Trans. JSASS Aerospace Tech. Japan Vol. 17, No. 3, pp. 315-320, 2019 DOI: 10.2322/tastj.17.315

Space-based Observation of Lunar Impact Flashes

1) 1) 2) 3) 4) By Ryota FUSE, Shinsuke ABE, Masahisa YANAGISAWA, Ryu FUNASE, and Hajime YANO

1)Department of Aerospace Engineering, Nihon University, Chiba, Japan 2)Department of Engineering Science, The University of Electro-Communications, Tokyo, Japan 3)Department of Aeronautics and Astronautics, The University of Tokyo, Tokyo, Japan 4)Institute of Space and Astronautical Science, JAXA, Sagamihara, Japan

(Received June 30th, 2017)

When a meteoroid impacts the Moon at several tens of km/s, a brilliant flash, referred to as a lunar impact flash, can be observed at the point of impact by ground-based telescope. Lunar impact flashes observed from the ground are biased due to atmospheric extinction, background illumination of earthshine, and short observation windows, typically a few hours per day during a period of approximately 10 days. NASA’s meteoroid impact program took 10 years to detect 400 lunar impact flashes. EQUULEUS will demonstrate low-energy trajectory control techniques, such as multiple lunar flybys, proposed by the University of Tokyo and JAXA, within the Earth-Moon region. The spacecraft will be launched by NASA-SLS in 2019. The DELPHINUS camera system will be placed onboard EQUULEUS to observe lunar impact flashes while the spacecraft stays in halo orbit around an Earth-Moon L2 point. Thus, the lunar distance from the spacecraft is approximately one tenth that from ground-based observation. We estimate that DELPHINUS will detect 1,607, 2,699, and 4,534 lunar impact flashes during its six-month mission phase by assuming the limiting magnitude of its camera to be the 4.5th, 5.0th, and 5.5th magnitudes, respectively. The present study describes the DELPHINUS camera system and the first method for space-based observation of lunar impact flashes.

Key Words: Lunar Impact Flash, Meteoroid, Moon, Halo Orbit

Nomenclature meteoroid impacts the lunar surface, part of the kinetic energy of the meteoroid is converted into light energy.1) Figure 1 : observable area shows a lunar impact flash on the night side of the Moon. : DELPHINUS magnitude 𝐴𝐴 : limiting magnitude 𝐷𝐷𝐷𝐷𝐷𝐷 : night side of the Moon 𝐿𝐿 : surface area 𝑁𝑁 : distance 𝑆𝑆 : flux of the lunar impact flash 𝑑𝑑 : Moon-EQUULEUS-Sun angle 𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 Subscripts 𝛼𝛼 E : Earth M : Moon cam : camera Fig. 1. Lunar impact flash. (The image was recorded by M. Ishida on eq : EQUULEUS Dec. 15, 2007 (JST).) mag : magnitude EQUULEUS (EQUilibriUm Lunar-Earth point 6U 1. Introduction Spacecraft) is a deep-space 6U-class spacecraft developed by the University of Tokyo and JAXA, which will be launched High-velocity meteoroid impacts are dangerous to humans by NASA’s new rocket SLS (Space Launch System) in 2019. in the lunar environment. Even if a meteoroid does not EQUULEUS will demonstrate low-energy trajectory control directly strike a person, fragments ejected by an impact on the techniques, such as multiple lunar flybys, within the Moon can travel fast enough to be hazardous. As such, it is Earth-Moon region, and observe the Earth and the Moon very important to know the meteoroid flux and size through three science mission instruments. distribution on the Moon. When a meteoroid impacts the DELPHINUS (DEtection camera for Lunar impact Moon at several tens of km/s, a brilliant flash (in the visible PHenomena IN 6U Spacecraft) is a science mission and near-infrared wavelength regions), referred to as a lunar instrument that was developed for the purpose of observing impact flash, can be observed at the point of impact by lunar impact flashes and near-Earth asteroids. DELPHINUS is ground-based telescope. The duration of a lunar impact flash being developed by Nihon University, the University of is short (approximately 0.01 to 0.1 seconds), and when a Electro-Communications, the University of Tokyo, and JAXA in collaboration with COSINA Co., Ltd., WATEC Co., Ltd.,

Copyright© 2019 by the Japan Society for Aeronautical and Space Sciences and ISTS. All rights reserved. J-STAGE Advance published date: January 31st, 2019 315 Trans. JSASS Aerospace Tech. Japan Vol. 17, No. 3 (2019) and IMAGETECH Co., Ltd. The observation of lunar impact space-based observation is expected to enhance impact flash flashes makes it possible to estimate the flux and size detection efficiency and will enable evaluation of the lunar distribution of meteoroids ranging in diameter from surface impact environment. centimeters to a few meters.2) Evaluation of the flux of 2.2. Development of the DELPHINUS camera system meteoroids impacting the Moon is expected to contribute to In order to achieve the lunar impact flash observation by lunar surface environment assessment for human lunar space-based observation, DELPHINUS camera was activities. developed with a focus on the following three points: In the present paper, we examine the feasibility of • Blocking reflected and stray light from the Moon, Sun, and space-based lunar impact flash observation by EQUULEUS Earth. containing DELPHINUS camera system. • Implementation of two camera systems to reduce false A diagram of EQUULEUS containing DELPHINUS detections. camera system is shown in Fig. 2. • A high frame rate (1/60 s) that maximizes the detection rate for impact flashes. A design that satisfies the above conditions is shown in Fig. 3 and Tables 1 and 2. Moreover, the permissible size for incorporating DELPHINUS camera system in EQUULEUS is 0.5U (= 10 cm [W] × 5 cm [D] × 10 cm [H]). DELPHINUS camera system consists of the following components: • Two WATEC T065 CCD camera modules • Shielding incident light (> 45 degrees) by hoods • A FPGA (Field Programmable Gate Array) for high-speed image processing

Fig. 2. Diagram of EQUULEUS.

2. Examination of the DELPHINUS Camera System

2.1. Ground-based and space-based observation of lunar impact flashes Through ground-based observations, NASA has detected approximately 400 flashes over a period of 10 years.3) In general, in order to distinguish false detections caused by cosmic rays and electronic noises from true impact flash events, at least two telescopes are essential for the ground-based observation of lunar impact flashes. However, the following three factors lower the detection efficiency of Fig. 3. Image of DELPHINUS. ground-based observation. First, due to the geometrical restriction of the Table 1. Camera module specifications. Moon-Sun-Earth system, the night side of the Moon is Items WATEC T065 CCD camera module observable for only a few hours per day from the Image size 1/3” CCD area sensor Crescent-Moon phase to the Half-Moon phase This Valid pixels 659 (H) × 494 (V) observable moon phase period is approximately ten days per Pixel size µm (H) × µm (V) Exposure time 1/60 to 1/4,000 [s] lunar cycle. 7.4 7.4 Second, earthshine is sunlight reflecting from the Earth Wavelength 380 to 750 [nm] which illuminates the unilluminated night side of the Moon. Dimensions 50.0 [mm] × 50.0 [mm] × 6.0 [mm] Earthshine provides a 13th magnitude/arcsec2 background, which reduces the S/N ratio of impact flash events. Table 2. Optical lens specifications. Third, due to the Earth's atmospheric extinction, the Items COSINA lens detectable number of flash events will be decreased. In Focal length 50 [mm] addition, cloudy weather makes observation impossible. Lens aperture 35.7 [mm] F-number 1.4 Space-based observation solves these problems. Long-term continuous Moon observation can be achieved by the attitude control of the spacecraft. Observation from the Earth-Moon The fields of view of DELPHINUS at distances of 60,000 L2 point can reduce the effect of earthshine. Atmospheric km, 40,000 km, and 20,000 km from the lunar surface are extinction and weather conditions can be ignored. Therefore, shown in Fig. 4.

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Captured Orbiters (TCOs).

3. Deriving the Optimum Halo Orbit

3.1. Method of deriving the optimum halo orbit We derived an optimum Earth-Moon Lagrangian (L2) halo orbit for lunar impact flash observations from eight halo orbit candidates currently being investigated by the EQUULEUS orbital team.4) The Moon-EQUULEUS-Earth angle, the Moon- EQUULEUS-Sun angle, and the altitude from the lunar surface can be derived from the geometrical relationship among EQUULEUS, the Earth, Sun, and the Moon. As shown in Fig. 5, assuming the diameter of the Moon ( ) is 1, the distance of the night-side as seen from EQUULEUS is expressed as follows: 𝐴𝐴𝐴𝐴 Fig. 4. Field of view of DELPHINUS.

Stray light that originated from the Earth, Sun, and the Moon is estimated using a ray-tracing method in order to propagate light scattered from multiple locations. The results are shown in Table 3.

Table 3. Stray light simulation.

Conditions: Altitude 20,000 [km] Earth flux [W/m2] Fig. 5. Moon-EQUULEUS-Sun angle. Sun flux ?; [W/m2] 2.30×10 Sunlit Moon flux @ [W/m2] 1.40×10 Allowance flux ?; [W/m2] (10% of sensor saturation) 9.00×10 Earth stray light ?@ 4.30×10 𝑁𝑁 = 𝐴𝐴𝐴𝐴 = 𝐴𝐴𝐴𝐴 + 𝑀𝑀𝑀𝑀 ?E ; 25° = 9.80×10 [W/m ] = 0.23% 1 1 Sun stray light ?E ; = + cos𝛼𝛼 45° = 1.40×10 [W/m ] = 0.033% ?@ ; 2 2 45° = 4.09×10 [W/m ] = 9.5% 1 ?@ ; = 1 + cos 𝛼𝛼 50° = 3.64×10 [W/m ] = 8.5% (1) ?@ ; 2 ; 55° = 2.47×10 [W/m ] = 5.7% 𝛼𝛼 Moon stray light ?@ ; = cos 60° = 2.52×10 [W/m ] = 5.9% A period that satisfies four2 observation requirements of the ?@ ; 1.33×10 [W/m ] = 3.1% halo orbit candidate is regarded as the observable period. Stray light flux in the case of 45-degree phase angles for the Moreover, the observable ratio is derived as the ratio of the Sun, Earth, and the Moon satisfied the requirement whereby observable period to the total time on the halo orbit (mission the acceptable value of the stray-light flux had to be smaller phase). We defined the optimum halo orbit as the orbit with than 10% of the sensor saturation value. Based on the the highest observable ratio. spacecraft orbit, the attitude requirement, and the detectable 3.2. Results and discussion regarding the optimum halo efficiency of lunar impact flash events, the following four orbit observation requirements were specified for DELPHINUS The observable ratio obtained from eight halo orbit operations. candidates is shown in Table 4. • The Moon-EQUULEUS-Earth angle is greater than 45

degrees Table 4. Halo orbit candidates. • The Moon-EQUULEUS-Sun angle is greater than 45 Halo orbit Mission phase Observable period Observable degrees candidate ratio • The night side of the Moon is more than 25% of the Moon 1040_26N 194 days 18 hours 75 days 11 hours 38.74% as seen from EQUULEUS 1040_26S 194 days 19 hours 74 days 16 hours 38.33% • The altitude from the lunar surface is less than 60,000 km 1120_14N 176 days 1 hours 32 days 15 hours 18.53% We intend to observe lunar impact flashes only when all of 1120_14S 176 days 3 hours 31 days 11 hours 17.86% the above requirements are satisfied. Based on these 1160_12N 171 days 10 hours 24 days 6 hours 16.48% requirements, DELPHINUS plans to observe recently 1160_12S 171 days 7 hours 24 days 3 hours 16.42% discovered (or known) Near-Earth Objects (NEOs), 1177_12N 182 days 18 hours 0 hours 0% Potentially Hazardous Objects (PHOs), and Temporarily 1177_12S 182 days 19 hours 0 hours 0%

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In the halo orbit candidate column of Table 4, the four-digit number, the two-digit number, and S or N are the ratio of the distance between the Earth and the halo orbit when the Earth-Moon distance is 1 (e.g., 1040 is 1.040), the number of cycles on the halo orbit for 180 days, and the South/North position of the spacecraft relative to the Moon, respectively. As shown in Table 4, 1040_26N is the optimum orbit for lunar impact flash observation because the observable ratio of the 1040_26N orbit is the highest. Moreover, there are two orbits for which the observable ratio is 0%, i.e., orbits that do not satisfy the observation requirements. Graphs of the time variation of the four observation requirements of 1040_26N are shown in Fig. 6 (Moon-EQL- Earth and Moon-EQL-Sun angles) and Fig. 7 (lunar altitude Fig. 8. Moon-EQUULEUS-Earth angle. and night-side ratio). Moreover, graphs of the time variation of the Moon-EQL-Earth angle and the lunar altitude of 1040_26N, 1120_12N, 1160_12N, and 1177_12N are shown in Figs. 8 and 9.

Fig. 9. Lunar altitude.

Regarding the optimum orbit, i.e., 1040_26N, Fig. 6 shows that the Moon-EQUULEUS-Earth angle always exceeds 45 Fig. 6. Moon-EQL-Earth and Moon-EQL-Sun angles of the degrees. Hence, lunar impact flash observation becomes optimum halo orbit. possible by satisfying the other three requirements. Moreover, from Fig. 7, since the EQUULEUS lunar altitude is always less than 72,000 km, the lunar altitude requirement is easy to satisfy. Regarding the calculation of the observable ratio, as shown in Fig. 8, the Moon-EQUULEUS-Earth angle increases as the distance between the Earth (Moon) and the halo orbit (the four-digit number of the halo orbit candidate) decreases. Moreover, as indicated by the increase in the lunar low altitude point of each halo orbit candidate in Fig. 9, the overall lunar altitude of each halo orbit candidate is increasing. Since the Earth and the Moon are on the same orbital plane and we observe the Moon from the EML2 halo orbit (far side of the Moon as seen from the Earth), the smaller the distance between the Moon and halo orbit, the larger the angle at which the Earth enters the DELPHINUS field of view. The increase in the lunar altitude is natural, as the distance Fig. 7. Lunar altitude and night-side ratio of the optimum halo orbit. between the Moon and the halo orbit gradually increases. Therefore, as the distance between the Moon and the halo orbit decreases, the observation requirements become easier to satisfy.

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detection number estimation by DELPHINUS, it is necessary 4. Estimation of the Impact Flash Detection Number to extract only sporadic meteor origin data from the ground-based observation data. Suggs et al.5) detected a total 4.1. Method of estimating the detection number of 126 flashes, including 46 flashes originating from sporadic We estimated the impact flash detection number on the meteors for an observation time of and an optimum orbit from ground-based observations of lunar observation area of . With magnitude 5) th th 266.88 [hours] impact flash. There are three important factors in estimation. information between the 5 and 10e magnitudes; , a cumulative 3.8×10 [km ] The first factor is the observation area. The night-side area magnitude distribution function adapted to space-based of the Moon increases and decreases, and the observation area observations is created from ground-based observation data. increases and decreases accordingly. Moreover, as the lunar By dividing this approximation by the total observation time altitude changes, the observation area entering the and the observation area, the flux of sporadic meteors is DELPHINUS field of view changes. The observation areas at expressed as follows: each time are derived from the three cases shown in Fig. 10 and Eq. (2), (3), and (4) for these time changes. In Eq. (4), (6) and are the DELPHINUS viewing angles. _.fE_g? ^; ; 𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 𝑥𝑥 = 3.95×10 [flashes/km /h] Therefore, in the period satisfying the four observation 4.19 5.58 requirements, by multiplying Eq. (6) by the observation area of Eq. (2) through (4) and using the of Eq. (5) as , it is possible to estimate the detection number of lunar impact 𝐷𝐷𝐷𝐷𝐷𝐷ONZ flashes per hour. 𝑥𝑥

4.2. Results and discussion regarding the estimation of the detection number First, we conducted the DELPHINUS first light experiment of the DELPHINUS in March 2017, and revealed the 5.5th Fig. 10. Observation area. magnitude as its limiting magnitude. Automatic detection th onboard EQUULEUS with a threshold of 5.5 magnitude,

(2) however, could cause numerous false detections. Therefore, QRRS 𝑆𝑆 ; the threshold value adopted in the flight operation may be 𝐴𝐴MNO ≈ 𝑁𝑁 [km ] (3) 2 slightly smaller in magnitude (brighter). QRRS ; MNO 𝑆𝑆 ° ° (4) From Eq. (5), the on the optimum orbit 1040_26N 𝐴𝐴 ≈ [km ] th th th ; 4 ; for the camera limiting magnitudeONZ of the 4.5 , 5.0 and 5.5 𝐴𝐴MNO ≈ 2𝑑𝑑UVQ tan 4.19 tan 5.58 km 𝐷𝐷𝐿𝐿𝐿𝐿 The second factor is that the sensitivity of the brightness magnitude is shown in Fig. 11. Moreover, from Eq. (6), the (magnitude) of the flash depends on the lunar altitude. Based results of detection of the number of events per day on the lunar altitude changes, the EQUULEUS observations (calculated from one-hour time resolution data) satisfying the indicate that the detection number and the detectable observation requirements on the optimum orbit 1040_26N is magnitude of DELPHINUS camera increase closer to the shown in Fig. 12, and the results for the total detection lunar surface. In contrast, the detection number and the number during the mission phase and the detection efficiency detectable magnitude decrease farther from the lunar surface. per hour during the observable period are shown in Table 5. Using the following equation, we obtained the limiting magnitude of the lunar impact flash from the viewpoint of the Earth:

; (5) 𝑑𝑑`Q 𝐷𝐷𝐷𝐷𝐷𝐷ONZ − 𝐿𝐿ONZ = 2.5log^_ [Rmag] 𝑑𝑑UVQ The third factor is ground-based observation data for estimating the detection number. A typical source of lunar impact flashes is thought to be originated from sporadic meteors that are considered random impactors not associated with any particular meteor shower. While the Earth encounters a meteoroid stream, an enhanced activity from a certain cometary dust trail can be observed as a meteor shower typically for a few hours. Since observation by Fig. 11. DLP magnitude. DELPHINUS involves few observation periods during meteor showers, most detected events are thought to originate from sporadic meteors. Therefore, in order to make a suitable

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5. Conclusion

In the present paper, we investigated the observational strategy of lunar impact flashes from the Earth-Moon L2 point in space using the DELPHINUS camera system, which will be installed on the EQUULEUS deep-space 6U spacecraft. EQUULEUS has eight halo orbit candidates, and in order to derive the optimum halo orbit suitable for lunar impact flash observation, four observation requirements are specified for DELPHINUS. Moreover, the observable ratio of the lunar impact flash increases as the distance between the Moon and the halo orbit decreases. However, being too close to the Moon is disadvantageous. The number of lunar impact flashes detected from the optimum halo orbit is estimated by taking into account the Fig. 12. Cumulative number of detections per day. magnitude distribution function calculated by ground-based observations. As a result, assuming the 4.5th, 5.0th, and 5.5th Table 5. Total number of flashes detected per mission phase. magnitudes as the limiting magnitudes of DELPHINUS, Lmag = 4.5 Lmag = 5.0 Lmag = 5.5 1,607, 2,699, and 4,534 flashes, respectively, can be detected Total number of flashes 1,607 2,699 4,534 in 0.5 years. Considering that NASA detected 400 flashes in detected [flashes] Detection efficiency 10 years, the average detection efficiency for space-based 0.887 1.490 2.504 [flashes/hour] observation is increased by 80 times, and if a sufficient number of flashes can be detected, the meteoroid flux and size From Fig. 9, the optimum orbit is lower (around 5,000 km) distribution can be evaluated. during the low-altitude portion than the other candidate orbits. Therefore, as shown in Fig. 11, when the spacecraft Acknowledgments transitions to low altitude, even the limiting magnitude of the th th 4.5 magnitude becomes the of the 14 magnitude. The authors would like to thank all EQUULEUS members

Because the limiting magnitudeONZ of the ground-based from Nihon University, the University of Electro 𝐷𝐷𝐷𝐷𝐷𝐷 th observation of the lunar impact flash is approximately the 10 Communications, JAXA, and the University of Tokyo for magnitude, which indicates that it is possible to detect a their support. Special thanks are due to COSINA Co., Ltd., darker flash by DELPHINUS, which is nearly impossible by WATEC Co., Ltd., and IMAGETECH Co., Ltd. ground-based observation.

From Fig. 12 and Table 5, the total number of detections is References 1,607 to 4,534 in the mission phase for 195 days. As shown in

Fig. 12, there is periodicity in the period of the larger number 1) Yanagisawa, M. and Kisaichi, N.: Lightcurves of 1999 Leonid of detections per day. This period coincides with the period of Impact Flashes on the Moon, Icarus, 159 (2002), pp. 31-38. the lunar low altitude, and the reason for the increase in the 2) Abe, S., Yanagisawa, M., Yano, H., and Funase, R.: Mining Earth's detection number is that it is possible to detect dark flashes Mini-moons near Cislunar Space by 6U Spacecraft, Proceedings of because is advantageous. However, if the lunar the 60th Space sciences and technology conference, Japan, Hakodate, 2016, pp. 1-3 (in Japanese). altitude is too low, as shown in Figs. 4 and 10, it will be too 𝐷𝐷𝐷𝐷𝐷𝐷ONZ 3) Moser, D.E.: Lunar Impacts NASA, [online] https://www.nasa.gov/ close to the lunar surface and the observation area will centers/marshall/news/lunar/lunar_impacts.html/ decrease. Therefore, there is an optimum distance for the lunar alamo_lunar_impact_observations405.pdf, (accessed June 19, altitude, which is neither too high nor too low. 2017). In addition, considering that NASA's ground-based 4) Oguri, K., Kakihara, K., Campagnola, S., Ozaki, N., Oshima, K., Yamaguchi, T., and Funase, R.: EQUULEUS Mission Analysis: observation has obtained 400 flashes in 10 years, and the Design of the Science Orbit Phase, Proceedings of the 31st estimation of the present study involving space-based International Symposium on Space Technology and Science, Japan, observations is expected to obtain at least 1,600 flashes in 0.5 Matsuyama, 2017. years, the temporal detection efficiency is increased by more 5) Suggs, R.M., Moser, D.E., Cooke, W.J., and Suggs R.J.: The flux of kilogram-sized meteoroids from lunar impact monitoring, than 80 times. Icarus, 238 (2014), pp. 23-36.

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