Five-color photoelectric photometry of asteroid 433 Eros
Abstract
Five-color photoelectric lightcurves of asteroid 433 Eros were obtained on 9 nights during the 1974/75 apparition. Although color differences due to changing solar phase angle were detected, color differences during a single rotation of Eros are less than 1%. Amplitudes of up to 1m.44 were measured, and there are indications that three reversals in the relative depths of the two minima occured between late December 1974 and late January 1975. The absolute visual magnitude at primary maximum, corrected to zero phase and to one AU from Earth and Sun, is about V0(1,0) = 10m.8.
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Surface properties of asteroids: A synthesis of polarimetry, radiometry, and spectrophotometry
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Lightcurves and the axis of rotation of asteroid 433 Eros
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Photometry from 0.65 to 2.2 microns of asteroid 433 Eros
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Cited by (19)
We present visible-wavelength (0.435–0.925 μm) spectroscopic measurements for 1341 main-belt asteroids observed during the second phase of the Small Main-belt Asteroid Spectroscopic Survey (SMASSII). The purpose of this survey is to provide a new basis for studying the compositional structure of the asteroid belt. Through the large sample size and the relatively high spectral resolution (R∼100) of the SMASSII data, we find that values of the spectral parameters describing these data form more of a continuum than previously realized. Objects with intermediate spectral characteristics are bridging gaps that once separated distinct spectral classes. In some cases, newly revealed small-scale spectral features may be indicative of previously unrecognized mineral constituents. Here we present the data and principal component analyses that describe the SMASSII spectra. A companion paper utilizes these principal component scores, along with other measures of the spectral features, to develop a new taxonomy that takes advantage of the information contained within charge-coupled device spectra.
We present near-infrared spectrometer (NIS) observations (0.8 to 2.4 μm) of the S-type asteroid 433 Eros obtained by the NEAR Shoemaker spacecraft and report results of our Hapke photometric model analysis of data obtained at phase angles ranging from 1.2° to 111.0° and at spatial resolutions of 1.25×2.5 to 2.75×5.5 km/spectrum. Our Hapke model fits successfully to the NEAR spectroscopic data for systematic color variations that accompany changing viewing and illumination geometry. Model parameters imply a geometric albedo at 0.946 μm of 0.27±0.04, which corresponds to a geometric albedo at 0.550 μm of 0.25±0.05. We find that Eros exhibits phase reddening of up to 10% across the phase angle range of 0–100°. We observe a 10% increase in the 1-μm band depth at high phase angles. In contrast, we observe only a 5% increase in continuum slope from 1.486 to 2.363 μm and essentially no difference in the 2-μm band depth at higher phase angles. These contrasting phase effects imply that there are phase-dependent differences in the parametric measurements of 1- and 2-μm band areas, and in their ratio. The Hapke model fits suggest that Eros exhibits a weaker opposition surge than either 951 Gaspra or 243 Ida (the only other S-type asteroids for which we possess disk-resolved photometric observations). On average, we find that Eros at 0.946 μm has a higher geometric albedo and a higher single-scatter albedo than Gaspra or Ida at 0.56 μm; however, Eros's single-particle phase function asymmetry and average surface macroscopic roughness parameters are intermediate between Gaspra and Ida. Only two of the five Hapke model parameters exhibit a notable wavelength dependence: (1) The single-scatter albedo mimics the spectrum of Eros, and (2) there is a decrease in angular width of the opposition surge with increasing wavelength from 0.8 to 1.7 μm. Such opposition surge behavior is not adequately modeled with our shadow-hiding Hapke model, consistent with coherent backscattering phenomena near zero phase.
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.
The shape of Eros
1990, IcarusThe convex hull of Eros' polar silhouette, estimated from radar echo spectra obtained in 1975 by R. F. Jurgens and R. M. Goldstein (1976, Icarus 28, 1–15), provides new information about this asteroid's shape. Monte Carlo simulations are used to optimize the estimation, to explore the nature and severity of associated errors, and to guide bias-correction procedures. Eros' hull is shaped like a rounded trapezoid, whose long and short bases faced Earth during epochs of primary and secondary maxima, respectively, in the January 1975 optical lightcurves. The nonaxisymmetric shape helps to explain odd harmonics in Eros' echo spectral signature as a function of rotation phase, whose presence cannot be accounted for by homogeneous ellipsoid models. The extreme breadths of Eros' polar silhouette are within a few kilometers of 35 and 16 km. Additional constraints on Eros' figure are obtained by inverting an optical lightcurve to estimate the asteroid's “mean cross section,” which is a two-dimensional average of three-dimensional shape. Eros' mean cross section and polar silhouette have similar elongations. The hull estimate permits previously reported radar time-delay and Doppler-frequency measurements to be referenced directly to Eros' center of mass.
Albedo and color contrasts on asteroid surfaces
1979, IcarusAsteroids 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.
Data are presented for the 182 asteroids whose rotational properties are available in the literature. Plots are provided for the asteroid rotational frequency f and lightcurve amplitude Δm versus asteroid size; the latter is determined using standard methods if data are available but otherwise is estimated from asteroid albedos, selected depending on taxonomic type or orbital position. A linear least-squares fit to all the data shows that f increases with decreasing size, confirming McAdoo and Burns' (1973) result; this is demonstrated to be primarily caused by relatively more small non-C than C asteroids in our sample, coupled with a slower mean rotation rate for C asteroids (P ≈ 11 hr) than non-C asteroids (P ≈ 9 hr). In terms of the collisional theory of Harris (1979a), this means that the C's are less dense than the other minor planets. Any slight tendency for smaller asteroids to spin faster, even within a taxonomic type, could be due to selection effects; our data are not extensive enough to determine whether the very smallest (⪅ 10-km diameter) spin especially fast. The minor planets of our survey become more irregular at smaller sizes, disputing the conclusions of Bowell (1977b), Degewij (1977), and Degewij et al. (1978), based on other, perhaps more complete, data; selection effects may account for this disagreement. Shapes do not appear to depend on taxonomic type. The dispersion of asteroid rotation rates from the mean is found to be in excellent agreement with a three-dimensional Maxwellian distribution, such as would be developed in a collisionally evolved system. The rotation axes, therefore, appear to be randomly oriented in space. Rotation pole positions are also tabulated and calculated to likely be constant in space over the extent of past observation. Observers are encouraged to measure the rotational properties of faint objects and asteroids of unusual taxonomic types, and to carry out long-time studies of asteroids which over short periods do not seem to vary.