In the moments leading up to NASA’s DART spacecraft’s impact with the asteroid Dimorphos in 2022, high-resolution images of Dimorphos and its larger companion, Didymos, were captured. These images have allowed scientists to delve into the complex history of these binary asteroids—Didymos, a primary asteroid, and Dimorphos, its secondary moonlet.
Analyses of the craters and surface strength on Didymos suggest it formed around 12.5 million years ago, while Dimorphos likely formed about 300,000 years ago. Didymos is believed to have originated in the main asteroid belt between Mars and Jupiter before being displaced into the inner solar system.
Research into the largest boulders on both asteroids has provided insights into their origins. “Both asteroids are aggregates of rocky fragments resulting from the catastrophic destruction of a parent asteroid,” explained Maurizio Pajola of the National Institute for Astrophysics (INAF) in Italy, who led one of five studies published in Nature Communications. He noted that the large boulders could not have formed from impacts on Didymos and Dimorphos themselves, as such impacts would have shattered the bodies.
Didymos, with a diameter of approximately 780 meters, is classified as a near-Earth asteroid, while Dimorphos is about 170 meters wide. Both are “rubble pile” asteroids, consisting of rocky debris held together by gravity. The largest boulder on Dimorphos is the size of a school bus, whereas the largest on Didymos is as large as a soccer field.
Olivier Barnouin, a planetary geologist at Johns Hopkins University Applied Physics Laboratory and lead author of another study, described the surfaces of both asteroids as weak, even weaker than loose sand. He noted that Dimorphos has surface cracks and boulders, while Didymos might have finer-grained soils at the equator, though the images available provide limited detail.
The research indicates that Dimorphos likely formed from material ejected from Didymos’s equatorial region due to its past rapid rotation. This faster rotation, influenced by the YORP effect (spin acceleration from uneven sunlight exposure), caused Didymos to expel boulders that eventually coalesced into Dimorphos. Currently, Didymos spins once every 2.25 hours, with fewer boulders observed at its equator compared to the rougher mid-latitudes and poles.
The DART (Double Asteroid Redirection Test) mission, which impacted Dimorphos on September 26, 2022, at a velocity of approximately 22,530 kph, demonstrated the feasibility of altering an asteroid’s trajectory to prevent potential Earth collisions. Although Didymos and Dimorphos pose no immediate threat to Earth, the collision slightly altered Dimorphos’s shape and provided valuable data on binary asteroid systems.
Barnouin noted that binary asteroids represent about 10-15% of near-Earth asteroids. “Every new observation helps us understand how asteroids form and evolve,” he said, emphasizing the importance of studying these complex systems, particularly those smaller than one kilometer.
