New study rules out binary hardening as cause of Dimorphos’s orbital period drop

A new study has challenged a popular explanation for the unexpected 30-second shortening of Dimorphos’s orbital period. The researchers found that the proposed mechanism would actually produce the opposite effect, given the gravitational dynamics of the small moon. The paper has been accepted for publication in Astronomy & Astrophysics and is currently available on the arXiv preprint server.

NASA’s DART (Double Asteroid Redirection Test) was launched into space in 2022 and crashed into Dimorphos, a small moon of the near-Earth asteroid Didymos. The mission was one of planetary defense, where scientists wanted to test whether deliberately hurling a satellite into a small space rock could nudge it off course, away from the planet.

Scientists around the world rejoiced when the crash shortened the binary system’s orbital period by about 33 minutes. However, in the weeks that followed, observations revealed an additional 30-second reduction—an unexpected twist that left researchers puzzled about its cause.

The initial hypothesis for this unexpected shift focused on a process known as binary hardening. In this scenario, debris ejected by the crash interacts with Dimorphos’s gravity and is eventually flung out of the system, causing the binary (Didymos-Dimorphos system) to lose angular momentum and end up with a tighter orbit.

The recent study by Harrison Agrusa and Camille Chatenet, from Université Côte d’Azur, France, showed that gravitational scattering of ejecta did not explain Dimorphos’s abnormal drop in orbital period. The researchers used the REBOUND N-body code to perform N-body simulations, a computational method that calculates the motion of multiple objects as they interact through each other’s gravitational forces. In these simulations, 10,000 ejecta particles were modeled to study their collective dynamics after the DART impact.

The results indicated that Dimorphos is too weak a scatterer (low escape speed, Safronov number ~0.14) to eject enough angular momentum from the system.

If the bound ejecta isn’t being scattered, the most plausible outcome would be re-accretion, where the debris falls back into either component of the binary system. Since DART’s impact on Dimorphos was nearly head-on, causing most of the ejected material to enter prograde orbits, where the debris is expected to move in the same direction as the mutual orbit of Dimorphos and Didymos.

It is known that when material in the prograde with the system re-accretes, it adds to the angular momentum of the binary system, which would result in a net increase of the orbital period instead of the observed decrease.

All the calculations carried out during this study point toward the material being subsequently reaccumulated into the system. This suggests the presence of an additional mechanism that can counteract this effect and cause the observed net decrease in orbital period.

The researchers proposed that rotation-induced reshaping of Dimorphos is the most plausible alternative explanation for the ~30-second drop in orbital period, although they acknowledged that additional studies are needed to reach a definitive conclusion.

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