A lander leg sinks 40 mm further in the test bed than it will on the Moon. A drill draws 15% more torque than the flight unit ever will. A dust seal passes because the particles were too round. None of these are instrument failures. They are simulant failures, and they are only discovered after the hardware is built.
The question is not which simulant is best. It is which simulant is wrong in ways your test does not care about.
Simulant suppliers quote fidelity as a single percentage, and that number is almost always a mineralogical match against a reference. Mineralogy matters, but it is one of at least five properties that decide whether a test bed behaves like regolith.
Chemistry and mineralogy. What the material is made of, and in what proportions. This is what X-ray diffraction and X-ray fluorescence measure, and it is the figure most often quoted. It governs oxygen yield in ISRU work and it governs almost nothing in a mobility trial.
Particle size distribution. The single most consequential property for anything that touches the surface. NASA's assessment of its own NU-LHT series marks one variant as not recommended purely because its distribution is unrealistically fine, despite acceptable chemistry.
Particle shape. Lunar grains are angular and they interlock. Rounded terrestrial analogues flow when real regolith would arch and bridge. Shape drives abrasiveness, seal performance and excavation force.
Glass and agglutinate content. Agglutinates are welded aggregates formed by micrometeorite impact. They have no terrestrial equivalent, so they are manufactured into a simulant deliberately. NASA's own highland simulants carry between 16% and 30% pseudo-agglutinate depending on the variant, and the difference has been shown to change geomechanical behaviour relevant to excavation.
Geotechnical state. Angle of repose, internal friction angle, cohesion and bulk density. These are the numbers a mobility or excavation model actually consumes. They also depend on how the bed was packed, which is why a geotechnical figure quoted without its density state is not usable for comparison.
A simulant can be excellent on the first property and useless on the fifth.
Most selection conversations start with geography. Highland or mare, equatorial or polar. That is the wrong first question.
NASA's Lunar Regolith Simulant User's Guide organizes its recommendations by application rather than by region: excavation and flow, drilling, abrasion and wear, oxygen production, human health. Its assessment of JSC-1A recommends the material for geotechnical testing with loose simulant, and for ISRU work that does not depend on iron redox or ilmenite. The same material, recommended for one purpose and qualified out of another.
The practical order is:
A wheel-soil interaction trial is governed by particle shape, distribution and geotechnical state. Chemistry is nearly irrelevant. An oxygen extraction trial inverts that completely.
Simulants are conventionally sorted into highland and mare. The Apollo sample set supports that split, and it remains a reasonable first cut. Standard highland simulants are typically built against the average chemistry of Apollo 16 regolith.
But the binary is under pressure. Recent work argues that sorting regolith into mare and highland does not capture the variation actually observed across the surface, and the programmes driving demand right now are heading for terrain the binary never described: permanently shadowed regions, crater floors, and deposits where volatiles are the point of the mission.
If your test bed represents a south polar site, a highland simulant matched to an equatorial Apollo 16 average is an approximation you should be able to defend in a design review. Sometimes it is defensible. Often it is not, and nobody asked.
Five questions, and the answers should be documents rather than adjectives.
Which reference sample, by number. Not "highland regolith" but a specific Apollo return sample. A supplier who cannot name the reference cannot tell you what the concordance figure is measured against.
How was concordance verified. X-ray diffraction and X-ray fluorescence are the baseline. Ask for the method, not just the percentage.
What is the particle size distribution of the lot I will receive. Not the product line. The lot. Distributions drift between production runs, and a mobility result is not reproducible across a drift you were not told about.
What are the geotechnical values, and at what density state. Angle of repose and friction angle without the packing condition are not comparable to anything.
Can the distribution be set to my protocol. A stock grade optimized for excavation is the wrong bed for a dust seal, and buying two stock grades is often more expensive than specifying one.
The useful mental shift is to stop treating a simulant as a product you select and start treating it as a test article you specify. The bed is part of the experiment. If it is not documented to the same standard as the instrumentation, the result is not reproducible, whatever the fidelity percentage on the invoice says.
NASA's Simulant Advisory Committee is working toward exactly this: guidelines that separate bulk-use, moderate-fidelity and high-fidelity material so that a programme can state which class it needs. Until those guidelines are settled, the burden sits with the specifying engineer.
Regolithia is supplied against a test protocol rather than from a grade list. Highland reproduces Apollo 16 soil 64501 at 96.4 % phase match and Mare reproduces Apollo 17 soil 72501 at 99.2 % phase match, each computed over four modelled phases from XRD and SEM-EDS compositions, with every phase and every oxide reported against its target value. The particle size distribution and size fractions are published on the datasheet, and the reference soil, phase match figure and lot number travel on the certificate of analysis. Alternative size fractions can be produced on request.
Full specification: DS-003.
