The DART impact was an historical moment, which was the outcome of 20 years of efforts for some of us, starting in Europe with a concept called Don Quijotte, which eventually led to the birth of DART and Hera. So, for the few of us who were at the origin of this amazing adventure, both on US and European sides, it was a very emotional moment.
In fact, what I appreciated the most is that the public could discover with us what Didymos and Dimorphos look like. It’s the first time, I think, that both the experts and the public had the same initial level of ignorance. Except for its size, we had no other information about Dimorphos. Would it be spherical or very elongated, would its surface be bare rock or full of boulders? This is what we waited for so long to discover. When we thought about Didymos as a target in 2011, we only had a shape model for the primary, thanks to radar observations, but nothing about Dimorphos except its size. So for 11 years, we just had a mathematical model of the binary asteroid, relying on pure assumptions. I cannot put in words the feeling when we go from a mathematical model, which we worked with for so long, to real images! This is amazing and it is even more so when the reality is very different from the expectations!!!
The first surprise was the shape of the primary, which is not the perfect spinning top shape that we expected!!! We were just … Whoa! Then, in the room with the team, we were all discussing what it meant, waiting to start seeing Dimorphos. We had to wait the last minutes before impact to see something. Then, its ellipsoidal shape became visible, and then rapidly, its surface full of boulders and then the impact. It was crazy, it was like discovering a new world we never saw before. I screamed like I never had, because this is the moment that was a dream 20 years ago, turned into a reality. At the end of it, a few of us just cried.
These images showed again what past missions like Hayabusa2 and OSIRIS-REx revealed, which is that asteroids are not boring rocks in space. They are real and very complex small geological worlds! On a single image, like the one before impact, we see many boulders with different morphologies, and we can spend hours studying them (we actually do so)! This is fascinating and tells us something about how they form and the processes acting in our Solar System during its whole history.
Then came the observations by LICIACube and from the ground. And then, another whoa!!! So much ejecta! We made a mess! I was even fearing that we destroyed Dimorphos, and fortunately this is not the case! But although we knew we could be surprised by the outcome, this was a confirmation that surprises wait for us. The increase of brightness is spectacular, the fact that JWST [James Webb Space Telescope] and HST [Hubble Space Telescope] could also observe it simultaneously is another historical moment as this is the first time both telescopes point to the same object at the same time. So, we have many good reasons to jump on our chairs, as scientists, since what we observe is clearly much more exciting than what could have happened, although it is still too early to really know what it means in terms of the deflection.
This is therefore a very exciting moment. DART is a huge success. It demonstrated that we are able to impact a very small body located at 11 millions km from us by knowing only its size. Now, we have to see in terms of deflection. The DART team is currently doing huge efforts to extract from the data the orbital change, the ejecta properties and all information that can allow us to understand what happened. However, what we see makes us even more motivated (although we already were at 100%) to develop and launch Hera in 2024, because despite the great effort of the DART team, we will be left with many unknowns. In particular, what is the final state of Dimorphos? Did we leave a crater, did we reshape it entirely? What is its internal structure? What is its mass, which allows measuring the momentum transferred by the impact and which is the quantity we need to measure to quantify and validate the deflection? Hera will give us this crucial information.
DART was thus a huge success. It provides the initial conditions of the impact, including the spectacular images of the impact site before impact, LICIACube provides crucial information on the early ejecta from the impact, ground based observations give us the orbital period change of Dimorphos, and Hera will provide the final outcome in great details as well as the physical and compositional properties of the asteroid. With DART, LICIACube and Hera, which constitute the AIDA International Cooperation, we will have a fully documented deflection test, which is needed for impact models to be fully validated at the asteroid scale. This is international cooperation at its best!!
