Target asteroid subsurface mechanical property characterization

Background

The rq-0-26 resolution validated dual counter-rotating bucket-wheel excavation as the baseline mechanism, with 90-95% torque cancellation demonstrated in models. However, the bucket-wheel design parameters (tooth geometry, rotation speed, penetration depth) depend critically on the mechanical properties of the material being excavated. OSIRIS-REx's TAG operation at Bennu revealed an unexpectedly low-density, weak surface that the spacecraft sank into — demonstrating that surface observations alone cannot predict subsurface behavior.

Why This Matters

Subsurface mechanical properties determine:

• Bucket tooth design (cutting vs. scooping geometry)
• Required excavation force and power consumption
• Material flow into buckets (cohesive vs. granular behavior)
• Housing and containment requirements for ejected particles
• Whether the excavator can operate continuously or must adapt to heterogeneous zones

Without characterization data, bucket-wheel designs are based on assumed properties that may be wrong by an order of magnitude. Bennu and Ryugu sample returns provide surface data but limited subsurface information.

Key Considerations

• Rubble pile asteroids may have very different subsurface properties than monolithic bodies
• Porosity varies from near-surface (50-60%) to depth (20-40%)
• Cohesion from van der Waals forces dominates at small particle sizes
• Thermal processing of near-surface layers by solar heating may create sintered crusts
• Volatile content affects mechanical behavior (ice-cemented vs. dry regolith)
• Returned samples from Bennu and Ryugu provide ground truth for limited depths

Research Directions

1. Returned sample mechanical testing: Conduct comprehensive mechanical testing (shear strength, cohesion, compressive strength) on Bennu and Ryugu samples at various confining pressures.

2. Radar tomography mission design: Specify prospecting satellite radar capabilities for subsurface density and structure mapping of target asteroids.

3. Regolith simulant development: Create improved asteroid regolith simulants calibrated against returned sample data, including volatile-bearing variants.

4. Penetrometer mission concept: Design a low-cost precursor mission deploying penetrometers on a target asteroid to measure subsurface strength profiles to 1-2 meter depth.

5. Adaptive excavation algorithm: Develop control algorithms that allow bucket-wheel systems to adapt in real-time to varying material properties, reducing dependence on pre-characterization accuracy.