We observed near-Earth Asteroid (NEA) 2002 CE26 in August and September 2004 using the Arecibo S-band (2380-MHz, 12.6-cm) radar and NASA's Infrared Telescope Facility (IRTF). Shape models obtained ...based on inversion of our delay-Doppler images show the asteroid to be
3.5
±
0.4
km
in diameter and spheroidal; our corresponding nominal estimates of its visual and radar albedos are 0.07 and 0.24, respectively. Our IRTF spectrum shows the asteroid to be C-class with no evidence of hydration. Thermal models from the IRTF data provide a size and visual albedo consistent with the radar-derived estimate. We estimate the spin-pole to be within a few tens of degrees of
λ
=
317
°
,
β
=
−
20
°
. Our radar observations reveal a secondary approximately 0.3 km in diameter, giving this binary one of the largest size differentials of any known NEA. The secondary is in a near-circular orbit with period
15.6
±
0.1
h
and a semi-major axis of
4.7
±
0.2
km
. Estimates of the binary orbital pole and secondary rotation rate are consistent with the secondary being in a spin-locked equatorial orbit. The orbit corresponds to a primary mass of
M
=
1.95
±
0.25
×
10
13
kg
, leading to a primary bulk density of
ρ
=
0.9
+
0.5
/
−
0.4
g
cm
−3
, one of the lowest values yet measured for a main-belt or near-Earth asteroid.
We observed near-Earth asteroid (NEA) 2100 Ra-Shalom over a six-year period, obtaining rotationally resolved spectra in the visible, near-infrared, thermal-infrared, and radar wavelengths. We find ...that Ra-Shalom has an effective diameter of
D
eff
=
2.3
±
0.2
km
, rotation period
P
=
19.793
±
0.001
h
, visual albedo
p
v
=
0.13
±
0.03
, radar albedo
σ
ˆ
OC
=
0.36
±
0.10
, and polarization ratio
μ
c
=
0.25
±
0.04
. We used our radar observations to generate a three-dimensional shape model which shows several structural features of interest. Based on our thermal observations, Ra-Shalom has a high thermal inertia of ∼10
3 J m
−2 s
−0.5 K
−1, consistent with a coarse or rocky surface and the inferences of others Harris, A.W., Davies, J.K., Green, S.F., 1998. Icarus 135, 441–450; Delbo, M., Harris, A.W., Binzel, R.P., Pravec, P., Davies, J.K., 2003. Icarus 166, 116–130. Our spectral data indicate that Ra-Shalom is a K-class asteroid and we find excellent agreement between our spectra and laboratory spectra of the CV3 meteorite Grosnaja. Our spectra show rotation-dependent variations consistent with global variations in grain size. Our radar observations show rotation-dependent variations in radar albedo consistent with global variations in the thickness of a relatively thin regolith.
► We observed Asteroids 64 Angelina and 69 Hesperia with the Arecibo radar. ► E-class Angelina is tied for the highest radar polarization ratio observed in main-belt asteroids. ► M-class Hesperia has ...a radar albedo consistent with high metal content. ► Hesperia is 20% smaller than the IRAS diameter.
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,
D
eff
=
110
±
15
km, approximately 20% smaller than the reported IRAS value. We estimate Hesperia to have a radar albedo of
σ
ˆ
OC
=
0.45
±
0.12
, 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.
•A revised shape model for Asteroid (216) Kleopatra is presented.•Kleopatra is bi-lobate and one of the most elongated asteroids observed.•Based on radar properties, Kleopatra appears to be ...dominantly metallic.•Radar data suggest variations in the extent of silicate regolith.•Geopotential modeling indicates regolith will accumulate at the asteroid neck.
We used three different sets of Arecibo delay-Doppler radar images and five well-covered occultations to generate a revised three-dimensional shape model of asteroid (216) Kleopatra with a spatial resolution of ∼10 km. We find Kleopatra to be a bi-lobate contact binary of overall dimensions 276 × 94 × 78 km ± 15% and equivalent diameter Deq = 122 ± 30 km; our uncertainties are upper and lower bounds. Separated binary models are ruled out by multi-chord occultations. Our model is 27% longer than the “dog-bone” model originally published by Ostro et al. (2000) but is similar to their model in the minor and intermediate axes extents. Our model's dimensions are also consistent with more recent ones based on lightcurves, adaptive-optics, and interferometric imaging. We confirm a rotational period of P = 5.385280 h ± 0.000001 h and a rotation pole at ecliptic longitude and latitude (λ, β) = (74°, + 20°) ± 5°. Over its southern hemisphere (the one most frequently observed on Earth), Kleopatra's radar albedo is 0.43 ± 0.10, consistent with a high near-surface bulk density and, by inference, the high metal content expected for M-class asteroids. However, the radar albedo for equatorial observations is considerably lower and more typical of a dominantly silicate composition. This observation could readily be explained by a relatively thin (1–2 m) silicate mantle over equatorial latitudes. Kleopatra's surface is relatively smooth with a mean slope of 12° at the ∼10 km baseline scale. Analysis of its geopotential surface suggests loose material will preferentially migrate to the neck, and this is supported by our radar observations.
We have conducted a radar-driven observational campaign of main-belt asteroids (MBAs) focused on X/M class asteroids using the Arecibo radar and NASA Infrared Telescope Facilities (IRTF). M-type ...asteroids have been identified as metallic, enstatite chondrites and/or heavily altered carbonaceous chondrites Bell, J.F., Davis, D., Hartmann, W.K., Gaffey, M.J., 1989. In: Binzel, R.P., Gehrels, T., Matthews, M.S. (Eds.), Asteroids II. Univ. of Arizona Press, Tucson, pp. 921–948; Gaffey, M.J., McCord, T.B., 1979. In: Gehrels, T., Matthews, M.S. (Eds.), Asteroids. Univ. of Arizona Press, Tucson, pp. 688–723; Vilas, F., 1994. Icarus 111, 456–467. Radar wavelength observations can determine whether an asteroid is metallic and provide information about the porosity and regolith depth. Near-infrared observations can help determine the grain size, porosity and composition of an object. Concurrent observations with these tools can give us a wealth of information about an object. Our objectives for this observation program were to (a) determine if there are any consistent relationships between spectra in the near-infrared wavelengths and radar signatures and (b) look for rotationally resolved relationships between asteroid radar properties and near-infrared spectral properties. This paper describes preliminary results of an ongoing survey of near-infrared observations of M-type asteroids and is a companion paper to radar observations reported by Shepard Shepard, M.K., and 19 colleagues, 2008a. Icarus 195, 184–205. In the analysis of 16 asteroid near-infrared spectra and nine radar measurements, we find a trend indicating a correlation between continuum slope from 1.7 to 2.45 μm and radar albedo—an asteroid with a steep continuum slope also has a bright radar albedo, which suggests a significant metal content. This may provide a means to use near-IR observations to predict the most likely metallic candidates for radar studies.
► Radar observations reveal that NEA 1994 CC is the second-to-date confirmed triple system. ► The primary looks similar to 1999 KW4 Alpha with sloped hemispheres and an equatorial ridge. ► The inner ...satellite appears to be in spin–orbit lock, while the outer satellite is rotating asynchronously. ► At least 25% of the binaries/triples with D
>
200
m have a satellite that is asynchronous.
We report radar, photometric, and spectroscopic observations of near-Earth Asteroid (136617) 1994 CC. The radar measurements were obtained at Goldstone (8560
MHz, 3.5
cm) and Arecibo (2380
MHz, 12.6
cm) on 9
days following the asteroid’s approach within 0.0168
AU on June 10, 2009. 1994 CC was also observed with the Panchromatic Robotic Optical Monitoring and Polarimetry Telescopes (PROMPT) on May 21 and June 1–3. Visible-wavelength spectroscopy was obtained with the 5-m Hale telescope at Palomar on August 25. Delay-Doppler radar images reveal that 1994 CC is a triple system; along with (153591) 2001 SN263, this is only the second confirmed triple in the near-Earth population. Photometry obtained with PROMPT yields a rotation period for the primary
P
=
2.38860
±
0.00009
h and a lightcurve amplitude of ∼0.1
mag suggesting a shape with low elongation. Hale telescope spectroscopy indicates that 1994 CC is an Sq-class object. Delay-Doppler radar images and shape modeling reveal that the primary has an effective diameter of 0.62
±
0.06
km, low pole-on elongation, few obvious surface features, and a prominent equatorial ridge and sloped hemispheres that closely resemble those seen on the primary of binary near-Earth Asteroid (66391) 1999 KW4. Detailed orbit fitting reported separately by Fang et al. (Fang, J., Margot, J.-L., Brozovic, M., Nolan, M.C., Benner, L.A.M., Taylor, P.A. 2011. Astron. J. 141, 154–168) gives a mass of the primary of 2.6
×
10
11
kg that, coupled with the effective diameter, yields a bulk density of 2.1
±
0.6
g
cm
−3. The images constrain the diameters of the inner and outer satellites to be 113
±
30
m and 80
±
30
m, respectively. The inner satellite has a semimajor axis of ∼1.7
km (∼5.5 primary radii), an orbital period of ∼30
h, and its Doppler dispersion suggests relatively slow rotation, 26
±
12
h, consistent with spin–orbit lock. The outer satellite has an orbital period of ∼9
days and a rotation period of 14
±
7
h, establishing that the rotation is not spin–orbit locked. Among all binary and triple systems observed by radar, at least 25% (7/28) have a satellite that rotates more rapidly than its orbital period. This suggests that asynchronous configurations with
P
rotation
<
P
orbital are relatively common among multiple systems in the near-Earth population. 1994 CC’s outer satellite has an observed maximum separation from the primary of ∼5.7
km (∼18.4 primary radii) that is the largest separation relative to primary radius seen to date among all 36 known binary and triple NEA systems. 1994 CC, (153591) 2001 SN263, and 1998 ST27 are the only triple and binary systems known with satellite separations >10 primary radii, suggesting either a detection bias, or that such widely-separated satellites are relatively uncommon in NEA multiple systems.
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,
μ
c
=
0.50
±
0.02
and Hungaria,
μ
c
=
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
D
eff
=
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,
D
max
⩾
11
km
consistent with previous size estimates.
We examine the relative importance of coherent and incoherent echo components in near‐nadir radar scattering from self‐affine surfaces. The coherent component, important only for observations at ...small incidence angles (nadir‐looking sounders and altimeters), is a function of both surface roughness and scattering area. Models for the incoherent component require a minimum scattering area for validity; for smaller regions, the radar echo is overestimated. We propose a simple criterion for the horizontal scale that defines this shift in applicable scattering model and show the range of roughness parameters for which MARSIS, SHARAD, and possibly JIMO radar sounder measurements may be modeled using the two approaches.
We advocate the use of fractal surface statistics as a simple, quantitative, and general model for planetary surface roughness. We determine the shadowing behavior of a wide range of fractal surfaces ...using computer simulations, and present an empirical function that reproduces their observed behavior within statistical uncertainties. We compare the shadowing behavior of fractal surfaces to four analytic shadowing models for random surfaces and find that three of these, including the Hapke (1984,Icarus59, 41–59) model, are well approximated by specific cases of a general fractal surface model. In addition, we demonstrate that a fractal surface model provides a way of quantitatively verifying and extending previous interpretations of the Hapke (1984) roughness parameter. We hypothesize that the scale which dominates surface shadowing, and by extension photometric roughness, is the smallest surface scale for which shadows exist and that this scale is a function of intrinsic physical parameters such as the single scattering albedo and particle phase function. If correct, a major implication of this hypothesis is that photometric roughness may have different physical meanings on different surfaces.
Interpretations of visible/near-infrared reflectance spectra of Mars are often complicated by the effects of dust coatings that obscure the underlying materials of interest. The ability to separate ...the spectral reflectance signatures of coatings and substrates requires an understanding of how their individual and combined reflectance properties vary with phase angle. Toward this end, laboratory multispectral observations of rocks coated with different amounts of Mars analog dust were acquired under variable illumination and viewing geometries using the Bloomsburg University Goniometer (BUG). These bidirectional reflectance distribution function (BRDF) data were fit with a two-layer radiative transfer model, which replicated BUG observations of dust-coated basaltic andesite substrates relatively well. Derived single scattering albedo and phase function parameters for the dust were useful in testing the model's ability to derive the spectrum of a “blind” substrate (unknown to the modeler) coated with dust. Subsequent tests were run using subsets of the BUG data restricted by goniometric or coating thickness coverage. Using the entire data set provided the best constraints on model parameters, although some reductions in goniometric coverage could be tolerated without substantial degradation. Predictably, the most thinly coated samples provided the best information on the substrate, whereas the thickest coatings best replicated the dust. Dust zenith optical thickness values ∼0.6–0.8 best constrain the substrate and coating simultaneously, particularly for large ranges of incidence or emission angles. The lack of sufficient angles can be offset by having a greater number and range of coatings thicknesses. Given few angles and thicknesses, few constraints can be placed concurrently on the spectral properties of the coating and substrate.
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