In 2022, NASA’s Double Asteroid Redirection Test (DART) collided with the asteroid Dimorphos in a test of planetary defense technology. Its success was measured by a significant change in Dimorphos’ orbit around the larger asteroid Didymos. Since then, various observatories have been analyzing the data to try to piece together what the impact debris tells us about the asteroid’s structure.
All these observations were made far away from the impact. But DART carried his small CubeSat, called LICIACube, and dropped it into a tracking orbit several weeks before impact. It took time to bring all of LICIACube’s images back to Earth and analyze them, but the results have now arrived, and they reveal the composition and history of Dimorphos, and the significant impact the collision had on its orbit. We’re giving you a hint as to why.
track debris
LICIACube was equipped with both narrow-field and wide-field imagers (named LEIA and LUKE by a few carefully chosen backronyms). DART was tracked within the collision area for approximately 3 minutes, and images were taken starting approximately 1 minute before the collision and continuing for more than 5 minutes after the collision.
These indicate that the impact created a complex debris field. Rather than a simple cone of material, there were masses of filaments and ejecta, all moving at different speeds. A paper published today in Nature attempts to catalog many of them. So, for example, he can identify one stream of ejected material that appears in the first post-impact image and follow it until imaging stops. At this point, the area has spread over eight kilometers from the crash site. In other words, it is about 50 meters per second.
Separately, a mass of material traveling at about 75 meters per second was visible for about a minute and a half. The second mass moved at about half that speed.
The fastest moving material they were able to track was ejected at about 500 meters per second, which equates to about 1,800 kilometers per hour (1,100 miles per hour). This helps increase the value of LICIACube. That’s because the best observations from a distance were made by Hubble, which detected only objects moving at half that speed.
Strangely enough, the injected material initially appears to have a reddish tint, but gradually becomes more bluish over time. The researchers suggest this could mean that the asteroid’s surface was exposed to radiation, turning red, and the first material from the impact was emerging from the surface. After that, the redness decreased as more material came out from inside.
Another paper was published late last year that focused on the dimensions of the debris cone. These were used to back-calculate and assess where that cone reached the surface of Dimorphos. Based on that, the researchers involved estimated that the material came from a crater about 65 meters in diameter.
weak interior
Tracking all complex debris is important as it plays a role in DART’s effectiveness. We know exactly how much momentum the DART spacecraft brought to the collision, and we can compare it to estimates of the amount needed to change Dimorphos’ orbit. Based on the magnitude of the orbital change and the estimate of Dimorphos’ initial mass, it is clear that DART’s momentum cannot explain all of the change. Therefore, a significant portion of the momentum exchange occurred because the collisional debris carried momentum away from the dimorphos.
Additional papers attempt to obtain LICIACube data on ejected material and use it to estimate the internal properties of Dimorphos. A physics model of the impact was used to test different internal compositions of the asteroid, which vary based on the asteroid’s density, amount of solid rock and free material, and other properties. The best results were obtained from relatively low-density porous bodies without many large rocks near the surface.
Given its structure, the researchers conclude that DART likely caused the global destruction of the targeted structure.
Dimorphos’ weak, fragmentary structure is very similar to what we’ve seen on visits to so-called “rubble pile asteroids” like Bennu and Ryugu. What is surprising is that it is much weaker than its larger neighbor, the structure of Didymus. But it’s consistent with a model of how Dimorphos formed. These assumptions assume that Didymos ejected material, some of which ended up in orbit while remaining bound by gravity.
One possible way this could happen is through collisions, which would be expected to provide enough energy to liberate various materials from Didymos. However, an alternative idea is that solar heat could increase Didymus’ rotation until its gravity can no longer hold all the matter. In this case, the lighter material may flake off the surface first, which may explain the relatively small size of the material in Dimorphos.
The good news is that within the next few years we will have a much better understanding of post-impact systems. ESA plans to launch the Hera probe at the end of 2024, which will orbit the Didymos/Dimorphos system and provide detailed data on the aftermath of the collision.
Planetary Science Journal, 2023. DOI: 10.3847/PSJ/ad09ba (About DOI).
Nature, 2024. DOI: 10.1038/s41586-023-06998-2
Natural Astronomy, 2024. DOI: 10.1038/s41550-024-02200-3