An international team led by researchers at the Purple Mountain Observatory, Chinese Academy of Sciences, has completed a one year observing campaign to better classify these objects. From October 2023 to October 2024 they used the Johnson Cousins BVRI broadband photometric system on two facilities, the Purple Mountain Observatory Yaoan High Precision Telescope in China and the Kottamia Astronomical Observatory 1.88 meter telescope in Egypt, to observe dozens of near Earth asteroids.

After calibration and data reduction, the team derived multicolor photometric indices for 84 near Earth asteroids and then carried out taxonomic classification for 80 of them. The new dataset significantly expands the number of small, faint objects with secure taxonomic types, which have been difficult to classify because they are only observable for short periods after discovery.

The survey finds that nearly half of the classified objects, 46.3 percent, belong to the S complex, which is associated with stony, silicate rich compositions. Another 26.3 percent are in the darker, carbon rich C complex, 15.0 percent fall into the X complex, and 6 percent are in the D complex, with the remaining few identified as A type and V type asteroids.

By examining the distribution of types with size, the researchers show that C and X complex asteroids are more common among smaller near Earth asteroids with absolute magnitude greater than 17.0. In this size range the fraction of C and X complex objects is roughly double that found among larger bodies, suggesting different source regions or evolutionary pathways for small near Earth asteroids.

The team also notes that X complex objects tend to cluster at sub kilometer diameters, while C and S complex asteroids appear more evenly distributed across the sampled size range. These contrasting patterns point to differences in collisional evolution, surface processing, or delivery mechanisms from the main belt.

Analysis of orbital parameters highlights further structure in the population. For near Earth asteroids with Jovian Tisserand parameter less than 3.1, C and D complex objects dominate, hinting at a possible link to cometary reservoirs. This dynamical signature, combined with their dark, primitive surfaces, supports the idea that some near Earth asteroids originated as extinct or dormant comets.

One object in the sample, near Earth asteroid 385268, stands out as a likely example of this pathway. Its spectral properties resemble those of Jupiter family comets, and dynamical modeling indicates it may have been delivered from the Themis family in the outer main belt via Jupiter's 2:1 mean motion resonance, illustrating how main belt material can be transported into Earth crossing orbits.

The work also examines 13 potentially hazardous asteroids contained in the sample, objects whose orbits bring them close enough to Earth to pose an elevated impact risk. Among these, C complex and S complex asteroids each contribute five bodies, a roughly equal split that differs from some earlier expectations focused more heavily on S type objects.

Because C complex asteroids are typically more porous and structurally weaker than S complex bodies, their presence among potentially hazardous asteroids raises important questions for planetary defense planning. Kinetic impact deflection strategies may couple less efficiently to such porous targets, and mission designers will need to account for this when planning future mitigation tests and campaigns.

Beyond the immediate implications for impact risk, the survey adds a new layer of detail to our picture of near Earth asteroid origins and evolution. The taxonomic and dynamical patterns offer constraints on how material is transported from different source regions, reshaped by collisions, and delivered into Earth crossing space over Solar System history.

The team plans to extend this work by targeting even fainter near Earth asteroids and by combining optical photometry with infrared measurements. Adding thermal infrared data will refine albedo and size estimates and improve taxonomic classification, further tightening the link between composition, size, and orbital characteristics in the near Earth population.

Research Report:Taxonomic classification of 80 near-Earth asteroids

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