Penetrometry in Microgravity- From Brie to Bennu

In this abstract we discuss a proposal for a microgravity flight campaign within which we will investigate penetrometry in a microgravity environment. Understanding the mechanical properties of solar system minor bodies is essential for understanding their origin and evolution. Past missions such as Hayabusa-2 and OSIRIS-REX have landed on asteroids and taken samples to discover what these bodies are made of. However, there has been conflicting evidence and reports into the physical properties of the granular surface material of these bodies. With future missions such as JAXA’s MMX mission travelling to Phobos to take a sample of the body the results from this campaign will be very important to that and future missions. Penetrometry, i.e. the determination of the reaction force an object experiences as it penetrates into a surface, can help to understand the essential properties regarding regolith such as grain size, grain shape, cohesion and bulk density. The usage of penetrometry however has mostly been limited ground-based studies such as soil sciences or even cheese maturation. Very little is known about the underlying physics of penetrometry. Results of penetrometry experiments are largely analysed based on empirical models, which presents us with a challenge if we want to apply the same parameters to understand granular materials on asteroid surfaces. Obviously, gravity cannot be eliminated in the laboratory. Hence, it is essential to verify penetrometry as a method and validate penetrometry instrument designs in microgravity.

For this purpose, we propose a parabolic flight campaign. Our experiment will test the use of penetrometry in asteroid-analogue environments by investigating samples with varying properties such as grain size and shape. The microgravity aspect of the experiment is one of the most important factors because it enables us to correlate laboratory experiments at 1g with identical setups in a gravity regime relevant to asteroids. The proposed experimental setup will include a variety of samples with varying grain sizes, grain shapes, porosities and grain size distributions. The penetrometer used will also have varying properties such as the diameter, shape, and velocity of penetration. A robotic arm will push a penetrometer into the samples to measure the reaction force which can then be used to determine the mechanical properties of the samples. By varying the samples and penetrometer properties it will be possible to better understand the relevant parameters affecting reaction force. The suitability of the setup will also be reviewed to understand its usage and applicability in microgravity environments such as the robotic arm that will be used. All of the experiments carried out during the parabolic campaign will also be done at 1g to compare the tests in varying gravity levels. With a better understanding of the science behind penetrometry and the effects of microgravity, future missions will be better prepared and be able to use penetrometry more effectively to understand small-body surfaces.