A giant crater on 90 Antiope?

Mutual event observations between the two components of 90 Antiope were carried out in 2007-2008. The pole position was refined to λ₀ = 199.5 ± 0.5° and β₀ = 39.8 ± 5° in J2000 ecliptic coordinates, leaving intact the physical solution for the components, assimilated to two perfect Roche ellipsoids, and derived after the 2005 mutual event season (Descamps, P., Marchis, F., Michalowski, T., Vachier, F., Colas, F., Berthier, J., Assafin, M., Dunckel, P.B., Polinska, M., Pych, W., Hestroffer, D., Miller, K., Vieira-Martins, R., Birlan, M., Teng-Chuen-Yu, J.-P., Peyrot, A., Payet, B., Dorseuil, J., Léonie, Y., Dijoux, T., 2007. Figure of the double Asteroid 90 Antiope from AO and lightcurves observations. Icarus 187, 482-499). Furthermore, a large-scale geological depression, located on one of the components, was introduced to better match the observed lightcurves. This vast geological feature of about 68 km in diameter, which could be postulated as a bowl-shaped impact crater, is indeed responsible of the photometric asymmetries seen on the “shoulders” of the lightcurves. The bulk density was then recomputed to 1.28 ± 0.04 g cm⁻³ to take into account this large-scale non-convexity. This giant crater could be the aftermath of a tremendous collision of a 100-km sized proto-Antiope with another Themis family member. This statement is supported by the fact that Antiope is sufficiently porous (∼50%) to survive such an impact without being wholly destroyed. This violent shock would have then imparted enough angular momentum for fissioning of proto-Antiope into two equisized bodies. We calculated that the impactor must have a diameter greater than ∼17 km, for an impact velocity ranging between 1 and 4 km/s. With such a projectile, this event has a substantial 50% probability to have occurred over the age of the Themis family.     

Nature of the small main belt Asteroid 3169 Ostro

We present a set of rotational lightcurve measurements of the small main belt Asteroid 3169 Ostro. Our observations reveal an unambiguous, double-peaked rotational lightcurve with a peak-to-peak variation up to 1.2±0.05 mag and a synodic period of 6.509±0.001 h. From the large flux variation and the overall shape of the lightcurves, we suggest that 3169 Ostro could be a tightly bound binary or a contact binary, similar to the Trojan Asteroid 624 Hektor. A shape model of this system is proposed on the assumption that 3169 Ostro is a Roche binary described by a pair of homogeneous elongated bodies, with a size ratio of 0.87, in hydrostatic equilibrium and in circular synchronous motion around each other. The direction of the spin axis is determined modulo 180° by its J2000 ecliptic coordinates λ₀=50±10°, β₀=±54±5°. The binary interpretation and the pole solution adequately fit the earlier photometric observations made in 1986 and 1988. However, additional supporting lightcurves are highly desirable especially in the next mutual events occurrence of 2008 and 2009 in order to remove the pole ambiguity and to confirm unambiguously the binary nature of 3169 Ostro.     

Radar and optical observations and physical modeling of near-Earth Asteroid 10115 (1992 SK)

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.     

Direct measurement of the size, shape, and pole of 511 Davida with Keck AO in a single night

Using the high-quality data set of 165 images taken at 11 epochs over the 5.13 h rotation of the large C-type Asteroid 511 Davida, we find the dimensions of its triaxial ellipsoid model to be 357±2×294±2×231±50 km. The images were acquired with the adaptive optics system on the 10 m Keck II telescope on December 27, 2002. The a and b diameters are much better determined than previously estimated from speckle interferometry and indirect measurements, and our mean diameter, (abc)^1/3=289±21 km, is 19% below previous estimates. We find the pole to lie within 2° of [RA=295°; Dec=0°] or in Ecliptic coordinates [λ=297°; β=+21°], a significant improvement to the pole direction. Otherwise, previous determinations of the axial ratios agree with our new results. These observations illustrate that our technique of finding the dimensions and pole of an asteroid from its changing projected size and shape is very powerful because it can be done in essentially one night as opposed to decades of lightcurves. Average departures of 3% (5 km) of the asteroid's mean radius from a smooth outline are detected, with at least two local positive-relief features and at least one flat facet showing approximately 15 km deviations from the reference best-fit ellipsoid. The facet is reminiscent of large global-scale craters on Asteroid 253 Mathilde (also a C-type) when seen edge-on in close-up images from the NEAR mission flyby. We show that giant craters (up to 150 km diameter, the size of the largest facets seen on Davida) can be expected from the impactor size distribution, without likelihood of catastrophic disruption of Davida.     

Radar and infrared observations of binary near-Earth Asteroid 2002 CE26

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±0.25×¹⁰¹³ kg, leading to a primary bulk density of , one of the lowest values yet measured for a main-belt or near-Earth asteroid.     

Radar observations and a physical model of Asteroid 1580 Betulia

We report Arecibo (2380-MHz, 13-cm) observations of Asteroid 1580 Betulia in May-June 2002. We combine these continuous-wave Doppler spectra and delay-Doppler images with optical lightcurves from the 1976 and 1989 apparitions in order to estimate Betulia's shape and spin vector. We confirm the spin vector solution of Kaasalainen et al. [Kaasalainen, M., and 21 colleagues, 2004. Icarus 167, 178-196], with sidereal period P=6.13836 h and ecliptic pole direction (λ,β)=(136°,+22°), and obtain a model that resembles the Kaasalainen et al. convex-definite shape reconstruction but is dominated by a prominent concavity in the southern hemisphere. We find that Betulia has a maximum breadth of 6.59±0.66 km and an effective diameter of 5.39±0.54 km. These dimensions are in accord with reanalyzed polarimetric and radar data from the 1970s. Our effective diameter is 15% larger than the best radiometric estimate of Harris et al. [Harris, A.W., Mueller, M., Delbó, M., Bus, S.J., 2005. Icarus 179, 95-108], but this difference is much smaller than the size differences between past models. Considering orbits of test particles around Betulia, we find that this asteroid's unusual shape results in six equilibrium points close to its equatorial plane rather than the usual four points; two of these six points represent stable synchronous orbits while four are unstable. Betulia's close planetary encounters can be predicted for over four thousand years into the future.     

Spin axis of (2953) Vysheslavia and its implications

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.     

Multi-wavelength observations of Asteroid 2100 Ra-Shalom

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 Deff=2.3±0.2 km, rotation period P=19.793±0.001 h, visual albedo pv=0.13±0.03, radar albedo , and polarization ratio μc=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 ∼10³ 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.     

Mass and density of Asteroid 121 Hermione from an analysis of its companion orbit

We report on Adaptive Optics observations of the satellite of Asteroid 121 Hermione with the ESO-Paranal UT4 VLT and the Keck AO telescopes. The binary system, belonging to the Cybele family, was observed during two observing campaigns in January 2003 and January 2004 aiming to confirm its trajectory and accurately determine its orbital elements. A precessing Keplerian model was used to describe the motion of S/2002 (121) 1. We find that the satellite of Hermione revolves at a=768±11 km from the primary in P=2.582±0.002 days with a roughly circular and prograde orbit (e=0.001±0.001, i=3±2° w.r.t. equator primary). These extensive astrometric measurements enable us to determine the mass of Hermione to be 0.54±0.03×¹⁰¹⁹ kg and its pole solution (λ₀=1.5°±2.00, β₀=10°±2.0 in ecliptic J2000). Additional Keck AO observations taken close to the asteroid opposition in December 2003 give us direct insight into the structure of the primary which presents a bilobated shape. Since the angular resolution is limited to the theoretical angular resolution of the telescope (43 mas corresponding to a spatial resolution of 80 km), two shape models (called snowman and peanut) are proposed based on the images which were deconvolved with MISTRAL deconvolution process. Assuming a purely synchronous orbit and knowing the mass of the primary, the peanut shape composed of two separated components is quite unlikely. Additionally the J₂ calculated from the analysis of the secondary orbit is not in agreement with the peanut model, but close to the snowman shape. The bulk density of the primary as derived from the observed size of the snowman shape is estimated to ρ∼1.8±0.2 g/cm³ implying a porosity ∼14% for this C-type asteroid, corresponding to a fractured asteroid. Considering the IRAS diameter, the density is lower (ρ=1.1±0.3 g/cm³) leading to a high porosity (p=30-60%) with a nominal value of p=48%, which indicates a completely loose rubble-pile structure for the primary. Further work is necessary to better constrain the size, shape, and then internal structure of Hermione's primary.     

Radar and photometric observations and shape modeling of contact binary near-Earth Asteroid (8567) 1996 HW1

We observed near-Earth Asteroid (8567) 1996 HW1 at the Arecibo Observatory on six dates in September 2008, obtaining radar images and spectra. By combining these data with an extensive set of new lightcurves taken during 2008-2009 and with previously published lightcurves from 2005, we were able to reconstruct the object’s shape and spin state. 1996 HW1 is an elongated, bifurcated object with maximum diameters of 3.8 × 1.6 × 1.5 km and a contact-binary shape. It is the most bifurcated near-Earth asteroid yet studied and one of the most elongated as well. The sidereal rotation period is 8.76243 ± 0.00004 h and the pole direction is within 5° of ecliptic longitude and latitude (281°, −31°). Radar astrometry has reduced the orbital element uncertainties by 27% relative to the a priori orbit solution that was based on a half-century of optical data. Simple dynamical arguments are used to demonstrate that this asteroid could have originated as a binary system that tidally decayed and merged.     

Radar observations of E-class Asteroids 44 Nysa and 434 Hungaria

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.     

Radar observations and a physical model of contact binary Asteroid 4486 Mithra

Arecibo (2380 MHz, 12.6 cm) and Goldstone (8560 MHz, 3.5 cm) delay-Doppler radar images obtained in July and August of 2000 reveal that 4486 Mithra is an irregular, significantly bifurcated object, with a central valley ∼380 m deep and a long axis potentially exceeding 2 km. With its bimodal appearance, Mithra is a strong candidate for a contact binary asteroid. Sequences of Goldstone images spanning up to 3 h per day show very little rotation and establish that Mithra is an unusually slow rotator. We used Goldstone and Arecibo data to estimate Mithra’s 3D shape and spin state. We obtain prograde (λ = 337°, β = 19°) and retrograde (λ = 154°, β = −19°) models that give comparable fits, have very similar shapes roughly resembling an hourglass, and have a rotation period of 67.5 ± 6.0 h. The dimensions of these two models are very similar; for the prograde solution the maximum dimensions are X = 2.35 ± 0.15 km, Y = 1.65 ± 0.10 km, Z = 1.44 ± 0.10 km. Dynamical analysis of our models suggests that in the past, Mithra most likely went through a period of even slower rotation with its obliquity close to 90°. The spin rate is predicted to be increasing due to thermal torque (YORP), while the obliquity, which is currently +68° and +106° for the prograde and retrograde models, respectively, is predicted to move away from 90°.     

Physical modeling of near-Earth Asteroid (29075) 1950 DA

Near-Earth Asteroid (29075) 1950 DA may closely encounter Earth in 2880. The probability of Earth impact may be as high as 1/300, but the outcome of the encounter depends critically on the physical properties of the asteroid [Giorgini et al., 2002. Science 196, 132-136]. We have used Arecibo and Goldstone radar data and optical lightcurves to estimate the shape, spin state, and surface structure of 1950 DA. The data allow two distinct models. One rotates prograde and is roughly spheroidal with mean diameter 1.16±0.12 km. The other rotates retrograde and is oblate and about 30% larger. Both models suggest a nickel-iron or enstatite chondritic composition. Ground-based observations should be able to determine which model is correct within the next several decades.     

Photometric survey of binary near-Earth asteroids

Photometric data on 17 binary near-Earth asteroids (15 of them are certain detections, two are probables) were analysed and characteristic properties of the near-Earth asteroid (NEA) binary population were inferred. We have found that binary systems with a secondary-to-primary mean diameter ratio Ds/Dp?0.18 concentrate among NEAs smaller than 2 km in diameter; the abundance of such binaries decreases significantly among larger NEAs. Secondaries show an upper size limit of Ds=0.5-1 km. Systems with Ds/Dp?0.5 are abundant but larger satellites are significantly less common. Primaries have spheroidal shapes and they rotate rapidly, with periods concentrating between 2.2 to 2.8 h and with a tail of the distribution up to ∼4 h. The fast rotators are close to the critical spin for rubble piles with bulk densities about 2 g/cm³. Orbital periods show an apparent cut-off at Porb∼11 h; closer systems with shorter orbital periods have not been discovered, which is consistent with the Roche limit for strengthless bodies. Secondaries are more elongated on average than primaries. Most, but not all, of their rotations appear to be synchronized with the orbital motion; nonsynchronous secondary rotations may occur especially among wider systems with Porb>20 h. The specific total angular momentum of most of the binary systems is similar to within ±20% and close to the angular momentum of a sphere with the same total mass and density, rotating at the disruption limit; this suggests that the binaries were created by mechanism(s) related to rotation near the critical limit and that they neither gained nor lost significant amounts of angular momentum during or since formation. A comparison with six small asynchronous binaries detected in the main belt of asteroids suggests that the population extends beyond the region of terrestrial planets, but with characteristics shifted to larger sizes and longer periods. The estimated mean proportion of binaries with Ds/Dp?0.18 among NEAs larger than 0.3 km is 15±4%. Among fastest rotating NEAs larger than 0.3 km with periods between 2.2 and 2.8 h, the mean proportion of such binaries is (66⁺¹⁰₋₁₂)%.     

Cassini ISS astrometric observations of the inner jovian satellites, Amalthea and Thebe

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 R_J, 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].     

New insights on the binary Asteroid 121 Hermione

We report on the results of a 6-month photometric study of the main-belt binary C-type Asteroid 121 Hermione, performed during its 2007 opposition. We took advantage of the rare observational opportunity afforded by one of the annual equinoxes of Hermione occurring close to its opposition in June 2007. The equinox provides an edge-on aspect for an Earth-based observer, which is well suited to a thorough study of Hermione’s physical characteristics. The catalog of observations carried out with small telescopes is presented in this work, together with new adaptive optics (AO) imaging obtained between 2005 and 2008 with the Yepun 8-m VLT telescope and the 10-m Keck telescope. The most striking result is confirmation that Hermione is a bifurcated and elongated body, as suggested by Marchis, et al. [Marchis, F., Hestroffer, D., Descamps, P., Berthier, J., Laver, C., de Pater, I., 2005. Icarus 178, 450-464]. A new effective diameter of 187 ± 6 km was calculated from the combination of AO, photometric and thermal observations. The new diameter is some 10% smaller than the hitherto accepted radiometric diameter based on IRAS data. The reason for the discrepancy is that IRAS viewed the system almost pole-on. New thermal observations with the Spitzer Space Telescope agree with the diameter derived from AO and lightcurve observations. On the basis of the new AO astrometric observations of the small 32-km diameter satellite we have refined the orbit solution and derived a new value of the bulk density of Hermione of 1.4 + 0.5/−0.2 g cm⁻³. We infer a macroscopic porosity of ∼33 + 5/−20%.     

Photometric analysis of Eros from NEAR data

A photometric model of (433) Eros at wavelengths from 450 to 1050 nm is constructed using the combination of the images from the multispectral imager (MSI) obtained during the one-year long orbital phase of the NEAR mission, ground-based lightcurves from earlier observations, and our theoretical forward modeling simulations coupled with the NEAR shape model. The single scattering albedo is found to be 0.33±0.03 at 550 nm, which is smaller than past findings by 30%. The amplitude and width of the opposition effect are 1.4±0.1 and 0.010±0.004 from ground based lightcurves. It is confirmed that the asymmetry factor of the single-particle phase function and the surface roughness parameter do not depend on wavelength from 450 to 1050 nm, and their values are estimated to be −0.25±0.02 and 28°±3°, respectively, comparable with the earlier measurements from the NEAR NIS data. The geometric albedo and the Bond albedo at 550 nm are calculated to be 0.23 and 0.093, respectively, which make Eros less reflective than previous models, but still slightly more reflective than average S-type asteroids. The lower albedos of Eros are more consistent with our forward modeling simulations, as well as with its spectrum. Eros is a typical S-type asteroid like (951) Gaspra and (243) Ida, and has similar surface regolith properties. Combining the single-scattering albedo with the olivine composition of ordinary chondrites, taking into account space weathering darkening, we constrain the grain size of the regolith particles on Eros to a range of 50 to 100 μm.     

The rotational and physical properties of the Centaur (32532) 2001 PT13

We present observations of the Centaur (32532) 2001 PT₁₃ taken between September 2000 and December 2000. A multi-wavelength lightcurve was assembled from V-, R- and J-band photometry measurements. Analysis of the lightcurve indicates that there are two peaks of slightly different brightness, a rotation period of 0.34741±0.00005 day, and a maximum photometric range of 0.18 mag. We obtained VRJHK colors (V-R=0.50±0.01, V-J=1.69±0.02, V-H=2.19±0.04, and V-K=2.30±0.04) that are consistent with the grey KBO/Centaur population. The V-R color shows no variation as a function of rotational phase; however, we cannot exclude the possibility that rotational variations are present in the R-J color. Assuming a 4% albedo, we estimate that 2001 PT₁₃ has an effective diameter of 90 km and a minimum axial ratio a/b of 1.18. We find no evidence of a coma and place an upper limit of 15 g s⁻¹ on the dust production rate.     

A three-dimensional solution for the orbit of the asteroidal satellite of 22 Kalliope

We carried out new observations of the binary asteroid 22 Kalliope (S2/2001) with the Shane 3-m telescope of the Lick observatory in October and November 2001. With a FWHM (full width at half maximum) of 0″.2, Kalliope (apparent size of about 0″.15) was not resolved but it was possible to separate the secondary from its primary whose apparent separation was of the order of 0″.7 with a magnitude difference of 3.22±0.20. As each set of observations spanned a few days of time, they are well distributed along the secondary's orbit, enabling us to accurately estimate its orbit.The satellite orbits 22 Kalliope in a prograde manner with respect to Kalliope's rotational spin (which is in a retrograde sense relative to its orbit around the Sun), on a highly inclined (i=19.8±2.0 with respect to the equator of 22 Kalliope) and moderately eccentric orbit (e=0.07±0.02) with an orbital period of 3.58±0.08 days. The semi-major axis is 1020±40 km. Using Kalliope's diameter as determined from IRAS data, the asteroid's bulk density is about 2.03±0.16 g cm⁻³, suggestive of a highly porous body with a porosity of 70% considering that the grain density of its meteoritic analog is of ∼7.4 g cm⁻³. This suggests a rubble pile, rather than solid, body. The measured nodal precession rate of the secondary's orbit seems to be much higher than expected from Kalliope's oblateness, assuming a homogeneous body (constant density). This suggests that Kalliope may be 60% more elongated or 35% larger than presently believed or/and that its internal structure is highly inhomogeneous with a denser outer shell.     

Optical and thermal infrared observations of six near-Earth asteroids in 2002

We present thermal infrared photometry and spectrophotometry of six Near-Earth Asteroids (NEAs) using the 3.8 m United Kingdom Infrared Telescope (UKIRT) together with quasi-simultaneous optical observations of five NEAs taken at the 1.0 m Jacobus Kapteyn Telescope (JKT). For Asteroid (6455) 1992 HE we derive a rotational period P=2.736±0.002 h, and an absolute visual magnitude H=14.32±0.24. For Asteroid 2002 HK₁₂ we derive . The Standard Thermal Model (STM), the Fast Rotating Model (FRM) and the Near-Earth Asteroid Thermal Model (NEATM) have been fitted to the measured fluxes to derive albedos and effective diameters. The derived geometric albedos and effective diameters are (6455) 1992 HE: pv=0.26±0.08, Deff=3.55±0.5 km; 1999 HF₁: pv=0.18±0.07, ; 2000 ED₁₀₄: pv=0.18±0.05, Deff=1.21±0.2 km; 2002 HK₁₂: , Deff=0.62±0.2 km; 2002 NX₁₈: pv=0.031±0.009, Deff=2.24±0.3 km; 2002 QE₁₅: , Deff=1.94±0.4 km. The limitations of using the NEATM to observe NEAs at high phase angles are discussed.