A change of the rotation period of asteroid (65803) Didymos caused by the DART impact

TL;DR

Didymos–Dimorphos' DART impact altered the binary's angular momentum, and the authors detect a small but statistically significant slowdown of the primary's rotation using two decades of photometric data. They apply light-curve inversion with a convex shape model to pre- and post-impact observations, obtaining $P_1 = 2.260389$ h and $P_2 = 2.260440$ h, i.e., a change of $ abla P = 0.000051$ h ≈ $0.18$ s. Bootstrap analyses and systematic checks confirm the result is robust against data treatment and secondary signal subtraction. The most plausible explanation is reshaping that increases the moment of inertia along the spin axis due to localized mass movements from ejecta, consistent with Didymos's near-spin-limit shape; this small spin-down provides a direct link between the momentum transfer and the primary's rotation, with Hera observations planned to further constrain any ongoing spin evolution. Overall, the study demonstrates how a kinetic-impact event can imprint a measurable rotational response on an asteroid and informs expectations for planetary-defense scenarios and post-impact evolution modeling.

Abstract

On 26 September 2022, the NASA Double Asteroid Redirection Test (DART) spacecraft impacted Dimorphos, the secondary component of the binary asteroid (65803)~Didymos. This experiment tested the Kinetic Impactor technology for diverting dangerous asteroids. Due to the impact, the binary system's angular momentum has changed, resulting in a significant change in the orbital period of Dimorphos. Precise values of the pre- and post-impact orbital periods were derived from a large set of photometric light curves measured for the Didymos-Dimorphos system during six apparitions from 2003 to 2023. We used these data to detect a possible change in the rotation period of the primary as a consequence of the impact. We analyzed the binary system's light curves using the binary asteroid light curve decomposition method. We selected parts of the light curves covering orbital phases outside mutual events, which represent the primary rotational light curves. We applied the light curve inversion method to construct a convex shape model of Didymos and determine its rotation period before and after the impact. These two periods were treated as independent free parameters of the modeling. We found a value of 2.2603891 +/- 0.0000002 h for the pre-impact period and 2.260440 +/- 0.000008 h for the post-impact period. Their difference 0.18 +/- 0.03 s is small yet significant, indicating that the rotation of Didymos became slower after the DART impact. The most plausible physical explanation is Didymos's post-impact reshaping, making its shape more oblate.

Figures (5)

• Figure 1: The correlation between $\Delta P$ and $t_\mathrm{change}$. The red points represent the results based on the original data set. The blue points are results based on bootstrap data.
• Figure 2: The dependence between $\Delta P$ and $\Delta\varphi$. The red circle represents the best-fit values for the original data set, with the spin pole of Didymos fixed in the direction of the orbital pole of Dimorphos. Blue points are based on bootstrap samples. When the spin axis direction is optimized, the best-fit solution for $\Delta P$ and $\Delta\varphi$ moves to values represented by the red triangle. Bootstrap samples with free pole direction are shown as green triangles.
• Figure 3: Convex shape model of Didymos reconstructed from its light curves.
• Figure 4: Example light curves at which the difference between the constant-period model (dashed black curve) and the model with the two different periods (solid red curve) is most prominent. The blue points are measured photometric data normalized to the unit mean brightness.
• Figure 5: Histograms of periods determined from bootstrap samples for four subsets of light curve data. The post-impact periods are separated from the pre-impact periods.