In a first-of-its-kind planetary defense test, NASA’s Double Asteroid Redirection Test (DART) mission has provided scientists with unparalleled insights into the complicated history and formation of the near-Earth binary asteroid system comprising Didymos and Dimorphos. In October 2022, the DART spacecraft rammed into Dimorphos, an event that has by now produced high-resolution images of Dimorphos instrumental in unraveling mysteries about these celestial bodies.
A team of scientists, with Olivier Barnouin and Ronald-Louis Ballouz, from the Maryland-based Johns Hopkins University Applied Physics Laboratory (APL) have studied the geology of both asteroids. By taking images from the Italian Space Agency, the DART and its CubeSat companion LICIACube found that on the surface of Dimorphos, there are boulders that vary in size. Conversely, Didymos was low in elevation and smooth but jagged and more cratered once one went up. The scientists concluded that Dimorphos probably spewed out from Didymos due to a significant mass-shedding event.
Sources of Didymos and Dimorphos have also been revealed by the analyses. Didymos is estimated to be approximately 780 meters in diameter and was created some 12.5 million years ago in the central asteroid belt between Mars and Jupiter, then being ejected into the inner solar system. Compare this with the very much younger dimorphos, which formed about 300,000 years ago after material from Didymos itself was kicked clear of it by rapid rotation, a process significantly accelerated by the YORP effect, which just means sunlight unevenly beating on its surface.
“Both asteroids are aggregates of rocky fragments formed from the catastrophic destruction of a parent asteroid,” noted Maurizio Pajola of the National Institute for Astrophysics in Italy. Large boulders on both asteroids back up this idea; they could not have formed from impacts on their surfaces alone.
“They are these rubble pile asteroids where all of the boulders are held together by gravity from pieces of rocky debris, so it’s this very delicate balance,” said Barnouin. The largest boulder on Dimorphos is that of a school bus, while the largest on Didymos is the size of a soccer field. “Their surface is covered with boulders,” Barnouin noted, adding that the surfaces on both asteroids are much weaker than loose sand.
The impact of the DART mission produced an alteration in the orbit of Dimorphos at a speed of 14,000 miles per hour around Didymos. This has proven that kinetic force can be used to alter a space object’s route effectively. This has considerably enhanced our understanding of binary asteroid systems, the members of which constitute about 10-15% of all near-Earth asteroids.
The data, once returned by the DART mission, will inform the next steps for planetary defense strategies and improve understanding of asteroid formation and their evolution. “With each new observation of an asteroid or asteroid system, we learn more and more about the formation and evolution of asteroids,” Barnouin said. These results emphasize the complexity of asteroid systems and the critical importance of continued observation and study for providing vital information toward planetary defense and our general understanding of the history of the solar system.