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1.
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.  相似文献   

2.
Absorption bands near 0.43 and 0.60–0.80 m, with a relative intensity of approximately 5–10%, were discovered in the reflectance spectra of five M asteroids (21, 75, 161, 201, and 497) and two S asteroids (11 and 198). It is highly probable that these absorption bands are related to oxidized and/or hydrated silicates that incorporate OH structural groups. Absorbed water in the surface material of about 24% of the known M asteroids and three E asteroids has been identified by a group of astronomers at the University of Arizona (Rivkin et al., 1995; Rivkin, 1997; Rivkin et al., 2000) on the basis of the characteristic 3-m absorption band. In addition to this, four of the six E-type asteroids observed spectrally in the visible region quite recently were found to have an absorption band at 0.5 m (Burbine et al., 1998; Fornasier and Lazzarin, 2001), which could likewise be associated with the presence of hydrated silicates in their material. The above data contradict the generally accepted viewpoint on the nature of M, S, and E asteroids, which is based on their common observed characteristics as bodies that arose at high temperatures (in the 1000–2000°C range). This contradiction, however, can be eliminated if use is made of the cosmogonic model developed by V.S. Safronov and co- workers. According to this model, the initial evolution of the main-belt asteroids was directly influenced by the process of the growth of Jupiter and was later controlled by its gravitational field.  相似文献   

3.
We present a total of 289 new astrometric observations of the inner jovian satellites, Amalthea and Thebe, obtained using the Cassini ISS narrow angle camera. Observations were made using image sequences from 2000 December 11-12 (inbound) and 2001 January 15-16 (outbound), at phase angles of approximately 2° and 122°, respectively. Target distances were of order 284 RJ, giving a maximum resolution of approximately 100 km/pixel. Centroided line and sample values for 239 observations of Amalthea and 50 of Thebe are provided, together with estimated camera pointing information for each image. Orbit fitting using a uniformly precessing Keplerian ellipse model, taking into account the oblateness of Jupiter up to terms in J6, gave RMS fit residuals of 0.364 and 0.443 pixel for Amalthea and Thebe, respectively (equivalent to 0.450 and 0.547 arcsec). RMS residuals relative to the JPL JUP230 ephemeris were 0.306 and 0.604 pixel (equivalent to 0.378 and 0.746 arcsec), for Amalthea and Thebe. The fitted orbital parameters confirm the relatively high inclinations of these satellites (0.374°±0.002° and 1.076°±0.003°, respectively), equivalent to maximum vertical displacements above Jupiter's equatorial plane of 1188±6 and 4240±12 km, respectively, consistent with current estimates of the half-thicknesses of the Amalthea and Thebe gossamer rings [Ockert-Bell, M.E., Burns, J.A., Dauber, I.J., Thomas, P.C., Veverka, J., Belton, M.J.S., Klaasen, K.P., 1999. Icarus 138, 188-213].  相似文献   

4.
Color Variations on Eros from NEAR Multispectral Imaging   总被引：1，自引：0，他引：1  
NEAR multispectral imaging was obtained at seven wavelengths (450-1050 nm) to characterize hemispheric and regional color properties of Eros. The highest-resolution whole-disk data, 180 m/pixel, were obtained during the last pre-orbit insertion sequence on 12 February 2000. The same areas were imaged again in color at 10-20 m/pixel from high orbit in March-April 2000, and selected targets have been studied in color at resolutions as high as 4 m/pixel from low orbit. Whole-disk spectra are in close agreement with ground-based observations. These and the disk-resolved measurements show little variation in visible-wavelength color, but they do reveal spatial variation of several percent in the 950-nm/760-nm reflectance ratio, used here as a proxy for depth of the 1-μm olivine-pyroxene absorption band. After photometric correction to i=30° e=0° using both a Hapke correction and a modified empirical phase function, the disk-resolved images show reproducible spatial variations in albedo and 950-nm/760-nm reflectance ratio. The northern hemisphere exhibits average reflectances at i=30°, e=0° of 0.136±0.007 at 760 nm and 0.115±0.006 at 950 nm, equivalent to geometric albedos of 0.30±0.02 and 0.26±0.02, respectively. There is more than factor of 2 spatial variation in reflectance, but only about 10% variation in the 950-nm/760-nm reflectance ratio. Reflectance and color ratio are highly correlated, with the highest reflectances in discrete, sharp-edged patches on slopes exceeding 20°, consistent with material being exposed by downslope movement. Eros is also conspicuously deficient in small, bright, spectrally distinctive craters which are found on the other two S asteroids, Gaspra and Ida, imaged by spacecraft at close range. Eros exhibits a larger range of albedos than other S asteroids, but its color variations are much more subtle: variation in the 950-nm/760-nm reflectance ratio with 760-nm reflectance is several times less than on those asteroids or in the lunar maria. Of the different mechanisms possibly responsible for reflectance and color differences on Eros, spatial differences in the extent of regolith aging by space weathering are most consistent with Eros's observed properties. However, the effects of this process are both qualitatively and quantitatively different than in the lunar maria.  相似文献   

5.
We report new radar observations of E-class Asteroid 64 Angelina and M-class Asteroid 69 Hesperia obtained with the Arecibo Observatory S-band radar (2480 MHz, 12.6 cm). Our measurements of Angelina’s radar bandwidth are consistent with reported diameters and poles. We find Angelina’s circular polarization ratio to be 0.8 ± 0.1, tied with 434 Hungaria for the highest value observed for main-belt asteroids and consistent with the high values observed for all E-class asteroids (Benner, L.A.M., Ostro, S.J., Magri, C., Nolan, M.C., Howell, E.S., Giorgini, J.D., Jurgens, R.F., Margot, J.L., Taylor, P.A., Busch, M.W., Shepard, M.K. [2008]. Icarus 198, 294-304; Shepard, M.K., Kressler, K.M., Clark, B.E., Ockert-Bell, M.E., Nolan, M.C., Howell, E.S., Magri, C., Giorgini, J.D., Benner, L.A.M., Ostro, S.J. [2008b]. Icarus 195, 220-225). Our radar observations of 69 Hesperia, combined with lightcurve-based shape models, lead to a diameter estimate, Deff = 110 ± 15 km, approximately 20% smaller than the reported IRAS value. We estimate Hesperia to have a radar albedo of , consistent with a high-metal content. We therefore add 69 Hesperia to the Mm-class (high metal M) (Shepard, M.K., Clark, B.E., Ockert-Bell, M., Nolan, M.C., Howell, E.S., Magri, C., Giorgini, J.D., Benner, L.A.M., Ostro, S.J., Harris, A.W., Warner, B.D., Stephens, R.D., Mueller, M. [2010]. Icarus 208, 221-237), bringing the total number of Mm-class objects to eight; this is 40% of all M-class asteroids observed by radar to date.  相似文献   

6.
The highest resolution (pixel scale 30 km) images of Ceres to date have been acquired by the Advanced Camera for Surveys onboard Hubble Space Telescope, through three wide band filters, centered at 535, 335, and 223 nm, covering more than one rotation of Ceres. The lightcurve at 535 nm agrees with earlier observations at V-band [Tedesco, E.F., Taylor, R.C., Drummond, J., Harwood, D., Nickoloff, I., Scaltriti, F., Schober, H. J., Zappala, V., 1983. Icarus 54, 23-29] in terms of magnitude, amplitude, and shape. The 0.04 magnitude lightcurve amplitude cannot be matched by Ceres' rotationally symmetric shape, and is modeled here by albedo patterns. The geometric albedos at the above three wavelengths are measured to be 0.087±0.003, 0.056±0.002, and 0.039±0.003, respectively. V-band geometric albedo is calculated to be 0.090±0.003, consistent with earlier observations [Tedesco, E.F., 1989. In: Binzel, R.P., Gehrels, T., Matthews, M.S. (Eds.), Asteroids II. Univ. of Arizona Press, Tucson, pp. 1090-1138]. A strong absorption band (30%) centered at about 280 nm is observed, but cannot be identified with either laboratory UV spectra or the spectra of Europa or Ganymede. The single-scattering albedo has been modeled to be 0.070±0.002, 0.046±0.002, and 0.032±0.003, respectively. The photometric roughness of Ceres' surface is found to be about 44°±5° from photometric modeling using Hapke's theory, consistent with earlier radar observations [Mitchell, D.L., Ostro, S.J., Hudson, R.S., Rosema, K.D., Campbell, D.B., Velez, R., Chandler, J. F., Shapiro, I.I., Giorgini, J.D., Yeomans, D.K., 1996. Icarus 124, 113-133]. The first spatially resolved surface albedo maps of Ceres at three wavelengths have been constructed from HST observations, as well as the corresponding color maps. Eleven surface albedo features are identified, ranging in scale from 40-350 km. Overall the range of these albedo and color variations is small compared to other asteroids and some icy satellites.  相似文献   

7.
D. Polishook  N. Brosch 《Icarus》2009,199(2):319-332
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.  相似文献   

8.
Wang  Xiao-bin  Shi  Ying 《Earth, Moon, and Planets》2002,91(3):181-186
Photometric observations of the four selected asteroids, obtained at Yunnan Observatory, China during recent two years, are presented and analysed. We have determined the synodic periods of (174)\, Phaedra and (276)\, Adeldheid as 5.74h ± 0.01 and 6.29h ± 0.01,which confirmed the previous results. For (38)\, Leda and (346)\, Hermentria, we derived their period values, 10.171h ± 0.007 and 19.408h ± 0.005 for the first time. The color-indices of 38, were re-determined as 0.73 ± 0.02 for B-V and 0.41 ± 0.02for U-B, respectively, which are the same as the ones given by Tedesco (1989).  相似文献   

9.
A. Carbognani 《Icarus》2011,211(1):519-527
A rotating frequency analysis in a previous paper, showed that two samples of C and S-type asteroids belonging to the Main Belt, but not to any families, present two different values for the transition diameter to a Maxwellian distribution of the rotation frequency, respectively 48 and 33 km. In this paper, after a more detailed statistical analysis, aiming to verify that the result is physically relevant, we found a better estimate for the transition diameter, respectively DC = 44 ± 2 km and DS = 30 ± 1 km. The ratio between these estimated transition diameters, DC/DS = 1.5 ± 0.1, can be supported with the help of the YORP (Yarkovsky-O’Keefe-Radzievskii-Paddack) effect, although other physical causes cannot be completely ruled out.In this paper we have derived a simple scaling law for YORP which, taking into account the different average heliocentric distance, the bulk density, the albedo and the asteroid “asymmetry surface factor”, has enabled us to reasonably justify the ratio between the diameters transition of C-type and S-type asteroids. The same scaling law can be used to estimate a new ratio between the bulk densities of S and C asteroids samples (giving ρS/ρC ≈ 2.9 ± 0.3), and can explain why the asteroids near the transition diameter have about the same absolute magnitude. For C-type asteroids, using the found density ratio and other estimates of S-type density, it is also possible to estimate an average bulk density equal to 0.9 ± 0.1 g cm−3, a value compatible with icy composition. The suggested explanation for the difference of the transition diameters is a plausible hypothesis, consistent with the data, but it needs to be studied more in depth with further observations.  相似文献   

10.
The photometric properties of the nucleus of Comet 9P/Tempel 1 are studied from the disk-resolved color images obtained by Deep Impact (DI). Comet Tempel 1 has typical photometric properties for comets and dark asteroids. The disk-integrated spectrum of the nucleus of Tempel 1 between 309 and 950 nm is linear without any features at the spectral resolution of the filtered images. At V-band, the red slope of the nucleus is 12.5±1% per 100 nm at 63° phase angle, translating to B-V=0.84±0.01, V-R=0.50±0.01, and R-I=0.49±0.02. No phase reddening is confirmed. The phase function of the nucleus of Tempel 1 is constructed from DI images and earlier ground-based observations found from the literature. The phase coefficient is determined to be β=0.046±0.007 mag/deg between 4° and 117° phase angle. Hapke's theoretical scattering model was used to model the photometric properties of this comet. Assuming a single Henyey-Greenstein function for the single-particle phase function, the asymmetry factor of Tempel 1 was fitted to be g=−0.49±0.02, and the corresponding single-scattering albedo (SSA) was modeled to be 0.039±0.005 at 550 nm wavelength. The SSA spectrum shows a similar linear slope to that of the disk-integrated spectrum. The roughness parameter is found to be 16°±8°, and independent of wavelength. The Minnaert k parameter is modeled to be 0.680±0.014. The photometric variations on Tempel 1 are relatively small compared to other comets and asteroids, with a ∼20% full width at half maximum of albedo variation histogram, and ∼3% for color. Roughness variations are evident in one small area, with a roughness parameter about twice the average and appearing to correlate with the complex morphological texture seen in high-resolution images.  相似文献   

11.
Near-Earth Asteroid 4660 Nereus has been identified as a potential spacecraft target since its 1982 discovery because of the low delta-V required for a spacecraft rendezvous. However, surprisingly little is known about its physical characteristics. Here we report Arecibo (S-band, 2380-MHz, 13-cm) and Goldstone (X-band, 8560-MHz, 3.5-cm) radar observations of Nereus during its 2002 close approach. Analysis of an extensive dataset of delay-Doppler images and continuous wave (CW) spectra yields a model that resembles an ellipsoid with principal axis dimensions X=510±20 m, Y=330±20 m and . The pole direction is approximately located at ecliptic pole longitude and latitude of λ=+25°, β=+80° with the uncertainty radius of 10°. Our modeling yields a refined rotation period of 15.16±0.04 h. Nereus has a circular polarization (SC/OC) ratio of 0.74±0.08, which implies substantial near-surface centimeter-to-decimeter scale roughness. Dynamical analysis of our model suggests that YORP alteration of the rotation period may become evident within a few years. Nereus has two stable synchronous orbits where natural material may remain in orbit, while most asteroids observed to date do not have such stable synchronous orbits. We also find that spacecraft orbits about Nereus are feasible.  相似文献   

12.
Systematic and uniform sets of photometric and polarimetric observations of comet 1P/Halley have been analyzed. The phase dependence of brightness for comet Halley was obtained at phase angles α ranging from 1.4° ≤ α ≤ 65°. The following parameters were determined: the amplitude of the opposition effect Δm = 0.75m ± 0.06m; the half-width at a half-maximum of intensity HWHM = 6.4° ± 1.6°; the linear phase coefficient β = 0.0045 ± 0.0001 mag/deg for α from 30° ≤ α ≤ 65°; and the phase angle at which a nonlinear increase in brightness starts, α opp ≈ 31°. For the first time, the phase-angle dependence was obtained for the color of the dust of comet Halley: the color index BC-RC systematically decreases with increasing phase angle. Such a phase behavior of the dust color can be caused by the decrease in the mean size of dust particles that occurs when the comet approaches the Sun. For comet Halley, the negative polarization branch is almost symmetric; the minimal value of polarization is P min = −1.54% at a phase angle αmin = 10.5°, and the inversion angle is αinv = 21.7°. A comparative analysis of the phase functions of brightness and polarization has been performed for the cometary dust and atmosphereless bodies. Among the latter are low-albedo asteroids of the P and C types (102 Miriam and 47 Aglaja, respectively), as well as Deimos; high-albedo objects, such as the E-type asteroid 64 Angelina and the icy satellite of Jupiter Europa; and the Moon with its intermediate albedo. The possibility of a weak depression in the negative polarization branch of comets Halley and 47P/Ashbrook-Jackson at phase angles smaller than 2° is discussed.__________Translated from Astronomicheskii Vestnik, Vol. 39, No. 4, 2005, pp. 353–363.Original Russian Text Copyright © 2005 by Rosenbush.  相似文献   

13.
Photometric observations made during the years 2000-2005 are used to determine the pole orientation of (2953) Vysheslavia, a ?15-km size member of the Koronis family. We find admissible solutions for ecliptic latitude and longitude of the rotation pole P3: βp=−64°±10° and λp=11°±8° or P4: βp=−68°±8° and λp=192°±8°. These imply obliquity values γ=154°±14° and γ=157°±11°, respectively. The sidereal rotation period is Psid=0.2622722±0.0000018 day. This result is interesting for two reasons: (i) the obliquity value between 90° and 180° is consistent with a prediction done by Vokrouhlický et al. [Vokrouhlický, D., Bro?, M., Farinella, P., Kne?evi?, Z., 2001. Icarus 150, 78-93] that Vysheslavia might have been transported to its unstable orbit by the Yarkovsky effect, and (ii) with the obliquity close to 180°, Vysheslavia seems to belong to one of the two distinct groups in the Koronis family found recently by Slivan [Slivan, S.M., 2002. Nature 419, 49-51], further supporting the case of dichotomy in the spin axis distribution in this family. We also argue against the possibility that Vysheslavia reached its current orbit by a recent collisional breakup.  相似文献   

14.
We present observations of magnitude-phase dependences of three low-albedo asteroids down to phase angles of 0.1–0.2°. Data were obtained during 40 nights from 1994 to 1995 within the joint observational program at ESO and Kharkiv Astronomical Observatories with the aim to reach as low phase angles as possible. All three low-albedo asteroids may display a small nonlinear increase in magnitude-phase dependence at subdegree phase angles. The phase curves of 50 Virginia and 102 Miriam are poorly approximated by the HG function. Rotation periods of the asteroids were also determined: 14.310±0.010 hours for 50 Virginia, 6.030±0.001 h for 91 Aegina and 15.789± 0.003 h for 102 Miriam.  相似文献   

15.
The potentially hazardous Asteroid (33342) 1998 WT24 approached the Earth within 0.0125 AU on 2001 December 16 and was the target of a number of optical, infrared, and radar observing campaigns. Interest in 1998 WT24 stems from its having an orbit with an unusually low perihelion distance, which causes it to cross the orbits of the Earth, Venus, and Mercury, and its possibly being a member of the E spectral class, which is rare amongst near-Earth asteroids (NEAs). We present the results of extensive thermal-infrared observations of 1998 WT24 obtained in December 2001 with the 3-m NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii and the ESO 3.6-m telescope in Chile. A number of thermal models have been applied to the data, including thermophysical models that give best-fit values of 0.35±0.04 km for the effective diameter, 0.56±0.2 for the geometric albedo, pv, and 100-300 J m−2 s−0.5 K−1 for the thermal inertia. Our values for the diameter and albedo are consistent with results derived from radar and polarimetric observations. The albedo is one of the highest values obtained for any asteroid and, since no other taxonomic type is associated with albedos above 0.5, supports the suggested rare E-type classification for 1998 WT24. The thermal inertia is an order of magnitude higher than values derived for large main-belt asteroids but consistent with the relatively high values found for other near-Earth asteroids. A crude pole solution inferred from a combination of our observations and published radar results is β=−52°, λ=355° (J2000), but we caution that this is uncertain by several tens of degrees.  相似文献   

16.
Three selected asteroids, numbered 58, 360 and405, were observed during 2000at Yunnan Observatory, China. Thenew lightcurves are presented inthis paper. The synodic period of (58)Concordia is estimated as 9.90 ± 0.01hfor the first time. The synodic period of (405) Thia derived from ourobservation is 9.96 ± 0.01h, which is slightly different from the previousresult. For 360, we obtained the synodic period of 6.18 ± 0.02h which isconsistent with previous values. Additionally, the BV and UB-colourindexes for the three asteriods were also determined.  相似文献   

17.
We present the results of observational campaigns of asteroids performed at Asiago Station of Padova Astronomical Observatory and at M.G. Fracastoro Station of Catania Astrophysical Observatory, as part of the large research programme on Solar System minor bodies undertaken since 1979 at the Physics and Astronomy Department of Catania University. Photometric observations of six Main-Belt asteroids (27 Euterpe, 173 Ino, 182 Elsa, 539 Pamina, 849 Ara, and 984 Gretia), one Hungaria (1727 Mette), and two Near-Earth Objects (3199 Nefertiti and 2004 UE) are reported. The first determination of the synodic rotational period of 2004 UE was obtained. For 182 Elsa and 1727 Mette the derived synodic period of 80.23±0.08 and , respectively, represents a significant improvement on the previously published values. For 182 Elsa the first determination of the H-G magnitude relation is also presented.  相似文献   

18.
We estimate Asteroid 1992 SK's physical properties from delay-Doppler images and Doppler-only echo spectra obtained during March 22-27, 1999, at Goldstone and from optical lightcurves obtained during February-March 1999 at Ond?ejov Observatory. The images span only about 15° of sky motion and are not strong, but they place up to twenty 40 m by 160 m pixels on the asteroid and have complete rotational phase coverage. Our analysis establishes that the radar observations are confined to subradar latitudes between −20° and −40°. The echo spectra and optical lightcurves span ∼80° of sky motion, which provides important geometric leverage on the pole direction. The lightcurves are essential for accurate estimation of the asteroid's shape and spin state. We estimate the asteroid's period to be 7.3182±0.0003 h and its pole direction to be at ecliptic longitude, latitude=(99°±5°,−3°±5°). The asteroid is about 1.4 km in maximum extent and mildly asymmetric, with an elongation of about 1.5 and relatively subdued topography. The OC radar albedo is 0.11±0.02 and the SC/OC ratio is 0.34±0.05. The current orbital solution permits accurate identification of planetary close approaches during 826-2690. We use our model to predict salient characteristics of radar images and optical lightcurves obtainable during the asteroid's March 2006 approach.  相似文献   

19.
The Asteroids 5 Astraea, 19 Fortuna, 51 Nemausa, 68 Leto, 138 Tolosa, 196 Philomela and 409 Aspasia have been studied using Strömgren photometry observations made in 1997. Simultaneous lightcurves in the uvby Strömgren filters of synodic periods of 0.70004±0.00020, 0.31013±0.00003, 0.32400±0.00020, 0.61910±0.00100, 0.42087±0.00009, 0.34750±0.00020 and 0.37576±0.00060 days, and amplitudes, in the y filter, of 0.m16±0.m05, 0.m30±0.m02, 0.m15±0.m04, 0.m29±0.m02, 0.m43±0.m03, 0.m30±0.m04 and 0.m15±0.m06 have been found for 5 Astraea, 19 Fortuna, 51 Nemausa, 68 Leto, 138 Tolosa, 196 Philomela and 409 Aspasia, respectively. Additional observations of 138 Tolosa and 196 Philomela during February 2000, show lightcurve amplitudes equal or greater than 0.m15 for 138 Tolosa and of 0.m45 for 196 Philomela in 2000 opposition.Solutions for the sense of rotation, sidereal period, pole orientation and shape properties have been proposed for the first time for 138 Tolosa and improved solutions have been obtained for 5 Astraea, 19 Fortuna, 51 Nemausa, 196 Philomela and 409 Aspasia.  相似文献   

20.
We observed the E-class main-belt Asteroids (MBAs) 44 Nysa and 434 Hungaria with Arecibo Observatory's S-band (12.6 cm) radar. Both asteroids exhibit polarization ratios higher than those measured for any other MBA: Nysa, μc=0.50±0.02 and Hungaria, μc=0.8±0.1. This is consistent with the high polarization ratios measured for every E-class near-Earth asteroid (NEA) observed by Benner et al. [Benner, L.A.M., and 10 collegues, 2008. Icarus, submitted for publication] and suggests a common cause. Our estimates of radar albedo are 0.19±0.06 for Nysa and 0.22±0.06 for Hungaria. These values are higher than those of most MBAs and, when combined with their high polarization ratios, suggest that the surface bulk density of both asteroids is high. We model Nysa as an ellipsoid of dimension 113×67×65 km (±15%) giving an effective diameter Deff=79±10 km, consistent with previous estimates. The echo waveforms are not consistent with a contact binary as suggested by Kaasalainen et al. [Kaasalainen, M., Torppa, J., Piironen, J., 2002. Astron. Astrophys. 383, L19-L22]. We place a constraint on Hungaria's maximum diameter, Dmax?11 km consistent with previous size estimates.  相似文献   

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