Nordic-based observations of binary asteroid (163693) Atira reveal a hydrated CM chondrite composition
Abstract
We present photometric, spectroscopic, and polarimetric observations of (163693) Atira collected on the night of 05 March 2024. A dual-band lightcurve acquired in the V and R_C filters using the Metsähovi Observatory (L08) in Kirkkonummi, Finland spans nearly 8~hours. From these measurements we estimate a V-R_C = 0.43 +/- 0.07 mag. A visible spectrum and linear polarisation measurement were obtained with the Nordic Optical Telescope in La Palma, Spain. We employ a semi-empirical model to recover the spectral flux that was lost due to a mis-aligned spectroscopic slit. The corrected model reflectance spectrum indicates a primitive Cgh taxonomic type with a distinct absorption feature from 0.60 to 0.85 um with a depth of ~7.4 %. Direct comparison with laboratory spectra of meteorites indicates close matches with CM carbonaceous chondrites. We detect a large degree of linear polarisation at a phase angle of 83 degrees which is consistent with (3200) Phaethon and (155140) 2005 UD, but lower than (162173) Ryugu and (152679) 1998 KU2. We use a CM carbonaceous chondrite density to estimate its bulk porosity of 51 +/- 31 % from previously estimated bulk density. The uncertainty in bulk density and bulk porosity is dominated by the volume of the primary. Future near-infrared spectroscopic and thermal infrared observations can be used to refine the composition and bulk density. Finally, we discuss implications for ground-based physical characterisation studies of the Atira population.
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BibTeX
@misc{maclennan2026nordicbased,
title = {{Nordic-based observations of binary asteroid (163693) Atira reveal a hydrated CM chondrite composition}},
author = {Eric M. MacLennan and Grigori Fedorets and Zuri Gray and Teemu Willamo and Tuomas Salo and Anne Virkki and Mikael Granvik and Karri Muinonen},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.27722},
eprint = {2609.27722},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.27722},
}
RIS
TY - UNPB
TI - Nordic-based observations of binary asteroid (163693) Atira reveal a hydrated CM chondrite composition
AU - Eric M. MacLennan
AU - Grigori Fedorets
AU - Zuri Gray
AU - Teemu Willamo
AU - Tuomas Salo
AU - Anne Virkki
AU - Mikael Granvik
AU - Karri Muinonen
PY - 2026
DA - 2026/09/23
PB - arXiv
DO - 10.48550/arxiv.2609.27722
UR - https://arxiv.org/abs/2609.27722
AB - We present photometric, spectroscopic, and polarimetric observations of (163693) Atira collected on the night of 05 March 2024. A dual-band lightcurve acquired in the V and R_C filters using the Metsähovi Observatory (L08) in Kirkkonummi, Finland spans nearly 8~hours. From these measurements we estimate a V-R_C = 0.43 +/- 0.07 mag. A visible spectrum and linear polarisation measurement were obtained with the Nordic Optical Telescope in La Palma, Spain. We employ a semi-empirical model to recover the spectral flux that was lost due to a mis-aligned spectroscopic slit. The corrected model reflectance spectrum indicates a primitive Cgh taxonomic type with a distinct absorption feature from 0.60 to 0.85 um with a depth of ~7.4 %. Direct comparison with laboratory spectra of meteorites indicates close matches with CM carbonaceous chondrites. We detect a large degree of linear polarisation at a phase angle of 83 degrees which is consistent with (3200) Phaethon and (155140) 2005 UD, but lower than (162173) Ryugu and (152679) 1998 KU2. We use a CM carbonaceous chondrite density to estimate its bulk porosity of 51 +/- 31 % from previously estimated bulk density. The uncertainty in bulk density and bulk porosity is dominated by the volume of the primary. Future near-infrared spectroscopic and thermal infrared observations can be used to refine the composition and bulk density. Finally, we discuss implications for ground-based physical characterisation studies of the Atira population.
ER -