How do granular media behave in a low-gravity environment? We can't match the answer to the analysis!

The characteristics of the celestial surface layer can only be understood by going to the actual site and touching the surface, but we cannot wait until then for the development of the spacecraft. To tackle this issue, the Hourglass mission was developed to study the behavior of different granular media in low gravity environments onboard the International Space Station. The goal was to acquire the set of parameters that can be used in analysis to reduce the required resources needed to realize a spacecraft design.

What are the big challenges?

Numerical simulations have long been used to verify the design of landers and rovers, which are indispensable for lunar and planetary exploration, as well as for the preliminary validation of various missions. However, the gravity of the celestial surface layer at the landing target for the spacecraft is less than 1G, making it difficult to confirm these results in ground experiments on Earth. While parabolic flight in an airplane and free fall within a drop tower are often used as test environments for low gravity, it is difficult to reproduce a stable and long-term low gravity environment with these methods, and therefore challenging to obtain sufficient measurement results to verify the gravitational dependence of the flow behavior of the surface granular media.

This has meant that current spacecraft designs are limited to using parameters acquired by predicting the characteristics of the surface layer from remote observation and knowledge about the formation of celestial bodies, and the uncertainty for an analysis is large. There are many records from previous spacecraft activities on the surface of celestial bodies, and observational results of the macroscopic interaction forces. However, to observe the behavior of regolith particles, it is necessary to continuously capture video while tracking several tens of micron-sized floating regolith particles, which is difficult. If a stable, long-term low-gravity environment can be established in a closed space and the behavior of small regolith particles can be recorded in a video, then a resulting parameter set that can be used to form the basis of future calculations can be obtained. 

How was it cleared?

With this in mind, the Hourglass mission was developed to conduct flow experiments using an hourglass-shaped specimen container holding sand from Earth and simulated regolith. The set-up was placed in an artificial gravity environment of less than 1G generated using the centrifugal force created by the turntable-type cell culture experimental equipment (CBEF: Centrifuge-equipped Biological Experiment Facility) installed in the Kibo module of the International Space Station (ISS).

CBEF has been previously used in biological experiments, and can stably supply an arbitrary low-gravity environment over a long time. The facility was therefore considered suitable for providing a suitable gravity environment for specifying granular flow characteristics. This is a new approach in which experiments in the near-Earth space environment are used for preliminary demonstration of landing exploration in deep space.

What did they find?

As a result of the Hourglass mission, more than 3000 continuous image data sets were acquired to observe the behavior and accumulation state of 8 types of granular media (simulated regolith and natural sand) under a controlled gravity environment (10 levels of gravitational strength) in a confined space. Based on this data and the results of this study, the characteristics of the gravity-dependent flow and bulk density of sand can be used as a basis for a mechanical model and parameter set necessary for predicting the interaction between machines and regolith under low gravity. For example, it will be possible to create a good initial ground state for a celestial body that is the target for future exploration, and contribute to improving the accuracy of numerical analysis of the ground reaction force to the lander and the rover's traversability, which strongly depends on the magnitude of gravity in which the regolith is placed. By proceeding with multi-level parametric studies, we can break away from designs that include many margins associated with the worst-case possible case, and improve the efficiency and optimal designs with more limited parameters. As a result, this study will improve the reliability of design verification, reduce the weight of the related subsystems, and shorten the development period, and will be one of the stepping stones for high-frequency exploration activities.

What is the future impact?

The behavior of the granular media in low gravity, which has never been seen before at the particle level, will also attract interest from non-researchers, providing non-speculative answers to questions such as “How long would it take to drain the sand if you flipped an hourglass on the moon?”. The limitation of numerical modeling is that it can only consider the acting force that has been taken into account, so the extent to which forces other than gravity work in low gravity, and whether the structure of the model itself changes when gravity is removed are now gradually becoming clear through further analysis of the data acquired through the Hourglass mission. The Hourglass data is also likely to contribute to  identification of planet formation processes and more future of spacecraft design.

Experimental movies and simulation results here: https:/osf.io/3zcm2/