In the final moments before NASA’s DART spacecraft collided with the asteroid Dimorphos in a significant planetary defense test in 2022, it captured high-resolution images of Dimorphos and its larger companion, Didymos.
These images have allowed scientists to explore the complex history of these two near-Earth rocky bodies and understand the formation of binary asteroid systems, where a primary asteroid is orbited by a smaller moonlet.
An analysis of the craters and surface strength on Didymos suggests it formed around 12.5 million years ago, while a similar analysis indicates Dimorphos formed about 300,000 years ago.
Researchers believe Didymos likely originated in the main asteroid belt between Mars and Jupiter before being displaced into the inner solar system.
Examining the largest boulders on Didymos and Dimorphos provided insights into the origins of the two asteroids.
“Both asteroids are collections of rocky fragments resulting from the catastrophic destruction of a parent asteroid,” said astronomer Maurizio Pajola of the National Institute for Astrophysics (INAF) in Italy, lead author of one of five studies on the asteroids published on Tuesday in the journal Nature Communications.
“These large boulders could not have formed from impacts on the surfaces of Didymos and Dimorphos themselves, as such impacts would have disintegrated these bodies,” Pajola noted.
Didymos, with a diameter of about half a mile (780 meters), is classified as a near-Earth asteroid. Dimorphos is approximately 560 feet (170 meters) wide.
Both are considered “rubble pile” asteroids, made up of rocky debris that has coalesced due to gravitational forces.
“Their surfaces are covered with boulders. The largest on Dimorphos is the size of a school bus. In contrast, the largest on Didymos is as big as a soccer field,” said Olivier Barnouin, a planetary geologist and geophysicist at the Johns Hopkins University Applied Physics Laboratory in Maryland and lead author of another of the studies.
The researchers concluded that Dimorphos is made up of material ejected from the equatorial region of Didymos due to its rapid rotation.
“Regarding Didymos, it is believed that it once rotated more quickly around its axis because of the YORP effect (spin acceleration caused by sunlight impacting its uneven surface).
This caused boulders to be expelled from its equatorial region, forming Dimorphos,” Pajola explained.
Didymos currently completes a rotation every 2-1/4 hours.
Few boulders were observed in the equatorial region of Didymos.
“Its equator is much smoother, while the mid-latitudes up to the poles are rougher, with large boulders on the surface,” Pajola said.
NASA’s DART (Double Asteroid Redirection Test) conducted a proof-of-concept mission, showing that a spacecraft could use kinetic force to alter the trajectory of a space object potentially on a collision course with Earth. Didymos and Dimorphos are not actual threats to Earth.
On September 26, 2022, DART impacted Dimorphos at about 14,000 miles per hour (22,530 kph) from approximately 6.8 million miles (11 million km) from Earth, drastically altering its path.
The collision also subtly changed the shape of Dimorphos.
Data from the DART mission has enhanced the understanding of binary asteroid systems.
“Binary asteroid systems make up about 10-15% of all near-Earth asteroids,” Barnouin stated. “Overall, each new observation of an asteroid or asteroid system increases our knowledge of asteroid formation and evolution.
They are complex systems but share key similarities, especially among smaller asteroids less than a kilometer (0.62 miles) in size.”
