Physical and dynamical analysis of the potential binary near-Earth asteroid (243566) 1995 SA

Introduction

Photometric lightcurves observations have been intensively applied to derive many important physical information of asteroids, such as rotational properties (rotational period and spin direction) and shape model (Kaasalainen et al., 2004; Durech et al., 2015). Asteroids lightcurves also have been extensively used to detect and characterize binary systems (Pravec et al., 2006).  The current observational evidence indicates that small binary systems (D = 0.2–10 km), both in the main belt asteroid and in the near-Earth region, can form by rotational fission of reaccumulated bodies, likely as a result of the YORP effect. Here, we will present the lightcurves analysis for a possible binary asteroid, denominated (243566) 1995 SA, as well as a dynamic analysis of its surface. This work is part of our extensive observational campaign for the physical characterization for a large sample of asteroids, in particular, of near-Earth asteroids (NEAs) within the scope of the IMPACTON Project (Rondón et al., 2019, 2022; Monteiro et al., 2020, 2021, 2023).

Observations and data reduction

Photometric observations of binary systems are being performed using the 1.0-m f/8 telescope of the Observatório Astronômico do Sertão de Itaparica (OASI, Brazil) of the IMPACTON project (Rondón et al., 2020), as well as two small instruments (0.61 m and 0.36 m telescopes) of the Blue Mountain Observatory (BMO, Australia). Lightcurve observations were carried out using sidereal tracking and an R-Johnson-Cousins filter. The images were taken with an exposure time according to the object magnitude and sky motion. To obtain photometric spectra, observations were made using the g, r, i, z Sloan Digital Sky Survey (SDSS) filters, using differential tracking mode. The science images were calibrated following the standard procedures, including bias, dark and flat-field images.

The rotation periods were derived using a Fourier series analysis (Harris et al., 1989) while the spin direction and shape model were obtained by applying the lightcurve inversion method (Kaasalainen and Torppa, 2001; Kaasalainen et al., 2001). To investigate binary periods, we used the "Dual Period Search" tool in MPO Canopus software which is based on the method described by Pravec et al. 2006. To obtain the color indices and the photometric spectrum of some objects, we used standard field stars to calculate the zero-point of the night and, consequently, the calibrated magnitude of the asteroid. Thus, the photometric spectrum was derived from the observed color indices minus the solar color indices and transformed to normalized reflectance at the r filter.

Results

We investigated the lightcurves of about 20 NEA with rotational periods between 2 and 3 h. For 8 of them, binarity signatures were found in their lightcurves (Monteiro et al., 2023). Fig. 1 shows the primary and secondary lightcurves of a possible binary NEA denominated (243566) 1995 SA. For this NEA, in addition to the primary and secondary lightcurves, we derived the minimum relative size of the components (D2/D1) of 0.37 (from the depth of the mutual event). The lightcurve obtained during its apparition in March 2014 showed a secondary lightcurve, but no clear mutual events, with a period of about 16 h. 

Figure 1 - Lightcurves of (243566) 1995 SA. Left: the primary lightcurve component. Right: the secondary lightcurve showing a possible mutual event observed on the 25^th July 2014. The primary lightcurve component was subtracted.

By applying the lightcurve inversion method, we derived the ecliptic longitude and latitude of the pole direction (𝜆, 𝛽 = 228°, -20°), as well as an approximately oblate shape for this NEO, with axes ratio of a/b = 1.07, a/c = 1.34,and b/c = 1.26. Fig. 2 shows the convex shape model for the NEA 1995 SA (Monteiro et al. submitted). Photometric data were also obtained using the SDSS g-, r-, i-, and z-band filters, allowing to derive the colour indices and photometric spectra. 1995 SA was classified as an S-type asteroid, using Carvano’s taxonomy.

Figure 2 – Convex shape model for the NEO (243566) 1995 SA reconstructed from the lightcurves for the best-fit pole (𝜆, 𝛽 = 228°, -28°).

These results were used to investigate the surface dynamics of NEA 1995 SA in search of evidence for binary formation. Specifically, we analyzed its geopotential surface, surface accelerations, slopes, and escape velocity across its entire surface (Monteiro et al., submitted). As an example, Fig. 3 presents a geopotential map of this object. The dynamics assessment was conducted using a shape model derived from lightcurve data. Consequently, these findings provide initial insights into the potential formation mechanism of its possible satellite.

Fig. 3 - Map of the geopotential computed across the surface of the NEA 1995 SA.

Conclusions

Our physical characterization of the NEA 1995 SA includes its rotational and orbital periods, spin direction, shape model, color indices, and photometric spectrum. Additionally, we build upon previous findings that identified satellite signatures in its lightcurves (Monteiro et al. 2023), providing new evidence that supports the binary nature of 1995 SA. Notably, the surface dynamics of 1995 SA offer insight into the material loss from a progenitor body. Based on these results, we reinforce that binary objects in this population can originate from rotational fission (Monteiro et al., submitted).

Acknowledgements

F.M. thanks the financial support given by the São Paulo Research Foundation (FAPESP), Brasil - Process Number 2024/16260-6. Support by CNPq (310964/2020-2) and FAPERJ (E-26/202.841/2017 and E-26/201.001/2021) is acknowledged by D.L.

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