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.

Differential Ablation of Organic Coatings From Micrometeoroids Simulated in the Laboratory

“…Figure 4 shows an example of an observed event compared to the model using these parameters. The best‐fit heating coefficient is consistent with the results of DeLuca and Sternovsky (2019), who found that the heating coefficient can be less than 1. Using Equation (), the best‐fit heating coefficient corresponds to a drag coefficient Γ = 1.24 , which is very similar to the drag coefficients measured by DeLuca and Sternovsky (2019).…”

“…(2013), who found organic coatings of approximately 100 nm thickness on the individual submicron grains that form micron‐size aggregate particles. The particle entered the atmosphere at an angle of 0° to the zenith, and the heating coefficient was set to Λ = 0.6 , which was the heating coefficient found by DeLuca and Sternovsky (2019). The ablating particle deposited its entire PPy content into the atmosphere over a relatively narrow altitude range of 93–94 km.…”

“…This facility has been used to study the basic physics of ablation. Past measurements included measuring the ionization efficiencies of Al and Fe particles (DeLuca et al., 2018; Thomas et al., 2016) and measuring the drag and heating of Al particles (DeLuca & Sternovsky, 2019). To extend such studies to more complex compositions, we used organic‐coated particles in the present study.…”

Atomic‐Scale Simulations of Meteor Ablation

“…Briani et al (2013) calculate the Λ as an output of their numerical model (0.9 for low velocities), and Popova et al (2007) calculates Λ with energy ratios from Monte Carlo simulations, finding Λ between 0.75 and 1.0. Experimentally, DeLuca and Sternovsky (2019) used a measured drag coefficient to constrain the energy transfer coefficient to Λ = 0.58 ± 0.37 for a low velocity aluminum target in air. MD simulations with physical interatomic potentials can provide an estimate for the energy transfer coefficient as well, and this is the subject of the paper.…”

“…However, the single particle impacts influence the heating rate of the meteoroid, which determines the thermal mass loss rate as well. Thermal ablation is sometimes referred to as evaporation (Briani et al, 2013;Campbell-Brown & Koschny, 2004;Čapek & Borovička, 2017;Čapek et al, 2019;Ceplecha et al, 1998;Girin, 2017;Popova, 2004) or thermal mass loss (DeLuca & Sternovsky, 2019;Hill et al, 2004;Rogers et al, 2005;Thomas, 2017;Vondrak et al, 2008). We have chosen to use the term thermal ablation as it encapsulates all possible phase transitions due to the thermal energy imparted on the meteoroids surface from atmospheric particles.…”