This incredible video from September 26, 2022, shows the perspective of the DART spacecraft as it approached and then struck the asteroid Dimorphos. The final 5 1/2 minutes are condensed into the shortened video. Via NASA/ Johns Hopkins APL/ YouTube.
- NASA’s DART mission successfully impacted the asteroid moonlet Dimorphos in 2022. The collision altered the asteroid’s orbit.
- Now, new research suggests that Dimorphos likely formed from material spun off from its larger binary companion, Didymos.
- Future missions to Dimorphos and Didymos, like ESA’s Hera mission, will further explore this binary asteroid system.
A closer look at DART’s impact
In 2022, NASA’s DART (Double Asteroid Redirection Test) spacecraft intentionally collided with the asteroid moonlet Dimorphos. NASA wanted to see how much the impact could alter the orbit of Dimorphos, as a test of our human ability to deflect a possible asteroid on a collision course with Earth. Initial analyses did show that DART succeeded in changing the asteroid’s path. So it was a successful test! But now there’s more. On July 30, 2024, NASA scientists said they’ve learned more about why DART was so effective. The knowledge relates to the origins of Dimorphos and its larger asteroid companion, Didymos.
The researchers published their peer-reviewed findings in five new papers in Nature Communications. (See the list at end of this post.)
DART and the origins of Dimorphos and Didymos
DART didn’t just strike Dimorphos. It studied the geology of both Dimorphos and Didymos, to learn more about their origins and evolution. The two asteroids orbit each other as a binary pair, although you can also think of Dimorphos as a smaller “moonlet” of Didymos. Didymos is about 2,500 feet (760 meters) in diameter, while Dimorphos is about 525 feet (160 meters) in length. Thomas Statler is the lead scientist for Solar System Small Bodies at NASA Headquarters in Washington. He said:
These findings give us new insights into the ways that asteroids can change over time. This is important not just for understanding the Near-Earth Objects that are the focus of planetary defense, but also for our ability to read the history of our solar system from these remnants of planet formation. This is just part of the wealth of new knowledge we’ve gained from DART.
In one of the new papers, Olivier Barnouin and Ronald Ballouz of Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, examined images of both asteroids to study their geology. The images came from both DART and the Italian LICIACube cubesat that accompanied it. The images showed boulders of various sizes on smaller Dimorphos. Didymos, however, was different. The larger asteroid was rocky at higher elevations, but smoother at lower elevations. It also had more craters than Dimorphos.
Was Dimorphos spun off from Didymos?
The findings suggested Dimorphos was likely “spun off” from Didymos. That is, material was forced off Didymos’ surface to form Dimorphos. This may have occurred due to the spin of Didymos being accelerated.
The researchers found both asteroids have weak surfaces. The finding also suggests the two asteroids differ significantly in age. Didymos’ age is estimated to be 12.5 million years. On the other hand, the age of Dimorphos is less than 300,000 years.
Dimorphos formed in stages
Another paper, led by Maurizio Pajola of the National Institute for Astrophysics (INAF) in Rome, compared the shapes and sizes of the boulders and their distribution patterns on both asteroids’ surfaces. The analysis showed Dimorphos likely formed in stages, from remnant material spun off Didymos.
In addition, the study results from Alice Lucchetti, also of INAF, suggested Dimorphos experienced thermal fatigue. That process – the gradual weakening of material by fluctuations in temperature – could have broken up boulders on Dimorphos’ surface quite quickly. This in turn would also alter the appearance of the asteroid overall faster than previously thought.
As previously noted, the DART mission results indicate the surfaces of both asteroids are weak. In fact, for Didymos, the bearing capacity of its surface is at least 1,000 times less than either dry sand on Earth or regolith “soil” on the moon. That study was led by Jeanne Bigot and Pauline Lombardo at ISAE-SUPAERO in Toulouse, France. The bearing capacity is the surface soil’s ability to support loads applied to the ground, measured as the average contact pressure.
Finally, Colas Robin of ISAE-SUPAERO in France compared the surface boulders of Dimorphos to those of asteroids Itokawa, Ryugu and Bennu. They are what scientists call rubble-pile asteroids, where chunks of rock and debris are loosely bound together by gravity rather than a more solid single mass. Notably, the boulders on Dimorphos were found to be quite similar. This suggests Dimorphos likely formed in a similar way to other rubble-pile asteroids.
Why the DART mission was so successful
The researchers said it was Dimorphos’ low surface strength that likely contributed to DART’s significant effect on its orbit. Simply put, it was easy to move.
Barnouin said:
The images and data that DART collected at the Didymos system provided a unique opportunity for a close-up geological look of a near-Earth asteroid binary system. From these images alone, we were able to infer a great deal of information on geophysical properties of both Didymos and Dimorphos and expand our understanding on the formation of these two asteroids. We also better understand why DART was so effective in moving Dimorphos.
The European Space Agency (ESA) is planning to go back to Dimorphos and Didymos with its Hera mission. It is scheduled to launch in October 2024 and reach the asteroids in October 2026. Hera will not impact either of the asteroids. Rather, it will study the effects of the DART mission, in particular the crater left behind on Dimorphos from the impact.
Bottom line: Scientists analyzing data from the DART mission in 2022 have found new evidence of how the binary asteroid system of Didymos and Dimorphos originated.
Sources:
The bearing capacity of asteroid (65803) Didymos estimated from boulder tracks
The geology and evolution of the Near-Earth binary asteroid system (65803) Didymos
Fast boulder fracturing by thermal fatigue detected on stony asteroids
Read more: How DART deflected an asteroid (but released a boulder swarm)
Read more: ‘After’ photos of DART’s collision with an asteroid
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