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1.
Abstract— We observed 25143 Itokawa, the target of Japan's Hayabusa (MUSES‐C) sample‐return mission, during its 2001 close approach at Arecibo on twelve dates during March 18‐April 9 and at Goldstone on nine dates during March 20‐April 2. We obtained delay‐Doppler images with range resolutions of 100 ns (15 m) at Arecibo and 125 ns (19 m) at Goldstone. Itokawa's average circular polarization ratio at 13 cm, 0.26 ± 0.04, is comparable to that of Eros, so its cm‐to‐m surface roughness probably is comparable to that on Eros. Itokawa's radar reflectivity and polarization properties indicate a near‐surface bulk density within 20% of 2.5 g cm?3. We present a preliminary estimate of Itokawa's shape, reconstructed from images with rather limited rotation‐phase coverage, using the method of Hudson (1993) and assuming the lightcurve‐derived spin period (12.132 hr) and pole direction (ecliptic long., lat. = 355°, ?84°) of Kaasalainen et al. (2003). The model can be described as a slightly asymmetrical, slightly flattened ellipsoid with extents along its principal axes of 548 times 312 times 276 m ± 10%. Itokawa's topography is very subdued compared to that of other asteroids for which spacecraft images or radar reconstructions are available. Similarly, gravitational slopes on our Itokawa model average only 9° and everywhere are less than 27°. The radar‐refined orbit allows accurate identification of Itokawa's close planetary approaches through 2170. If radar ranging planned for Itokawa's 2004 apparition succeeds, then tracking of Hayabusa during its 2005 rendezvous should reveal Yarkovsky perturbation of the asteroid's orbit.  相似文献   

2.
Ben Zellner 《Icarus》1976,28(1):149-153
Newly available photometric, polarimetric, spectroscopic, thermal-radiometric, radar, and occultation results are synthesized in order to derive a coherent model for Eros. The geometric albedo is 0.19±0.01 at the visual wavelength, and the overall dimensions are approximately 13 × 15 × 36km. The rotation is about the short axis, in the direct sense, with a sidereal period of 5h16m13s.4. The pole of rotation lies within a few degrees of ecliptic coordinates λ = 16° and β = + 11°.Eros is uniformly coated with a particulate surface layer several millimeters thick. It has an iron-bearing silicate composition, similar to that of a minority of main-belt asteroids, and probably identifiable with H-type ordinary chondrites.  相似文献   

3.
UBV observations of asteroid 433 Eros were conducted on 17 nights during the winter of 1974/75. The peak-to-peak amplitude of the lightcurve varied from about 0.3 mag to nearly 1.4mmag. The absolute V mag at maximum light, extrapolated to zero phase, is 10.85. Phase coefficients of 0.0233 mag/degree, 0.0009 mag/degree and 0.0004 mag/degree were derived for V, B-V, and U-B, respectively. The zero-phase color of Eros (B?V = 0.88, U?B = 0.50) is representative of an S (silicaceous) compositional type asteroid. The color does not vary with rotation. The photometric behavior of Eros can be modeled by a cylinder with rounded ends having an axial ratio of about 2.3:1. The asteroid is rotating about a short axis with the north pole at λ0 = 15° and β0 = 9°.  相似文献   

4.
Ronald C. Taylor 《Icarus》1985,61(3):490-496
Refinements to the pole-determination method photometric astrometry (PA) were completed in 1983 (R. C. Taylor and E. F. Tedesco, 1983, Icarus54, 13–22). A goal is to redo the pole analysis for every asteroid whose pole had been determined from earlier versions of PA: Previous PA poles are reviewed in this paper. Asteroid 433 Eros is in that collection and has redone. The result are prograde rotation; a sidereal period of 0.219588 ± 0.000005 day; and a north pole at 22° longitude, +9° latitude. The uncertainty of the pole is 10°. The pole position of Eros determined by C.D. Vesely (1971, In Physical Studies of Minor Planets (T. Gehrels, Ed.), pp. 133–140, NASA SP-267) and Dunlap (1976, Icarus28, 69–78), using earlier versions of photometric astrometry, were within 21 and 7°, respectively, of the present result.  相似文献   

5.
Analytic expressions for the semimajor and semiminor axes and an orientation angle of the ellipse projected by a triaxial ellipsoid (an asteroid) and of the ellipse segment cast by a terminator across the ellipsoid as functions of the dimensions and pole of the body and the asterocenteric position of the Earth and Sun are derived. Applying these formulae to observations of the Earth-approaching asteroid 433 Eros obtained with the speckle interferometry system of Steward Observatory on December 17–18, 1981, and January 17–18, 1982, the following dimensions are derived: (40.5 ± 3.1 km) × (14.5 ± 2.3 km) × (14.1 ± 2.4 km) Eros' north pole is found to lie within 14° of RA = 0h16m Dec. = +43° (ecliptic longitude 23°, latitude +37°). Other than knowing the rotation period of Eros, these results are completely independent of any other data, and in the main confirm the results obtained in the 1974–1975 apparition by other methods. These dimensions, together with a lightcurve from December 18, 1981, lead to a geometric albedo of 0.156 ± 0.010. A series of two-dimensional power spectra and autocorrelation functions of the resolved asteroid clearly show it spinning in space.  相似文献   

6.
E.F. Tedesco 《Icarus》1976,28(1):21-28
UBV photometry and lightcurves of Eros were obtained on nine dates between October 1974 and March 1975. The absolute V magnitude at photometric maximum extrapolated to a solar phase angle of zero is 10.75 while the linear phase coefficient is 0.026mag deg?1. The mean colors at solar phase angles greater than 30° are B?V = 0.92 and U?B = 0.52mag. No significant color variations over the surface were detected.  相似文献   

7.
The brightness temperature of 433 Eros was less than 460°K during the close opposition in January 1975.  相似文献   

8.
Radar observations of 433 Eros were made at the Arecibo Observatory using a wavelength of 70 cm during the close approach of Eros to Earth in mid-January, 1975. A peak radar cross section of 39 ± 15 km2 was observed. The spectral broadening obtained was approximately 30 Hz, which is consistent with a value of 16 km for the maximum radius of the asteroid. The surface of Eros appears to be relatively rough at the scale of a wavelength as compared to the surfaces of the terrestrial planets and the Moon. The composition of the surface is not well determined, except that it cannot be a highly conducting metal. A single measurement each of round-trip echo times delay and doppler shift was made.  相似文献   

9.
We report on radiometric and reflected light observations of 433 Eros at high time resolution, high accuracy, and broad spectral coverage. We use a thermal inertia model to estimate the thermal inertia, albedo, and size of Eros. We find an albedo of 0.125 ± 0.025 with axes of 39.3 ± 2.0 × 16.1 ± 0.8 km. Our estimate of the albedo is about 30% lower than previous estimates.  相似文献   

10.
J.S. Hall  L.A. Riley 《Icarus》1974,23(2):144-156
Intensity and polarization measures have been made of Saturn in the ultraviolet (0.37 μm) and green (0.57 μm) since 1968, at phase angles which ranged from 0°.0 to 6°.1. During this interval, the tilt of the planet's equatorial plane changed from 11°.2 to 26°.4, with the south pole toward the Earth.The polarization of the disk in the ultraviolet shows a strongly radial pattern at all observed phase angles, with a maximum of about 3.7 × 10?2 at all limbs and a gradual decrease to nearly zero at the center. Where the rings overlap the disk, the better observations show their polarization to be small and in the direction of the equatorial plane.In green light and close to opposition the relatively small polarization of the disk appeared to be radial in 1968, but by 1973 polarization (still radial ) was detected only in the southern hemisphere. In all years the polarization appears to be strongest near the south pole, particularly when the pole is nearest the limb. When the pole is furthest from the limb (tilt = 26°), there is a suggestion that maximum polarization may occur at the pole itself. At large phase angles the polarization along the E-W diameter shows a prominent phase-dependent component of 1.0 × 10?2 in a N-S direction.The intensity profiles in the ultraviolet suggest some transmission of light by ring A and less than about 20% by ring B. A limiting value of 39% was obtained from the polarization data.  相似文献   

11.

The results of the first polarimetric measurements of near-Earth asteroid 2014 JO25 and comet 41P/Tuttle-Giacobini-Kresák performed on April 19, 2017, with a CCD sensor at the prime focus (f/3.85) of the 2.6-m Shajn Telescope of the Crimean Astrophysical Observatory in the R filter are reported. The degree of linear polarization of the asteroid is P = 2.69 ± 0.44% at a phase angle of 55.6°, which is typical of an S-type asteroid. Its geometric albedo is ρv ≈ 0.2. A digital filter applied to the direct image of the comet reveals a jet and a tail directed toward the Sun (PA = 45.1°) and away from it (PA = 241.2°), respectively, in the coma. The maximum degree of linear polarization in the near-nucleus region of the comet is 18% at a phase angle of 69.8°. The polarization decreases to 16.2–10.7% in coma regions with a radius of 865–4856 km. Various factors affecting the maximum degree of polarization and the polarization-degree distribution over the coma are discussed.

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12.
Fourteen photometric lightcurves of 433 Eros were made at the Astronomical Observatory of Torino during the 1974–75 close passage. The absolute magnitude of the primary maximum (10m78), the phase coefficient (0.023 mag/degree), the synodic and sidereal period of rotation (0d.21956 and 0d.21959, respectively) and the ecliptic coordinates of the pole (λ = 17°, β = 10°) were deduced.  相似文献   

13.
J.L. Dunlap 《Icarus》1976,28(1):69-78
Ten lightcurves and UBV photometry of 433 Eros were obtained between August 1972 and May 1975. The absolute magnitude of the lightcurve maximum is 10.75 and the phase coefficient is 0.025 mag/deg. There may be a small difference in B-V color between the northern and southern hemispheres. The pole of the axis of rotation is directed toward λ0 = 16°, β0 = 12°, ecliptic longitude and latitude, respectively, and the rotation is direct with a sidereal period of 0.d219599 or 5h16m13s4 ± 0.s2. The dimensions derived from the polarimetric albedo and the lightcurve amplitudes are 12km × 12km × 31km for a smooth cylinder with hemispherical ends.  相似文献   

14.
David Morrison 《Icarus》1976,28(1):125-132
Radiometry of Eros at 10 and 20 μm demonstrates that the thermal conductivity of the upper centimeter of the surface is approximately as low as that of the Moon, suggesting that the asteroid has a regolith of highly porous rocky material. When combined with photoelectric photometry, these infrared measurements yield an effective diameter of Eros at maximum light of D0 = 22 ± 2 km and a geometric albedo of pv = 0.18 ± 0.03.  相似文献   

15.
Analysis of the disk-integrated solar phase curve of 433 Eros, as derived from ground-based telescopic and NEAR Shoemaker spacecraft measurements, shows that Eros's surface properties are typical of average S-type asteroids. Eros displays the same single-particle scattering characteristics and porosity vs theoretical grain size relationships as typical S-asteroids, as does Ida. Eros's single-scattering albedo, however, is higher. The geometric albedo at 550 nm derived for Eros (0.29±0.02) is higher than Ida's but is equivalent to Gaspra's within the error bars. The phase integral (0.39±0.02) and Bond albedo (0.12±0.02) for Eros are higher than those estimated for average S-type asteroids but commensurate with the values obtained for Gaspra.  相似文献   

16.
Abstract— Radar data enable us to estimate an asteroid's near‐surface bulk density, thus providing a joint constraint on near‐surface porosity and solid density. We investigate two different approaches to simplifying this joint constraint: estimating solid densities by assuming uniform porosities for all asteroids; and estimating porosities by assuming uniform mineralogy within each taxonomic class. Methods used to estimate asteroids' near‐surface solid densities from radar data have not previously been calibrated via independent estimates. Recent spacecraft results on the chondritic nature of 433 Eros now permit such a check, and also support porosity estimation for S‐class objects. We use radar albedos and polarization ratios estimated for 36 main‐belt asteroids and nine near‐Earth asteroids to estimate near‐surface solid densities using two methods, one of which is similar to the uncalibrated algorithms used in previous studies, the other of which treats Eros as a calibrator. We also derive porosities for the same sample by assigning solid densities for each taxonomic class in advance. Density‐estimation results obtained for Eros itself are consistent with the uncalibrated method being valid in the mean; those derived for the full sample imply that uncalibrated solid densities are, at most, a few tens of percent too large on average. However, some derived densities are extremely low, whereas most porosity estimates are physically plausible. We discuss the relative merits of these two approaches.  相似文献   

17.
Polarimetric reflectance measurements have been made at a wavelength of for a suite of predominantly lunar regolith simulants in support of development efforts for the BepiColombo Laser Altimeter (BELA). Measurements were made using an instrument at the University of Bern, Switzerland, that has been modified to accommodate photometric characterizations of laboratory samples down to 0° phase angle (g) with linearly polarized illumination and a linearly polarized receiver. The data reveal opposition surges that are dominated by polarization state-conserving reflectance terms for all samples. Relative strengths of the trans-state reflectance terms are lowest for the darkest samples, suggesting that multiple scattering is responsible for state conversions. Normal albedo measurements of the lunar simulants range from 0.13 to 0.22 at zero illumination angle (i=0°). The total reflectance of the regolith simulants at g=0° were found to decrease with increasing i, which is inconsistent with predictions of reflectance models for Mercury. However, the g=0° reflectance remains higher at i>0° than would be expected for a gray Lambert surface that is scaled to the g=i=0° reflectance value. Polarization ratios for reflectance under polarized illumination but unconstrained emission show that the samples scatter light in the in-plane polarization state more efficiently than in the transverse state at g=0° and i>0°. The opposite is true for g>0° polarization ratios, which indicate that transverse polarized illumination scatters more efficiently at high g. The polarization effect appears to correlate with the sample's characteristic grain size, but the statistical basis of this trend is weak. The implications of these measurements upon the performance of the BELA instrument are discussed.  相似文献   

18.
J. Veverka 《Icarus》1973,18(4):657-660
The disk-integrated polarization of Titan has been measured at phase angles ranging from 0°.4 to 6°.l. The observed polarization is positive throughout this interval. This fact, when combined with published photometric data, suggests a model in which an optically thin Rayleigh atmosphere overlies an opaque cloud deck.  相似文献   

19.
We analyze the polarization effects of the radiation scattered in conical optically thin plasma envelopes. The density of free electrons in the envelope is assumed to decrease in inverse proportion to the square of the distance from the radiation source. The magnetic field, radial or azimuthal, is also assumed to vary in inverse proportion to the square of the distance from the center of the system. We take into account the fact that the scattered radiation near the surface of a star or a quasar is virtually unpolarized (the model of a nonpoint star). The spectra of linear polarization and its position angle are given for conical-envelope opening half-angles of 7.5°, 15°, and 30°. The inclination of the cone axis with respect to the observer’s direction took on values of 30°, 45°, 60°, 90°, 120°, 135°, and 150°. We allowed for the fact that part of the envelope is screened from the observer by the star itself. We also give polarization spectra for the radiation scattered in two mutually opposite conical envelopes. We use the results of our theoretical calculations to analyze the polarimetric observations of relativistic jets in cosmic gamma-ray bursts and active galactic nuclei. As a result, we estimated the magnetic fields in these objects. The constraint on the density of relativistic electrons is <107 cm?3.  相似文献   

20.
Lightcurves of 433 Eros are reported for 11 bandpasses ranging from 0.65 to 2.2 μm in wavelength. The relative spectral reflectance, R(λ), was not seen to vary during our observations. Eros has R(1.6 μm) = 1.5 ± 0.1 and R(2.2 μm) = 1.7 ± 0.1, where R(λ) is the spectral reflectance scaled to unity at λ = 0.56 μm. This spectral reflectance is suggestive of a mixture of silicates and material with high infrared reflectance, perhaps a metallic phase such as meteoritic “iron”.  相似文献   

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