and for the target bulk density we have considered ρ
t
= 1.6 g
cm
-3
. By entering in this model the previously-obtained value of the kinetic
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energy E
k
, the diameter D for impactor bulk densities ρ
p
of 0.3, 1.8 and 3.7
g cm
-3
yields 10.1 ± 0.5 m, 13.6 ± 0.6 m, and 15.3 ± 0.7 m, respectively.
We have also derived the crater size from the following equation, which was
proposed by Holsapple (1993):
.6.2
3/1
=
t
v
r
M
KD
ρ
π
(14)
D is again the rim-to-rim diameter, and π
v
is an adimensional factor which
has the following form:
µ+
µ−
µ+
µ
−ν
µ
µ−−ν
ρ
ρ
θρ
+
ρ
ρ
θ
=π
2
3
2
2
3
26
P
t
2
t
2
3
26
P
t
2
1v
))sin(V(
Y
K
))sin(V(
ag
K
(15)
with K
1
=0.2, K
2
=0.75, K
r
=1.1, µ=0.4, ν=0.333 and Y = 1000 Pa. The value
of the gravity on the lunar surface is g = 0.162 m s
-2
; the parameters a, M,
and V are the impactor radius, mass, and impact velocity, respectively. For
meteoroid bulk densities ρ
p
of 0.3, 1.8 and 3.7 g cm
-3
, Eq. (14) yields for the
rim-to-rim crater diameter D 10.4 ± 0.5 m, 13.3 ± 0.6 m, and 15.8 ± 0.7 m,
respectively, for a sporadic meteoroid hitting the Moon with an average
collision velocity of 17 km s
-1
.
Accepted for publication in Monthly Notices of the Royal Astronomical Society
(MNRAS) on 2019 March 29
Values derived from our analysis of the crater diameter are summarized in
Table 3. Both above-mentioned scaling models predict a similar rim-to-rim
diameter D for the same impactor bulk density, with D ranging from about
10 to 15 m. Because of its small size, this crater cannot be observed by
telescopes from our planet. But probes in orbit around the Moon can spot it,
provided that these can take pre- and post- impact images of the area where
the meteoroid collision takes place. For instance, craters produced by
previous collisions that gave rise to observed impact flashes were
successfully identified by cameras onboard the Lunar Reconnaissance
Orbiter (LRO), which orbits the Moon in a polar orbit since 2009 (Madiedo
et al. 2014, 2019; Suggs et al. 2014, Robinson et al. 2015). These
observations are or a paramount importance, since they would allow us to
compare the actual and predicted crater diameters to check the validity of
our assumptions. This would also provide a better constraint for the
luminous efficiency associated with the collision of meteoroids on the
Moon.
5. CONCLUSIONS
We have focused here on a lunar impact flash recorded during the Moon
eclipse that occurred on 2019 January 21. This is the first impact flash
unambiguously recorded on the Moon during a lunar eclipse and discussed
in the scientific literature. The event, spotted and confirmed in the
framework of the MIDAS survey, was also imaged by casual observers in
Accepted for publication in Monthly Notices of the Royal Astronomical Society
(MNRAS) on 2019 March 29
Europe, America and Africa. The peak V magnitude of the flash was 4.2 ±
0.2, and its duration was of 0.28 s. According to our analysis, the most
likely scenario with a probability of 99% is that the impactor that generated
this flash was a sporadic meteoroid. By considering a value for the luminous
efficiency of 3·10
-3
and an impact speed of 17 km/s, the estimated mass of
the impactor yields 45 ± 8 kg. By employing the Gault scaling law, the rim-
to-rim diameter of the crater generated during this collision ranges from
10.1 ± 0.5 m (for an impactor bulk density of 0.3 g cm
-3
) to 15.3 ± 0.7 m
(for a bulk density of 3.7 g cm
-3
). The Holsapple model predicts a similar
size. The crater could be measured by a probe in orbit around the Moon,
such as for instance the Lunar Reconnaissance Orbiter. The comparison
between the predicted and the experimental crater size could be very
valuable to allow for a better constraint of the luminous efficiency for
meteoroids impacting the lunar ground.
This is also the first time that lunar impact flash observations in more than
two wavelengths are reported. The impact plume blackbody temperature has
been estimated by analyzing the R, G and B channels of the color camera
employed to record the event. This multiwavelength analysis has resulted in
a peak temperature of 5700 ± 300 K.
ACKNOWLEDGEMENTS
Accepted for publication in Monthly Notices of the Royal Astronomical Society
(MNRAS) on 2019 March 29
We acknowledge funding from MINECO-FEDER project AYA2015-
68646-P, and also from project J.A. 2012-FQM1776 (Proyectos de
Excelencia Junta de Andalucía).
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(MNRAS) on 2019 March 29
TABLES
Date and time 2019 January 21 at 4h 41m 38.09 ± 0.01s UT
Peak brightness (magnitude) 4.2 ± 0.2 in V band
Impact location Lat.: 29.2 ± 0.3 ºS, Lon.: 67.5 ± 0.4 ºW
Duration (s) 0.28
Impactor kinetic energy (J) (6.55 ± 0.63)·10
9
Impactor mass (kg) 45 ± 8
Table 1. Characteristics of the lunar impact flash analysed here.
ZP
R
+ K
R
A 10.81 ± 0.06
ZP
V
+ K
V
A 11.07 ± 0.01
ZP
B
+ K
B
A 11.71 ± 0.02
C
R
-0.398 ± 0.11
C
V
-0.018 ± 0.006
C
B
0.157 ± 0.05
Table 2. Results obtained from the photometric calibration of the Sony A7S
camera, as defined by Equations (4 to 6).
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(MNRAS) on 2019 March 29
Scaling
law
Impact
angle (º)
Meteoroid
Density
(g cm
-3
)
Meteoroid
Mass
(kg)
Impact
Velocity
(km s
-1
)
Crater
Diameter
(m)
Gault 45 0.3 45±8 17 10.1±0.5
Gault 45 1.8 45±8 17 13.6±0.6
Gault 45 3.7 45±8 17 15.3±0.7
Holsapple 45 0.3 45±8 17 10.4±0.5
Holsapple 45 1.8 45±8 17 13.3±0.6
Holsapple 45 3.7 45±8 17 15.8±0.7
Table 3. Diameter of the fresh crater, according to the Gault and the
Holsapple models.
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(MNRAS) on 2019 March 29
FIGURES
Figure 1. Lunar impact flash recorded on 2019 January 21 by the 0.36 m SC
(up) and the 0.10 m refractor (down) telescopes.
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(MNRAS) on 2019 March 29
Figure 2. Lightcurve (evolution of V-magnitude as a function of time) of the
impact flash recorded by the 0.36 m telescope.
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(MNRAS) on 2019 March 29
Figure 3. Decomposed image of the lunar impact flash into the three basic
colour channels R, G, and B, during the peak luminosity of the event.
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(MNRAS) on 2019 March 29
Figure 4. Photometric calibration for R band performed by employing 30
reference stars in Messier 67. The solid line corresponds to the best fit
obtained from measured data.
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(MNRAS) on 2019 March 29
Figure 5. Photometric calibration for V band performed by employing 30
reference stars in Messier 67. The solid line corresponds to the best fit
obtained from measured data.
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(MNRAS) on 2019 March 29
Figure 6. Photometric calibration for B band performed by employing 30
reference stars in Messier 67. The solid line corresponds to the best fit
obtained from measured data.
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(MNRAS) on 2019 March 29
Figure 7. Flux densities obtained in R, V, and B bands. The solid line
represents the best fit of these data to the flux emitted by a blackbody at a
temperature T=5700 K.
