Intentional Collision and In-Situ Measurements
In September 2022, NASA's DART spacecraft approached a near-Earth asteroid named Didymos and its moon, Dimorphos. With cameras trained on Dimorphos specifically, DART took pictures with ever-increasing resolution as the distance to the moon's surface steadily shrank.
Unfortunately for both DART and Dimorphos, though, this was not a flyby mission. Instead of changing course, DART plowed straight into the moon with enough momentum to dig a fresh crater and to change the satellite's orbital period by more than 30 minutes.
The main purpose of this intentional collision was to test if it'd be possible to redirect an asteroid's path through a brute-force impact. This is a potentially humanity-saving skill, should astronomers ever discover a large asteroid destined to collide with Earth absent a dramatic intervention, and scientists involved with the mission want to be confident that everything they learn from this single collision is applicable to any future high-stakes redirections.
To more confidently extrapolate the results of this test to larger asteroids with structures and compositions that might differ from Dimophos's, astronomers need to measure every material property of this one asteroid that they can.
Some of these properties are best measured from up close to the asteroid, and the images beamed back by DART moments before its abrupt end contain much useful information. Others, though, such as how the surface scatters light, are better constrained by observing the asteroid from many different angles, which one can do by taking observations from Earth over many months. By combining these different types of measurements, a fuller picture emerges than could be built by either dataset alone.
Adding in Ground-Based Observations
This combination is what Bonnie Buratti, Jet Propulsion Laboratory, and collaborators accomplished in a recent study. The researchers took observations from Table Mountain Observatory of the Didymos/Dimorphos system over about five months in order to build a solar phase curve, or how the brightness of the two asteroids changes with respect to our viewing angle.
This allowed them to build a particle phase function, or a model of how the surface scatters light. Then, by poring over and reprocessing the DART images, Buratti and colleagues were able to quantify the surface roughness of the asteroid, a property that would have been impossible to disentangle from the phase function without the extra information gathered from the ground.
This synthesis demonstrates that although the missions we send up to gather exquisite in-situ data, sometimes important, fundamental properties require patient observations from back home as well. In 2026, the European Space Agency's Hera spacecraft will arrive at Didymos to survey the damage, and analysis of its data will rely heavily on context learned through studies like these in the intervening years.
Research Report:Pre-impact Albedo Map and Photometric Properties of the (65803) Didymos Asteroid Binary System from DART and Ground-Based Data
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