Universe Today
Binary asteroid systems, in which a large asteroid is orbited by a smaller satellite, are a growing field of interest. In recent years, this has included the asteroid Didymos, a 765-meter-diameter (2,510 ft) Near-Earth Asteroid (NEA) with an orbiting companion (Dimorphos). This moonlet was targeted by the Double Asteroid Redirect Test (DART), a kinetic impactor designed to test a promising technique for planetary defense. Similar binaries have been found across a wide array of small body populations in the Solar System and are being characterized by observatories and spacecraft.
To date, 13 asteroids measuring more than 100 km (62 mi) in diameter have been detected that have confirmed satellites. Interestingly, asteroid satellites are generally found around those with rapid rotations and an elongated shape. Previous models suggest that these satellites could be generated by impacts, but much remains unknown about how these systems came to be. By combining impact simulations, the team was able to track how spin and shape are related to collisional circumstances.
The research was led by Kevin J. Walsh, a Senior Research Scientist at the Southwest Research Institute(SwRI) in Boulder, Colorado. He was joined by researchers from the Johns Hopkins University Applied Physics Laboratory (JHUAPL), the Observatoire de la Côte d'Azur, Charles University, the Space Research and Planetary Sciences at the University of Bern, and the University of Tokyo. The paper describing his team's findings recently appeared online and is being reviewed for publication in theAstrophysical Journal Letters.
Asteroids are essentially material left over from the formation of the Solar System roughly 4.6 billion years ago. Therefore, the study of binary asteroid systems can provide important information regarding the collisional history of different asteroid populations. Changes in the orbital properties of the satellites can also provide insight into the internal properties of the primary. Similarly, differences in binary properties between asteroid classes may provide insight into the internal properties of different types of large asteroids and their formation process.
It is generally accepted that impacts are responsible for binary asteroid systems, since collisions are inevitable for large asteroids. Furthermore, investigations of Main Belt Asteroids like Ceres and Vesta(performed by theDawnspacecraft between 2011 to 2018) have noted impact craters and basins. Previous research that modeled asteroid impacts, disruption, and reaccumulation has shown that satellite formation is a possible consequence. As Walsh told Universe Today via email:
By 'impacts,' I simply mean that a large asteroid gets hit by some smaller asteroid. It builds a nice, big crater, and some of the ejected debris gets caught in orbit. The concept is sound, as we know, big asteroids get hit all the time. We also know that some get hit so hard that they literally break up – we see that in large families of asteroids that have very similar orbits and identical physical properties. However, the models never really explained why the ejecta doesn't just come back and hit the large asteroid. That is what simple physics suggests should happen if the target is a big, spherically symmetric object and the crater is relatively small.
Research has shown that the properties of asteroid satellites are highly dependent on the primary's size. For example, asteroids larger than 100 km (62 mi) in diameter typically have small satellites (<0.1 times the size of the primary). In addition, five of the 13 known systems have multiple satellites, with 130 Elektra having three. Meanwhile, fewer satellites have been observed around asteroids that are ~10 and 100 km (6.2 to 62 mi) in diameter, while those that are more than ~300 km (186.5 mi) appear to have none.
What really caught our attention was a few things: first, the asteroids with satellites were all elongated and NOT spherical, and second, they were all rotating pretty fast," said Walsh. "We also noticed which asteroids didn't have satellites, which is most of the ones with really big families of asteroids that would have been liberated in huge impacts. This data combined suggested that 'impacts' wasn't enough to explain things, but that we needed to understand what types of impacts and the key mechanism that led some ejecta to end up as satellites and some to escape and become part of a larger asteroid family.
For their study, the team conducted simulations of asteroid impacts, which resulted in a wide range of post-impact shapes. These relied on a hydrodynamics code, which models the big shockwave and initial breaking of the target object. This was followed by an N-body granular dynamics code that simulated the gravity of the new fragments, how they interacted with each other, and the resulting shape and spin of the post-impact primaries. Lastly, they performed a long-term simulation to see how the smaller fragments orbited their primaries over time.
Ultimately, their results showed a correlation between satellite formation and the shape and spin of the largest remaining remnant. As Walsh explained:
They revealed that it wasn't about how big/energetic the impacts are, but rather about the angular momentum (from pre-impact spin of the target, or applied rotation from an oblique impact) that can produce the odd-shaped primary and a bunch of satellites on stable orbits. We could also dig deeper and figure out where the material was likely to originate from inside the parent body, and found that it was typically all located 10-20 km (6.2 to 12.4 mi) deep.
These last findings are particularly significant since they predict how the observation and study of asteroid satellites will provide valuable insight into material deep within their primaries. Since an asteroid's interior is naturally shielded from solar and cosmic radiation and the vacuum of space, scientists would be studying material as it was when the Solar System was still forming. In addition, their study clarifies many unanswered questions about this less-understood population of asteroids.
It tells us why we don't see satellites around every big asteroid (they do all get impacted a lot!), and why we don't see them around parent asteroids with huge asteroid families," said Walsh. "It also helps us understand the genetic relationship between the primary and the satellite. Finally, it could help us search deeper for more satellites that have not yet been observed since we now know what the key properties of the primary body should be.
Several space missions are planned for the near future that will rendezvous with binary asteroids to learn more about the origin and evolution of the Solar System. These missions will also provide more information on the orbital mechanics of asteroid populations, which will help inform planetary defense. In addition, facilities like the Vera C. Rubin Observatory are expected to dramatically increase the list of candidate systems in the coming years.
Further Reading:arXiv
