We review the results of the spectrophotometric and multi-band photometric observations of main-belt asteroids aimed at searching for signs of sublimation-driven dust activity (SDA) and the analysis of these signs with numerical simulations of light scattering. The reflectance spectra of 17 main-belt asteroids out of 112 objects, which were observed near perihelion after 2012, exhibited the signs of activity as a temporary dust exosphere. Among these 17 asteroids, there are 12 primitive asteroids of C, F, B, P, and D spectral types with low-temperature mineralogy and a low geometric albedo (<0.10) and 5 asteroids predominantly of X-type with mixed mineralogy and a relatively high geometric albedo (>0.10). In our view, the most important result of these observations is that SDA was almost simultaneously detected in several asteroids for each of the observation periods, which apparently suggests that this phenomenon is frequent and widespread in the Solar System. We also discuss why in ground-based observations the spectral method is more sensitive than the direct imaging method in detecting the signs of a thin dust exosphere on main-belt asteroids. The registered reflectance spectra of asteroids were analyzed in comparison with numerically modeled spectra of an active asteroid enveloped by a dust exosphere containing particles of various composition and morphology. This comparison made it possible to obtain an insight into the properties of a formed temporary dust exosphere. Particularly, it has been found that the dust exosphere of an active asteroid may contain aggregate particles of different composition and structure rather than just homogenous submicron ice and dust particles. A relatively stable though temporary SDA in a primitive-type asteroid is apparently indicative of water-ice deposits located near the surface of the body. As our investigations suggest, on main-belt primitive asteroids, the mechanism of SDA often operates in conjunction with meteoroid impacts and the electrostatic field formed near the sunlit surface of the bodies. At the same time, flares and eruptive events on the Sun may considerably influence the shape and extension of the dust exosphere.

This study explores the diversity and similarity of linear polarization among low-albedo Near-Earth Asteroids (NEAs), focusing on the exceptionally high polarization of NEA 25330 (1999 KV4). Polarimetric observations are essential for understanding the physical properties of NEAs, providing insights into the nature of scattering particles and ambient physical processes that cannot be achieved using other observational methods.

UBVR polarimetric observations of 12 main-belt mostly primitive asteroids located near perihelion heliocentric distances were carried out from December 2022 to April 2023 with Zeiss-2000 telescope at the Terskol Peak observatory. The purpose of the monitoring program was to search for changes in the polarimetric parameters of the asteroids caused by possible sublimation-dust activity, as a result of which the formation of rarefied dust exospheres of asteroids is possible. The objects of the program were asteroids: (1) Ceres, (53) Kalypso, (117) Lomia, (164) Eva, (214) Ashera, (324) Bamberga, (419) Aurelia, (505) Cava, (554) Peraga, (654) Zelinda, (704) Interamnia, (1021) Flammario. Polarimetric observations of asteroids (117) Lomia, (164) Eva and (505) Kava were made for the first time, the remaining asteroids were observed earlier. Only for two asteroids (1) Ceres and (704) Interamnia, according to spectrophotometric observations, temporal spectrophotometric variability was noted earlier. Analysis of temporal changes in the degree of polarization of asteroids and comparison of the results of observations with the data available in the literature showed that the stability of the observed degree of polarization is comparable with measurement errors of ∼ 1pt (0.02- 0.1)

In the period December 12, 2022, to February 28, 2023, we performed a series of UBVRI observations of 65 asteroids, including 50 primitive-type objects, which could be observed near perihelion at that time, and calculated their approximated reflectance spectra. The observations were aimed at searching for spectral signs of sublimation-driven dust activity in the asteroids and formation of a transient dust exosphere, which are caused by the presence of Н2О ice in their interior and the highest subsolar temperature. Based on the analysis of the accuracy and shape of approximated reflectance spectra of the primitive-type asteroids, a set of which was conventionally restricted by the geometric albedo value not exceeding 0.10, we found sublimation-driven dust activity in asteroids 164 Eva, 360 Carlova, and 750 Oskar by distinct short-term variations in the reflectance of these asteroids in the U band. During one of the nights, similar spectral changes and sublimation-driven dust activity related to them were detected in asteroid 629 Bernardina; the geometric albedo of this asteroid is somewhat higher (0.14 or 0.19), which may be indicative of a heterogeneous composition of its material and, apparently, a local outcropping of water ice on its surface. Moreover, in three asteroids of a high-temperature type, 757 Portlandia, 1121 Natascha, and 1687 Glarona, noticeable variations were detected in the entire spectral range; they may be connected with ejection of submicron dust particles from the surface by some other mechanisms. To substantiate and interpret the results of observations, we numerically modeled the reflectance spectra of conditional asteroids of taxonomic types C and S (with low- and high-temperature mineralogy, respectively), which are enveloped by an optically thin dust exosphere containing aggregate or homogeneous submicron particles of different composition. On the basis of this modeling, we introduce a quantitative criterion to identify a weak sublimation-driven dust activity in primitive-type asteroids in the visible range.

Активные астероиды (АА) представляют собой уникальный класс объектов с орбитами, типичными для астероидов, но демонстрирующих кометоподобные характеристики, такие как кома. Открытие объектов, которые классифициру- ются как астероиды и кометы, привело к концепции астероидно-кометного континуума, что предполагает размытые границы между этими объектами, которые могут иметь общее происхождение и пути эволюции. Активность астеро- идов может быть вызвана различными механизмами, основным отличием которых является периодичность явлений, выявляемая в ходе регулярных наблюдений. Изучение физико-химических свойств поверхностных материалов, а так- же определение наличия и состава разреженной экзосферы активных астероидов проводится с использованием трех методов: спектрофотометрии, UBVRI-фотометрии и поляриметрии. Исследования проводятся в ИНАСАН в сотруд- ничестве с ГАИШ МГУ и Крымской астрофизической обсерваторией. Изменения в спектре отражения, полученные с помощью спектрофотометрии, такие как возрастание отражательной способности и/или изменение общего градиента спектра, могут свидетельствовать о рассеянии света на частицах в экзосфере, что способствует выявлению объек- тов с активностью сублимационно-пылевого характера. UBVRI-фотометрия, особенно в U-диапазоне, чувствительна к таким изменениям и позволяет быстро исследовать множество астероидов. С помощью этих двух методов было выявлено около 30 АА. Поляриметрия предоставляет информацию о размере и структуре частиц. Наблюдения при больших фазовых углах и анализ зависимости поляризации от фазового угла помогают определить кандидатов в активные астероиды. Несмотря на значительный интерес к АА, существующих данных недостаточно для проведения всестороннего статистического анализа и выявления факторов, влияющих на их активность. Поэтому международ- ное сотрудничество между научными учреждениями имеет решающее значение для эффективного изучения этих объектов. Active asteroids (AAs) are a unique class of objects with asteroid-like orbits that exhibit comet-like characteristics, such as comae. The discovery of objects classified as both asteroids and comets has led to the concept of an asteroid-comet continuum, which suggests blurred boundaries between asteroids and comets that may share common origins and evolu- tionary paths. The activity of asteroids can be caused by various mechanisms, the primary distinction between which is the periodicity of the phenomena, identified through regular observations. The study of the physico-chemical properties of surface materials, as well as the presence and composition of a tenuous exosphere of AAs, is conducted using three methods: spectrophotometry, UBVRI-photometry, and polarimetry. Research is carried out at the INASAN in collabora- tion with the SAI MSU, and the CrAO. Changes in the reflection spectrum obtained through spectrophotometry, such as an increase in reflectivity and/or a change in the overall spectrum gradient, can indicate light scattering on particles in the exosphere, aiding in the identification of objects with sublimation-dust activity. UBVRI-photometry, especially in the U-band, is sensitive to such changes and allows for the rapid study of many asteroids. About 30 AAs have been identified using these two methods. Polarimetry provides information about the size and structure of particles. Observations at large phase angles and analysis of phase-angle dependence of polarization help identify candidates for AAs. Although AAs are of great interest, there is still insufficient data for comprehensive statistical analysis and identification of influencing factors. Therefore, international collaboration between scientific institutions is crucial for the effective study of these objects.

Spectral signs of sublimation-driven dust activity of main-belt asteroids with primitive mineralogy (of C, B, F, G, and X types) are considered. The reflectance spectra of active asteroids of primitive types are compared with model ones of a conditionally active C-type asteroid surrounded by a dust exosphere consisting of submicron aggregate particles of different composition. This phenomenon in primitive asteroids may be impact-induced and become subsequently periodic near perihelion if connected with considerable water ice deposits of the asteroids. Significant additional factors influencing the primitive-type active asteroids, although secondary by random nature, are flares and eruptive events on the Sun, and electrostatic field of photoemission nature on the sunlit side of the bodies. We argue that the formation of aggregate dust particles in a dust exosphere of active asteroids may be one of the fundamental processes.

This paper focuses on the application of UBVRI-photometry to detect signs of sublimation-dust activity in Main Belt asteroids. Survey observations have been conducted at the Caucasian Mountain Observatory of Moscow State University since 2021, using the RC600 telescope, which has provided data for 133 asteroids. Special attention is given to primitive-type asteroids (with low-temperature mineralogy), many of which may still retain water ice in their interiors. Numerical models of light reflection by active C-type asteroids with a dust exosphere have confirmed spectral differences depending on particle composition. A qualitative criterion for the activity of a primitive asteroid was established, based on the occurrence of a negative gradient in its reflection spectrum in an active state due to ice sublimation and the formation of a dust exosphere. As a result of observations and data analysis, spectral signs of dust activity were detected in 14 primitive asteroids, confirming the effectiveness of the method used.

The detection and study of spectral signs of sublimation-driven dust activity (SDA) in 20 primitive-type asteroids of the Main Belt observed in the visible range allowed us to ascertain its temporal (or periodic) character. The asteroids exhibit SDA of relatively low intensity at minimal heliocentric distances, when their subsolar temperatures are, accordingly, highest (i.e., the surface temperature of the asteroid at the center of its sunlit hemisphere is close to the absolute maximum). Most likely, the major cause of SDA in these asteroids is that their interior contains H2O ice, which becomes outcropped due to recent impact events. As follows from the numerical modeling of the reflectance spectra of active asteroids under consideration, the optically thin dust exosphere (DE), which was induced by SDA and envelopes these bodies, consists of submicron homogeneous particles of different composition and/or fractal-like aggregates of such particles. Taking into account an extremely weak gravitational field of asteroids that cannot hold this optically thin DE even for a short time (especially, if their sizes are less than 10 km), we conclude that the DE of active primitive-type asteroids is supported by two jointly operating mechanisms. Namely, the electrostatic field of photoemission nature creates a plasma–dust sheath on a sunlit side of the asteroid (as in all atmosphereless celestial bodies), and SDA provides the additional ejection of dust particles from the surface into this layer, which enhances their amount to a detectable level; at the same time, the self-rotation of the body contributes to the spreading of dust around the asteroid.

Обнаружение и изучение спектральных признаков сублимационно-пылевой активности (СПА) двадцати астероидов примитивных типов Главного пояса, наблюдаемых в видимом диапазоне, позволило нам установить ее временный (или периодический) характер. СПА астероидов проявляет себя с относительно низкой интенсивностью преимущественно вблизи их минимальных гелиоцентрических расстояний и, соответственно, при их наиболее высоких подсолнечных температурах (т. е. вблизи абсолютного максимума поверхностной температуры астероида в центре его освещенного полушария). С высокой вероятностью основной причиной СПА этих астероидов является наличие льда Н2О в их недрах, который оказывается вскрытым благодаря недавним ударным событиям. Как следует из численного моделирования спектров отражения рассматриваемых активных астероидов, окружающая эти тела оптически тонкая пылевая экзосфера (ПЭ), обусловленная СПА, состоит из субмикронных однородных частиц разного состава и/или фракталоподобных агрегатов таких частиц. С учетом крайне слабого гравитационного поля астероидов (особенно при их размерах менее ~10 км), неспособного удерживать такую оптически-тонкую ПЭ даже непродолжительное время, мы пришли к выводу о том, что ПЭ активных астероидов примитивных типов поддерживается совместной работой двух механизмов: электростатическое поле фотоэмиссионной природы создает плазменно-пылевой слой на подсолнечной стороне астероида (как и всех безатмосферных небесных тел), а СПА обеспечивает дополнительный выброс пылевых частиц с поверхности в этот слой до обнаружимого уровня; при этом собственное вращение тела способствует их распределению вокруг астероида.

The reflectance spectra of active asteroids (AAs) measured in the visible and near-UV ranges exhibit unusual features, which are apparently caused by the light scattering in an exosphere formed under active processes on an asteroid. To estimate the prospects for a quantitative interpretation of these features, we numerically simulated reflectance spectra of an AA enveloped by an exosphere composed of aggregate submicron particles of various composition and morphology, as well as homogeneous submicron particles. We assumed that the sizes of aggregates’ constituents correspond to those of grains in agglomerates of cometary and interplanetary dust. It has been shown that the scattering on aggregates of submicron grains produces interference features at wavelengths shorter than 0.6 μm, and the positions of these features are determined by both the sizes of these grains (rather than the aggregates themselves) and the real part of their refractive index. The structure of an aggregate and variations (up to ±20%) in the sizes of constituents weakly influence the position of these features. The shape of the spectrum at longer wavelengths also depends on the sizes of grains in the aggregates and serve as an additional criterion for estimating this parameter. Calculations performed for aggregate particles absorbing in a short-wavelength range (which is typical of many materials that one may expect to find on AAs) show that the absorption significantly weakens the interference details appeared in this range. Hence, the attempts to detect strongly absorbing particles in the exosphere and to estimate their properties by these spectral features cannot yield reliable results, as opposed to the simulations for weakly absorbing particles. The presence of homogeneous weakly absorbing submicron particles in the exosphere of an AA results in a steady growth of the intensity at wavelengths shorter than 0.4−0.5 μm. Spectral measurements at the wavelengths shorter than 0.35 μm may help to estimate more reliably the properties of weakly absorbing particles, both aggregate and homogeneous, in the exospheres of AAs.

— In December 2021 to February 2022, the UBVRI photometry of 29 primitive-type asteroids of the Main Belt, being near the perihelion distances, was carried out at the 0.6-meter RC600 semiautomatic telescope of the Caucasus Mountain Observatory (CMO) of the Sternberg Astronomical Institute, Moscow State University. These observations, as well as the data processing and analysis, were aimed at searching for supposed sublimation-driven dust activity on asteroids under maximal subsolar temperatures. Among the tasks to be accomplished was the comparison of the physical and dynamic parameters of active and inactive asteroids. The main result is that substantial spectral signs of quasi-simultaneous sublimation-driven dust activity were detected on six primitive-type asteroids of the Main Belt—145 Adeona, 302 Clarissa, 322 Phaeo, 435 Ella, 690 Wratislavia, and 779 Nina (on 302 Clarissa, 322 Phaeo, 435 Ella, and 690 Wratislavia—for the first time). These six asteroids represent ~21% of all bodies included in the observation program. Probable spectral manifestations of activity at a lower intensity level were detected for the first time on five more asteroids—424 Gratia, 751 Faina, 762 Pulcova, 778 Theobalda, and 859 Bouzaréah. On 145 Adeona and 779 Nina, sublimation-driven dust activity near perihelion was registered for the third time for the last ten years; this time interval corresponds to about three orbital periods of these asteroids around the Sun. We consider the circumstances due to which five of the listed asteroids possess families. General processes and conditions that could and/or can influence the portion of primitive-type asteroids that quasi-simultaneously show signs of sublimation-driven dust activity are discussed.

The MATLAB program was developed to estimate composition of the asteroid material. The program models reflectance spectra of asteroids using reflectance spectra of meteorite analogues. Modeling results and estimation of the material composition for 10 asteroids (1 Ceres, 4 Vesta, 51 Nemausa, 65 Cybele, 102 Miriam, 250 Bettina, 266 Aline, 379 Huenna, 704 Interamnia, 1474 Beira) are discussed. A good agreement between the model spectra and the asteroids reflectance spectra is achieved for a number of asteroids (1 Ceres, 4 Vesta, 51 Nemausa, 250 Bettina, 704 Interamnia).

— Physical parameters and characteristics of asteroids as solid atmosphereless celestial bodies are traditionally studied with the same methods as those used for investigating most of the other celestial objects, though they have certain specific features. The main attention is paid to spectrophotometry, as the most effective tool to study remotely the composition, evolution, and origin of asteroids. However, very important information about asteroids was also obtained by other observational methods, such as photometry, polarimetry, radiometry, and radar. Because of this, in addition to spectrophotometry, we discuss here photometry, polarimetry, and radiometry, which, on the one hand, are very close in methodology and, on the other hand, there has been a trend to their integrated use. In connection with the discovery of sublimation–dust activity on a number of asteroids and the periodic formation of a dust exosphere around these asteroids near perihelion (see, e.g., Busarev et al., 2021), we also consider a methodologically new approach to estimating the chemical and mineralogical composition of particles in the exosphere of these asteroids and, indirectly, of their surface material.

As was shown in papers [1, 2] and other studies of these authors, sublimation–dust activity in some primitive-type asteroids of the Main asteroid belt (MAB) correlates with the near-perihelion position of asteroids in orbit. These results suggest that activity of this kind is induced by the sublimation (cometary) mechanism, i.e., release of dust particles from the surface of evaporating ice-bearing layers. It is considered that these layers become bared due to collisions between asteroids in the MAB. However, collisions may also directly result in ejecting the dust matter (the impact mechanism). Here, we consider the both mechanisms. The frequency and effectiveness of impacts have been quantitatively estimated. It has been shown that the collision frequency of projectile asteroids (impactors), the kinetic energy of which is higher than (1−3) × 10 10 J (sufficient to eject a significant amount of dust), with a target asteroid ~100 km across (by an example of asteroid 145 Adeona) is up to ~2 yr –1 . For the characteristic time of dust activity assumed at 0.01 yr, we found that, at any given time moment, among ~300 MAB asteroids larger than 100 km in diameter, several asteroids may be active due to the impact mechanism action. It is noted that this estimate is consistent with observations. To make the cometary mechanism effective, the collisions should be more powerful (the characteristic energy is 10 13 J) for excavating the ice-bearing layers over a sufficiently large area (up to 0.1 km 2 ). The frequency of powerful collisions is low, but the regions of uncovered ice-bearing layers exist for a long time. Outbursts of solar activity and heating during the asteroid’s passage along the perihelion part of the orbit may provide for the observed frequency of sublimation activity in large primitive asteroids of the MAB (there are about 200 of them). According to our model, approximately one large asteroid is active at any given time moment. Further observations are required to confirm this estimate.

— In December 2020, at the 2-m telescope of the Terskol branch of the Institute of Astronomy, we carried out spectrophotometric observations in the range of ~0.36–0.95 μm of nine asteroids of the Main Belt (19 Fortuna, 52 Europa, 102 Miriam, 177 Irma, 203 Pompeja, 250 Bettina, 266 Aline, 379 Huenna, and 383 Janina), which were near perihelia of their orbits with significant eccentricities, in order to determine the effect of maximum subsolar temperatures on them. The study of the reflecance spectra of asteroids showed that the mineralogy of their matter is low-temperature and basically corresponds to the previously established classification (Tholen, 1989; Bus and Binzel, 2002). But in the spectra of eight asteroids (with the exception of 102 Miriam), for the first time, significant deviations were found that go beyond the spectral boundaries of their taxonomic types in the absence of noticeable changes in the spectral transparency of the Earth’s atmosphere at time intervals shorter and longer than the exposure time. Such features can be interpreted as light scattering by a mobile (or inhomogeneous) dusty exosphere that forms near these asteroids near perihelion in the process of ice sublimation at the highest subsolar temperatures. In addition, as follows from the data obtained from the GOES-16 and SOHO satellites, the considered asteroids at the end of November 2020 (10 days before the start of our observations) were affected by a strong solar flare in the X-ray range, and then by a shock wave in solar wind caused by a coronal mass ejection on the Sun, which was quasi-synchronous with the X-ray flare. This probably led to an additional increase in the sublimation activity of the asteroids and manifestations of a derivative dusty exosphere.

The mineralogy of the surface matter of asteroids can be judged using a qualitative interpretation of the reflection spectraof these bodies. The features of the reflectance spectra (absorption bands) are wide enough for correct interpretation even ata low resolution of the recording device. It should be noted that the number of events of registration of sublimation activityamong primitive asteroids has been increasing recently. The emergence of a temporary exosphere affects the asteroid’sreflection spectrum. The reflection spectra of 12 asteroids of the Main Belt, mainly of primitive type, have been obtainedand analyzed. The aim of the research was to study the influence of the temporal exosphere on the correct determinationof the spectral type of the asteroid. In addition, for asteroid 203 Pompeia, the spectral type has been determined for thefirst time(class C).

С декабря 2021 по март 2022 г. на полуавтоматическом телескопе RC-600 КГО ГАИШ (Кавказской горной обсерватории ГАИШ МГУ) были проведены UBVRI-наблюдения 56 астероидов (45 астеоридов Главного Пояса и 11 астероидов, сближающихся с Землей, троянских астероидов и марс-кроссеров) преимущественно примитивных типов, движущихся вблизи перигелия. Целью наблюдательной программы был охват максимального числа астероидов для уточнения их спектрального класса, а также выявления признаков вероятной сублимационной активности. Приблизительно пятая часть астероидов не имела определенного спектрального класса, который удалось оценить благодаря нашим спектральным наблюдениям. Некоторые астероиды, а именно 145 Адеона, 779 Нина, 521 Бриксия и 322 Фео, имели явные признаки сублимационной активности, а у астероидов 751 Фаина, 762 Пулкова и 778 Теобальда можно предполагать наличие активности. Для всех включенных в данную наблюдательную программу астероидов проведен сравнительный анализ физических и динамических параметров, таких как диаметр, период вращения вокруг собственной оси, большая полуось орбиты, эксцентриситет и наклонение орбиты. From December 2021 to March 2022, observations of 56 asteroids (45 Main Belt asteroids and 11 near-Earth asteroids, Trojan asteroids and Mars crossers) of mainly primitive types moving near perihelion were carried out on the semi-automatic RC- 600 telescope of the CMO SAI MSU (Caucasian Mountain Observatory of the SAI MSU). The purpose of the observation program was to cover the maximum number of asteroids to clarify their spectral class, as well as to identify signs of probable sublimation activity. About a fifth of the asteroids did not have a certain spectral class, which was estimated thanks to our spectral observations. Some asteroids, namely 145 Adeona, 779 Nina, 521 Brixia and 322 Phaeo, had obvious signs of sublimation activity, and asteroids 751 Faina, 762 Pulkova and 778 Theobalda can be assumed to have activity. For all asteroids of the Main Belt included in this observation program, a comparative analysis of physical and dynamic parameters, such as diameter, rotation period around its own axis, the major semi-axis of the orbit, eccentricity and inclination of the orbit, was carried out.