ALMA Observations of the DART Impact: Characterizing the Ejecta at Sub-Millimeter Wavelengths

TL;DR

This study used ALMA to observe the Didymos-Dimorphos asteroid system before and after the DART impact, measuring ejecta mass and properties at millimeter wavelengths to enhance understanding of impact outcomes.

Contribution

First ALMA observations of asteroid ejecta at millimeter wavelengths, providing detailed ejecta mass estimates and spectral emissivity data post-DART impact.

Findings

Ejecta mass estimated at 1.3–6.4×10^7 kg

Ejecta mass accounts for 0.3–1.5% of Dimorphos' mass

Spectral emissivity consistent with siliceous and carbonaceous asteroids

Abstract

We report observations of the Didymos-Dimorphos binary asteroid system using the Atacama Large Millimeter/Submillimeter Array (ALMA) and the Atacama Compact Array (ACA) in support of the Double Asteroid Redirection Test (DART) mission. Our observations on UT 2022 September 15 provided a pre-impact baseline and the first measure of Didymos-Dimorphos' spectral emissivity at $\lambda=0.87$ mm, which was consistent with the handful of siliceous and carbonaceous asteroids measured at millimeter wavelengths. Our post-impact observations were conducted using four consecutive executions each of ALMA and the ACA spanning from T$+$3.52 to T$+$8.60 hours post-impact, sampling thermal emission from the asteroids and the impact ejecta. We scaled our pre-impact baseline measurement and subtracted it from the post-impact observations to isolate the flux density of mm-sized grains in the ejecta. Ejecta dust masses were calculated for a range of materials that may be representative of Dimorphos' S-type asteroid material. The average ejecta mass over our observations is consistent with 1.3--6.4$\times10^7$ kg, with the lower and higher values calculated for amorphous silicates and for crystalline silicates, respectively. Owing to the likely crystalline nature of S-type asteroid material, the higher value is favored. These ejecta masses represent 0.3--1.5\% of Dimorphos' total mass and are in agreement with lower limits on the ejecta mass based on measurements at optical wavelengths. Our results provide the most sensitive measure of mm-sized material in the ejecta and demonstrate the power of ALMA for providing supporting observations to spaceflight missions.