In 2022, NASA's Double Asteroid Redirect Test (DART) smashed into the asteroid Dimorphos in a successful test of planetary defense technology. That success was measured by a significant shift in Dimorphos' orbit around the larger asteroid Didymos. Since then, various observatories have been analyzing the data to try to piece together what the debris from the impact tells us about the structure of the asteroid.

All of those observations have taken place at great distances from the impact. But DART carried a small CubeSat called LICIACube along for the ride and dropped it onto a trailing trajectory a few weeks before impact. It took a while to get all of LICIACube's images back to Earth and analyzed, but the results are now coming in, and they provide hints about Dimorphos' composition and history, along with why the impact had such a large effect on its orbit.

Tracing debris

LICIACube had both narrow and widefield imagers on board (named LEIA and LUKE via some carefully chosen backronyms). It trailed DART through the impact area for about three minutes and captured images starting about a minute before the impact and continuing for over five minutes afterward.

These showed that the impact created a complex field of debris. Rather than a simple cone of material, there were filaments and clumps of ejecta, all moving at different speeds. One paper, released in Nature today, tries to catalog a lot of it. So, for example, it identifies one stream of ejected material that shows up in the first post-impact images and can be tracked until imaging stops. By this point, it has extended over eight kilometers from the impact site. That works out to be a velocity of about 50 meters a second.

Separately, there was a clump of material that was visible for about a minute and a half and traveling at about 75 meters a second; a second clump moved at about half that rate.

The fastest moving material they could track was ejected at about 500 meters a second, which is about 1,800 kilometers an hour (1,100 mph). And that helps drive home LICIACube's value, since the best observations we have at a distance were done by Hubble, and it only detected objects moving at half that speed.

Oddly, the ejected material initially looks reddish in tint but gradually shifts to more blue over time. The researchers suggest that this could mean that the surface of the asteroid had been reddened by exposure to radiation, and the first material to exit the impact came from the surface. Later, as more of the material came from the interior, the redness dropped.

Late last year, a separate paper focused on the dimensions of the debris cone. Using those, it worked backward to evaluate where that cone reached the surface of Dimorphos. Based on that, the researchers involved estimated that the material was coming from a crater that was about 65 meters in diameter.

A weak interior

Tracking all the complex debris is important partly because it played a role in the effectiveness of DART. We know exactly how much momentum the DART spacecraft carried into the collision, and we can compare that to 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's pretty clear that DART's momentum can't account for all of the change. So, a significant amount of the exchange of momentum came about as debris from the impact carried momentum away from Dimorphos.

An additional paper takes the LICIACube data on the material ejected and uses it to try to estimate the internal properties of Dimorphos. A model of the physics of the collision was used to test a variety of internal compositions for the asteroid that varied based on their density, amount of solid rock vs loose material, and other characteristics. The best results came from a relatively low-density porous body that doesn't have a lot of large boulders near its surface.

Given that structure, the researchers conclude that DART likely caused global disruption of its target's structure.

Dimorphos' weak, fragmented structure looks a lot like we've seen in visits to what are called "rubble pile asteroids" like Bennu and Ryugu. The striking thing about it is that it's much weaker than the structure of its larger neighbor, Didymos. That's consistent, however, with models of how Dimorphos must have formed. These posit that Didymos shed material, some of which stayed gravitationally bound and ended up in orbit.

One way that could happen is via a collision, but that might be expected to be energetic enough to liberate a wide range of materials from Didymos. An alternative, however, is that solar heating could increase the spin of Didymos until it no longer had enough gravitational pull to hold on to all its material. In this case, lighter material is likely to be shed from the surface first, possibly accounting for the relatively small size of the material in Dimorphos.

The good news is that we're scheduled to have an even better view of the post-impact system in a few years. In late 2024, the ESA plans to launch a probe called Hera that will go into orbit around the Didymos/Dimorphos system and provide detailed data on the collision's aftermath.

The Planetary Science Journal, 2023. DOI: 10.3847/PSJ/ad09ba  (About DOIs).

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

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