New determination of the size and bulk density of the binary Asteroid 22 Kalliope from observations of mutual eclipses

In 2007, the M-type binary Asteroid 22 Kalliope reached one of its annual equinoxes. As a consequence, the orbit plane of its small moon, Linus, was aligned closely to the Sun's line of sight, giving rise to a mutual eclipse season. A dedicated international campaign of photometric observations, based on amateur-professional collaboration, was organized and coordinated by the IMCCE in order to catch several of these events. The set of the compiled observations is released in this work. We developed a relevant model of these events, including a topographic shape model of Kalliope refined in the present work, the orbit solution of Linus as well as the photometric effect of the shadow of one component falling on the other. By fitting this model to the only two full recorded events, we derived a new estimation of the equivalent diameter of Kalliope of 166.2±2.8 km, 8% smaller than its IRAS diameter. As to the diameter of Linus, considered as purely spherical, it is estimated to 28±2 km. This substantial “shortening” of Kalliope, gives a bulk density of 3.35±0.33 g/cm³, significantly higher than past determinations but more consistent with its taxonomic type. Some constraints can be inferred on the composition.     

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.     

Albedos and diameters of three Mars Trojan asteroids

We observed the Mars Trojan Asteroids (5261) Eureka and (101429) 1998 VF₃₁ and the candidate Mars Trojan 2001 FR₁₂₇ at 11.2 and 18.1 microns using Michelle on the Gemini North telescope. We derive diameters of 1.28, 0.78, and <0.52 km, respectively, with corresponding geometric visible albedos of 0.39, 0.32, and >0.14. The albedos for Eureka and 1998 VF₃₁ are consistent with the taxonomic classes and compositions (S(I)/angritic and S(VII)/achondritic, respectively) and implied histories presented in a companion paper by Rivkin et al. Eureka's surface likely has a relatively high thermal inertia, implying a thin regolith that is consistent with predictions and the small size that we derive.     

Constraints on the surface composition of Trojan asteroids from near-infrared (0.8-4.0 μm) spectroscopy

We present new near-infrared spectra of 20 Trojan asteroids. The spectra were recorded at the NASA Infrared Telescope Facility (IRTF) using the recently commissioned medium-resolution spectrograph SpeX and at the Multiple Mirror Telescope (MMT) using the instrument FSPEC. Spectra of all of these objects were measured in K-band (1.95-2.5 μm), 8 of these in L-band (2.8-4.0 μm), and 14 in the range 0.8-2.5 μm. These observations nearly double the number of published 0.8-2.5 μm spectra of Trojan asteroids and provide the first systematic study of the L-band region for these distant asteroids. The data show that the red spectral slope measured in the near-IR extends through the L-band, though it is not as steep here as at shorter wavelengths. A significant diversity is apparent in the near-IR spectral slopes of this sampling of objects. Most of the spectra do not contain any definitive absorption features characteristic of surface composition (e.g., H₂O, organics, silicates) as seen on main-belt asteroids and several Centaur and Kuiper Belt objects. A few objects may display spectral activity, and the reliability of these possible features is discussed. While these spectra are generally compatible with silicate surfaces to explain the spectral slope mixed with some fraction of low albedo material, there is no absolute indication of silicates. The spectral slope could also be explained by the presence of hydrocarbons, but the lack of absorption features, especially in L-band where very strong fundamental absorptions from these molecules appear, constrains the character and abundance of these materials at the surface.     

Composition of the L5 Mars Trojans: Neighbors, not siblings

Mars is the only terrestrial planet known to have Trojan (co-orbiting) asteroids, with a confirmed population of at least 4 objects. The origin of these objects is not known; while several have orbits that are stable on Solar System timescales, work by Rivkin et al. [Rivkin, A.S., Binzel, R.P., Howell, E.S., Bus, S.J., Grier, J.A., 2003. Icarus 165, 349–354] showed they have compositions that suggest separate origins from one another. We have obtained infrared (0.8–2.5 μm) spectroscopy of the two largest L5 Mars Trojans, and confirm and extend the results of Rivkin et al. We suggest that the differentiated angrite meteorites are good spectral analogs for 5261 Eureka, the largest Mars Trojan. Meteorite analogs for 101429 1998 VF31 are more varied and include primitive achondrites and mesosiderites.     

Near-Earth asteroid surface roughness depends on compositional class

Radar observations of 214 near-Earth asteroids (NEAs) reveal a very strong correlation of circular polarization ratio with visible-infrared taxonomic class, establishing distinct differences in the centimeter-to-several-decimeter structural complexity of objects in different spectral classes. The correlation may be due to the intrinsic mechanical properties of different mineralogical assemblages but also may reflect very different formation ages and collisional histories. The highest ratios are measured for groups associated with achondritic igneous rocky meteorites: the E class, whose parent body may be 3103 Eger, and the V class, derived from the mainbelt asteroid (and Dawn mission target) 4 Vesta.     

Constraining near-Earth object albedos using near-infrared spectroscopy

Low-albedo near-Earth objects (NEOs) are warm enough to emit detectable thermal flux at 2.5 μm when near perihelion. Thermal radiation can account for 33% or more of the total flux for an object with an albedo ?0.04 at 1.0 AU. This is measurable using near-infrared spectroscopic instruments enabling albedos to be constrained for a larger sample of NEOs.     

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.     

The cool surfaces of binary near-Earth asteroids

Here we show results from thermal-infrared observations of km-sized binary near-Earth asteroids (NEAs). We combine previously published thermal properties for NEAs with newly derived values for three binary NEAs. The η value derived from the near-Earth asteroid thermal model (NEATM) for each object is then used to estimate an average thermal inertia for the population of binary NEAs and compared against similar estimates for the population of non-binaries. We find that these objects have, in general, surface temperatures cooler than the average values for non-binary NEAs as suggested by elevated η values. We discuss how this may be evidence of higher-than-average surface thermal inertia. This latter physical parameter is a sensitive indicator of the presence or absence of regolith: bodies covered with fine regolith, such as the Earth’s moon, have low thermal inertia, whereas a surface with little or no regolith displays high thermal inertia. Our results are suggestive of a binary formation mechanism capable of altering surface properties, possibly removing regolith: an obvious candidate is the YORP effect.We present also newly determined sizes and geometric visible albedos derived from thermal-infrared observations of three binary NEAs: (5381) Sekhmet, (153591) 2001 SN₂₆₃, and (164121) 2003 YT₁. The diameters of these asteroids are 1.41 ± 0.21 km, 1.56 ± 0.31 km, and 2.63 ± 0.40 km, respectively. Their albedos are 0.23 ± 0.13, 0.24 ± 0.16, and 0.048 ± 0.015, respectively.     

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.     

Figure of the double Asteroid 90 Antiope from adaptive optics and lightcurve observations

A long-term adaptive optics (AO) campaign of observing the double Asteroid (90) Antiope has been carried out in 2003-2005 using 8-10-m class telescopes, allowing prediction of the circumstances of mutual events occurring during the July 2005 opposition [Marchis, F., Descamps, P., Hestroffer, D., Berthier, J., de Pater, I., 2004. Bull. Am. Astron. Soc. 36, 1180]. This is the first opportunity to use complementary lightcurve and AO observations to extensively study the (90) Antiope system, an interesting visualized binary doublet system located in the main belt. The orbital parameters derived from the AO observations have served as input quantities for the derivation of a whole set of other physical parameters (namely shapes, surface scattering, bulk density, and internal properties) from analysis of collected lightcurves. To completely model the observed lightcurves, we employed Roche figures to construct an overall shape solution. The combination of these complementary observations has enabled us to derive a reliable physical and orbital solution for the system. Our model is consistent with a system of slightly non-spherical components, having a size ratio of 0.95 (with Ravg=42.9±0.5 km, separation=171±1 km), and exhibiting equilibrium figures for homogeneous rotating bodies. A comparison with grazing occultation event lightcurves suggests that the real shapes of the components do not depart from Roche equilibrium figures by more than 10%. The J2000 ecliptic coordinates of the pole of the system are λn=200°±2° and αn=38°±2°. The orbital period was refined to P=16.5051±0.0001 h, and the density is found to be slightly lower than previous determinations, with a value of 1.25±0.05 g/cm³, leading to a significant macro-porosity of 30%. Possible scenarios for the origin of the system are also discussed.     

Rotationally-resolved spectroscopy of Vesta I: 2-4 μm region

We present new infrared (2-4 μm) spectroscopic observations of Vesta obtained in 2001, 2003, and 2004. Together with previously published work, these present a picture of how Vesta's spectrum changes with sub-Earth latitude and longitude. Vesta's albedo and 2-μm band vary regularly with its rotational phase. While establishing the continuum level for Vesta in the 3-μm region is not straightforward, Vesta appears to have a spectrum consistent with the HED meteorites and not requiring a 3-μm water of hydration band. We cannot formally rule out a shallow (∼1%) band, however. We place limits on the extent to which solar-wind implantation and contamination by CM-like impactors has changed the surface spectrum of Vesta.     

Thermal emission spectroscopy (5.2-38 μm) of three Trojan asteroids with the Spitzer Space Telescope: Detection of fine-grained silicates

We present thermal emission spectra (5.2-38 μm) of the Trojan asteroids 624 Hektor, 911 Agamemnon, and 1172 Aneas. The observations used the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. Emissivity spectra are created by dividing the measured Spectral Energy Distribution (SED) by a model of the thermal continuum. We employ the Standard Thermal Model (STM), allowing physical parameters (e.g., radius and albedo) to vary in order to find the best thermal continuum fit to the SED. The best-fit effective radius (R) and visible geometric albedo (pv) for Hektor (R=110.0±7.3, ) and Aneas (R=69.1±5.1, ) agree very well with previous estimates, and for Agamemnon (R=71.5±5.2, ) we find slightly a smaller size and higher albedo than previously derived. Other thermal models (e.g., thermophysical) result in estimates of R and pv that vary a few percent from the STM, but the resulting emissivity spectra are identical. The emissivity spectra of all three asteroids display an emissivity plateau near 10-μm and another broader rise from ∼18 to 28 μm. We interpret these as indications of fine-grained silicates on the surfaces of these asteroids. The emissivity spectra more closely resemble emission spectra from cometary comae than those from solid surfaces and measured in the laboratory for powdered meteorites and regolith analogs. We hypothesize that the coma-like emission from the solid surfaces of trojans may be due to small silicate grains being imbedded in a relatively transparent matrix, or to a very under-dense (fairy-castle) surface structure. These hypotheses need to be tested by further laboratory and theoretical scattering work as well as continued thermal emission observations of asteroids.     

Young Asteroid 832 Karin shows no rotational spectral variations

We have made near-IR spectral observations of the very young (5.75 Myr) S-type asteroid 832 Karin, well sampled in rotational phase over its 18.35-h period. We find no significant variations in its reflectance spectrum. Karin, the brightest member of the Karin cluster (a sub-family of the larger, older Koronis dynamical family), was shown to be exceptionally young by Nesvorný et al. [Nesvorný, D., Bottke, W.F., Dones, L., Levison, H., 2002. Nature 417, 720-722], using backward numerical integration of orbital elements of cluster members. Their precise dating of the collisional breakup gives us an opportunity, for the first time and without age-dating of physical samples, to monitor time-evolution of processes, like space weathering, that operate on timescales of ∼1-10 Myr. Sasaki et al. [Sasaki, T., Sasaki, S., Watanabe, J., Sekiguchi, T., Yoshida, F., Kawakita, H., Fuse, T., Takato, N., Dermawan, B., Ito, T., 2004. Astrophys. J. 615, L161-L164; Sasaki, T., Sasaki, S., Watanabe, J., Sekiguchi, T., Yoshida, F., Ito., T., Kawakita, H., Fuse, T., Takato, N., Dermawan, B., 2005. Lunar Planet. Sci. XXXVI. Abstract #1590] had made similar measurements of Karin, although more sparsely sampled than ours, and claimed dramatically different colors as a function of rotational phase. Sasaki et al. interpreted their data to be showing the reddish, space-weathered exterior surface of the precursor asteroid, as well as an interior face, which had not had time to become space-weathered. On five nights over 2006 January 7-14 UT, we observed Karin with the SpeX (0.8-2.5 μm) spectrometer of the IRTF. We analyze data in 30° intervals of rotational longitude, some of which we sampled on two different nights. The spectra are consistent with little or no spectral variation as the asteroid rotates; certainly there are no changes as large as previously reported. The previous observations were probably spurious. Our average spectrum resembles the “blue” spectrum of Sasaki et al., which they interpreted to be the “fresh” surface. Karin is not quite as red as typical S-types, yet has rather shallow absorption bands. We surmise that the space-weathering process affecting Karin has had time to reduce spectral contrast, but has not operated long enough to redden its spectrum—an intermediate case of space weathering, which has gone to completion for most main-belt asteroids. This work sets an important constraint on the timescale for the ubiquitous space-weathering process affecting S-types, namely that its effects are evident, but not yet complete, at ∼6 Myr.     

A peculiar family of Jupiter Trojans: The Eurybates

The Eurybates family is a compact core inside the Menelaus clan, located in the L₄ swarm of Jupiter Trojans. Fornasier et al. (Fornasier, S., Dotto, E., Hainaut, O., Marzari, F., Boehnhardt, H., De Luise, F., Barucci, M.A. [2007]. Icarus 190, 622-642) found that this family exhibits a peculiar abundance of spectrally flat objects, similar to Chiron-like Centaurs and C-type main belt asteroids. On the basis of the visible spectra available in literature, Eurybates family’s members seemed to be good candidates for having on their surfaces water/water ice or aqueous altered materials.To improve our knowledge of the surface composition of this peculiar family, we carried out an observational campaign at the Telescopio Nazionale Galileo (TNG), obtaining near-infrared spectra of 7 members. Our data show a surprisingly absence of any spectral feature referable to the presence of water, ices or aqueous altered materials on the surface of the observed objects. Models of the surface composition are attempted, evidencing that amorphous carbon seems to dominate the surface composition of the observed bodies and some amount of silicates (olivine) could be present.     

Near-infrared spectroscopy of primitive asteroid families

We compare 13 near-infrared (0.8-2.4 μm) spectra of two low albedo C complex outer-belt asteroid families: Themis and Veritas. The disruption ages of these two families lie at opposite extremes: 2.5 ± 1.0 Gyr and 8.7 ± 1.7 Myr, respectively. We found striking differences between the two families, which show a range of spectral shapes and slopes. The seven Themis family members (older surfaces) have “red” (positive) slopes in the 1.6-2.4 μm region; in contrast, the six Veritas members (younger surfaces) have significantly “flatter” slopes at these same wavelengths. Moreover, the two families are characterized by different concavity at shorter (1.0-1.5 μm) wavelengths with the Themis group being consistently flat or concave up (smile) and the Veritas group being consistently concave down (frown). Each family contains a broad range of diameters, suggesting our results are not due to comparisons of asteroids of different sizes. The statistically significant clustering of the two spectral groups could be explained by one of the following three possibilities or a combination of them: (1) space weathering effects, (2) differences in original composition, or (3) differences in thermal history perhaps as a result of the difference in parent body sizes. As a result of our analyses, we propose a new method to quantify broad and shallow structures in the spectra of primitive asteroids. We found reasonable matches between the observed asteroids and individual carbonaceous chondrite meteorites. Because these meteoritic fits represent fresh surfaces, space weathering is neither necessary nor ruled out as an explanation of spectral differences between families. The six Veritas family near-infrared (NIR) spectra represent the first NIR analysis of this family, thus significantly increasing our understanding of this family over these wavelengths.     

Thermal inertia of main belt asteroids smaller than 100 km from IRAS data

Recent works have shown that the thermal inertia of km-sized near-Earth asteroids (NEAs) is more than 2 orders of magnitude higher than that of main belt asteroids (MBAs) with sizes (diameters) between 200 and 1000 km. This confirms the idea that large MBAs, over hundreds millions of years, have developed a fine and thick thermally insulating regolith layer, responsible for the low values of their thermal inertia, whereas km-sized asteroids, having collisional lifetimes of only some millions years, have less regolith, and consequently a larger surface thermal inertia.Because it is believed that regolith on asteroids forms as a result of impact processes, a better knowledge of asteroid thermal inertia and its correlation with size, taxonomic type, and density can be used as an important constraint for modeling of impact processes on asteroids. However, our knowledge of asteroids’ thermal inertia values is still based on few data points with NEAs covering the size range 0.1-20 km and MBAs that .Here, we use IRAS infrared measurements to estimate the thermal inertia values of MBAs with diameters and known shapes and spin vector, filling an important size gap between the largest MBAs and the km-sized NEAs. An update to the inverse correlation between thermal inertia and diameter is presented. For some asteroids thermophysical modeling allowed us to discriminate between the two still possible spin vector solutions derived from optical lightcurve inversion. This is important for (720) Bohlinia: our preferred solution was predicted to be the correct one by Vokrouhlický et al. [2003. The vector alignments of asteroid spins by thermal torques. Nature 425, 147-151] just on theoretical grounds.     

Thermal inertia of near-Earth asteroids and implications for the magnitude of the Yarkovsky effect

Thermal inertia determines the temperature distribution over the surface of an asteroid and therefore governs the magnitude the Yarkovsky effect. The latter causes gradual drifting of the orbits of km-sized asteroids and plays an important role in the delivery of near-Earth asteroids (NEAs) from the main belt and in the dynamical spreading of asteroid families. At present, very little is known about the thermal inertia of asteroids in the km size range. Here we show that the average thermal inertia of a sample of NEAs in the km-size range is . Furthermore, we identify a trend of increasing thermal inertia with decreasing asteroid diameter, D. This indicates that the dependence of the drift rate of the orbital semimajor axis on the size of asteroids due to the Yarkovsky effect is a more complex function than the generally adopted D⁻¹ dependence, and that the size distribution of objects injected by Yarkovsky-driven orbital mobility into the NEA source regions is less skewed to smaller sizes than generally assumed. We discuss how this fact may help to explain the small difference in the slope of the size distribution of km-sized NEAs and main-belt asteroids.     

Thermal infrared (8-13 μm) spectra of 29 asteroids: the Cornell Mid-Infrared Asteroid Spectroscopy (MIDAS) Survey

We report the results of the Cornell Mid-IR Asteroid Spectroscopy (MIDAS) survey, a program of ground-based observations designed to characterize the 8-13 μm spectral properties of a statistically significant sample of asteroids from a wide variety of visible to near-IR spectral classes. MIDAS is conducted at Palomar Observatory using the Spectrocam-10 (SC-10) spectrograph on the 200-in Hale telescope. We have measured the mid-infrared spectra of twenty-nine asteroids and have derived temperature estimates from our data that are largely consistent with the predictions of the standard thermal model. We have also generated relative emissivity spectra for the target asteroids. On only one asteroid, 1 Ceres, have we found emissivity features with spectral contrast greater than 5%. Our spectrum of 4 Vesta suggests emissivity variation at the 2-3% level. Published spectra of several of the small number of asteroids observed with ISO (six of which are also included in our survey), which appeared to exhibit much stronger emissivity features, are difficult to reconcile with our measurements. Laboratory work on mineral and meteorite samples has shown that the contrast of mid-IR spectral features is greatly reduced at fine grain sizes. Moreover, the NEAR mission found that 433 Eros is covered by a relatively thick fine-grained regolith. If small bodies in general possess such regoliths, their mid-IR spectral features may be quite subtle. This may explain the evident absence of strong emissivity variation in the majority of the MIDAS spectra.     

Equilibrium figures of inhomogeneous synchronous binary asteroids

The present paper deals with the application of the classical theory of equilibrium figures of two rotating liquid masses to the case where bodies exhibit a radially stratified internal density distribution so that they can be considered as inhomogeneous bodies. The derived ellipsoidal shape solutions are applied to five real systems of equal-sized synchronous asteroids. Furthermore, internal inhomogeneity puts strong constraints on the surface grain density. A satisfactory model fit is achieved with internal densities of asteroids steadily increasing outwards. In particular, from such an approach we derived grain densities of the considered systems in agreement with their mineralogical composition inferred from reflectance spectroscopy. According to this new approach, 4492 Debussy, presently of unknown spectral type, is predicted to appear as a C-type object with a grain density on the order of 2 g/cm³.