Between a Moon Rock and a Hard Place

Imagine your favorite video game- perhaps it has nice greenery, trees, rocks and all of the small physical details that make it realistic. Simulation modeling for real-world application is not too different. Simulation modeling helps make a specific physical environment, in this case, the south pole of the Moon, available for testing on a computer. It helps predict performance of an entity, MoonRanger, and its activities, trekking the moon for ice. MoonRanger’s simulation team is integral to the mission’s success in helping test autonomy software and surface operations.

Simulation is a very accessible testing method allowing the team to test different conditions and paths a number of times. Using the virtual tools, they can shift and alter the terrain to see how MoonRanger would fare on slightly varied missions. Since not a lot is known about the south pole in terms of rock distribution and topography, the team must utilize information from other missions to inform their work. The simulations help shape the rover’s software, and ultimately its decision-making facilities or autonomy. The more information and potential outcomes the team provides, the more prepared the rover is to handle hazards such as, slopes, craters, rocks, and different optical properties, lunar lighting, shadowing and gravity.

Since MoonRanger has four suspensionless wheels and is skid-steered, its mean-free path is dependent on rock size distributions and its affected mobility. Mean free path is the straight-line distance that a vehicle can attain 50 percent of the time without being impeded by a probabilistically-generated terrain. A vehicle with significant mobility, suspension, scale and ground clearance (i.e. a 4-wheel-drive farm tractor) will have a much longer mean free path in a given terrain than a suspensionless micro-rover like MoonRanger. Using something called the Monte-Carlo method, the team is able to generate many different trials, ultimately examining the rover’s innate ability to traverse without any technological safeguards, such as perception.

The team also uses probability models and analysis to provide a more concrete picture of the simulations in regard to the south pole’s rock distributions. Mobility, perception and navigation are all intertwined with simulation. At CMU, there is a sandbox to test prototypes with similar specifications to that of MoonRanger. For instance, there was some concern over MoonRanger’s safety in driving away after drop-deploying to the moon’s surface. While driving away from the lander, the rover’s perception and planning will circumvent any obstacles, but the terrain that underlies its stowage position on the lander is not known. It is possible that the rover may not be able to maneuver these underlying obstacles, so the team analyzed different rock sizes and position combinations to determine the risks. 

Terrains that the rover could drop-deploy onto are probabilistically classified into high, moderate and low risk to egress mobility or the rover’s ability to drive away intact. Although the rover has successfully driven over some single rocks of 10 cm protrusion height, some combinations of such rocks thwart MoonRanger’s ability to recover, and that height threshold is deemed high risk. The rover negotiates 8 cm rocks and some combinations of them, so that MoonRanger remains unharmed most of the time. Since egress is deemed likely in the vast majority of cases, egress risk is deemed to be moderate. Egress succeeds with 6 cm protrusion heights, so risk is deemed low.  Risk of high-centering or internal rover damage due to a high rock contacting the rover’s belly is deemed high (red) for a 14 cm protrusion, moderate (yellow) for a 12 cm protrusion, and low (green) for a 10 cm protrusion. 

The process of simulations allows for more replication of the environment, prototyping software, a sanity check for software, repetitive testing, and treks longer than anything in the field. This work also meshes well with in-field testing for true, physical application, but there is limited opportunity for that given Pittsburgh’s sporadic weather in comparison to the moon. Using simulation and virtual tools, the team is able to identify anything hazardous and quantify MoonRanger’s journey. There is an old saying, “It’s not the destination, but the journey” but in this case, it’s both.

By: Kerry Mills