The rotation period of asteroid (65803) Didymos before and after the DART impact

On 26 September 2022, the NASA Double Asteroid Redirection Test (DART) spacecraft impacted Dimorphos, the secondary component of the binary asteroid (65803) Didymos (Daly et al. 2023). This experiment aimed to test the kinetic impactor deflection strategy. Due to the impact, the binary system’s angular momentum has changed, resulting in a significant change in the orbital period of Dimorphos. The orbital period was measured precisely by observing mutual events in the light curves of the system. An extensive ground-based photometric campaign resulted in a rich data set of light curves covering six apparitions between 2003 and 2023. Analysis of this data set and independent radar observations have shown that the orbital period has decreased by about 33 minutes (Thomas et al. 2023). The data set is described in detail in Pravec et al. (2022) and Moskovitz et al. (2024). We aimed to use these data to constrain a possible change in the rotation period of the primary as a consequence of the impact. Applying the binary asteroid lightcurve decomposition method (Pravec et al. 2022, 2024), we selected parts of the light curves taken at orbital phases outside the mutual events, subtracted the signal of the secondary component, and applied the light curve inversion method of Kaasalainen & Torppa (2001) and Kaasalainen et al. (2001) to reconstruct a convex shape model of Didymos and precisely determine its rotation period before and after the impact. We introduced another free parameter to the model – a change ∆ω of the rotation rate ω at the time of the DART impact. We assumed that before the impact, the model rotated with the angular frequency ω1, and then, after the impact but before the first post-impact observations, the frequency changed to ω2 = ω1 + ∆ω. The rotation phase φ of Didymos at some epoch t is then described as φ(t) = φ0 + (ω1 + ∆ω)(t − t0) , where φ0 is an initial rotation phase at the time t0 , ∆ω is zero for pre-impact data and nonzero for post-impact data. Both ω1 and ∆ω were parameters of the optimization that defined the pre-impact rotation period P1 = 2π/ω1 and the post-impact rotation period P2 = 2π/ω2 = 2π/(ω1 + ∆ω). We found the best-fit values 2.260 389 1 ± 0.000 000 2 h for the pre-impact period and 2.260 451 ± 0.000 008 h for the post-impact period. Their difference 0.22 ± 0.03 s is small yet significant, meaning that the rotation of Didymos has decelerated after the DART impact. A possible physical explanation might be the accretion of impact ejecta on Didymos, which affected its angular momentum. Although the impactor hit Dimorphos, some angular momentum may have been transferred to the primary component by accretion of ejecta (Richardson et al. 2022). However, numerical simulations need to show this as a realistic scenario.References:Daly, R. T., Ernst, C. M., Barnouin, O. S., et al. 2023, Nature, 616, 443Kaasalainen, M., & Torppa, J. 2001, Icarus, 153, 24Kaasalainen, M., Torppa, J., & Muinonen, K. 2001, Icarus, 153, 37Moskovitz, N., Thomas, C., Pravec, P., et al. 2024, PSJ, 5, 35Pravec, P., Meyer, A. J., Scheirich, P., et al. 2024, Icarus, 418, 116138Pravec, P., Thomas, C. A., Rivkin, A. S., et al. 2022, PSJ, 3, 175Richardson, D. C., Agrusa, H. F., Barbee, B., et al. 2022, PSJ, 3, 157Thomas, C. A., Naidu, S. P., Scheirich, P., et al. 2023, Nature, 616, 448