In 2022, the DART probe collided with Dimorph, a moon of the asteroid Didymus. Scientists wanted to check if we could throw a small but potentially dangerous space object off course. It turns out that DART not only changes the trajectory of a small object, but also completely “moves” it.

Astronomers and other scientists are watching closely for the DART probe’s intentional collision with Dimorph in 2022. Instruments aboard the LICIACube mission’s six cubesats observed the nearby system in the first minutes after the event, and ground-based and space-based telescopes recorded the effects of the impact for weeks.

Thus it was possible to collect a lot of data. We know the exact location and angle of the impact, and we know that Dimorph’s orbital period around the asteroid Didyma was shortened by 33 minutes compared to the previous 11 hours and 55 minutes. CubeSats captured the complex structure of matter flying in “threads” kilometers from where DART crashed.

However, the mass, density, surface properties and internal structure of the dimorph continue to be the subject of research. It was not possible to measure these directly with DART instruments or LICIACube cubesats. To successfully deflect potentially hazardous asteroids, scientists need to know these parameters.

Additionally, after examining the Didyma system, we will better understand the evolution and properties of such cosmic bodies. As you may remember, the existence of moons on asteroids was thought to be impossible until a third of a century ago, because even the theoretical mechanisms that would enable their formation were not known.

Therefore, an international group of scientists created computer models of the collision of DART with the Dimorph for various parameters regarding the properties of matter and the internal structure of this cosmic body. Meanwhile, they used the Bern SPH codex, where they simulated the reset of the impactor, to collect soil samples from the Ryugu asteroid as part of the Hayabusa-2 mission.

After comparing the results of the simulations with observational data, the researchers concluded that the Dimorph was extremely “fluffy.” The adhesion force between soil and rocks is below 50 pascals, such as the asteroids Ryugu and Bennu. Dimorph’s bulk density is less than 2.4 thousand kilograms per cubic meter. So it is much more porous than scientists thought.

The total volume of large stones larger than 2.5 meters in diameter does not exceed 40%, at least near the surface. And this matches what we saw in the last images taken by the DART probe before the impact.

As can be seen in the simulation, with such a structure, a crater resulting from the fall of the DART probe would not have formed on the surface of Dimorph. Instead the entire body was deformed and “confused”.

Based on the evaluation of the authors of the study published in the journal Nature AstronomyThis caused a change in the gravitational interaction between Dimorph and the asteroid Didymus, adding a few seconds to a few minutes to the final reduction in the satellite’s orbital period.

Judging by the loose structure, the Dimorph is composed of matter that was “thrown down” by Didymus’ own rotation or by a collision with another object. A similar mechanism of satellite formation on asteroids and planets was previously predicted by physicists and astronomers. Under these conditions, Dimorph can form within a few days. However, it may take several years. Crucially, this means that in other similar systems, asteroid moons can easily change shape and their surfaces may be young.

In a few years, scientists will have the opportunity to test these results. In October 2024, the European Space Agency plans to launch the Hera mission to the Didim system. The station, which includes cameras, an altimeter, a spectroscope and a pair of accompanying cube satellites, will be able to directly examine the composition, structure and physical properties of Didymus and Dimorph matter. The station is expected to arrive at the asteroid in December 2026.