April 15, 2024

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Close-up images of the DART asteroid crash reveal complex debris

Close-up images of the DART asteroid crash reveal complex debris

In 2022, NASA's Double Asteroid Redirection Test (DART) collided with the asteroid Demorphos in a successful test of planetary defense technology. This success was measured by the significant shift in Dimorphos' orbit around the larger asteroid Didymos. Since then, different observatories have analyzed the data to try to piece together what the debris from the impact tells us about the asteroid's structure.

All of these observations occurred over large distances from the impact. But DART carried a small CubeSat called LICIACube during the flight and dropped it on a later trajectory a few weeks before impact. It took some time to return all of the LICIACube images to Earth and analyze them, but the results are now in, and they offer hints about the formation and history of Dimorphos, along with why the impact had such a large impact on its orbit.

Debris tracking

The LICIACube had narrow- and wide-field imagers (called LEIA and LUKE by some carefully chosen back names). He tracked the DART across the impact zone for about three minutes and took images starting about a minute before impact and continuing for more than five minutes afterward.

These showed that the impact created a complex debris field. Instead of a simple cone of matter, there were strings and clumps of projectiles, all moving at different speeds. One paper, published in Nature today, attempts to classify many of them. So, for example, it identifies a single stream of ejected material that appears in the first images after impact and can be tracked until imaging stops. At this point, it had extended more than eight kilometers from the impact site. This means that the speed is about 50 meters per second.

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Separately, there was a mass of material that was visible for about a minute and a half and moving at a speed of about 75 meters per second; The second group moved at about half that rate.

The fastest moving material they could track was ejected at a speed of 500 meters per second, or about 1,800 kilometers per hour (1,100 mph). This helps determine the value of LICIACube, since the best observations we have at a distance were made by Hubble, and it has only detected objects moving at half that speed.

Curiously, the ejected material initially appears red but gradually turns blue over time. The researchers suggest that this may mean that the surface of the asteroid was reddened due to exposure to radiation, and that the first material that emerged from the collision came from the surface. Later, as more material arrived from inside, the redness decreased.

Late last year, a separate paper focused on the dimensions of the debris cone. Using these elements, we worked backwards to evaluate where this cone reached the surface of the dimorphos. Based on this, the participating researchers estimated that the material was coming from a crater approximately 65 meters in diameter.

Weak interior

Keeping track of all the complex debris is important in part because it played a role in the effectiveness of DART. We know exactly how much momentum the DART spacecraft carried during the collision, and we can compare that with estimates of the amount needed to change Dimorphos' orbit. Based on estimates of the magnitude of the orbital change, as well as the initial mass of dimorphos, it is very clear that DART's momentum cannot explain all of the change. Therefore, a significant amount of momentum exchange occurred when debris from the collision carried momentum away from Dimorphos.

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An additional paper takes LICIACube data on ejected matter and uses it to try to estimate the internal properties of dimorphos. An impact physics model was used to test a variety of asteroid's internal compositions that vary based on its density, the amount of solid rock versus loose material, and other properties. The best results came from a relatively low-density, porous body that didn't have a lot of large rocks near its surface.

Given this structure, the researchers concluded that DART likely caused global disruption of its target structure.

Dimorphos' weak, fragmented structure looks a lot like what we've seen on our visits to so-called “rubble pile asteroids” like Bennu and Ryugu. The amazing thing about it is that it is much weaker in structure than its larger neighbor, Didymos. However, this is consistent with models explaining how dimorphos form. This theory posits that Didymos shed material, some of which remained bound by gravity and ended up in orbit.

One way this could happen is via impact, but it is expected to be powerful enough to liberate a wide range of material from Didymos. But the alternative is that solar heating could increase Didymos's rotation until it doesn't have enough gravity to hold on to all its matter. In this case, the lighter material would likely be shed from the surface first, perhaps due to the relatively small volume of material in the dimorphos.

The good news is that we should get a better view of the post-crash system within a few years. In late 2024, the European Space Agency plans to launch a probe called Hera that will enter orbit around the Didymos/Demorphos system and provide detailed data on the effects of the collision.

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Journal of Planetary Science, 2023. DOI: 10.3847/PSJ/ad09ba (About digital IDs).

Nature, 2024. DOI: 10.1038/s41586-023-06998-2

Natural Astronomy, 2024. DOI: 10.1038/s41550-024-02200-3