Polarimetry and albedo of the near-Earth asteroid 2025 FA22;
: Rivet, J.-P.; Bagnulo, S.; Bendjoya, P.; Borisov, G.; Cellino, A.; Devogèle, M.; Gray, Z.; Ieva, S.; Kolokolova, L.; Kwon, Y. G.; Berdyugin, A.; Berdyugina, S. V.; Boulanger, L.; Dotto, E.; Fatka, P.; Frank E.; Lazzarin, M.; Piirola, V.; Pravec, P.; The NEOPOPs team
Publisher: EDP Sciences
: 2026
Astronomy and Astrophysics
: A358
: 710
: 0004-6361
: 1432-0746
DOI: https://doi.org/10.1051/0004-6361/202659015
: https://doi.org/10.1051/0004-6361/202659015
: https://research.utu.fi/converis/portal/detail/Publication/527005842
We report spectropolarimetric and broadband polarimetric observations of the near-Earth asteroid (NEA) 2025 FA22 during its close approach on 18 September 2025 (about two Moon distances). With a diameter estimated between 130 and 290 m, 2025 FA22 is among the largest NEAs observable at such proximity, prompting an International Asteroid Warning Network (IAWN) rapid-response campaign. Although early orbital solutions indicated a possible impact in 2089, further follow-up astrometric observations ruled out a collision hazard. The favourable geometry of this close encounter enabled dense coverage of the positive part of the phase-polarisation curve, from the high polarisation domain (high phase angles), to near the inversion angle where the linear polarisation fraction vanishes. The spectropolarimetric observations provided the wavelength dependence of the polarisation fraction. Using empirical relationships, an estimate of the geometric albedo could be drawn from the slope of the phase-polarisation curve at an inversion angle pv = 0.17 ± 0.04 in the V band. This value, together with the spectropolarimetric trend, provides constraints on the taxonomic class, with the results being most consistent with an M (or Xc)-type classification. These results demonstrate the value of polarimetry and spectropolarimetry for the rapid characterisation of newly discovered NEAs in planetary defence campaigns.
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Based on observations obtained with data collected at the Paranal Observatory under programs ID 115.29F0.002 and 115.29F0.003 (filter polarimetry and spectropolarimetry of FA22, PI: Ieva), 095.C-0925(A) (spectropolarimetry of Lutetia and Eugenia, PI: Bagnulo) and 092.C-0639(A) (spectropolarimetry of Eros, PI: Cellino). G.B. acknowledges partial support from grant KΠ-06-H88/5 by the Bulgarian National Science Fund. G.B. gratefully acknowledges observing grant support from the Institute of Astronomy and the National Astronomical Observatory, Bulgarian Academy of Sciences. BNAO Rozhen is a facility of the National Roadmap for Research Infrastructure 2020-2027 (RACIO project), financially coordinated by the Ministry of Education and Science of the Republic of Bulgaria. S.V.B. acknowledges the support of the ERC Advanced Grant HotMol ERC-2011-AdG-291659 for the construction of the DIPol-UF polarimeter, in collaboration with the University of Turku, Finland. This project is funded by the Horizon Europe Programme of the European Union and implemented by ESA. Views and opinion expressed are however those of the authors only and the European Commission cannot be held responsible for any use which may be made of the information contained therein.
