Successful kinetic impact into an asteroid for planetary defence

The study addresses whether a kinetic impactor can autonomously target and measurably deflect a small near-Earth asteroid, advancing planetary defense capabilities. Context is provided by incomplete catalogs of hazardous near-Earth objects and recommendations identifying a kinetic impact test as a top mitigation priority. NASA’s DART mission targeted Dimorphos, the small secondary in the (65803) Didymos binary, enabling ground-based measurement of any orbital change. The paper reports the impact timeline, targeting performance, impact geometry and site characteristics, and the dimensions and inferred properties of Dimorphos, demonstrating the feasibility of kinetic impact technology for asteroid deflection.

Prior work includes policy and strategy studies identifying kinetic impact tests as high priority for hazard mitigation. Earlier spacecraft impact experiments, such as Deep Impact at comet Tempel 1 and Hayabusa2’s Small Carry-on Impactor at Ryugu, explored small-body properties but were not designed to achieve or measure deflection. Observational constraints on the Didymos system before DART came from radar and telescopic photometry, yielding size and orbital properties estimates for Dimorphos. Comparative small-body exploration by NEAR Shoemaker (Eros), Hayabusa (Itokawa), OSIRIS-REx (Bennu), and Chang’e-2 (Toutatis) established the prevalence of rubble-pile structures and diverse shapes, informing expectations for surface morphology and cratering processes relevant to DART’s impact outcomes.

• Mission and instruments: DART launched 24 November 2021 carrying the DRACO narrow-angle imager for optical navigation, terminal guidance, and characterization. DRACO detected Didymos 61 days before impact.
• Pre-impact operations: From 27 August 2022 (30 days prior), DRACO obtained optical navigation images of Didymos every 5 hours for ground processing.
• Autonomous guidance: On 26 September 2022 at 19:09:24 UTC (4 h 5 min before impact), the SMART Nav autonomous navigation system took control, processing onboard DRACO images to identify Didymos and, once resolved, Dimorphos. Because Dimorphos was often hidden due to system dynamics and instrument resolution, SMART Nav initially guided toward Didymos, then, once Dimorphos was reliably detected, retargeted to Dimorphos 50 minutes before impact. Maneuvering ended at 23:11:52 UTC (2.5 minutes before impact) to minimize jitter and smear in final imaging.
• Imaging and data return: The spacecraft continuously downlinked images during terminal guidance. The final full image was acquired 1.818 s before impact at 5.5 cm/pixel; the last partial image 0.855 s before impact at 2.6 cm/pixel.
• Shape and size reconstruction: High-resolution DRACO images, despite partial coverage and ~60° phase angle, were used to construct a shape model of Dimorphos, deriving principal extents and a volume-equivalent diameter.
• Trajectory and pointing reconstruction: Spacecraft trajectory and attitude were reconstructed to determine the precise impact time, site location (latitude/longitude), impact angle relative to local horizontal and surface normal, and interactions between spacecraft components (bus and solar arrays) and local boulders at the site.
• Photometric/physical property derivations: A size estimate for Didymos from DRACO images, combined with prior telescopic observations, yielded an updated visible geometric albedo for the system. System mass and density estimates, together with an assumption of equal bulk densities for Didymos and Dimorphos, were used to infer Dimorphos’s mass and porosity.
• Surface analysis: Final images were analyzed for boulder size-frequency distribution, surface textures (e.g., blocky terrain, partially buried boulders, absence of smooth ponds), and evidence of craters within the resolution limits.

• Autonomous impact success: SMART Nav correctly distinguished between Didymos and Dimorphos and guided DART to impact Dimorphos, validating autonomous terminal guidance for a small, dim target.
• Impact event parameters (Table 1):
  • Time: 26 September 2022 at 23:14:24.183 ± 0.004 UTC.
  • Speed: 6.1449 ± 0.0003 km/s.
  • Impact angle: 73 ± 7° from local horizontal (17 ± 7° from surface normal).
  • Site: 8.84 ± 0.45° S, 264.30 ± 0.47° E; offset ~25 ± 1 m from center of figure.
  • Spacecraft mass: 579.4 ± 0.7 kg; kinetic energy: 10.94 ± 0.01 GJ.
  • SMART Nav maneuvering ended 2.5 minutes pre-impact; final full image at 5.5 cm/pixel 1.818 s pre-impact; final partial image at 2.6 cm/pixel 0.855 s pre-impact.
• Impact geometry and contact sequence: The +Y solar array struck “boulder 1” (~6.5 m long, ~2.2 m high), the −Y array grazed “boulder 2” (~6.1 m long, ~1.6 m high), and the spacecraft bus impacted between them; the bus contributed ~88% of spacecraft mass and thus most of the energy/momentum transfer.
• Dimorphos shape and size: Oblate spheroid; extents X: 177 ± 2 m, Y: 174 ± 4 m, Z: 116 ± 2 m; volume-equivalent diameter 151 ± 5 m.
• Didymos size: Volume-equivalent diameter 761 ± 26 m.
• System properties: Mass (5.6 ± 0.5) × 10^11 kg; bulk density 2,400 ± 300 kg/m^3; system visible geometric albedo 0.15 ± 0.02.
• Dimorphos inferred mass and porosity: Inferred mass ~4.3 × 10^9 kg; assuming composition similar to LL chondrites and grain densities ~3,520–3,580 kg/m^3 implies bulk porosity on the order of ~30% (uncertainty difficult to quantify), consistent with a rubble-pile structure.
• Surface morphology: Boulder-strewn, blocky terrain with boulders from ~0.16 m to 6.5 m; deficit of 0.2–0.5 m boulders relative to a single power-law expectation; no unambiguous impact craters detected; absence of extensive smooth regolith ponds.
• Planetary defense outcome: The impact produced a measurable change in Dimorphos’s orbit (quantified in a companion study), demonstrating kinetic impactor technology as a viable asteroid deflection technique.

The DART mission demonstrates that an autonomously guided kinetic impactor can successfully target and impact a small asteroid, achieving a measurable orbital deflection. This addresses the central planetary defense question of whether kinetic impact can be operationalized without a prior reconnaissance mission, provided adequate warning time. The impact geometry—near the center of figure with a shallow offset and a high incidence relative to the local surface—maximized momentum transfer efficiency. Surface observations indicating a rubble-pile, boulder-rich regolith suggest target properties that influence cratering, ejecta production, and momentum enhancement, in line with laboratory experiments and simulations. The updated albedo and size estimates, together with system mass and density, refine the physical characterization of the Didymos–Dimorphos system and inform models of impact outcomes and momentum transfer. Collectively, these results substantiate kinetic impact as a practical component of planetary defense, while highlighting the value of detailed characterization (e.g., by a follow-up mission) to improve predictions for specific targets.

DART achieved the first full-scale kinetic impact test on an asteroid, autonomously impacted Dimorphos, and produced a measurable orbital change, validating kinetic impactor technology for potential planetary defense. The mission constrained Dimorphos’s size, shape, surface morphology, and inferred bulk properties, and refined photometric properties of the Didymos system. These findings provide key inputs for impact modeling and future deflection mission design. Future work, notably ESA’s Hera mission arriving in early 2027, will directly measure masses, refine densities and porosity, map the crater and ejecta distribution, and further quantify momentum transfer, improving our understanding of impact physics on rubble-pile asteroids and enhancing deflection predictability.

• No direct mass measurement of Dimorphos by DART; mass and porosity inferences rely on system mass, volume estimates, and the assumption of equal bulk densities for Didymos and Dimorphos, which cannot be rigorously tested with DART data and carry difficult-to-quantify uncertainties.
• Imaging constraints: DRACO imaged only a portion of Dimorphos at ~60° phase angle; illumination and coverage limitations may obscure features (e.g., small craters) and bias surface statistics (e.g., boulder size distributions).
• Surface age and crater detection: No unambiguous craters were identified, but crater detection is challenging on boulder-covered terrains, potentially limiting geomorphological interpretations.
• Detailed momentum enhancement and ejecta characterization are not fully constrained in this paper and rely on complementary analyses and future in situ measurements (e.g., by Hera).