Polarimetry and albedo of the Near-Earth Asteroid 2025 FA22

Abstract

We report spectropolarimetric and broadband polarimetric observations of the near-Earth asteroid 2025 FA22 during its close approach of 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 AsteroidWarning Network (IAWN) rapid-response campaign. Although early orbital solutions indicated a possible impact in 2089, further follow-up astrometric observations ruled out collision hazard. The favourable geometry of this close encounter enabled a dense coverage of the positive part of the phase-polarisation curve, from the high polarisation domain (high phase angles), nearly to 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 inversion angle : $\rho_v = 0.17\pm0.04$ 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-type classification. These results demonstrate the interest of polarimetry and spectropolarimetry for rapid characterisation of newly discovered NEAs in planetary defence campaigns.