The interview was conducted by KAI DÜRFELD

Mr. Dr. Ulamec, the impact of NASA’s DART probe on Dimorphos, the small moon of the planetoid Didymos, showed that a celestial body can be deflected from its orbit by a targeted impact. That was a great success. What was the real challenge of the mission?

DART was a technology demonstration. The mission was intended to show that it is actually possible to precisely hit a body that is only around 160 meters in size with an autonomous probe. This is by no means trivial. Because the planetoid must be hit at the center of gravity if possible. Otherwise, not only will its trajectory change. An impact in the wrong place can cause it to tumble or affect its rotation. The fact that the planetoid was only partially illuminated from the probe’s perspective further increased the challenge. Only after the impact was successful could the actual effect be determined. To do this, we measured how the orbital period of the moon Dimorphos changed around the larger planetoid Didymos. And that’s exactly where it got interesting: the effect was significantly stronger than expected.

Why?

Because such an impact not only transmits the momentum of the space probe. The impact also throws large amounts of material into space. This ejected material creates an additional recoil effect. We are talking about the beta factor. In the case of DART, this factor was approximately three. This means: Dimorphos was deflected about three times more than if only the probe’s impulse had worked. This suggests that the planetoid has a very loose structure. It looks more like a pile of rubble than a compact boulder. Many researchers now even assume that Hera may no longer find a classic impact crater there.

Can the effect of such distraction now be reliably predicted?

This is one of the central open questions. The DART strike was successful and we learned a lot. But so far it has only been a single experiment. Dimorphos is probably typical of certain small planetoids, but it is not yet possible to say for sure. Other bodies could be constructed completely differently. Some are porous, others compact and others made of metal. They would probably react completely differently to an impact.