Polarimetry and albedo of the Near-Earth Asteroid 2025 FA22

TL;DR

The paper presents a dense, multi-instrument polarimetric study of the NEA 2025 FA22 during its close approach, combining spectropolarimetry and broadband filter polarimetry to densely sample the positive branch of the phase-polarisation curve at large phase angles. A trigonometric representation of the phase-polarisation data yields an inversion angle $\alpha_0 \approx 19.5^{\circ}$ and a slope at inversion $h \approx 0.12\%/\deg$, from which a geometric albedo $a_V = 0.16 \pm 0.02$ is inferred, while $P_{\rm max}$ occurs near $\alpha \approx 110^{\circ}$. The spectropolarimetric results reveal a nearly wavelength-independent polarisation with subtle color trends, and the polarisation spectrum resembles that of asteroid Lutetia, hinting at an M-type affinity. Overall, the work demonstrates the utility of polarimetry for rapid surface characterization of newly discovered NEAs in planetary-defense campaigns, providing constraints on size and taxonomy that can guide follow-up observations and potential mitigation planning. $P_r(\alpha)$, $\alpha_0$, $h$, and $a_V$ are expressed with $...$ equations as needed to support the conclusions, and the approach shows promise for real-time threat assessment and surface-property inference in future NEA surveys.

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). The diameter of 2025 FA22 is estimated between 130 and 290 meters. It is among the largest NEAs observable at such proximity, prompting an International Asteroid Warning Network (IAWN) rapidresponse 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 phasepolarisation curve, from the high polarisation domain (high phase angles), nearly to the inversion angle where the linear polarisation rate vanishes. The spectropolarimetric observations provided the wavelength dependence of the polarisation rate. Using empirical relationships, an estimate of the geometric albedo could be drawn from the slope of the phase-polarisation curve at inversion angle. Moreover, the spectropolarimetric trends yielded constraints on the taxonomic class. These results demonstrate the interest of polarimetry and spectropolarimetry for rapid characterisation of newly discovered NEAs in planetary defence campaigns.

Figures (3)

• Figure 1: Observing geometry of FA22 relative to the Moon and Earth in the projected ecliptic $x$--$y$ (left) and $x$--$z$ (right) planes. Star symbols mark the UTs of the first observation on each date listed in Table \ref{['Table_Log']}. The vertical bar indicates the projected point at closest Earth approach (UT 2025-09-18T07:39:48.108+00:00; distance $\approx$842,060 km). The solid black arrow indicates the anti-solar vector at the median epoch of the observing campaign.
• Figure 2: Phase-polarisation curve. Blue solid circles, green squares and red empty circles refer to the polarisation measurements obtained with the B, V, and R filters, respectively. Solid lines corresponds with the best-fits obtained with Eq. (\ref{['Eqn:trigo']}) and dotted lines their 3-$\sigma$ boundaries.
• Figure 3: Normalised polarisation spectrum of FA22 obtained on 2025-09-18 at $\alpha \simeq 97^\circ$ (blue solid circles) compared with the spectra of other asteroids (obtained in the positive branch).