The Surface Structure of the M-Type Asteroid 21 Lutetia: Spectral and Frequency Analysis

A preliminary study of the surface of the asteroid 21 Lutetia with ground-based methods is of significant importance, because this object is included into the Rosetta space mission schedule. From August 31 to November 20, 2000, about 50 spectra of Lutetia and the same number of spectra of the solar analog HD10307 (G2V) and regional standards were obtained with a resolution of 4 and 3 nm at the MTM-500 telescope television system of the Crimean astrophysical observatory. From these data, the synthetic magnitudes of the asteroid in the BRV color system have been obtained, the reflected light fluxes have been determined in absolute units, and its reflectance spectra have been calculated for a range of 370–740 nm. In addition, from the asteroid reflectance spectra obtained at different rotation phases, the values of the equivalent width of the most intensive absorption band centered at 430–440 nm and attributed to hydrosilicates of the serpentine type have been calculated. A frequency analysis of the values V(1, 0) confirmed the rotation period of Lutetia 0.^d3405 (8.^h172) and showed a two-humped light curve with a maximal amplitude of 0.^m25. The color indices B-V and V-R showed no noticeable variations with this period. A frequency analysis of the equivalent widths of the absorption band of hydrosilicates near 430–440 nm points to the presence of many significant frequencies, mainly from 15 to 20 c/d (c/d is the number of cycles per day), which can be caused by a heterogeneous distribution of hydrated material on the surface of Lutetia. The sizes of these heterogeneities (or spots) on the asteroid surface have been estimated at 3–5 to 70 km with the most frequent value between 30 and 40 km.     

Evidence of the complex structure of asteroid 21 Lutetia

We discuss the results of the analysis of three sets of observations of asteroid 21 Lutetia—spectrophotometry, simultaneous BVR photometry, and spectrometry—which show that the asteroid is not a monolithic body. The frequency analysis of the B-V and V-R color indices and the V values, which were obtained from simultaneous BVR measurements in 2004 and calculated from the spectrophotometric observations performed in 2000 (the synthetic values and the color indices), allowed us to demonstrate that the known rotation period of 8.^h172 of the asteroid does not exist at all. At a rather high confidence level, six new periods were found: 2.^h0, 2.^h93, 16.^h8, 1.^d25, 3.^d25, and 60^d. During spectral observations with a 1.25-m telescope at the southern laboratory of the Sternberg Astronomical Institute in Nauchnyi (Crimea) in 2004, the spectra of two components spaced 2.8″ apart were registered. In the short-wavelength spectral range, quick variations of the reflectance of the components were observed. They show the changes in their spectral types from S to C. The analysis of the synthetic values of the color indices determined from the spectrophotometric observations in 2000 confirmed the presence of quick spectral variations. We conclude that asteroid 21 Lutetia is a complex satellite system. This statement is confirmed by the analysis of data published in different sources.     

Results of Complex Observations of Asteroid (596) Scheila at the Sanglokh International Astronomical Observatory

Results of astrometric and BVRI photometric observations of the active asteroid (596) Scheila are presented. The observations were carried out at the Zeiss-1000 telescope of the Sanglokh International Astronomical Observatory of the Institute of Astrophysics of the Academy of Sciences of the Republic of Tajikistan on June 16?17 and from July 30 to August 1, 2017. The coordinates of the object and its orbit were determined; and the apparent brightness in four filters, the absolute brightness in the V and R filters, and the color indices were obtained. The light curves suggest that no substantial changes in the asteroid’s brightness occurred during the observations. The absolute brightness of the asteroid in the V and R filters was (9.1 ± 0.05)^m and (8.8 ± 0.03)^m, respectively. The mean value of the asteroid diameter was (119 ± 2) km. The mean values of the color indices (B?V = (0.72 ± 0.05)^m, V?R = (0.29 ± 0.03)^m, and R?I = (0.31 ± 0.03)^m) agree well with the values for asteroids of the P- and D-types and its averages. The rotation period of the asteroid estimated from photometric observations was 16.1 ± 0.2 h. The analysis of the data has shown that the asteroid continues to exhibit the same values of absolute brightness and other characteristics as those before the collision with a small body in December 2010, though the latter resulted in the outburst event and cometary activity of the asteroid. Most likely, the collision of asteroid (596) Scheila with a small body did not lead to catastrophic changes in the surface of the asteroid or to its compete break-up.     

Comparison of feature sizes on the surface of the asteroid 4 Vesta determined using ground-based and space observations

Vesta’s crater sizes that are based on images returned from the Dawn probe orbiting the asteroid 4 Vesta are compared with sizes of spots on its surface derived from hydrosilicate equivalent widths and asteroid color indices B-V and V-R observed spectrophotometrically on earth using a spectral-frequency method. The sizes of craters and spots at the asteroid poles prove to correspond to the sizes of spots that are assumed to be located at latitudes of 40°...45° N and S. Comparative results for the crater sizes ranging from 10 to 100 km are tabulated. We conclude that crater sizes on asteroid surfaces can be determined using the spectral-frequency method.     

New reflectance spectra of 40 asteroids: A comparison with the previous results and an interpretation

This paper presents and discusses selected reflectance spectra of 40 Main Belt asteroids. The spectra have been obtained by the author in the Crimean Laboratory of the Sternberg Astronomical Institute (2003–2009). The aim is to search for new spectral features that characterize the composition of the asteroids’ material. The results are compared with earlier findings to reveal substantial irregularities in the distribution of the chemical-mineralogical compositions of the surface material of a number of minor planets (10 Hygiea, 13 Egeria, 14 Irene, 21 Lutetia, 45 Eugenia, 51 Nemausa, 55 Pandora, 64 Angelina, 69 Hesperia, 80 Sappho, 83 Beatrix, 92 Undina, 129 Antigone, 135 Hertha, and 785 Zwetana), which are manifest at different rotation phases. The vast majority of the analyzed high-temperature asteroids demonstrate subtle spectral features of an atypical hydrated and/or carbonaceous chondrite material (in the form of impurities or separate units), which are likely associated with the peculiarities of the formation of these bodies and the subsequent dynamic and impact processes, which lead, inter alia, to the delivery of atypical materials. Studies of 4 Vesta aboard NASA’s Dawn spacecraft have found that asteroids of similar types can form their own phyllosilicate generations provided that their surface material contains buried icy or hydrated fragments of impacting bodies. The first evidence has been obtained of a spectral phase effect (SPE) at small phase angles (≤4°) for 10 Hygiea, 21 Lutetia, and, possibly, 4 Vesta. The SPE manifests itself in an increasing spectral coefficient of brightness in the visible range with decreasing wavelength. This effect is present in the reflectance spectrum of CM2 carbonaceous material at a phase angle of 10° and absent at larger angles (Cloutis et al., 2011a). The shape of Hygeia’s reflectance spectra at low phase angles appears to be controlled by the SPE during the most part of its rotation period, which may indicate a predominantly carbonaceous chondrite composition on a part of the asteroid’s surface. For Vesta, the SPE may manifest itself in the flat or slightly concave shape of the asteorid’s reflectance spectra at some of the rotation phases, which is likely caused by the increased number of dark spots on corresponding parts of its surface.     

Albedo and color contrasts on asteroid surfaces

Asteroids in general display only small or negligible variations in spectrum or albedo during a rotational cycle. Color variations with rotation are described in the literature but are usually comparable to the noise in the measurements. Twenty-four asteroids have been systematically monitored for such color changes. Only 3 Juno, 4 Vesta, 6 Hebe, 71 Niobe, 349 Dembowska, and 944 Hidalgo display color variations larger than 0.03 mag. In each of these cases the asteroid appears redder near maximum brightness. Of seven asteroids monitored polarimetrically, only 4 Vesta shows a convincing variation, attributed to an albedo change with rotation. The lightcurve can be explained by albedo differences alone; Vesta apparently has a nearly spheroidal shape. Notwithstanding the above results, the degree of uniformity of most asteroid surfaces is remarkable. If asteroids exist with large discrete domains of ferrosilicate, metallic, and/or carbonaceous material together on their surfaces, they have not yet been identified.     

Spectral studies of asteroids 21 lutetia and 4 vesta as objects of space missions

Asteroid 21 Lutetia is one of the objects of the Rosetta mission carried out by the European Space Agency (ESA). The Rosetta spacecraft launched in 2004 is to approach Lutetia in July 2010, and then it will be directed to the comet Churyumov-Gerasimenko. Asteroid 4 Vesta is planned to be investigated in 2011 from the Dawn spacecraft launched by the National Aeronautics and Space Administration (NASA) in 2007 (its second object is the largest asteroid, 1 Ceres). The observed characteristics of Lutetia and Vesta are different and even contradictory. In spite of the intense and versatile ground-based studies, the origin and evolution of these minor planets remain obscure or not completely clear. The types of Lutetia and Vesta (M and V, respectively) determined from their spectra correspond to the high-temperature mineralogy, which agrees with their albedo estimated from the Infrared Astronomical Satellite (IRAS) observations. However, according to the opinion of some researchers, Lutetia is of the C type, and, therefore, its mineralogy is of the lowtemperature type. In turn, hydrosilicate formations have been found in some places on the surface of Vesta. Our observations also testify that at some relative phases of rotation (RP), the reflectance spectra of Lutetia and Vesta demonstrate features confirming the presence of hydrosilicates in the surface material. However, this fact can be reconciled with the magmatic nature of Lutetia and Vesta if the hydrated material was delivered to their surfaces by falling primitive bodies. Such small bodies are probably present everywhere in the main asteroid belt and can be the relicts of silicate-icy planetesimals from Jupiter’s formation zone or the fragments of primitive-type asteroids. When interpreting the reflectance spectra of Lutetia and Vesta, we discuss the spectral classification by Tholen (1984) from the standpoint of its general importance for the estimation of the mineralogical type of the asteroids and the study of their origin and evolution.     

Grain size of the surface regolith of asteroid 4 Vesta estimated from its reflectance spectrum in comparison with HED meteorites

Abstract— The grain-size distribution of the regolith of asteroid 4 Vesta has been estimated by comparing its reflectance spectra (0.3–2.6 μm) with those of HED meteorites. The finest grain-size separate (<25 μm) of a particular howardite has a reflectance spectrum most similar to Vesta's. In order to better simulate Vesta's surface mineralogy, reflectance spectra of those finest HED meteorite powders were linearly combined, and Vesta's spectrum was scaled for the best fit between them. Both the albedo and the shape of reflectance spectrum of Vesta were well reproduced by regional mixtures of the finest (<25 μm) powders of HED meteorites. The result suggests the heterogeneity of Vesta's surface and provides an estimate of the visible reflectance of Vesta that is close to its IRAS albedo. Thus, this suggests that fine grains can be generated and retained by relatively small bodies (Vesta is approximately 500 km in diameter).     

The rotation period and pole orientation of asteroid 4 Vesta

In 1971 asteroid Vesta was observed in a region of the sky in which it had never been observed before. Its photometric lightcurve had two distinct maxima. Those observations have been the only strong evidence to support a rotation period of about 10 hr 41 min. Lightcurves made in 1982, when Vesta was at the same aspect as 1971, do not show two different maxima. It is concluded that there was a systematic error in the 1971 observations. At this time a definitive statement cannot be made about the true period of Vesta, although the 5 hr 20 min period does appear more plausible. Radar echoes in 1988 and 1992 should resolve the problem. The shorter rotation period was assumed and the photometric astrometry method applied. The sidereal period is 5 hr 20 min 31.68 sec 0.2225889 ± 0.0000002 days, the rotation is prograde, and the coordinates of the north pole are 103° longitude and +43° latitude with an uncertainty of abour 6°.     

Vesta,Vestoids, and the howardite,eucrite, diogenite group: Relationships and the origin of spectral differences

Abstract— Spectra of asteroid 4 Vesta and 21 small (estimated diameters less than 10 km) asteroids with Vesta‐like spectral properties (Vestoids) were measured at visible and near‐infrared wavelengths (～0.44 to ～1.65 μm). All of the measured small asteroids (except for 2579 Spartacus) have reflectance spectra consistent with surface compositions similar to eucrites and howardites and consistent with all being derived from Vesta. None of the observed asteroids have spectra similar to diogenites. We find no spectral distinction between the 15 objects tabulated as members of the Vesta dynamical family and 6 of the 7 sampled “non‐family” members that reside just outside the semi‐major axis (a), eccentricity (e), and inclination (i) region of the family. The spectral consistency and close orbital (a‐e‐i) match of these “non‐family” objects to Vesta and the Vesta family imply that the true bounds of the family extend beyond the subjective cut‐off for membership. Asteroid 2579 Spartacus has a spectrum consistent with a mixture of eucritic material and olivine. Spartacus could contain olivine‐rich material from Vesta's mantle or may be unrelated to Vesta altogether. Laboratory measurements of the spectra of eucrites show that samples having nearly identical compositions can display a wide range of spectral slopes. Finer particle sizes lead to an increase in the slope, which is usually referred to as reddening. This range of spectral variation for the best‐known meteoritic analogs to the Vestoids, regardless of whether they are actually related to each other, suggests that the extremely red spectral slopes for some Vestoids can be explained by very fine‐grained eucritic material on their surfaces.     

Radar observations of the asteroid 2011 UW158

In July 2015 intercontinental bistatic radar observations of the potentially dangerous asteroid 2011 UW158 during its close approach to the Earth were carried out. The asteroid was illuminated at a frequency of 8.4 GHz with the 70-m DSS-14 antenna of the Goldstone Deep Space Communications Complex, while the signal reflected from the asteroid was received with the 32-m radio telescopes of the Quasar VLBI network at the Zelenchukskaya and Badary Observatories. The spectra of the reflected radio signals were obtained. The sizes and rotation period of the asteroid consistent with photometric observations and the ratio of the powers of the reflected signals with left- and right-hand circular polarizations were determined. The derived values suggest that the asteroid has an inhomogeneous surface and a prolate shape. The observations of the Doppler shift of the reflected signal frequency were obtained, which allowed the orbital parameters of the asteroid to be improved.     

The origin of Vesta’s crust: Insights from spectroscopy of the Vestoids

High quality VNIR spectra of 15 Vestoids, small asteroids that are believed to originate from Vesta, were collected and compared to laboratory spectra and compositional data for selected HED meteorites. A combination of spectral parameters such as band centers, and factors derived from Modified Gaussian Model fits (band centers, band strengths, calculation of the low to high-Ca pyroxene ratio) were used to establish if each Vestoid appeared most like eucrite or diogenite material, or a mixture of the two (howardite). This resulted in the identification of the first asteroid with a ferroan diogenite composition, 2511 Patterson. This asteroid can be used to constrain the size of diogenite magma chambers within the crust of Vesta. The Vestoids indicate that both large-scale homogeneous units (>5 km) and smaller-scale heterogeneity (<1 km) exist on the surface of Vesta, as both monomineralogic (eucrite or diogenite material alone) and mixed (both eucrite and diogenite) spectra are observed. The small-scale of the variation observed within the Vestoid population is predicted by the partial melting model, which has multiple intrusions penetrating into the crust of Vesta. It is much more difficult to reconcile the observations here with the magma ocean model, which would predict much more homogeneous layers on a large-scale both at the surface and with depth.     

Reliability of estimates for the sizes of spots on asteroid surfaces made by the spectral-frequency method

A new spectral-frequency method (SFM) for the study of solid body surfaces is briefly described. This method allows estimation of the sizes of various spots. Estimates for the sizes of spots on asteroid surfaces made by the SFM and other methods are compared and discussed. The sizes of spots on the surface of asteroid 1620 Geographos determined by the SFM are well consistent with those of the craters obtained from radar data. The sizes of hydrosilicate spots on the surface of asteroid 21 Lutetia found by the SFM agree with those of the craters determined by the Rosetta spacecraft. The size of a blue spot on the surface of asteroid 4 Vesta found by the SFM is consistent with the size of the well-known crater on the south pole of the asteroid. It is inferred that the SFM is a promising method for the estimation of the sizes of spots on asteroid surfaces.     

Water combinations on the surface of the asteroid 4 Vesta

The spectral-frequency method (Busarev et al., 2007b) allowed us to obtain data on 16 sizes of hydrosilicate spots on the surface of the asteroid 4 Vesta. Large sizes (800 and 750 km) show clusters of small hydrosilicate spots near a well-known crater. Small spots of 50–13 km cover more than 50% of the surface. The predominant number of small-sized spots suggests their recent origin. Our data confirm the presence of water combinations on the surface of 4 Vesta and allow us to draw a conclusion on their recent appearance in collisions of this asteroid with primitive bodies arriving from the zone of Jupiter.     

Astrometry and photometry of asteroid (308635) 2005 YU55

Astrometric and photometric observations of asteroid (308635) 2005 YU55 were carried out at the computer-aided ZA-320M and MTM-500M telescopes of the Pulkovo observatory in the period of its closest approach to the Earth, from November 9 to 20, 2011. The new orbit of the asteroid changed after its close approach to the Earth was determined. From this orbit, a close approach to Venus on January 19, 2029, was precalculated, and the distance at the closest approach—359000 km—was estimated. From the analysis of the acquired photometric data, the axial rotation period of the asteroid was more accurately determined, and it amounted to 16.3 ± 0.4 h. The color indexes of the asteroid, B-V, V-R, and R-I, were estimated, which allowed the taxonomic class of the asteroid, B, to be determined from them. In addition, from our photometric observations of the asteroid, the earlier unknown change in its brightness with a period of 0.9–1.2 h was detected; the cause of this change has not been completely studied yet.     

Presidential Address: Presented 2000 August 28, Chicago,Illinois, USA The eucrite/Vesta story

Abstract— Many lines of evidence indicate that meteorites are derived from the asteroid belt but, in general, identifying any meteorite class with a particular asteroid has been problematical. One exception is asteroid 4 Vesta, where a strong case can be made that it is the ultimate source of the howardite‐eucrite‐diogenite (HED) family of basaltic achondrites. Visible and near‐infrared reflectance spectra first suggested a connection between Vesta and the basaltic achondrites. Experimental petrology demonstrated that the eucrites (the relatively unaltered and unmixed basaltic achondrites) were the product of approximately a 10% melt. Studies of siderophile element partitioning suggested that this melt was the residue of an asteroidal‐scale magma ocean. Mass balance considerations point to a parent body that had its surface excavated, but remains intact. Modern telescopic spectroscopy has identified kilometer‐scale “Vestoids” between Vesta and the 3:1 orbit‐orbit resonance with Jupiter. Dynamical simulations of impact into Vesta demonstrate the plausibility of ejecting relatively unshocked material at velocities consistent with these astronomical observations. Hubble Space Telescope images show a 460 km diameter impact basin at the south pole of Vesta. It seems that nature has provided multiple free sample return missions to a unique asteroid. Major challenges are to establish the geologic context of the HED meteorites on the surface of Vesta and to connect the remaining meteorites to specific asteroids.     

Rotations of the sun and stars and their activity

An equation for determining the magnetic activity cycle for the lower main-sequence stars is derived on the basis of the results of an analysis of magnetic structure drift with the solar activity cycle. The equation correlates the star activity cycle with the period of its rotation T _rot, the B?V color index, and the average chromospheric emission level 〈R′_HK>. The activity cycles for 30 stars (14 young and 16 old) entering the Wilson sample are calculated. The calculated magnetic activity cycles are found to be in good agreement with the observed ones.     

Olivine or impact melt: Nature of the “Orange” material on Vesta from Dawn

NASA’s Dawn mission observed a great variety of colored terrains on asteroid (4) Vesta during its survey with the Framing Camera (FC). Here we present a detailed study of the orange material on Vesta, which was first observed in color ratio images obtained by the FC and presents a red spectral slope. The orange material deposits can be classified into three types: (a) diffuse ejecta deposited by recent medium-size impact craters (such as Oppia), (b) lobate patches with well-defined edges (nicknamed “pumpkin patches”), and (c) ejecta rays from fresh-looking impact craters. The location of the orange diffuse ejecta from Oppia corresponds to the olivine spot nicknamed “Leslie feature” first identified by Gaffey (Gaffey, M.J. [1997]. Icarus 127, 130–157) from ground-based spectral observations. The distribution of the orange material in the FC mosaic is concentrated on the equatorial region and almost exclusively outside the Rheasilvia basin. Our in-depth analysis of the composition of this material uses complementary observations from FC, the visible and infrared spectrometer (VIR), and the Gamma Ray and Neutron Detector (GRaND). Several possible options for the composition of the orange material are investigated including, cumulate eucrite layer exposed during impact, metal delivered by impactor, olivine–orthopyroxene mixture and impact melt. Based on our analysis, the orange material on Vesta is unlikely to be metal or olivine (originally proposed by Gaffey (Gaffey, M.J. [1997]. Icarus 127, 130–157)). Analysis of the elemental composition of Oppia ejecta blanket with GRaND suggests that its orange material has ∼25% cumulate eucrite component in a howarditic mixture, whereas two other craters with orange material in their ejecta, Octavia and Arruntia, show no sign of cumulate eucrites. Morphology and topography of the orange material in Oppia and Octavia ejecta and orange patches suggests an impact melt origin. A majority of the orange patches appear to be related to the formation of the Rheasilvia basin. Combining the interpretations from the topography, geomorphology, color and spectral parameters, and elemental abundances, the most probable analog for the orange material on Vesta is impact melt.     

Fugitives from the Vesta family

Spectroscopic observations of Asteroid (4) Vesta and numerous members of the Vesta family located in the inner asteroid belt have determined that these objects have reflectance properties of basaltic material. A plausible hypothesis is that the surface of Vesta was punctured by large impacts in the past which dispersed fragments of its basaltic crust into space and produced one of the most prominent asteroid families ever created in the belt. Until recently, Vesta was the only known object in the asteroid belt which underwent differentiation and survived to the present epoch. Since 2000, many new small basaltic asteroids have been discovered in the inner and outer parts of the asteroid belt, possibly representing fragments from distinct differentiated bodies. These discoveries may help us to better understand the number and nature of objects in the inner Solar System that underwent geological differentiation. To investigate these issues we performed extensive numerical simulations whose aim was to reproduce, as precisely as possible, the dynamical evolution of Vesta's ejected fragments over timescales comparable to the family's age. Specifically, we numerically integrated the orbital evolution of 6600 test bodies with orbits that started within the Vesta family and dynamically evolved over 2 Gy. Our model included gravitational perturbation of all planets (except Mercury) and the Yarkovsky effect. The results show that a relatively large fraction of the original Vesta family members may have evolved out of the family borders defined by clustering algorithms and are now dispersed over the inner asteroid belt. We compared the orbital distribution of our model fragments with the orbital locations of known basaltic asteroids in various parts of the inner main belt to find that: (i) Most basaltic asteroids with semimajor axis located outside the Vesta family's borders in the inner main belt, including (809) Lundia and (956) Elisa, are most likely fugitives from the Vesta family that have evolved to their current orbits via various identified dynamical pathways. Our results also suggest that the Vesta family is at least ∼1 Gy old. (ii) Interestingly, orbits of many basaltic asteroids with , like those of (4796) Lewis and (5379) Abehiroshi, are displaced from the Vesta family to low inclinations and are not obtained in our simulations with sufficient efficiency. We propose that: (i) these small basaltic asteroids may be fragments of differentiated bodies other than (4) Vesta; or (ii) they were liberated from the Vesta's surface before (or during) the Late Heavy Bombardment epoch ∼3.8 Gy ago and their orbital inclinations separated from that of Vesta when secular resonances swept through the region.     

Photometry of the low-luminosity spiral galaxy NGC 4136

Multicolor BVRI surface photometry of the low-luminosity (M _V ≈?18^m) spiral galaxy NGC 4136 is presented. The photometric parameters of its components and the color distribution over the galactic disk are estimated. The color indices and the corresponding effective ages are determined for the brightest star-forming regions. The disk-to-dark halo mass ratio is derived from the measured rotation curve of the galaxy. The disk mass dominates within the optical boundaries of the galaxy, so its disk can be considered as a self-gravitating system.