Terramechanics vs. Geotechnical Characterization on the Moon

Why Mobility Data Are Not Design Parameters

Clarifying a Critical Boundary for Lunar Engineering and Construction

Lunar surface activity is no longer limited to exploration. Current programs openly discuss landing infrastructure, surface mobility corridors, power systems, excavation, and long-term occupation. The moment construction enters the conversation, the ground stops being a scientific curiosity and becomes an engineering constraint.

In parallel, a growing body of literature and public commentary refers to rover mobility data as “geotechnical characterization.” The intent is often benign. The consequence is not.

Terramechanics data are being described using geotechnical language, and in some cases treated as if they were suitable inputs for engineering decisions. This is a category error. It collapses system behavior and material properties into a single narrative, and in doing so, removes the very checks that engineering relies on to manage risk.

Engineering Boundary

Terramechanics produces system response under uncontrolled stress paths.

Geotechnical characterization produces material properties under controlled loading.

One informs operations. The other governs design.

On Earth, this distinction is well understood. Vehicle performance data do not replace site investigation. Wheel sinkage does not define bearing capacity. Slip does not define stiffness. Energy consumption does not define excavation resistance. The Moon does not change these fundamentals.

This article draws a clear boundary between terramechanics and geotechnical characterization. Not to diminish mobility science, but to place it correctly within the engineering process. If lunar infrastructure is to be designed with the same rigor expected in any terrestrial project, terminology must be precise and roles must be defined.

What follows is not a critique of missions or disciplines. It is a clarification of responsibility. When words drift, risk follows. Engineering exists to stop that drift before it becomes embedded in design.

What Terramechanics Actually Is

Terramechanics is the study of vehicle–terrain interaction. Its purpose is to describe how a specific mechanical system performs while moving across a ground surface under a defined configuration.

It is not a discipline concerned with intrinsic ground properties. It does not seek to characterize the material independently of the system acting upon it. Its outputs are inseparable from the vehicle that generates them.

Terramechanics evaluates system response through metrics such as traction, sinkage, slip, drawbar pull, energy consumption, and passability. These responses are governed by wheel or track geometry, vehicle mass distribution, suspension behavior, control logic, and operational mode, as much as by the ground itself.

All terramechanics measurements operate within a narrow mechanical envelope. The stress state imposed on the ground is low, transient, and shallow. The depth of influence is typically limited to the upper decimeters. The loading path is dynamic and uncontrolled. Boundary conditions are neither fixed nor repeatable.

As a result, terramechanics produces performance indicators, not material parameters. It answers operational questions: whether a vehicle can traverse a surface, climb a slope, avoid immobilization, or maintain efficiency. It does not answer engineering questions related to load transfer, deformation under sustained stress, or failure mechanisms.

A change in wheel diameter, tread geometry, mass, or control strategy can alter terramechanics outcomes without any change in the ground. That alone demonstrates that terramechanics describes system behavior rather than material state.

For the industry, the implication is straightforward. Terramechanics data are essential for rover design and surface operations. They are not substitutes for site investigation, foundation assessment, or excavation planning. Treating them as such introduces assumptions that the data cannot support.

What Geotechnical Characterization Requires

Geotechnical characterization is the process of determining material properties that govern how the ground carries load, deforms, and fails. Its objective is to provide parameters that can be used, with stated confidence, in engineering design.

A geotechnical parameter is not an observation. It is a property derived from a controlled test conducted under known conditions. The stress path is defined. The boundary conditions are stated. The depth and scale of influence are explicit. Without those elements, a parameter does not exist.

Geotechnical characterization therefore requires controlled loading. Loads must be applied deliberately, measured accurately, and interpreted within a known mechanical framework. The response of the ground must be observed independently of the system applying the load, not inferred through a vehicle whose geometry, control, and operational mode vary continuously.

The outputs of geotechnical characterization are well established. They include bearing capacity, stiffness or modulus, stress strain response, settlement behavior under sustained or cyclic loading, excavation and cutting resistance, and the mechanical state of the material as expressed through parameters such as overconsolidation ratio. These properties are depth dependent and must be resolved as such.

Equally important is what geotechnical characterization does not allow. Back calculated values derived from uncontrolled loading cannot be treated as material properties. Performance metrics cannot be elevated to design inputs. Correlations without defined stress paths cannot be generalized.

This discipline exists precisely to prevent those leaps. In every terrestrial infrastructure project, the separation between observation and parameter is enforced because failure to do so transfers uncertainty directly into design.

If anything, the absence of historical construction experience makes it more critical. Without controlled tests and defined stress conditions, claims of geotechnical characterization are not conservative. They are simply undefined.

Why Mobility Missions Do Not Provide Geotechnical Parameters

Mobility missions are often cited as sources of ground characterization because they interact continuously with the surface. That interaction is real and the data are valuable. The limitation lies not in the quality of the data, but in what the data represent.

Mobility telemetry measures system behavior. Wheel rotation, slip, sinkage, torque, power draw, and vehicle resistance are recorded as a rover traverses the surface. These signals describe how a specific vehicle responds to a specific terrain under a specific operating mode. They do not isolate the mechanical properties of the ground.

Any attempt to extract ground properties from mobility data relies on back calculation. That process is inherently model dependent. It assumes contact mechanics, load distribution, stress transfer, and deformation modes that are not directly measured and cannot be independently verified. Different terramechanics models applied to the same telemetry can produce different inferred values, even when the raw data are identical.

The mechanical envelope is also limited. Rover wheels apply low normal stresses relative to those associated with foundations, pads, excavations, or structural loads. The stress state is transient and shallow, with influence typically confined to the upper decimeters. There is no control over stress path, no ability to hold load, unload, reload, or vary confinement in a systematic way.

Configuration dependence further constrains interpretation. Changes in wheel diameter, tread geometry, suspension stiffness, vehicle mass, or control algorithm alter sinkage and slip without any change in the ground. When system modifications change the outcome, the result cannot be treated as a material property.

This distinction can be stated directly:

Aspect	Terramechanics	Geotechnical
Output	System response	Material property
Stress path	Unknown	Defined
Depth	Decimeters	Defined profile
Design use	Operational	Structural

Mobility data are indispensable for rover design, navigation, and risk management. They inform where a vehicle can go and how it should behave. They do not define bearing capacity, stiffness, settlement response, excavation resistance, or mechanical state with depth.

Using mobility missions as representative examples is appropriate but treating them as geotechnical investigations is not. The data answer operational questions. They were never intended to answer design questions and forcing them into that role introduces assumptions that cannot be defended under engineering scrutiny.

Why the Distinction Has Been Blurred

The blurring between terramechanics and geotechnical characterization did not occur because of poor work or careless intent. It is a structural outcome of how lunar surface activities have evolved.

For decades, lunar missions were not conceived with construction in mind. Surface interaction was limited to mobility, sampling, and short duration operations. Within that context, any observation related to ground behavior was sufficient, and precision in terminology carried little consequence. There was no requirement to separate system performance from material property because no design decisions depended on it.

Planetary science therefore became the dominant frame. Ground response was discussed qualitatively, often through rover behavior, imagery, or inferred resistance. Over time, the term geotechnical began to be used broadly to describe any information related to surface interaction, regardless of whether it met engineering definitions.

At the same time, no lunar geotechnical standards were established. Unlike terrestrial practice, there is no accepted framework defining test types, parameter validity, stress paths, or depth resolution for lunar ground. In the absence of standards, terminology drifted toward convenience.

Review processes reinforced this trend. Publications and mission assessments are typically evaluated by scientists and mobility specialists rather than geotechnical engineers. Within those communities, the distinction between system response and material property is not central, and the language passes without challenge.

The shift toward infrastructure has been faster than the shift in vocabulary. As a result, terms developed for exploration are now being applied to engineering contexts where they no longer hold. The language persists not because it is correct, but because it has not yet been forced to confront design responsibility.

Understanding why this happened matters. It makes clear that the issue is not disciplinary failure, but disciplinary transition. The boundary that was once optional has now become necessary.

Engineering Consequences of Blurred Language

When terminology drifts, uncertainty is not eliminated. It is displaced. In engineering, that displacement always has a cost.

Describing mobility data as geotechnical characterization creates a false sense of ground understanding. Site readiness is inferred where it has not been demonstrated. Ground behavior is assumed to be constrained when it remains largely undefined. These assumptions rarely fail immediately. They surface later, during design convergence, integration, or construction planning, when options are limited and changes are expensive.

One consequence is premature foundation thinking. Load bearing concepts are discussed using rover scale observations, even though the applied stresses, depth of influence, and loading duration are not comparable. Another is the misuse of mobility performance metrics in trade studies for landing pads, power systems, or excavation approaches, where deformation and stability govern performance rather than traversal.

Schedule risk follows naturally. When ground behavior is assumed rather than measured, programs advance without a clear basis for design margins. When geotechnical uncertainty eventually reappears, it does so late, often under schedule pressure, where risk is absorbed rather than engineered out.

Cost risk follows schedule risk. Redesign driven by ground uncertainty is rarely local. It propagates through structural sizing, construction sequencing, equipment selection, and operations. On Earth, this pattern is well documented. The Moon offers no exception.

Most importantly, blurred language erodes accountability. When a parameter is undefined, no one owns its validity. When system response is treated as material property, responsibility for performance becomes diffuse. Engineering relies on clear ownership of assumptions. Precision in terminology is the first step in maintaining that ownership.

This is why the distinction matters. Not because words are wrong, but because designs depend on what those words are allowed to imply.

What Proper Lunar Geotechnical Characterization Looks Like

If lunar infrastructure is to move beyond concept imagery and into credible design, geotechnical characterization must be intentional, explicit, and independent of mobility systems. It cannot be an incidental byproduct of traversal. It must be a defined objective with dedicated instrumentation and controlled tests.

At its core, lunar geotechnical characterization requires controlled interaction with the ground. Loads must be applied deliberately, not inferred. The magnitude, duration, and distribution of stress must be known. The depth of influence must be resolved. Without these elements, interpretation remains speculative.

A credible approach includes dedicated geotechnical payloads designed to interrogate the regolith mechanically. Shallow plate or footing tests provide direct information on bearing response and near surface stiffness. Penetration tests, when properly instrumented, constrain strength variation with depth and identify layering that mobility systems cannot resolve. Controlled excavation or cutting trials expose resistance mechanisms relevant to trenching, pad preparation, and subsurface access.

Equally important is depth awareness. Lunar regolith is not mechanically uniform with depth. Surface maturity, particle interlocking, and stress history evolve rapidly over the first meter. Any characterization that does not explicitly account for depth is incomplete for design purposes.

Within this framework, classification and interpretation must be structured. Regolith type alone is insufficient. Mechanical state matters.

This is where Lunar Regolith Classification (LRC), Lunar Overconsolidation Ratio (OCR*), and Construction Suitability Index (CSI) naturally belong. LRC provides a consistent classification of regolith units. OCR* captures the mechanical state and stress history that govern stiffness, strength mobilization, and deformation behavior. CSI integrates these inputs into a form that supports construction decision making rather than scientific description.

None of these replace mobility data. They complement it by addressing questions mobility systems were never intended to answer. Together, they form a progression from surface interaction to engineering understanding.

Takeaway

Mobility data describe how a vehicle performs on a surface. Geotechnical parameters describe how the ground will carry load, deform, and fail. One informs operations. The other governs design. Treating system response as material property removes the discipline that engineering depends on to manage uncertainty.

As lunar activity transitions from exploration to infrastructure, this boundary can no longer remain implicit. Foundations, pads, excavations, and subsurface works will not tolerate undefined assumptions. They require parameters obtained through controlled tests, known stress paths, and depth resolved interpretation.

Engineering responsibility depends on precise language. In a lunar environment, ambiguity carries immediate risk.

If lunar construction is to be credible, geotechnical characterization must be treated as its own discipline, not as an extension of mobility science. Terramechanics will remain essential. It simply must be used for what it is, not for what it is not.

Author

Roberto de Moraes

Senior Geotechnical Engineer and Engineering Geologist Advisor

Founder, SpaceGeotech.org

Author of The Moon Builders and Engineering the Lunar Sites for Construction