Toward orbital seismology: theory for speckle noise reduction in laser doppler vibrometer measurements on distant rough surfaces

Abstract

The interior structures of small planetary bodies such as asteroids and comets are an enigma, yet understanding them has immense value. Knowing the internal structure is important for understanding the formation and evolution of the solar system and also has implications for in-situ resource exploitation and planetary defense from asteroids. Despite the immense value, no detailed investigation of the interior properties of an asteroid has yet been made. Although orbital radar and lander based seismological approaches have been proposed to make direct interior observations, it has not yet been demonstrated that radar could transmit through a rocky asteroid, nor that landing multiple seismometers is feasible or affordable. An elegant alternative to landed seismometers is orbital laser Doppler vibrometry, which could record seismic shaking of a small body without contact with the surface. Laser Doppler vibrometers (LDVs) are mature instruments for terrestrial applications; and could function similarly in a space environment. However, when incident on a rough surface like an asteroid regolith, an LDV is subject to laser speckle noise which may be misinterpreted as seismic shaking. I address the challenge of making LDV measurements on naturally rough surfaces by quantifying the laser speckle noise that an orbital LDV would encounter during a hypothetical orbital measurement. Specifically, I simulate an LDV measurement of a seismic signal generated by an impact source on the asteroid 101955 Bennu. I demonstrate that speckle noise can be attenuated by combining multiple signals recorded by an orbital seismometer equipped with multiple LDV sensors. By mitigating laser speckle, I demonstrate that an orbital LDV can record seismic signals on a natural asteroid surface, which would enable an orbital seismometer to achieve the dense global coverage necessary for high resolution interior imaging.