2,3,4,6,7,8,- 1ESA Near-Earth Objects Coordination Center, Frascati, Italy (maxime.devogele@ext.esa.int)
- 2Planetary Defence Office, ESA ESOC, Robert-Bosch-Straße 5, 64293 Darmstadt, Germany
- 3University of Central Florida, Orlando, USA
- 4Caltech/IPAC, Pasadena, USA
- 5ESA ESAC / PDO, Bajo del Castillo s/n, 28692 Villafranca del Castillo, Madrid, Spain
- 6Institute of Astronomy and NAO, Bulgarian Academy of Sciences, Sofia, Bulgaria
- 7Armagh Observatory and Planetarium, Armagh, Northern Ireland, UK
- 8Armagh Observatory and Planetarium, Armagh, Northern Ireland, UK
- 9INAF – Osservatorio Astrofisico di Torino, I-10025 Pino Torinese, Italy
Context and aim. The diameter of a newly discovered near-Earth object (NEO) is usually inferred from its absolute magnitude, but this conversion is dominated by the assumed geometric albedo. For objects discovered shortly before a close approach, the use of a default albedo can therefore leave a wide range of possible sizes and impact consequences. We present a polarimetric approach designed to reduce this uncertainty with a minimal observational investment.
Concept. We present a phase-angle-dependent calibration that links the linear degree of polarization, Pr, to the geometric albedo, pV, for near-Earth objects observed at moderate to high phase angles. Classical polarimetric albedo estimates use the slope of the phase-polarization curve near the inversion angle [1], which requires several observations at low phase angles. Such observations are often not possible for NEOs, whose observing windows are short and whose discovery geometries frequently correspond to large Sun-object-observer angles. Our approach uses the positive polarization branch, typically above about 30 degrees, where the polarization amplitude is strongly anti-correlated with surface reflectivity through the Umow effect [2]. This makes it possible to obtain an albedo estimate from a single high-phase polarimetric measurement.
Observations and calibration. The calibration is built from a dedicated polarimetric survey of NEOs, combined with literature polarimetry and independent albedo estimates from thermal modelling, radar, stellar occultations, and spacecraft measurements. Most of the new observations were obtained with the Torino Polarimeter at the 1-m C2PU West telescope, Calern Observatory [3]. Additional measurements come from the FoReRo2 polarimeter at Rozhen Observatory [4] and from FORS2 in polarimetric mode at the VLT. For each calibration object with sufficient phase-angle coverage, we model the phase-polarization curve using either an exponential-linear function or a trigonometric function [5,6]. The model choice depends on the available phase-angle range and on whether the high-phase turnover is constrained. The phase-curve parameters and the Pr-pV relation at each selected phase angle are inferred with Markov Chain Monte Carlo sampling, allowing uncertainties in the polarization data and in the reference albedos to be propagated to the final calibration.
Application. For a target asteroid observed at a given phase angle, the calibration returns a posterior distribution for pV from the measured Pr and its uncertainty. If several observations are available, the individual posterior distributions can be combined to produce a final albedo estimate. The method is implemented in PANDA, Polarimetric ANalysis for the Determination of the Albedo, a database-backed tool that stores calibration objects, albedo references, selected phase-polarization models, and single- or multi-measurement predictions. Because the calibration depends on an evolving reference database, PANDA is designed to be updated when new polarimetric data or improved independent albedo determinations become available.
Implications. This work provides a practical route to rapid NEO albedo and size estimates from ground-based polarimetry. It is particularly relevant for objects discovered during close approaches, for targets with short visibility windows, and for cases in which thermal-infrared or radar characterization is not immediately available. The method does not replace independent diameter measurements, but it offers a fast complementary constraint that can reduce the uncertainty associated with assumed albedo values in planetary-defense follow-up.
References:
[1] Cellino A., et al. 2015, Monthly Notices of the Royal Astronomical Society, 451, 3473.
[2] Umow v. N., 1905, Phys. Z, 6, 674.
[3] Devogèle M., et al. 2017, Monthly Notices of the Royal Astronomical Society, 465, 4335.
[4] Nikolov Y., et al. 2026, Astronomy & Astrophysics, 708, A30.
[5] Muinonen K., et al. 2002, Memorie della Società Astronomica Italiana, 73, 716.
[6] Goidet-Devel B., et al. 1995, Planetary and Space Science, 43, 779.
How to cite: Devogele, M., Cano, J. L., Ferrais, M., Masiero, J., Selmi, E., Borisov, G., Bagnulo, S., Bendjoya, P., Rivet, J.-P., Abe, L., Cellino, A., and Peleato, E.: Rapid Albedo Estimation of Near-Earth Objects From Single High Phase Angle Polarimetric Observation, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-331, https://doi.org/10.5194/epsc2026-331, 2026.
