Brightness, Colour, Polarisation: A Multi-Instrument Observation Campaign of the Gorizont-6 Satellite

Abstract

We present a coordinated multi-instrument photometric and polarimetric study of the defunct geosynchronous satellite, Gorizont-6. This observation campaign combined wide-field multi-colour observations with simultaneous multi-site photometry and linear polarimetry. Our results demonstrate that the combined simultaneous colour and polarimetric measurements aid in breaking the degeneracy between the fundamental spin period and its harmonics, enabling light curve features to be associated with specific reflecting surfaces. Using phase-dispersion minimisation with bootstrap resampling, we then measure a steadily increasing rotation period across six epochs spanning nine months, well described by a damped exponential curve consistent with Yarkovsky O'Keefe Radzievskii Paddack (YORP) driven spin-down. A geometrical analysis of the multi-site observations provides additional constraints on the spin-axis orientation based on paired glint events. Overall, multi-colour photometry yields efficient, robust period measurements compared with single-band data, while polarimetry supplies the decisive constraints needed for unambiguous rotation-state determination, highlighting the value of combined photometric-polarimetric strategies for characterising defunct satellites.

Figures (8)

• Figure 1: An example illustration of one of the satellites belonging to the Gorizont family. The key features of the KAUR-3 bus design are annotated. This figure is reproduced and translated from kosmonavtika_gorizont.
• Figure 2: Top: Cartographic map illustrating the site locations for the NGTS (Paranal), STING (La Palma) and LT (La Palma) instruments. Bottom Left: A photograph of the Marana sCMOS camera, highlighted within the blue rectangle, mounted on one of the NGTS telescopes. This photograph is reproduced from 2026arXiv260316361A with permission. Bottom Right: A photograph of the ORM on La Palma, where STING and the LT are located.
• Figure 3: Left: Period--search statistic $\Theta(P)$ for the night 2023--12--12, shown as five panels (one per band: $B_{\mathrm{RGB}}$, $G_{\mathrm{RGB}}$, $R_{\mathrm{RGB}}$, $i$, and $B_{\mathrm{RGB}}-i$). In each panel we plot slices of $\Theta(P)$ centred on the trial PDM period from the $G_{\mathrm{RGB}}$ band, $P'_G = 481.0\,\mathrm{s}$, and its first three harmonics ($2P'_G$, $3P'_G$, $4P'_G$) to demonstrate how well the PDM is able to distinguish between the different fold periods. Right: Phase-folded de-trended light curves in the $B$, $G$, $R$, and $i$ bands (with $B_{\mathrm{RGB}}-i$ shown above each panel), folded on the same set of trial periods $hP'_G$. Light curves are median-centred and vertically offset between bands for clarity; no additional amplitude scaling is applied.
• Figure 4: Evolution of the PDM-derived rotation period of Gorizont-6 as a function of time (days since the night of the 12th December 2023). Points show the best-fit periods from each observing epoch with 1$\sigma$ uncertainties; the solid curve shows the best-fitting exponential approach model (Equation \ref{['eq:exp']}). The MOPTOP observation is shown by the orange scatter point.
• Figure 5: The six observations of Gorizont-6 with the STING telescope and polarimetry measurements (linear polarisation and polarisation angle) from the LT. The photometry (in all four STING bands) and polarimetry measurements are displayed as a function of rotational phase following a phase fold over the estimated bootstrapped rotational periods from the $B_{RGB} - i$ colour. In the case of the unreliable measurement on the night of the 21st January 2024, we take twice the bootstrapped estimate recovered from the $i$ band data. The individual STING band brightnesses were de-trended using the best fit linear baseline coefficients from the analysis prior, re-centred and offset for visual clarity. Like features across the photometry and polarimetry are annotated by the grey dashed vertical lines and are assigned A-D.