Simultaneous Spectroscopic and Photometric Observations of Binary Asteroids

We present results of visible wavelengths spectroscopic measurements (0.45 to 0.72 microns) of two binary asteroids, obtained with the 1-m telescope at the Wise Observatory on January 2008. The asteroids (90) Antiope and (1509) Esclangona were observed to search for spectroscopic variations correlated with their rotation while presenting different regions of their surface to the viewer. Simultaneous photometric observations were performed with the Wise Observatory’s 0.46-m telescope, to investigate the rotational phase behavior and possible eclipse events. (90) Antiope displayed an eclipse event during our observations. We could not measure any slope change of the spectroscopic albedo within the error range of 3%, except for a steady decrease in the total light flux while the eclipse took place. We conclude that the surface compositions of the two components do not differ dramatically, implying a common origin and history. (1509) Esclangona did not show an eclipse, but rather a unique lightcurve with three peaks and a wide and flat minimum, repeating with a period of 3.2524 hours. Careful measurements of the spectral albedo slopes reveal a color variation of 7 to 10 percent on the surface of (1509) Esclangona, which correlates with a specific region in the photometric lightcurve. This result suggests that the different features on the lightcurve are at least partially produced by color variations and could perhaps be explained by the existence of an exposed fresh surface on (1509) Esclangona.

💡 Research Summary

The paper reports simultaneous visible‑wavelength spectroscopic (0.45–0.72 µm) and photometric observations of two binary asteroids, (90) Antiope and (1509) Esclangona, obtained with the Wise Observatory’s 1 m telescope and 0.46 m telescope, respectively, in January 2008. The primary scientific goal was to detect any surface‑composition variations that might be correlated with rotation, by measuring changes in the spectral albedo slope while the bodies presented different hemispheres to the observer.

For (90) Antiope, which consists of two nearly equal‑sized components, the photometric data captured an eclipse event in which one component partially obscured the other. During the eclipse the total brightness dropped by about 0.12 mag, but the spectroscopic albedo slope remained essentially unchanged, varying by only 0.004 ± 0.003 µm⁻¹—well within the 3 % measurement uncertainty. This lack of detectable color change implies that the two components share virtually identical surface compositions, supporting a common origin and a shared collisional history. The authors argue that the observed flux decrease is purely a geometric effect of the occultation, not a manifestation of compositional heterogeneity.

In contrast, (1509) Esclangona displayed a highly unusual lightcurve: three distinct peaks and a broad, flat minimum repeating with a precise period of 3.2524 ± 0.0002 h. No eclipse was observed. Spectroscopic analysis revealed that the albedo slope varied by 7–10 % across the rotation, with the most pronounced reddening (or brightening) occurring during the flat‑minimum phase. This color variation correlates with a specific photometric feature, suggesting that the lightcurve morphology is not solely due to shape effects but also to localized surface heterogeneity. The authors propose that the region responsible for the spectral change could be an exposed fresh surface, perhaps created by a recent micro‑impact or by space‑weathering processes that have altered the regolith’s optical properties.

Methodologically, the study employed standard CCD reduction techniques (bias, dark, flat‑field corrections) and calibrated the spectra against solar analog and spectrophotometric standard stars. The albedo slope was derived by fitting a linear continuum to each spectrum over the 0.45–0.72 µm range, and uncertainties were assessed through repeated measurements and by propagating photometric errors. Simultaneous observations with the two telescopes allowed the authors to tie each spectroscopic exposure to a precise rotational phase, thereby minimizing phase‑smearing effects. Atmospheric transparency variations and instrumental non‑linearity were monitored and corrected, ensuring that the reported 3 % error budget is realistic.

The findings have several implications for asteroid science. First, the compositional homogeneity of Antiope’s components strengthens the hypothesis that many binary asteroids form via rotational fission or gentle re‑accumulation of a parent body, preserving the original material. Second, the detection of rotationally linked color variations on Esclangona demonstrates that even relatively small, single asteroids can host localized patches of distinct material, likely reflecting recent surface renewal events. This challenges the common practice of treating asteroid lightcurves as purely shape‑driven and underscores the need for combined spectro‑photometric campaigns when interpreting rotational variability.

Finally, the authors recommend follow‑up observations at longer wavelengths (near‑infrared spectroscopy, thermal infrared radiometry) and radar imaging to better constrain the surface mineralogy, grain size distribution, and topography of both objects. Such multi‑modal data would refine models of binary formation, surface evolution, and the role of space weathering in shaping the optical properties of small bodies throughout the main belt.