Based on our experience of development of a laser altimeter (LALT) aboard Kaguya lunar orbiter, we participate to the development of a laser altimeter called LIDAR aboard Japanese second asteroid mission Hayabusa-2 which will be launched in 2014. The development team is led by JAXA. Chiba Institute of Technology, University of Aizu and other universities and research institutes are members of LIDAR. LIDAR stands for ‘Light Detection and Ranging’, which is a technique to measure distance by using laser light. Laser light is emitted from the instrument, and the light bounced on the planetary surface is detected to calculate the time of flight of the laser light. Originally, the primary purpose of the LIDAR is optical navigation of the satellite with Optical Navigation Camera (ONC), however, the shape of the small asteroid is also measured in-situ by LIDAR, providing important information to decide where the satellite will touch down and where it takes samples of the asteroid. We are now discussing how to use the instrument more effectively in terms of scientific use and operation and planning of tests.
1. Estimation of density and porosity of the asteroid
When no maneuver is operated, a satellite is captured with the gravity of the asteroid and freely falls toward the asteroid. As the falling velocity is determined by the initial velocity and the gravity of the asteroid, the gravity field and the mass of the target asteroid can be estimated by measuring the distance to the asteroid when the spacecraft is freely falling toward the asteroid. The volume can be derived from shape models by LIDAR or camera topography. The averaged density can be obtained by the mass and volume. And if we assume a material which forms the asteroid, the average porosity of the whole asteroid can be estimated. The density and porosity will provide information of internal structure, and furthermore, the origin and evolution of the asteroid.
2. Measurement of reflectance at the laser wavelength
The surface reflectance is also measured by LIDAR, since this instrument illuminates the surface by laser light with the same condition of incident and emission angle (both is zero degree). The diversity of surface reflectance may be caused by the replacement of surface materials due to impacts by small bodies or tidal forces, or degrees of space weathering. Therefore the mixing process of the regolith may be estimated from the surface reflectance.
3. Existence of levitation dust particles
Light dusts with low initial velocity due to impacts or resurfacing are expected to levitate, because the mass and the resulting gravity field of the asteroid is small. We plan to detect such levitation dusts by catching the laser return signals from dusts. Hayabusa-2 is equipped the Small Carry-on Impactor (SCI) for the first time, which is to be used for impact experiment onto the asteroid surface. The dusts are expected to be detected by LIDAR.
Toward future explorations
The laser altimeter is one of mandatory instruments for the Earth-like planets, moons and asteroids. In Japan, we have experiences of development of laser altimeters aboard Hayabusa and Kaguya, followed by the third laser altimeter of Hayabusa-2. We are proposing another laser altimeter for the future planetary exploration. We will contribute to the planetary explorations both in Japan and other countries with our heritages.