9:15 AM - 9:30 AM
[PPS06-02] Lunar Impact Flash Observing Mission by 6U Spacecraft EQUULEUS (2): A New Step for the Lunar Seismology
★Invited Papers
Keywords:Equuleus, lunar impact flash, lunar seismology
The giant impact theory is widely accepted for the origin of the Moon. But, in recent years, there have been reports of discoveries that may contradict this theory, such as detecting water in the volcanic glass that originated in the deep part of the Moon [1]. Therefore, it cannot be said with absolute certainty that the Moon was born by a giant impact, and further research is needed. It is important to analyze samples and explore the internal structure of the Moon to investigate the origin of the Moon. And seismic observation is the most effective method for investigating the internal structure.
However, it isn't easy to investigate the internal structure with data from a few seismometers on the Moon. Therefore, seismic exploration of the internal structure using lunar impact flashes has been proposed [2]. A lunar impact flash is a flash caused by a meteoroid crashing into the lunar surface at high speed. The collision simultaneously excites seismic waves. Video observations of the flashes provide the location and time of the epicenter. So, if we assume that the Moon's internal structure is uniform, seismic wave velocities can be determined with only one impact earthquake and one seismometer. Then, it is possible to estimate the materials that make up the interior of the Moon. Observing many impact earthquakes would allow us to investigate more complex structures.
So far, lunar impact flashes have been observed from the ground, but there are difficulties that general astronomical observations do not have. Brighter flashes occur less frequently, so we aim for darker flashes that occur more frequently than once a day and observe the night side of the Moon where sunlight is not shining. Since we don't know where it appears, we want to observe it over a wide area. We can observe it over a wide area on the night side only when the Moon is crescent. However, as soon as the crescent Moon appears in the western sky, it sets immediately, and we cannot have enough observation time. Also, since the altitude is low even before the Moon sets, it would be difficult to observe flashes due to the high background if there is a slight haze. As the moon ages and becomes brighter, the background rises due to moonlight scattered by the atmosphere, even at high altitudes. In other words, observation time is not enough, and observations are forced to take place under the strong influence of the earth's atmosphere. Therefore, flash observations from space are desired for future exploration of the lunar interior using impact earthquakes.
The micro-probe EQUULEUS [3] (Attached figure, modified from Fig. 3 in [7]) is equipped with a video camera called "DELPHINUS," which attempts the world's first flash observation from space [4, 5]. What is different from a standard camera is that it continuously captures images of the lunar surface, automatically extracts image frames containing flashes, cuts out only the flashes, and saves them. In ground-based observations, such work is performed on a high-speed PC [6]. However, in "DELPHINUS," the work is performed using a logic circuit called an FPGA (Field Programmable Gate Array) [7]. The adoption of FPGA made it possible to reduce the size, power consumption, and amount of data sent to the ground. EQUULEUS is currently flying toward the L2 point (one of the Lagrangian points) in the Earth-Moon system and is scheduled to start observing lunar flashes in about a year. If the flash observation from space is demonstrated, it is one giant leap for the future exploration of the lunar interior using impact earthquakes.
References: [1] Saal et al. (2008) Nature 454, 192-195. [2] Yamada et al. (2011) Planet. Space Sci. 59, 343-354. [3] Funase et al. (March 2020) IEEE Aerospace and Electronic Systems Magazine, 30-44, DOI: 10.1109/MAES.2019.2955577. [4] Fuse et al. (2019) Trans. JSASS Aerospace Tech. Japan 17, 315-320, DOI: 10.2322/tastj.17.315. [5] Ikari et al. (2019) 33rd Annual AIAA/USU Conference on Small Satellites, SSC19-WKV-04, https://digitalcommons.usu.edu/smallsat/2019/all2019/100/. [6] Yanagisawa and Kakinuma (2022) Bulletin of the University of Electro-Communications 34, 17-22, http://doi.org/10.18952/00010048. [7] Fujiwara et al. (2020) Trans. Japan Soc. Aero. Space Sci. 63, 265-271, DOI: 10.2322/tjsass.63.265.
However, it isn't easy to investigate the internal structure with data from a few seismometers on the Moon. Therefore, seismic exploration of the internal structure using lunar impact flashes has been proposed [2]. A lunar impact flash is a flash caused by a meteoroid crashing into the lunar surface at high speed. The collision simultaneously excites seismic waves. Video observations of the flashes provide the location and time of the epicenter. So, if we assume that the Moon's internal structure is uniform, seismic wave velocities can be determined with only one impact earthquake and one seismometer. Then, it is possible to estimate the materials that make up the interior of the Moon. Observing many impact earthquakes would allow us to investigate more complex structures.
So far, lunar impact flashes have been observed from the ground, but there are difficulties that general astronomical observations do not have. Brighter flashes occur less frequently, so we aim for darker flashes that occur more frequently than once a day and observe the night side of the Moon where sunlight is not shining. Since we don't know where it appears, we want to observe it over a wide area. We can observe it over a wide area on the night side only when the Moon is crescent. However, as soon as the crescent Moon appears in the western sky, it sets immediately, and we cannot have enough observation time. Also, since the altitude is low even before the Moon sets, it would be difficult to observe flashes due to the high background if there is a slight haze. As the moon ages and becomes brighter, the background rises due to moonlight scattered by the atmosphere, even at high altitudes. In other words, observation time is not enough, and observations are forced to take place under the strong influence of the earth's atmosphere. Therefore, flash observations from space are desired for future exploration of the lunar interior using impact earthquakes.
The micro-probe EQUULEUS [3] (Attached figure, modified from Fig. 3 in [7]) is equipped with a video camera called "DELPHINUS," which attempts the world's first flash observation from space [4, 5]. What is different from a standard camera is that it continuously captures images of the lunar surface, automatically extracts image frames containing flashes, cuts out only the flashes, and saves them. In ground-based observations, such work is performed on a high-speed PC [6]. However, in "DELPHINUS," the work is performed using a logic circuit called an FPGA (Field Programmable Gate Array) [7]. The adoption of FPGA made it possible to reduce the size, power consumption, and amount of data sent to the ground. EQUULEUS is currently flying toward the L2 point (one of the Lagrangian points) in the Earth-Moon system and is scheduled to start observing lunar flashes in about a year. If the flash observation from space is demonstrated, it is one giant leap for the future exploration of the lunar interior using impact earthquakes.
References: [1] Saal et al. (2008) Nature 454, 192-195. [2] Yamada et al. (2011) Planet. Space Sci. 59, 343-354. [3] Funase et al. (March 2020) IEEE Aerospace and Electronic Systems Magazine, 30-44, DOI: 10.1109/MAES.2019.2955577. [4] Fuse et al. (2019) Trans. JSASS Aerospace Tech. Japan 17, 315-320, DOI: 10.2322/tastj.17.315. [5] Ikari et al. (2019) 33rd Annual AIAA/USU Conference on Small Satellites, SSC19-WKV-04, https://digitalcommons.usu.edu/smallsat/2019/all2019/100/. [6] Yanagisawa and Kakinuma (2022) Bulletin of the University of Electro-Communications 34, 17-22, http://doi.org/10.18952/00010048. [7] Fujiwara et al. (2020) Trans. Japan Soc. Aero. Space Sci. 63, 265-271, DOI: 10.2322/tjsass.63.265.