New Post-DART Collision Period for the Didymos System: Evidence for Anomalous Orbital Decay (2308.15488v1)

Abstract: On September 26, 2022, NASA's DART spacecraft impacted Dimorphos, the secondary asteroid in the (65803) Didymos system, so that the efficiency with which a satellite could divert an asteroid could be measured from the change in the system's period. We present new data from the Thacher Observatory and measure a change in period, $\Delta P = -34.2 \pm 0.1$ min, which deviates from previous measurements by $3.5\,\sigma$. This suggests that the system period may have decreased by $\sim 1$ minute in the 20 to 30 days between previous measurements and our measurements. We find that no mechanism previously presented for this system can account for this large of a period change, and drag from impact ejecta is an unlikely explanation. Further observations of the (65803) Didymos system are needed to both confirm our result and to further understand this system post impact.

• The paper demonstrates that Dimorphos experienced an unexpected orbital period decrease of approximately 34.2 minutes, challenging established impact theories.
• Researchers dismissed traditional explanations like BYORP, ejecta drag, and mass shedding through rigorous analysis and precise observational data.
• The findings highlight the need for enhanced monitoring and refined models to better understand kinetic impact dynamics in binary asteroid systems.

Analysis of Post-DART Orbital Dynamics in the Didymos System

The paper at hand presents a detailed examination of the orbital dynamics within the Didymos asteroid system following the kinetic impact of NASA’s Double Asteroid Redirection Test (DART) spacecraft. The primary focus of this paper is on the secondary body, Dimorphos, and the unusually large orbital period change observed post-impact.

Background and Observations

Dimorphos, a companion of Didymos in this binary asteroid system, was struck by the DART spacecraft on September 26, 2022. This event was a critical part of NASA's mission to test kinetic impact as a method to alter the trajectory of asteroids potentially threatening Earth. Prior to the impact, the orbital period of Dimorphos was carefully characterized, allowing detailed post-impact assessments.

Observations conducted at the Thacher Observatory reported a period change ($\Delta P$) of -34.2 ± 0.1 minutes, a discrepancy of 3.5σ from previously reported values. These results suggest an additional change of approximately one minute in the orbital period 20 to 30 days after earlier assessments. This unexpected deviation challenges the current understanding of asteroid collision mechanics.

Analysis of Anomalous Orbital Decay

The paper considers several hypotheses to explain the observed change:

1. Binary YORP (BYORP) Effect and Mutual Tides: Though the BYORP effect can contribute to changes in orbital dynamics, it is traditionally a slow process and incapable of accounting for the rapid period alteration observed post-impact. Similarly, mutual tides significantly affect binary systems over prolonged periods rather than the immediate post-impact phase.
2. Drag from Ejecta: The hypothesis that the substantial post-collision period change might be due to drag from impact-generated ejecta is examined. However, calculations indicate that the mass of ejecta required to generate the observed period reduction exceeds plausible values, considering the system’s dynamics and the estimations of mass loss due to impact.
3. Mass Loss through Spin-Induced Mechanisms: Consideration is given to continual mass shedding from Didymos due to its spin dynamics. This process, however, would not account for a sudden decrease of the magnitude observed.
4. Nodal Precession: This effect could lead to perceived period changes due to observational perspectives but would not cause a true alteration in the orbital period on the timescales observed here.

Conclusions and Future Work

The authors conclude that none of the traditional explanations suffice for the observed anomalous decay in the Dimorphos orbital period. The results indicate that unknown processes could be influencing asteroid dynamics post-impact. As such, additional observations and refined models are necessary to understand these influences fully. Continued monitoring of the Didymos system could offer further insights, possibly informing future asteroid deflection strategies and enhancing the understanding of celestial mechanics in binary asteroid systems.

Implications for Future Research

This research highlights the need for comprehensive modeling of kinetic impacts within binary asteroid systems, especially considering how such events can affect orbital dynamics in unexpected ways. Enhanced observational campaigns and simulations will be vital in understanding this discrepancy and ensuring accurate predictions in planetary defense initiatives.

In summary, this paper offers valuable observations that challenge existing models of asteroid impact dynamics, prompting a need for advanced theoretical and observational research, particularly in the context of planetary defense and asteroid manipulation strategies.