High-Speed Drag Measurements of Aluminum Particles in Free Molecular Flow

Abstract

High-speed drag in a free molecular flow is still poorly understood despite playing an important role in a variety of physical situations, including meteor ablation in the upper atmosphere, the orbits of satellites, and the dynamics of cosmic dust grains. To measure drag at high speeds, small aluminum spheres 0.1–2.1 μm in radius were shot at 1–10 km/s into air, N2, Ar, and CO2 using an electrostatic dust accelerator. The measured drag coefficient in air is Γ = 1.29 ± 0.13, with similar values for the other gases. The measurement constrains the heating coefficient to Λ = 0.58 ± 0.37 in air assuming the gas molecules reflect diffusely from the particles' surfaces. The drag appears to be independent of the molecular structures of the four gases tested but has a slight dependence on molecular mass. The drag is also higher than frequently assumed, which has implications for the modeling of high-speed objects in free molecular flows.