Photometry and physical characterization of near-Earth asteroid 2025 FA22 from one apparition

Abstract

We present comprehensive photometric characterisation of 2025 FA${}_{22}$, a Potentially Hazardous Asteroid (PHA) discovered on 29 March 2025 and observed during the seventh International Asteroid Warning Network (IAWN) coordinated campaign. The asteroid's close approach at 2 lunar distances on 18 September 2025 provided an opportunity for rapid physical characterisation in a simulated virtual impactor scenario. Photometric observations were conducted from 17 September to 1 October 2025, during which 2025 FA${}_{22}$ traversed a $150^{\circ }$ arc and spanned solar phase angles from $\sim 20^{\circ }$ to $\sim 70^{\circ }$. This geometry enabled detailed physical characterisation, including determination of the spin axis and shape. Convex inversion yields a sidereal rotation period of $P_{\mathrm{sid}}=13.07366\pm 0.00076$ h and a spin axis at ecliptic coordinates $(\lambda ,\beta )=(246^{\circ }\pm 9^{\circ },60^{\circ }\pm 9^{\circ })$. The absolute magnitude was derived as $H_{\mathrm{V}}=21.39_{-0.08}^{+0.07}$ mag ($G_1=0.8228$, $G_2=0.0194$), with colour indices $B-V=0.71\pm 0.05$ mag, $V-R=0.39\pm 0.03$ mag, and $R-I=0.39\pm 0.04$ mag, consistent with X-complex classification in the Bus--DeMeo taxonomy. Assuming a geometric albedo of $p_v=0.15_{-0.04}^{+0.05}$, representative of the moderate-albedo X-complex asteroids, we estimate $D_{\mathrm{eff}}=181_{-25}^{+31}$ m, consistent with "China Compound Eye" radar dimensions of $\sim 100\times 320$ m ($D_{\mathrm{eff}}\sim 186$ m), which also reveal a contact-binary morphology consistent with the axis ratios $a:b:c\sim 2.68:1.96:1.00$ derived from our photometric shape model. The bilobate morphology is consistent with YORP-driven spin-up and deformation during a previous YORP cycle, with the current slow rotation possibly explained by internal reconfiguration or tidal braking during close Earth encounters.

Accepted for publicaiton in MNRAS, 10 pages, 7 figures, 3 tables