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The near-Earth asteroids having orbits completely interior to Earth's are referred to as Atira class, named after the first discovered member (163693) Atira. This asteroid is the only confirmed binary asteroid in the Atira class with a primary component size of nearly 5 km orbited by a smaller $\sim$1 km secondary. Radar imaging (Deleon et al. 2024) used to detect the secondary has been used to model the shape and spin characteristics of the two components. The rotation period of the primary is 3.398 hours and the orbital period of the secondary is 15.577 hours. The compositional properties of Atira are unknown.
Due to their orbital configuration, they are challenging to observe from ground based Earth facilities. Observing facilities that are located at near the equator can only target Atira class objects for at most a few hours after sunset or before sunrise. However, high-latitude locations can observe Atira asteroids throughout an entire night, provided that they are seen at a high declination. In this work, we present over 8 hours of uninterrupted photometric observations of Atira collected at the 60-cm Metsähovi Observatory (IAU code L08) in southern Finland on the night of March 5, 2024. The resulting lightcurve, taken in $V$ and $R$ filters, represents over 2 rotations of the primary and half of the secondary's orbit.
We also performed optical spectroscopy and polarimetry of Atira using the 2.56-m Nordic Optical Telescope (IAU code Z23) located in La Palma, Canary Islands, Spain. The reflectance spectrum required some modeling to correct for the effects of improper spectroscopic slit orientation. The corrected spectrum is consistent with a Cg or Cgh-type in the SMASS taxonomy. The relatively high linear polarization at a large phase angle of 74$^\circ$ is consistent with a low albedo, primitive composition. A spectrum acquired in 2026 is also consistent with a primitive composition. Yet follow-up polarimetric measurements at different phase angles with the NOT in 2025-2026 suggest surface heterogeneity across different latitudes of Atira.
These observations (MacLennan et al. in prep) highlight the need for Nordic and Baltic observers in the physical characterization of the growing number of Atira class objects.