The role of shape-dependent flight stability in the origin of oriented meteorites

Khunsa Amin, Jinzi Mac Huang, Kevin J. Hu, Jun Zhang, and Leif Ristroph

PNAS August 13, 2019 116 (33) 16180-16185; first published July 26, 2019



Motivated by the conical shape of so-called oriented meteorites, we assess experimentally how the flight dynamics and postural stability of a body moving through a fluid depend on its shape. The family of cones of varying apex angle permits the systematic study across shapes from slender or sharp to broad or dull, and we find that only specific forms achieve the stably oriented flight needed to explain this class of meteorites. More generally, our study provides the map relating body shape to free fall motions, such as gliding, tumbling, straight descent, and fluttering, which will prove useful in aeronautics, the atmospheric sciences, and chemical engineering.”

“The atmospheric ablation of meteoroids is a striking example of the reshaping of a solid object due to its motion through a fluid. Motivated by meteorite samples collected on Earth that suggest fixed orientation during flight—most notably the conical shape of so-called oriented meteorites—we hypothesize that such forms result from an aerodynamic stabilization of posture that may be achieved only by specific shapes. Here, we investigate this issue of flight stability in the parallel context of fluid mechanical erosion of clay bodies in flowing water, which yields shapes resembling oriented meteorites. We conduct laboratory experiments on conical objects freely moving through water and fixed within imposed flows to determine the dependence of orientational stability on shape. During free motion, slender cones undergo postural instabilities, such as inversion and tumbling, and broad or dull forms exhibit oscillatory modes, such as rocking and fluttering. Only intermediate shapes, including the stereotypical form carved by erosion, achieve stable orientation and straight flight with apex leading. We corroborate these findings with systematic measurements of torque and stability potentials across cones of varying apex angle, which furnish a complete map of equilibrium postures and their stability. By showing that the particular conical form carved in unidirectional flows is also posturally stable as a free body in flight, these results suggest a self-consistent picture for the origin of oriented meteorites.”


Movie S1. This compilation shows separate experiments on the body motions and associated flows for cone and cone-like objects falling through water. The bodies are centimeter-scale, made of aluminum, and precision machined into conical forms of varying apex angles. Before releasing in a large water tank, their surfaces are coated with a thin layer of clay and fluorescein dye, which serves to visualize the flow.