Asteroid albedos deduced from stellar occultations

Albedos for 57 asteroids were determined using diameters obtained from stellar occultations. For 18 objects, the occultation albedos were determined to accuracies better than 5%. The effect on the occultation albedo due to errors in the asteroid absolute magnitude is discussed and correlations between the occultation albedos and IRAS and polarimetric albedos are presented. The higher-quality occultation albedos presented here are suitable for calibrating albedos obtained by indirect methods.     

Combining asteroid models derived by lightcurve inversion with asteroidal occultation silhouettes

Asteroid sizes can be directly measured by observing occultations of stars by asteroids. When there are enough observations across the path of the shadow, the asteroid’s projected silhouette can be reconstructed. Asteroid shape models derived from photometry by the lightcurve inversion method enable us to predict the orientation of an asteroid for the time of occultation. By scaling the shape model to fit the occultation chords, we can determine the asteroid size with a relative accuracy of typically ∼10%. We combine shape and spin state models of 44 asteroids (14 of them are new or updated models) with the available occultation data to derive asteroid effective diameters. In many cases, occultations allow us to reject one of two possible pole solutions that were derived from photometry. We show that by combining results obtained from lightcurve inversion with occultation timings, we can obtain unique physical models of asteroids.     

UESAC — The Uppsala-ESO Survey of Asteroids and Comets

The Uppsala-ESO Survey of Asteroids and Comets was undertaken to find previously undetected comets in the vicinity of Jupiter. Over 15000 positions of moving objects have been detected on 74 plates obtained from the European Southern Observatory in Chile and from the Anglo-Australian Observatory in Australia in 1992 and 1993. Two or more positions were secured for about 3300 asteroids and orbits have so far been calculated for 1944 asteroids. The main bulk of these asteroids are previously undetected. We present absolute magnitudes and diameters for asteroids which have an accurate orbit. The magnitude and diameter distributions are compared to the results of the Palomar-Leiden Survey of Faint Minor Planets.     

Absolute magnitudes of asteroids and a revision of asteroid albedo estimates from WISE thermal observations

We obtained estimates of the Johnson V absolute magnitudes (H) and slope parameters (G) for 583 main-belt and near-Earth asteroids observed at Ond?ejov and Table Mountain Observatory from 1978 to 2011. Uncertainties of the absolute magnitudes in our sample are <0.21 mag, with a median value of 0.10 mag. We compared the H data with absolute magnitude values given in the MPCORB, Pisa AstDyS and JPL Horizons orbit catalogs. We found that while the catalog absolute magnitudes for large asteroids are relatively good on average, showing only little biases smaller than 0.1 mag, there is a systematic offset of the catalog values for smaller asteroids that becomes prominent in a range of H greater than ～10 and is particularly big above H ～ 12. The mean (H_catalog ? H) value is negative, i.e., the catalog H values are systematically too bright. This systematic negative offset of the catalog values reaches a maximum around H = 14 where the mean (H_catalog ? H) is ?0.4 to ?0.5. We found also smaller correlations of the offset of the catalog H values with taxonomic types and with lightcurve amplitude, up to ～0.1 mag or less. We discuss a few possible observational causes for the observed correlations, but the reason for the large bias of the catalog absolute magnitudes peaking around H = 14 is unknown; we suspect that the problem lies in the magnitude estimates reported by asteroid surveys. With our photometric H and G data, we revised the preliminary WISE albedo estimates made by Masiero et al. (Masired, J.R. et al. [2011]. Astrophys. J. 741, 68–89) and Mainzer et al. (Mainzer, A. et al. [2011b]. Astrophys. J. 743, 156–172) for asteroids in our sample. We found that the mean geometric albedo of Tholen/Bus/DeMeo C/G/B/F/P/D types with sizes of 25–300 km is p_V = 0.057 with the standard deviation (dispersion) of the sample of 0.013 and the mean albedo of S/A/L types with sizes 0.6–200 km is 0.197 with the standard deviation of the sample of 0.051. The standard errors of the mean albedos are 0.002 and 0.006, respectively; systematic observational or modeling errors can predominate over the quoted formal errors. There is apparent only a small, marginally significant difference of 0.031 ± 0.011 between the mean albedos of sub-samples of large and small (divided at diameter 25 km) S/A/L asteroids, with the smaller ones having a higher albedo. The difference will have to be confirmed and explained; we speculate that it may be either a real size dependence of surface properties of S type asteroids or a small size-dependent bias in the data (e.g., a bias towards higher albedos in the optically-selected sample of asteroids). A trend of the mean of the preliminary WISE albedo estimates increasing with asteroid size decreasing from D ～ 30 down to ～5 km (for S types) showed in Mainzer et al. (Mainzer, A. et al. [2011a]. Astrophys. J. 741, 90–114) appears to be mainly due to the systematic bias in the MPCORB absolute magnitudes that progressively increases with H in the corresponding range H = 10–14.     

Mineralogical characterization of some basaltic asteroids in the neighborhood of (4) Vesta: first results

We present reflectance spectra of 19 V-type asteroids obtained at the 3.6 m Telescopio Nazionale Galileo covering 0.8 to 2.5 μm. For 8 of these asteroids we obtained also visible spectra in the same observational run. The range from 0.8 to 2.5 μm, encompassing the 1 and 2 μm pyroxene features, allows a precise mineralogical characterization of these asteroids. The obtained data suggests the possible coexistence of distinct mineralogical groups among the V-type asteroids, either probing different layers of (4) Vesta or coming from different bodies. No clear correlation was found between mineralogies and the objects being, or not, member of the Vesta dynamical family.     

Thermal emission spectroscopy (5.2-38 μm) of three Trojan asteroids with the Spitzer Space Telescope: Detection of fine-grained silicates

We present thermal emission spectra (5.2-38 μm) of the Trojan asteroids 624 Hektor, 911 Agamemnon, and 1172 Aneas. The observations used the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. Emissivity spectra are created by dividing the measured Spectral Energy Distribution (SED) by a model of the thermal continuum. We employ the Standard Thermal Model (STM), allowing physical parameters (e.g., radius and albedo) to vary in order to find the best thermal continuum fit to the SED. The best-fit effective radius (R) and visible geometric albedo (pv) for Hektor (R=110.0±7.3, ) and Aneas (R=69.1±5.1, ) agree very well with previous estimates, and for Agamemnon (R=71.5±5.2, ) we find slightly a smaller size and higher albedo than previously derived. Other thermal models (e.g., thermophysical) result in estimates of R and pv that vary a few percent from the STM, but the resulting emissivity spectra are identical. The emissivity spectra of all three asteroids display an emissivity plateau near 10-μm and another broader rise from ∼18 to 28 μm. We interpret these as indications of fine-grained silicates on the surfaces of these asteroids. The emissivity spectra more closely resemble emission spectra from cometary comae than those from solid surfaces and measured in the laboratory for powdered meteorites and regolith analogs. We hypothesize that the coma-like emission from the solid surfaces of trojans may be due to small silicate grains being imbedded in a relatively transparent matrix, or to a very under-dense (fairy-castle) surface structure. These hypotheses need to be tested by further laboratory and theoretical scattering work as well as continued thermal emission observations of asteroids.     

An Emulation Research on the Radio Occultation Exploration of Martian Ionosphere

The technical system of the Sino-Russian joint satellite-to-satellite Mars ionosphere occultation is analyzed and introduced. The analogue computation of the observed values of the radio waves of the ionosphere occultation event is carried out by adopting the three-dimensional ray tracking method and the electron density profile inversion is conducted by means of the simulated occultation observational data, with the result showing that the emulation algorithm is reliable. By taking advantage of the emulation method the case computation and analysis of the inversion errors caused by the observational error of the occultation radio wave phase and the satellite orbital error are respectively carried out, and it is obtained from the result that the effect of the phase measuring error of the 5% circle on the result of the daytime ionosphere occultation exploration may be neglected, while the absolute error of the night electron density measurement is less than 4 × 108 m^?3, and the main effect of the satellite orbital error on the occultation leads to the lifting or falling of the ionospheric height. The result shows that the technical system of the Sino-Russian joint Mars ionosphere occultation exploration is advanced. It can be expected that the high accuracy electron density profile is obtained and the technical system can be applied to the exploration of the lunar ionospheric environment.     

Constraints on the surface composition of Trojan asteroids from near-infrared (0.8-4.0 μm) spectroscopy

We present new near-infrared spectra of 20 Trojan asteroids. The spectra were recorded at the NASA Infrared Telescope Facility (IRTF) using the recently commissioned medium-resolution spectrograph SpeX and at the Multiple Mirror Telescope (MMT) using the instrument FSPEC. Spectra of all of these objects were measured in K-band (1.95-2.5 μm), 8 of these in L-band (2.8-4.0 μm), and 14 in the range 0.8-2.5 μm. These observations nearly double the number of published 0.8-2.5 μm spectra of Trojan asteroids and provide the first systematic study of the L-band region for these distant asteroids. The data show that the red spectral slope measured in the near-IR extends through the L-band, though it is not as steep here as at shorter wavelengths. A significant diversity is apparent in the near-IR spectral slopes of this sampling of objects. Most of the spectra do not contain any definitive absorption features characteristic of surface composition (e.g., H₂O, organics, silicates) as seen on main-belt asteroids and several Centaur and Kuiper Belt objects. A few objects may display spectral activity, and the reliability of these possible features is discussed. While these spectra are generally compatible with silicate surfaces to explain the spectral slope mixed with some fraction of low albedo material, there is no absolute indication of silicates. The spectral slope could also be explained by the presence of hydrocarbons, but the lack of absorption features, especially in L-band where very strong fundamental absorptions from these molecules appear, constrains the character and abundance of these materials at the surface.     

On the asteroid belt's orbital and size distribution

For absolute magnitudes greater than the current completeness limit of H-magnitude ∼15 the main asteroid belt's size distribution is imperfectly known. We have acquired good-quality orbital and absolute H-magnitude determinations for a sample of small main-belt asteroids in order to study the orbital and size distribution beyond H=15, down to sub-kilometer sizes (H>18). Based on six observing nights over a 11-night baseline we have detected, measured photometry for, and linked observations of 1087 asteroids which have one-week time baselines or more. The linkages allow the computation of full heliocentric orbits (as opposed to statistical distances determined by some past surveys). Judged by known asteroids in the field the typical uncertainty in the (a/e/i) orbital elements is less than 0.03 AU/0.03/0.5°. The distances to the objects are sufficiently well known that photometric uncertainties (of 0.3 magnitudes or better) dominate the error budget of their derived H-magnitudes. The detected asteroids range from HR=12-22 and provide a set of objects down to sizes below 1 km in diameter. We find an on-sky surface density of 210 asteroids per square degree in the ecliptic with opposition magnitudes brighter than mR=23, with the cumulative number of asteroids increasing by a factor of 10^0.27/mag from mR=18 down to the mR?23.5 limit of our survey. In terms of absolute H magnitudes, we find that beyond H=15 the belt exhibits a constant power-law slope with the number increasing proportional to ¹⁰0.30H from H?15 to 18, after which incompleteness begins in the survey. Examining only the subset of detections inside 2.5 AU, we find weak evidence for a mildly shallower slope for H=15-19.5. We provide the information necessary such that anyone wishing to model the main asteroid belt can compare a detailed model to our detected sample.     

Opposition effect of Trojan asteroids

CCD-photometry of three Jupiter Trojan asteroids were carried out to study their opposition effect. We obtained well-sampled magnitude–phase curves for (588) Achilles, (884) Priamus, and (1143) Odysseus in the maximal attainable phase angle range down to 0.1–0.2°. The magnitude–phase relations have a linear behavior in all observed range of phase angles and do not show any non-linear opposition brightening. We have not found any confident differences between phase slopes measured in B, V and R bands. The values of the measured phase slopes of Trojans are different from available data for Centaurs. They are within the range of phase slopes measured for some low-albedo main belt asteroids, also exhibit a linear behavior down to small phase angles. An absence of non-linear opposition brightening puts constraints on the surface properties of the studied objects, assuming very dark surfaces where single scattering plays dominating role.We also determined the rotation periods, amplitudes, the values of color indexes B–V and V–R, and the absolute magnitudes of these asteroids.     

Photometry and spin rate distribution of small-sized main belt asteroids

Photometry results of 32 asteroids are reported from only seven observing nights on only seven fields, consisting of 34.11 cumulative hours of observations. The data were obtained with a wide-field CCD (^40.5′×^27.3′) mounted on a small, 46-cm telescope at the Wise Observatory. The fields are located within ±1.5° from the ecliptic plane and include a region within the main asteroid belt. The observed fields show a projected density of ∼23.7 asteroids per square degree to the limit of our observations. 13 of the lightcurves were successfully analyzed to derive the asteroids' spin periods. These range from 2.37 up to 20.2 h with a median value of 3.7 h. 11 of these objects have diameters in order of two kilometers and less, a size range that until recently has not been photometrically studied. The results obtained during this short observing run emphasize the efficiency of wide-field CCD photometry of asteroids, which is necessary to improve spin statistics and understand spin evolution processes. We added our derived spin periods to data from the literature and compared the spin rate distributions of small main belt asteroids (5>D>0.15 km) with that of bigger asteroids and of similar-sized NEAs. We found that the small MBAs do not show the clear Maxwellian-shaped distribution as large asteroids do; rather they have a spin rate distribution similar to that of NEAs. This implies that non-Maxwellian spin rate distribution is controlled by the asteroids' sizes rather than their locations.     

Asteroids (65) Cybele, (107) Camilla and (121) Hermione: Infrared spectral diversity among the Cybeles

We find a clear diversity in the 3 μm and 10 μm features of three Cybele asteroids: (107) Camilla, (121) Hermione, and (65) Cybele. (121) Hermione exhibits a “check-like” 3 μm feature, which may be attributed to OH-dominated minerals and (107) Camilla shows a rounded “bowl” like feature closer to that of (65) Cybele, which may be attributed to H₂O-dominated minerals. The 10 μm features of these three asteroids are also different from each other.     

A spectral comparison of (379) Huenna and its satellite

We present near-infrared spectral measurements of Themis family Asteroid (379) Huenna (D ∼ 98 km) and its 6 km satellite using SpeX on the NASA IRTF. The companion was farther than 1.5″ from the primary at the time of observations and was approximately 5 magnitudes dimmer. We describe a method for separating and extracting the signal of a companion asteroid when the signal is not entirely resolved from the primary. The spectrum of (379) Huenna has a broad, shallow feature near 1 μm and a low slope, characteristic of C-type asteroids. The secondary’s spectrum is consistent with the taxonomic classification of C-complex or X-complex. The quality of the data was not sufficient to identify any subtle feature in the secondary’s spectrum.     

A photoelectric lightcurve survey of small main belt asteroids

A survey to obtain photoelectric lightcurves of small main-belt asteroids was conducted from November 1981 to April 1982 using the 0.91- and 2.1-m telescopes at the University of Texas McDonald Observatory. A total of 18 main-belt asteroids having estimated diameters under 30 km were observed with over half of these being smaller than 15 km. Rotational periods were determined or estimated from multiple nights of observation for nearly all of these yielding a sample of 17 small main-belt asteroids which is believed to be free of observational selection effects. All but two of these objects were investigated for very short periods in the range of 1 min to 2 hr using power spectrum analysis of a continuous set of integrations. No evidence for such short periods was seen in this sample. Rotationally averaged B(1,0) magnitudes were determined for most of the surveyed asteroids, allowing diameter estimates to be made. Imposing the suspected selection effects of photographic photometry on the results of this survey gives excellent agreement with the results from that technique. This shows that the inability of photographic photometry to obtain results for many asteroids is indeed due to the rotational parameters of those asteroids.     

A Specific Nongravitational Effect in the Asteroid Belt

The evolution of orbits of asteroids found in the IRAS and WISE albedo databases was calculated numerically from 2005 to 2016. It follows from the analysis of the obtained results that a certain nongravitational effect (NGE) currently affects the motion of a considerable fraction of main-belt asteroids with diameters up to 40 km. This conclusion agrees with the available data regarding the axial rotation of asteroids. The NGE manifests itself in an increase in the semimajor axes of orbits of low-albedo asteroids relative to the semimajor axes of orbits of high-albedo bodies. The NGE-induced rate of elongation of semimajor axes of asteroids with albedos р_v < 0.1 may be as high as (2–8) × 10^–8 AU/year. Errors in orbital elements of asteroids (unrelated to the accuracy of observational data used to determine these orbital elements) were found in one of the MPC catalogues for 2003 in the process of estimation of the accuracy of calculations.     

Some results from the National Observatory of Turkey, Kazan State University, and Nikolaev Astronomical Observatory on small bodies of the solar system

A scientific collaboration between TÜB?TAK National Observatory (Turkey), Kazan State University (Russia) and Nikolaev Astronomical Observatory (Ukraine) involves observations of minor planets and near-Earth asteroids (NEAs) with the 1.5 m Russian-Turkish telescope (RTT150). Regular observations of selected asteroids in the range of 11-18 magnitudes began in 2004 with the view of determining masses of selected asteroids, improving the orbits of the NEAs, and studying physical characteristics of selected asteroids from photometric observations. More than 3000 positions of 53 selected asteroids and 11 NEAs have been obtained with an internal error in the range of 30-300 mas for a single determination. Photometric reductions of more than 4000 CCD frames are in progress. Masses of 21 asteroids were estimated through dynamical method using the ground-based optical observations, mainly from the RTT150 and Minor Planet Center. A comparison of the observational results from the RTT150 in 2004-2005 with observations of the same objects at other observatories allows us to conclude that RTT150 can be used for ground-based support in astrometry for the space mission GAIA.     

Spin rate distribution of small asteroids

The spin rate distribution of main belt/Mars crossing (MB/MC) asteroids with diameters 3-15 km is uniform in the range from f=1 to 9.5 d⁻¹, and there is an excess of slow rotators with f<1 d⁻¹. The observed distribution appears to be controlled by the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect. The magnitude of the excess of slow rotators is related to the residence time of slowed down asteroids in the excess and the rate of spin rate change outside the excess. We estimated a median YORP spin rate change of ≈0.022 d⁻¹/Myr for asteroids in our sample (i.e., a median time in which the spin rate changes by 1 d⁻¹ is ≈45 Myr), thus the residence time of slowed down asteroids in the excess is ≈110 Myr. The spin rate distribution of near-Earth asteroids (NEAs) with sizes in the range 0.2-3 km (∼5 times smaller in median diameter than the MB/MC asteroids sample) shows a similar excess of slow rotators, but there is also a concentration of NEAs at fast spin rates with f=9-10 d⁻¹. The concentration at fast spin rates is correlated with a narrower distribution of spin rates of primaries of binary systems among NEAs; the difference may be due to the apparently more evolved population of binaries among MB/MC asteroids.     

Constraining albedo,diameter and composition of near-Earth asteroids via near-infrared spectroscopy

We present a method to constrain the albedo and diameters of near-Earth asteroids (NEAs) based on thermal flux in their near-infrared spectra (0.7–2.5 μm) using the Standard Thermal Model. Near-infrared spectra obtained with the SpeX instrument on NASA Infrared Telescope Facility are used to estimate the albedo and diameters of 12 NEAs (1992 JE, 1992 UY4, 1999 JD6, 2004 XP14, 2005 YY93, 2007 DS84, 2005 AD13, 2005 WJ56, 1999 JM8, 2005 RC34, 2003 YE45, and 2008 QS11). Albedo estimates were compared with average albedo for various taxonomic classes outlined by Thomas et al. (Thomas, C.A. et al. [2011]. Astron. J. 142(3)) and are consistent with their results. Spectral band parameters, like band centers, are derived and compared to spectra of laboratory mineral mixtures and meteorites to constrain their composition and possible meteorite analogs. Based on our study we estimate the albedos and diameters of these NEAs and compare them with those obtained by other techniques such as ground-based mid-infrared, Spitzer thermal infrared and Arecibo radar. Our results are broadly consistent with the results from other direct methods like radar. Determining the compositions of low albedo asteroids is a challenge due to the lack of deep silicate absorption features. However, based on weak absorption features and albedo, we suggest possible meteorite analogs for these NEAs, which include black chondrites, CM2 carbonaceous chondrites and enstatite achondrites. We did not find any specific trends in albedo and composition among the asteroids we observed.     

Asteroid observations at low phase angles: III. Brightness behavior of dark asteroids

The results of photometric observations of eight main-belt asteroids with low surface albedo are presented. The magnitude-phase dependences including low phase angles (<1 deg) have been obtained for Asteroids 76 Freia (down to phase angle 0.1 deg, P-type), 190 Ismene (0.3 deg, P-type), 303 Josephina (0.2 deg, C-type), 309 Fraternitas (0.1 deg, C-type), 313 Chaldaea (0.1 deg, C-type), 444 Gyptis (0.8 deg, P-type), 615 Roswitha (0.1 deg, C-type), and 954 Li (0.03 deg, FCX-type). The behavior of brightness in the range of opposition effect is found to be practically linear for 190 Ismene with amplitude of opposition effect only 0.03 mag. Amplitudes of the opposition effect for other asteroids are close to a mean for this type. The obtained data allowed us also to determine the rotation periods of asteroids: 303 Josephina (12.497±0.001 h), 309 Fraternitas (11.205±0.005 h), 615 Roswitha (4.422±0.001 h) and 954 Li (7.207±0.002 h). The color indexes B-V, V-R and R-I have been determined for some asteroids.