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Defining the Albedo and Photometric Properties of Dimorphos in Preparation for the Hera Encounter

Presentation #506.02 in the session Asteroids: Near-Earth Objects (Oral Presentation)

Published onOct 23, 2023
Defining the Albedo and Photometric Properties of Dimorphos in Preparation for the Hera Encounter

Even though the Double Asteroid Redirection Test (DART) mission’s main goal was to execute the first test of a kinetic impactor for planetary defense, the spacecraft returned unique data for understanding the interrelationships among small bodies. The prime goal of our investigation is to define as precisely as possible the albedo of Dimorphos and its photometric properties with the two-fold purpose of understanding where the Didymos-Dimorphos system sits within the S-family of asteroids, and of defining pre-impact conditions on Dimorphos prior to the Hera encounter. A ground-based data set of the solar phase curve of the system between ~5 and 77 degrees obtained at JPL’s Table Mountain Observatory and an image captured by DRACO, the DART camera, have enabled the attainment of these goals.

A map of the normal reflectance of Dimorphos shows the albedo of this secondary asteroid is well within the range of a typical S-type asteroid; this map also provides an accurate measure of the albedo and its distribution prior to the impact, which in turn will offer insights into space weathering and the nature of both the interior material and the post-impact ejecta. Perhaps the most intriguing feature of the albedo map is a series of striations that span almost the entire observed hemisphere. Possible explanations for them include slumping of fine-grained material, tidal stress marks, or the emplacement of impact ejecta. Preliminary modeling of the roughness of Dimorphos also suggests it is a typical asteroid. However, there appear to be areas of differing roughness that may offer clues to the origin of the striations.

In 2026, prior to the Hera encounter, key small phase angle observations less than one degree will occur, enabling a refinement of the photometric model and a better measurement of the geometric albedo. The opposition behavior of an airless body provides crucial information on the compaction state of the optically active portion of the regolith. However, two problems present challenges to any analysis: the ground-based phase curve is of the entire binary system, and measurements of the phase curve post-impact include the surface-effects of the impact.

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