Thumper and shotgun: Low velocity kinetic penetrometers to estimate regolith and rock properties for NASA's Asteroid Redirect Mission (ARM)

NASA's Asteroid Redirect Mission (ARM) is a challenging mission to capture and bring back either an entire asteroid (Option A) or a large boulder from the surface of an asteroid (Option B) to a cislunar orbit. Options A and B have a range of risks; one of them relates to the unknown strength of the asteroid or boulder. This paper describes methods of estimating asteroid regolith strength and density, and the strength of boulders, using kinetic impactors: Thumpers and Shotgun. Thumpers are large, instrumented kinetic impactors specifically designed to measure regolith strength during impact deceleration. The Shotgun system, on the other hand, uses a large number of small projectiles (“balls”) fired at low velocity at the surface of the asteroid or at the boulder. If a ball impacts regolith, it will create a crater whose size is a function of regolith strength and density. If a ball impacts a coherent boulder, it will bounce back at a certain speed, whose value is proportional to rock strength. If the rebound speed cannot be measured, hollow balls packed with retroreflectors could be used instead (similar to paintballs). The shell of balls can be designed to crack open and release retroreflectors when impacting rock above the threshold strength required for successful boulder retrieval. This paper describes the concepts of Thumpers and Shotgun and demonstrates their feasibility through a series of experiments. These methods leverage many of the advantages of in-situ measurements of target properties- particularly the ability to accurately determine geotechnical measurements- at a considerably reduced cost and implementation effort, and will enable significant risk buy-down on the scope of the ARM mission.