10:45 〜 11:00
[PCG20-06] Bi-static radar system for radio sounding of the cometary internal structures in Next Generation Small-Body Sample Return (NGSR) Mission
★Invited Papers
キーワード：バイスタティックレーダ、次世代小天体サンプルリターンミッション、彗星の内部構造
Investigations for a new mission of sample return from inactive comet as 289P/Blanpain are performed in the Next Generation small-body Sample Return Working Group (NGSR-WG). In addition to the sample return and in-situ analysis of the subsurface material of the comet nucleus, observations of the internal structures of the comet nucleus by radar and seismometer are also planned in NGSR mission. By using radar, we can know whether the comet nucleus is rubble pile formed through catastrophic disruptions and re-accumulation, or pebble pile formed through the accumulation of dust and pebbles.
In order to achieve the purpose, a new bi-static radar system is proposed in NGSR mission. The radar system consists of Radar-A (radar transceiver) installed on Deep Space Orbital Transfer Vehicle (DS-OTV) which stays at a distance of 10 km from the comet nucleus, and Radar-B (radar transponder, which receives radar pulses from Radar-A and immediately transmits them back) installed on the lander spacecraft which can get closer to the comet nucleus for touchdown operation. Radar system utilized in the Rosetta mission (CONSERT) was also a bi-static radar system whose radar transceiver was installed on the mother orbiter and whose radar transponder was installed on the lander. Since the lander of Rosetta is fixed on the surface of the comet nucleus, the paths of the radar pulses concentrate around the landing site. NGSR Radar-B will orbit around the comet nucleus. So, the paths of radar pulses can cover all over within the comet nucleus. NGSR Radar-A and B are chirp radar operated in a frequency range from 150 to 250 MHz. The range resolution is therefore 1.5 m in vacuum and ~1 m in the media with a permittivity of 2. For transmission in wide frequency range, we plan to apply a bowtie antenna with a length of 0.7 m and a width of 0.2 m, which can be stored on the panel of NGSR lander spacecraft with a size of 0.7 x 0.7 m2. In the operation plan of NGSR radar, DS-OTV stays at 10 km distance to the comet nucleus, and the lander spacecraft orbits at around 0.5 km distance to the comet nucleus with a period of about two days. The spin period of the comet nucleus of 289P/Blanpain is estimated to be about 9 hours. The radar pulses transmitted from Radar-A on DS-OTV are received at the Radar -B on the lander spacecraft as (a) radar pulses directly arrived, (b) radar pulses reflected at the surface and subsurface shallow reflectors, and (c) radar pulses passed through the comet nucleus. The radar pulses (c) are especially important for inversion analysis of the cometary internal structures, and they can be mainly received while the lander spacecraft is around the opposite side of DS-OTV in an azimuth range of 180+-14 degree from DS-OTV without being mixed with other radar pulse components. From the geometry of the paths of the radar pulse (c), a matrix for the inversion of the slowness distribution inside the comet nucleus can be derived. It was also confirmed that the matrix for inversion at a resolution of 3 m can be derived from the radar pulse (c) path data during 7 days at an interval of 1 s. Currently, the mass, size, and power consumptions of the bi-static radar system are roughly estimated on the basis of the estimations of those for similar ground penetrating radar (GPR) proposed for Lunar Polar Exploration (LUPEX) mission. Update of the estimations is now ongoing and the results will be reported by March 2024.
In order to achieve the purpose, a new bi-static radar system is proposed in NGSR mission. The radar system consists of Radar-A (radar transceiver) installed on Deep Space Orbital Transfer Vehicle (DS-OTV) which stays at a distance of 10 km from the comet nucleus, and Radar-B (radar transponder, which receives radar pulses from Radar-A and immediately transmits them back) installed on the lander spacecraft which can get closer to the comet nucleus for touchdown operation. Radar system utilized in the Rosetta mission (CONSERT) was also a bi-static radar system whose radar transceiver was installed on the mother orbiter and whose radar transponder was installed on the lander. Since the lander of Rosetta is fixed on the surface of the comet nucleus, the paths of the radar pulses concentrate around the landing site. NGSR Radar-B will orbit around the comet nucleus. So, the paths of radar pulses can cover all over within the comet nucleus. NGSR Radar-A and B are chirp radar operated in a frequency range from 150 to 250 MHz. The range resolution is therefore 1.5 m in vacuum and ~1 m in the media with a permittivity of 2. For transmission in wide frequency range, we plan to apply a bowtie antenna with a length of 0.7 m and a width of 0.2 m, which can be stored on the panel of NGSR lander spacecraft with a size of 0.7 x 0.7 m2. In the operation plan of NGSR radar, DS-OTV stays at 10 km distance to the comet nucleus, and the lander spacecraft orbits at around 0.5 km distance to the comet nucleus with a period of about two days. The spin period of the comet nucleus of 289P/Blanpain is estimated to be about 9 hours. The radar pulses transmitted from Radar-A on DS-OTV are received at the Radar -B on the lander spacecraft as (a) radar pulses directly arrived, (b) radar pulses reflected at the surface and subsurface shallow reflectors, and (c) radar pulses passed through the comet nucleus. The radar pulses (c) are especially important for inversion analysis of the cometary internal structures, and they can be mainly received while the lander spacecraft is around the opposite side of DS-OTV in an azimuth range of 180+-14 degree from DS-OTV without being mixed with other radar pulse components. From the geometry of the paths of the radar pulse (c), a matrix for the inversion of the slowness distribution inside the comet nucleus can be derived. It was also confirmed that the matrix for inversion at a resolution of 3 m can be derived from the radar pulse (c) path data during 7 days at an interval of 1 s. Currently, the mass, size, and power consumptions of the bi-static radar system are roughly estimated on the basis of the estimations of those for similar ground penetrating radar (GPR) proposed for Lunar Polar Exploration (LUPEX) mission. Update of the estimations is now ongoing and the results will be reported by March 2024.