The separation aerodynamics of idealised fragmenting meteoroids

For a meteoroid undergoing break-up within the atmosphere, the aerodynamic separation of fragments immediately following disruption plays a critical role in determining the risks posed at the terrestrial surface. In this work, experiments are performed in a hypersonic wind tunnel to determine the separation characteristics of idealised fragmented meteoroids, consisting of initially spherical clusters of up to 115 close-packed spherical bodies. For equal-sized fragments, the initial separation for populous ( upper N greater than or equivalent to 50 <mml:math xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:mnf="http://cambridge.org/core/manifest" xmlns:cup="http://contentservices.cambridge.org" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:m="http://cambridge.org/core/metadata" xmlns:core="http://cambridge.org/core" xmlns:c="http://cambridge.org/core/content"> <mml:mi>N</mml:mi> <mml:mo>≳</mml:mo> <mml:mn>50</mml:mn> </mml:math> $N\gtrsim 50$ ) clusters follows expected debris-cloud behaviour, with streamwise compression and lateral expansion, whereas smaller-population clusters exhibit a higher sensitivity to the precise initial configuration. For unequal fragments, more extreme separation for smaller fragments and an increased tendency towards subclustering are both observed. The mean separation velocity for equal clusters follows a power law as a function of population with an exponent tilde 0.4 <mml:math xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:mnf="http://cambridge.org/core/manifest" xmlns:cup="http://contentservices.cambridge.org" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:m="http://cambridge.org/core/metadata" xmlns:core="http://cambridge.org/core" xmlns:c="http://cambridge.org/core/content"> <mml:mo>∼</mml:mo> <mml:mn>0.4</mml:mn> </mml:math> $\sim0.4$ , while individual velocities are well modelled by a single-parameter Rayleigh distribution. In unequal clusters, mass retention in subclusters decreases the mean separation velocity, but individual velocities again follow approximate Rayleigh distributions (with the governing parameter now radius-dependent). The statistical model enabled by these measurements provides the first experimentally validated basis for predicting the aerodynamic disruption of small asteroids. An examination of the most ejected bodies in the dataset also reveals that the shock-surfing phenomenon (Laurence &amp; Deiterding, 2011, Shock-wave surfing, J. Fluid Mech ., vol. 676, pp. 396–431) can produce significant outliers and potentially explain a substantial portion of the discrepancy between airburst observations and previous predictions.