Evidence of Dust Activity in Main-Belt Asteroids from Their Reflectance Spectra. 1. The Multiband Photometric Method, Results of Observations of Asteroids in 2023–2025, and Their Analysis

Abstract In periods from December 2023 to March 2024 and from December 2024 to March 2025, we performed two series of UBVRI survey observations of 39 and 75 Main-belt asteroids, respectively, in order to find active objects among them. The surveys primarily included primitive-type asteroids moving near perihelion, which ensures the highest subsolar temperatures of the asteroids’ surface at that time. Based on short-term changes in the shape of the normalized approximated reflectance spectra of asteroids, as well as on differences from the spectra available in the SMASII database, we detected signs of dust activity in asteroids 210 Isabella, 271 Penthesilea, 372 Palma, 481 Emita, 569 Misa, and 1599 Giomus during the first survey and in asteroids 10 Hygiea, 11 Parthenope, 326 Tamara, 495 Eulalia, 973 Aralia, 1182 Ilona, and 1479 Inkeri during the second survey. The results of the second survey also suggest that asteroids 65 Cybele and 596 Scheila exhibit low activity. To explain the unusual features in the measured spectra, numerical simulations were performed for the reflectance spectra of a hypothetically active asteroid with a dust exosphere consisting of submicron- and micron-sized particles under various variants of their structure and chemical–mineral composition. It has been shown that the shape of the measured approximated reflectance spectra agrees well with the behavior of model spectra for asteroids surrounded by a dust exosphere. This paper is the first in a series devoted to various aspects of the study of asteroid activity. Here, we demonstrate the potential of the multiband photometric method for detecting active asteroids and discuss the results of the last observations and the signs of activity in the reflectance spectra of asteroids. In the subsequent papers of this series, we will examine the main mechanisms of dust activity in asteroids and the key factors that enable the detection of active asteroids and facilitate the analysis of this phenomenon.