Polarimetry and albedo of the near-Earth asteroid 2025 FA22

¹ Université Côte d’Azur, CNRS, OCA, LAGRANGE, France
² Armagh Observatory & Planetarium, College Hill, Armagh BT61 9DG, UK
³ Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences, 72 Tsarigradsko Chaussée Blvd., BG-1784 Sofia, Bulgaria
⁴ INAF – Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
⁵ ESA NEO Coordination Centre, European Space Agency, Largo Galileo Galilei, 1, Frascati, 00044, RM, Italy
⁶ Department of Physics, University of Helsinki, PO Box 64, 00014, Finland
⁷ INAF – Osservatorio astronomico di Roma, Via Frascati 33, 00078 Monteporzio Catone, Italy
⁸ Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
⁹ Caltech/IPAC, 1200 E California Blvd, MC 100-22, Pasadena, CA 91125, USA
¹⁰ Department of Physics and Astronomy, 20014 University of Turku, Turku, Finland
¹¹ Istituto Ricerche Solari Aldo e Cele Daccò (IRSOL), Faculty of Informatics, Università della Svizzera italiana, Via Patocchi 57, Locarno, Switzerland
¹² Euler Institute, Faculty of Informatics, Università della Svizzera italiana, Via Buffi 13, 6900 Lugano, Switzerland
¹³ Astronomical Institute of the Academy of Sciences of the Czech Republic, Fričova 298, Ondřejov, 25165 Czech Republic
¹⁴ INAF Padova, Padova, Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 17 January 2026
Accepted: 18 May 2026

Abstract

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 p_v = 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.