Successful kinetic impact into an asteroid for planetary defence

The potential threat of asteroid impact on Earth necessitates the development of effective planetary defense strategies. While no significant threat is anticipated in the next century, the incomplete catalog of near-Earth objects (NEOs) highlights the need for proactive measures. This research focuses on the Double Asteroid Redirection Test (DART) mission, a crucial step in validating kinetic impactor technology as a viable planetary defense method. The mission aimed to demonstrate the feasibility of deflecting an asteroid through a controlled kinetic impact, providing valuable data for future defense strategies. The choice of the Didymos binary system, with its smaller moon Dimorphos, allowed for precise measurement of the impact's effect on the asteroid's orbit using ground-based telescopes. This stands in contrast to previous missions utilizing impactors, which were primarily focused on scientific investigation rather than deflection, and thus lacked measurable orbital alterations. The DART mission, therefore, represents a paradigm shift – a full-scale test of a technology designed specifically for planetary defense.

Prior research on asteroid deflection has explored various approaches, including nuclear detonation, gravity tractors, and kinetic impactors. The National Academies of Sciences, Engineering, and Medicine have highlighted the importance of kinetic impactors as a high-priority planetary defense strategy. Previous missions like Deep Impact (investigating Comet Tempel 1) and Hayabusa2 (impacting asteroid Ryugu) provided valuable data on small-body properties but didn't prioritize deflection. The DART mission significantly advances this area by conducting a large-scale, targeted deflection experiment designed to yield quantifiable results. This study builds upon the existing literature by providing concrete data on the efficacy of kinetic impactor technology and characteristics of the impact and the target asteroid.

The DART spacecraft, launched on November 24, 2021, employed the Didymos Reconnaissance and Asteroid Camera for Optical navigation (DRACO) for optical navigation, terminal guidance, and asteroid characterization. The autonomous Small-body Maneuvering Autonomous Real Time Navigation (SMART Nav) system played a critical role, taking control 4 hours and 5 minutes before impact. SMART Nav utilized DRACO images to identify and target Dimorphos, despite its initially obscured position. The system maneuvered the spacecraft toward Didymos until Dimorphos became detectable, then switched to targeting Dimorphos. Maneuvering ceased 2.5 minutes before impact to minimize jitter. The impact occurred on September 26, 2022, at 23:14:24.183 UTC. High-resolution images from DRACO, streamed to Earth during the approach, allowed for post-impact analysis of Dimorphos's shape, size, and surface composition. A shape model was constructed from these images, and the spacecraft's trajectory and pointing were meticulously reconstructed to determine the impact location and angle. The impact's effect on Dimorphos's orbit was subsequently measured by ground-based telescopes (detailed in a companion paper). Dimorphos's mass was estimated indirectly using the orbital properties of the binary system, the system's total volume, and an assumption of equal bulk density for Didymos and Dimorphos. Spectral reflectance data were used to compare Didymos's composition to known meteorite types, facilitating porosity estimations for Dimorphos.

The DART mission successfully demonstrated the viability of kinetic impactor technology for asteroid deflection. The SMART Nav system autonomously navigated the spacecraft to impact Dimorphos, achieving a precise impact within 25 meters of the center of the asteroid's calculated figure. DRACO images revealed Dimorphos to be an oblate spheroid with a volume-equivalent diameter of 151 ± 5 m, significantly different in shape from other observed binary asteroid moons. The impact site was near two large boulders, with the spacecraft's solar arrays interacting with these before the bus impacted. The impact velocity was 6.1449 ± 0.0003 km/s, with an impact angle of 73 ± 7° from the local horizontal. The impact region showed a boulder-strewn surface resembling those of Itokawa, Bennu, and Ryugu, suggesting a rubble-pile structure for Dimorphos. The absence of a clearly defined impact crater suggests a young surface, though craters are difficult to identify on boulder-covered terrain. The estimated bulk density of Dimorphos is 2,400 kg m⁻³, implying a porosity of roughly 30%, consistent with a rubble-pile structure. This porosity likely comprises both macroporosity between rubble pieces and microporosity within them. The detailed characterization of Dimorphos’ surface provided crucial information on the material properties and behavior of this type of asteroid during an impact. Analysis of the impact region indicated a blocky terrain with variations in boulder sizes and distributions. A comparison with previous mission data from asteroid missions like Hayabusa2 and OSIRIS-REx provided additional context.

The DART mission's success directly addresses the research question of whether kinetic impactor technology is a viable method for asteroid deflection. The successful autonomous targeting and impact, resulting in a measurable change in Dimorphos's orbit, strongly affirm the viability of this technique. The detailed characterization of Dimorphos's physical properties further enhances the understanding of the impact process and informs future defense strategies. The findings highlight the importance of considering the target asteroid's composition and structure when planning a deflection mission. This emphasizes the necessity of future missions, such as the European Space Agency's Hera mission, to conduct detailed post-impact surveys. The results also show that a precursor reconnaissance mission, while beneficial, may not be strictly necessary for intercepting a sub-kilometer asteroid. However, the data obtained from such a mission would improve the optimization, planning, and outcome prediction significantly.

The DART mission represents a pivotal advancement in planetary defense capabilities. The successful kinetic impact into Dimorphos conclusively demonstrates the viability of this technology. Future research should focus on refining impact prediction models, incorporating diverse asteroid characteristics, and developing more sophisticated autonomous navigation systems. The comprehensive data gathered by DART will undoubtedly serve as a foundation for designing robust planetary defense protocols. The knowledge gained will contribute to improving our understanding of asteroid composition and behavior, crucial for mitigating future potential threats.

While the DART mission successfully demonstrated the concept of kinetic impact, limitations exist. The bulk density and porosity estimates for Dimorphos rely on assumptions about the bulk density of the entire Didymos binary system. The high-resolution images only covered a portion of Dimorphos's surface, limiting complete understanding of the global characteristics. The absence of a clear impact crater might be due to the nature of the rubble-pile structure and boulder-rich surface, making conclusive interpretations challenging. Furthermore, the long-term effects of the impact on Dimorphos's orbit require further observation and modeling.