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We report observations of the Didymos-Dimorphos binary asteroid system usingthe Atacama Large Millimeter/Submillimeter Array (ALMA) and the Atacama CompactArray (ACA) in support of the Double Asteroid Redirection Test (DART) mission.Our observations on UT 2022 September 15 provided a pre-impact baseline and thefirst measure of Didymos-Dimorphos' spectral emissivity at $\lambda=0.87$ mm,which was consistent with the handful of siliceous and carbonaceous asteroidsmeasured at millimeter wavelengths. Our post-impact observations were conductedusing four consecutive executions each of ALMA and the ACA spanning fromT$+$3.52 to T$+$8.60 hours post-impact, sampling thermal emission from theasteroids and the impact ejecta. We scaled our pre-impact baseline measurementand subtracted it from the post-impact observations to isolate the flux densityof mm-sized grains in the ejecta. Ejecta dust masses were calculated for arange of materials that may be representative of Dimorphos' S-type asteroidmaterial. The average ejecta mass over our observations is consistent with1.3--6.4$\times10^7$ kg, with the lower and higher values calculated foramorphous silicates and for crystalline silicates, respectively. Owing to thelikely crystalline nature of S-type asteroid material, the higher value isfavored. These ejecta masses represent 0.3--1.5\% of Dimorphos' total mass andare in agreement with lower limits on the ejecta mass based on measurements atoptical wavelengths. Our results provide the most sensitive measure of mm-sizedmaterial in the ejecta and demonstrate the power of ALMA for providingsupporting observations to spaceflight missions.

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