Asteroids 10 Hygiea, 135 Hertha,and 196 Philomela: Heterogeneity of the material from the reflectance spectra

The reflectance spectra of asteroids 10 Hygiea (C-type), 135 Hertha (M-type), and 196 Philomela (S-type) are obtained in a range of 0.40–0.91 μm with different time intervals. In this paper, the technique of the spectral measurements of asteroids is analyzed and the reflectance spectra of Hygiea, Hertha, and Philomela are interpreted. The main physical and chemical factors and processes influencing the spectral characteristics of asteroids are considered. It is determined that the spectra of Hertha and Hygiea contain variations exceeding the measurement errors several times at different relative rotation phases, whereas spectral variations of Philomela caused by its rotation hardly exceed the error limits. Most probably, these variations are caused by local manifestations of the impact metamorphism of the material of asteroids in serious impact events. Results show that, to determine the prevailing spectral type and the corresponding mineralogy of each asteroid, one should estimate and take into account the changes in its spectral characteristics for a time interval comparable to the rotation period.