Push an object through sand on Earth and you feel resistance that depends on speed, depth, grain size, and a dozen other variables. The physics is notoriously complex — granular drag is one of the oldest unsolved problems in mechanics, resisting clean theoretical description since Euler studied it.
Pongo et al. remove gravity and find simplicity. In microgravity experiments with embedded force sensors, granular drag reduces to a constant: a dimensionless drag coefficient of approximately 1.2, independent of velocity. The complicated behavior we observe on Earth is not intrinsic to granular resistance. It is gravity wearing a mask.
When gravity is present, an additional velocity-dependent term appears. This term decreases inversely with impact speed — faster projectiles feel proportionally less of it. The constant component reflects pure momentum transfer along the penetration axis: the projectile hits grains, the grains move, momentum is exchanged. The gravity-dependent component reflects stress accumulation from the weight of grains above — a force that depends on burial depth and gravitational loading, not on the collision dynamics.
The same decomposition appears in the initial peak drag force: a gravity-independent baseline plus a gravity-dependent correction.
The structural point: what we call granular drag is two things mixed together. One is the actual resistance to motion — momentum transfer, approximately constant. The other is gravity creating stress chains that the projectile must also overcome. Remove gravity and you see what drag actually is. The velocity dependence we thought was fundamental to granular media is an artifact of living at the bottom of a gravitational well.