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Loading contentThe principle that gravitational and inertial mass are identical, so that being in free fall is locally indistinguishable from floating in empty space. This insight led Einstein from special to general relativity, recasting gravity as the curvature of spacetime; it is tested to extraordinary precision by lunar laser ranging and dedicated satellites.
Facts on this topic will be cited from these primary and reference sources.
Mission data, planetary science, space telescopes, and public-domain imagery.
Most NASA-produced imagery is in the public domain; individual items are checked for usage terms before publication.