[P-CG18] Future missions and instrumentation for space and planetary science

[PCG18-P05] Studies of the multi-static subsurface radar for explorations of the Moon and solar system small bodies

*Atsushi Kumamoto¹, Hideaki Miyamoto², Toshiyuki Nishibori³, Ken Ishiyama⁴ (1.Tohoku University, 2.University of Tokyo, 3.JAXA, 4.National Institute of Technology, Tsuruoka College)

Keywords:Ground penetrating radar (GPR), Subsurface radar sounder, Bistatic/multistatic radar, Subsurface struscture in the lunar shallow layers, Internal structure of the comet nucleus, Finit-difference time domain (FDTD) simulation

In recent years, space agencies and some private companies are proposing intensive explorations of the Moon by using unmanned and manned spacecrafts, including space station around the Moon, and manned rover on the moon. In those lunar explorations in the next generation, it would be required to observe not only the surface of the Moon by camera, which brings us numerous information on the current Moon, but also below the surface, which brings us information on the past Moon. Subsurface radar observations were already performed in the previous missions as Apollo17, SELENE, and Chang'E-3/4/5. They were, however, observations by monostatic radar transmitting and receiving radar pulse at the same locations, with which the propagation velocity and the permittivity of the media was difficult to estimate. If we apply multi-static radar for the lunar subsurface exploration, it could be easier to apply common middle point (CMP) method and determine distribution of the propagation velocity and the permittivity below the lunar surface.
As a following mission of Hayabusa2 in the next generation, a sample return mission from inactive comet is discussed among researchers for proposal to JAXA. In the mission, it is planed that primitive solar system materials inside the comet are sampled and analyzed, and internal structures of the comet is imaged by radar and seismometer to discuss the comet was formed through the catastrophic disruption or through the accumulation of the pebbles. For imaging of the comet internal structures, multi-static radar installed on multiple spacecraft would be a better option than monostatic radar installed on single spacecraft. By applying analyses of migration or tomography, distribution of the permittivity in the comet is expected to be obtained in both cases of rubble-pile and pebble-pile structures in the comet.
In order to confirm the advantages of the multi-static radar mentioned above, finite-difference time domain (FDTD) simulations for radar sounding of the lunar shallow layers and comet nucleus. In the simulation for the lunar shallow layers, we set a simulation box with a size of 2.4 m x 3 m, five horizontal layers with different permittivity up to a depth of 2.5 m below the surface, one transmitting antenna at a height of 0.2 m transmitting chirp pulse in a frequency range from 0.5 to 3 GHz, and 11 receiving antennas at a height of 0.2 m and an interval of 0.2 m, and obtain waveform at the 11 receiving antennas. By applying CMP analysis, the permittivity of five layers could be estimated from the waveform data. In the simulation for the comet nucleus, we set comet nucleus with a diameter of 100 m including internal structures with different permittivity. Since the orbiter installing the transmitter and receiver would be operated at a distance of 10 km from the comet nucleus and simulation box with a size of 20 km x 20 km would be too large for FDTD simulation, we set simulation box with a size of 120 m x 120 m, plain wave source at a distance of 110 m from the comet nucleus center, and near fields of the electromagnetic wave at the rectangle boundary with a size of 100 m x 100 m are converted to the far field at the receiver installed on the spacecraft at a distance of 10 km. The estimation of the permittivity distribution in the comet nucleus from the obtained waveform at the spacecraft was tried by applying Kirchhoff migration and other analysis methods.