Near-Earth asteroid (275677) 2000 RS11 was observed over 5 d in March 2014 with both the Arecibo (2380 MHz, 12.6 cm) and Goldstone (8560 MHz, 3.5 cm) planetary radar systems. The continuous-wave spectra and delay-Doppler images collected revealed a sub-km-sized object with a strongly bifurcated shape. We used these radar observations, in combination with seven optical light curves collected in 2014 and one light curve from 2023, to create a comprehensive shape and spin-state model for RS11. We find a rotation period of P = (4 . 445 +/- 0 . 001) h around a pole of lambda = (225 +/- 80) degrees and beta = (-80 +/- 9) degrees relative to the plane of the ecliptic. The shape of RS11 is unusual in that it does not resemble many of the other near-Earth asteroids modelled with ground-based radar. Whilst RS11 consists of a largely spherical, smaller lobe attached to an elongated, larger lobe via a narrow neck, the smaller lobe is not aligned with the long axis of the larger lobe, but is closer to the larger lobe's shortest principal axis. In combination with a large concavity observed on the outer face of the larger lobe, this may point to an unusual formation or event in the object's past. We estimate that RS11 has a geometric albedo of pv = (0 . 16 +/- 0 . 06) and a radar albedo 0 . 08 < r7OC < 0 . 16 . Analysis of its gravitational environment reveals that for standard S-type asteroid densities, we would not expect rotational instability and it is possible for RS11 to be a low tensile strength rubble-pile asteroid.

The release of volatiles from comets is usually from direct sublimation of ices on the nucleus, but for very or hyperactive comets other sources have to be considered to account for the total production rates. In this work, we present new near-IR (NIR) imaging and spectroscopic observations of 46P/Wirtanen taken during its close approach to Earth on 2018 December 19 with the MMIRS instrument at the MMT Observatory to search for signatures of icy or ice-rich grains in its inner coma that might explain its previously reported excess water production. The morphology of the images does not suggest any change in grain properties within the field of view, and the NIR spectra do not show the characteristic absorption features of water ice. Using a new Markov Chain Monte Carlo–based implementation of the spectral modeling approach of Protopapa et al., we estimate the areal water ice fraction of the coma to be <0.6%. When combined with slit-corrected Af ρ values for the J , H , and K bands and previously measured dust velocities for this comet, we estimate an icy grain production rate of less than 4.6 kg s −1 . This places a strict constraint on the water production rate from pure icy grains in the coma, and in turn we find that for the 2018–2019 apparition approximately 64% of 46P’s surface was actively sublimating water near perihelion. We then discuss 46P’s modern properties within the context of other (formerly) hyperactive comets to understand how these complex objects evolve.

An observational study of Koronis family members' spin properties was undertaken with two primary objectives: to reduce selection biases for object rotation period and lightcurve amplitude in the sample of members' known spin vectors, and to better constrain future modeling of spin properties evolution. Here we report rotation lightcurves of nineteen Koronis family members, and derived results that increase the sample of determined spin vectors in the Koronis family to include 34 of the largest 36 family members, completing it to H approximate to 11.3 (D similar to 16 km) for the largest 32 members. The program observations were made during a total of 72 apparitions between 2005-2021, and are reported here along with several earlier unpublished lightcurves. All of the reported data were analyzed together with previously published lightcurves to determine the objects' sidereal rotation periods, spin vector orientations, and convex model shape solutions. The derived distributions of retrograde rotation rates and pole obliquities appear to be qualitatively consistent with outcomes of modification by thermal YORP torques. The distribution of spin rates for the prograde rotators remains narrower than that for the retrograde rotators; in particular, the absence of prograde rotators having periods longer than about 20 h is real, while among the retrograde rotators are several objects having longer periods up to about 65 h. None of the prograde objects newly added to the sample appear to be trapped in an s6 spin-orbit resonance that is characteristic of most of the largest prograde objects (Vokrouhlicky et al., 2003); these smaller objects either could have been trapped previously and have already evolved out, or have experienced spin evolution tracks that did not include the resonance.

We present the first measurements of asteroids in millimeter wavelength data from the South Pole Telescope (SPT), which is used primarily to study the cosmic microwave background (CMB). We analyze maps of two ∼270 deg 2 sky regions near the ecliptic plane, each observed with the SPTpol camera ∼100 times over 1 month. We subtract the mean of all maps of a given field, removing static sky signal, and then average the mean-subtracted maps at known asteroid locations. We detect three asteroids—(324) Bamberga, (13) Egeria, and (22) Kalliope—with signal-to-noise ratios (S/N) of 11.2, 10.4, and 6.1, respectively, at 2.0 mm (150 GHz); we also detect (324) Bamberga with an S/N of 4.1 at 3.2 mm (95 GHz). We place constraints on these asteroids’ effective emissivities, brightness temperatures, and light-curve modulation amplitude. Our flux density measurements of (324) Bamberga and (13) Egeria roughly agree with predictions, while our measurements of (22) Kalliope suggest lower flux, corresponding to effective emissivities of 0.64 ± 0.11 at 2.0 and < 0.47 at 3.2 mm. We predict the asteroids detectable in other SPT data sets and find good agreement with detections of (772) Tanete and (1093) Freda in recent data from the SPT-3G camera, which has ∼10× the mapping speed of SPTpol. This work is the first focused analysis of asteroids in data from CMB surveys, and it demonstrates we can repurpose historic and future data sets for asteroid studies. Future SPT measurements can help constrain the distribution of surface properties over a larger asteroid population.

There was an unprecedented opportunity to study the inner dust coma environments, where the dust and gas are not entirely decoupled, of comets 45P/Honda–Mrkos–Pajdus̆áková (45P/HMP) from 2016 December 26 to 2017 March 15, and 46P/Wirtanen from 2018 November 10 to 2019 February 13, both in visible wavelengths. The radial profile slopes of these comets were measured in the R and HB-BC filters most representative of dust, and deviations from a radially expanding coma were identified as significant. The azimuthally averaged radial profile slope of comet 45P/HMP gradually changes from −1.81 ± 0.20 at 5.24 days preperihelion to −0.35 ± 0.16 at 74.41 days postperihelion. Contrastingly, the radial profile slope of 46P/Wirtanen stays fairly constant over the observed time period at −1.05 ± 0.05. Additionally, we find that the radial profile of 46P/Wirtanen is azimuthally dependent on the sky-plane-projected solar position angle, while that of 45P/HMP is not. These results suggest that comets 45P/HMP and 46P/Wirtanen have vastly different coma dust environments and that their dust expansion properties are distinct. As evident from these two comets, well-resolved inner comae are vital for detailed characterization of dust environments.

Studying materials released from Jupiter-family comets (JFCs)—as seen in their inner comae, the envelope of gas and dust that forms as the comet approaches the Sun—improves the understanding of their origin and evolutionary history. As part of a coordinated, multiwavelength observing campaign, we observed comet 45P/Honda–Mrkos–Pajdušáková during its close approach to Earth in 2017 February. Narrowband observations were taken using the Bok 90″ telescope at Kitt Peak National Observatory on February 16 and 17 UT, revealing gas and dust structures. We observed different jet directions for different volatile species, implying source region heterogeneity, consistent with other ground-based and in situ observations of other comet nuclei. A repeating feature visible in CN and C2 images on February 16 was also observed on February 17 with an interval of 7.6 ± 0.1 hr, consistent with the rotation period of the comet derived from Arecibo Observatory radar observations. The repeating feature’s projected gas velocity away from the nucleus is 0.8 km s−1, with an outflow velocity of 0.5 km s−1. A bright compact spot adjacent to the nucleus provides a lower limit of the amount of material released in one cycle of ∼9.2 kg, depending on composition—a quantity small enough to be produced by repeated exposure of nucleus ices to sunlight. This repeating CN jet, forming within 400 km of the nucleus, may be typical of inner-coma behavior in JFCs; however, similar features could be obscured by other processes and daughter product species when viewed from distances further than the scale length of CN molecules.

Although numerous white dwarf stars host dusty debris disks, the temperature distribution of these stars differs significantly from the white dwarf population as a whole. Dusty debris disks exist exclusively around white dwarfs cooler than 27,000 K. This is all the more enigmatic given that the formation processes of dusty debris disks should favor younger, hotter white dwarfs, which likely host more dynamically unstable planetary systems. Here we apply a sophisticated material sublimation model to white dwarf systems to show that these statistics are actually a natural result of the interplay of thermal and tidal forces, and show how they define the circumstellar regions where dusty debris disks can form. We demonstrate that these processes tend to prevent stability against both sublimative destruction and reaccretion into planetesimals for rocky materials until white dwarfs cool to below ~25,000-32,000 K, in agreement with the observed limit of ~27,000 K. For pure water ice, this critical temperature is less than 2,700 K (requiring a cooling age older the universe); this precludes pure water ice-rich debris disks forming through the accepted two-step mechanism. The critical temperature is size-dependent; more massive white dwarfs could potentially host dusty debris disks at warmer temperatures.. Our model suggests that the location of the disks within the PG 0010+280, GD 56, GD 362, and PG 1541+651 systems are consistent with a forsterite-dominated olivine composition. We also find that very cool white dwarfs may simultaneously host multiple, independently formed dusty debris disks, consistent with observations of the LSPM J0207+3331 system.

Ground-based planetary radar systems are a crucial tool for post-discovery high-precision astrometry and physical characterization of near-Earth asteroids and comets.Radar ranging and imaging capabilities exceed those of any other ground-based instruments.As described in more detail in the Decadal white paper (henceforth WP) Mainzer et al. and WP Taylor et al., detection and characterization of potentially hazardous asteroids was mandated by the US Congress through George E. Brown, Jr. Near-Earth Object Survey Act in 2005.While optical telescopes survey for new asteroid detections, post-discovery characterization using planetary radar can secure the heliocentric orbit information, extending the interval of reliable orbit estimation by decades to centuries for most objects and preventing recently-discovered objects from being lost.Radar observations also provide constraints on the object's size, rotation state, morphology, satellites, and near-surface geology as well as regolith properties, making groundbased radar systems a powerful, low-cost complement to spacecraft destined for specific targets.Furthermore, they can be used for characterizing the nuclei of comets, which are typically obscured by the dust coma at optical wavelengths, and probe the decimeter-scale coma particle abundance, which is relevant especially for the study of disintegrating comets.These characteristics are invaluable information for understanding the formation and evolution of asteroids and comets, which are the building blocks of our Solar System, for evaluating impact risk and developing impact mitigation technologies, and for ensuring safe spacecraft encounters.In the next decade, in addition to continuing as a state-of-the-art astrometric tool for planetary defense, we expect ground-based radar observations to expand our knowledge of the physical characteristics of the near-Earth asteroid population in terms of their shape, binarity, and near-surface structure.We recommend that facilities with ground-based radar systems, planetary radar programs, and research and analysis of radar observations receive full support from NASA and NSF to ensure their availability to the planetary science community and as a resource for planetary defense initiatives through the next decade.

We present observations of five stellar occultations for (11351) Leucus and reports from two efforts on (21900) Orus. Both objects are prime mission candidate targets for the Lucy Discovery mission. Combined results for Leucus indicate a very dark surface with p V = 0.037 ± 0.001, which is derived from the average of the multichord occultations. Our estimate of the triaxial ellipsoidal shape is for axial diameters of 63.8 × 36.6 × 29.6 km assuming that the spin pole is normal to the line of sight. The actual shape of the object is only roughly elliptical in profile at each epoch. Significant topography is seen with horizontal scales up to 30 km and vertical scales up to 5 km. The most significant feature is a large depression on the southern end of the object as seen from a terrestrial viewpoint. For this work we developed a method to correct for differential refraction, accounting for the difference in color between the target object and the reference stars for astrometry derived from ground-based images.

The composition of comets in the solar system comes in multiple groups thought to encode information about their formation in different regions of the outer protosolar disk. The recent discovery of the second interstellar object, 2I/Borisov, allows for spectroscopic investigations into its gas content and a preliminary classification of it within the solar system comet taxonomies to test the applicability of planetesimal formation models to other stellar systems. We present spectroscopic and imaging observations from 2019 September 20 through October 26 from the Bok, MMT telescope (formerly the Multiple Mirror Telescope, Mount Hopkins, Arizona), and Large Binocular Telescopes. We identify CN in the comet's spectrum and set precise upper limits on the abundance of C(2)on all dates in October. We use a Haser model to convert our integrated fluxes to production rates and findQ(CN) = (1.1-1.9) * 10(24)mols s(-1)increasing over 2019 October 1 to 26, consistent with contemporaneous observations. We set our lowest upper limit on a C(2)production rate,Q(C-2) < 1.6 * 10(23)mols s(-1)on 2019 October 10. The measured upper limit ratio for that dateQ(C-2)/Q(CN) < 0.1 indicates that 2I/Borisov is strongly in the (carbon-chain) "depleted" taxonomic group if there is any C(2)production at all. Most "depleted" comets are Jupiter-family comets (JFCs), perhaps indicating a similarity in formation conditions between the most depleted of the JFCs and 2I/Borisov. More work is needed to understand the applicability of our knowledge of solar system comet taxonomies onto interstellar objects and we discuss future work that could help to clarify the usefulness of the approach.

Carbonaceous chondrite meteorites are some of the oldest Solar System planetary materials available for study. The CI group has bulk abundances of elements similar to those of the solar photosphere. Of particular interest in carbonaceous chondrite compositions are labile elements, which vaporize and mobilize efficiently during post-accretionary parent-body heating events. Thus, they can record low-temperature alteration events throughout asteroid evolution. However, the precise nature of labile-element mobilization in planetary materials is unknown. Here we characterize the thermally induced movements of the labile elements S, As, Se, Te, Cd, Sb, and Hg in carbonaceous chondrites by conducting experimental simulations of volatile-element mobilization during thermal metamorphism. This process results in appreciable loss of some elements at temperatures as low as 500 K. This work builds on previous laboratory heating experiments on primitive meteorites and shows the sensitivity of chondrite compositions to excursions in temperature. Elements such as S and Hg have the most active response to temperature across different meteorite groups. Labile element mobilization in primitive meteorites is essential for quantifying elemental fractionation that occurred on asteroids early in Solar System history. This work is relevant to maintaining a pristine sample from asteroid (101955) Bennu from the OSIRIS-REx mission and constraining the past orbital history of Bennu. Additionally, we discuss thermal effects on surface processes of near-Earth asteroids, including the thermal history of "rock comets" such as (3200) Phaethon. This work is also critical for constraining the concentrations of contaminants in vaporized water extracted from asteroid regolith as part of future in situ resource utilization for sustained robotic and human space exploration.

Impacts due to near-Earth objects (NEOs) are responsible for causing some of the great mass extinctions on Earth. While nearly all NEOs of diameter > 1 km, capable of causing a global climatic disaster, have been discovered and have negligible chance of impacting in the near future, we are far from completion in our effort to detect and characterize smaller objects. In an effort to test our preparedness to respond to a potential NEO impact threat, we conducted a community-led global planetary defense exercise with support from the NASA Planetary Defense Coordination Office. The target of our exercise was 2012 TC4, the similar to 10 m diameter asteroid that made a close pass by the Earth on 2017 October 12 at a distance of about 50,000 km. The goal of the TC4 observing campaign was to recover, track, and characterize 2012 TC4 as a hypothetical impactor in order to exercise the global planetary defense system involving observations, modeling, prediction, and communication. We made three attempts with the Very Large Telescope (VLT) on 2017 July 27, 31 and on 2017 August 5 and recovered 2012 TC4 within its ephemeris uncertainty at 2.2 arcmin from the nominal prediction. At visual magnitude V = 27, the recovery of 2012 TC4 is the faintest NEA detection thus far. If an impact during the 2017 close approach had been possible based on the 2012 astrometric data, these recovery observations would have been sufficient to confirm or rule out the impact. The first automatic detection by a survey (Pan-STARRS1) was on September 25, which is the earliest that 2012 TC4 would have been discovered in survey mode, if it had not been discovered in 2012. We characterized 2012 TC4 using photometry, spectroscopy and radar techniques. Based on photometric observations, we determined a rotation period of 12.2 min with an amplitude of 0.9 magnitudes. An additional lower amplitude period was detected, indicating that 2012 TC4 was in a state of non-principal axis rotation. The combined visible and near-infrared spectrum puts it in the taxonomic X-class. Radar images at 1.875 m resolution placed only a few range pixels on the asteroid, reveal an angular, asymmetric, and elongated shape, and establish that 2012 TC4 is less than 20 m on its long axis. We estimate a circular polarization ratio of 0.57 + -0.08 that is relatively high among NEAs observed to date by radar. We also performed a probabilistic impact risk assessment exercise for hypothetical impactors based on the 2012 TC4 observing campaign. This exercise was performed as part of ongoing efforts to advance effective impact risk models and assessment processes for planetary defense. The 2012 TC4 close approach provided a valuable opportunity to test the application of these methods using realistically evolving observational data to define the modeling inputs. To this end, risk assessments were calculated at several epochs before and during the close approach, incorporating new information about 2012 TC4 as it became available. Two size ranges were assessed-one smaller size range (H = 26.7) similar to the actual 2012 TC4, and one larger size range (H = 21.9) to produce a greater-damage scenario for risk assessment. Across the epochs, we found that only irons caused significant damage for smaller size. For the larger size case, however, hydrous stones caused the greatest damage, anhydrous stones caused the least damage, and irons caused moderate damage. We note that the extent of damage depends on composition in different size regimes and, after astrometry, size is the most important physical property to determine for an incoming object.

Linking meteorites to source regions in the main asteroid belt is important for understanding the conditions under which their parent bodies formed. Ordinary chondrites are the most abundant class of meteorites on Earth, totaling 86% of all collected samples. Some S-type asteroids/families have been proposed as sources for the three different (H, L, and LL) types of ordinary chondrites with Hebe, Agnia, Merxia, and Koronis families being the source for H chondrites, Gefion for H/L chondrites, and Flora family for LL chondrites. However, the composition and meteorite affinity of several large S-type main belt asteroids remains unconstrained leaving the possibility of additional source regions for ordinary chondrite meteorites. Here we investigate the surface composition of three large S-type asteroids, (3) Juno, (7) Iris, and (25) Phocaea, using their near-infrared spectra (0.7–2.55 μm) to identify the parent body of the H chondrites. We use a Bayesian inference model to confirm the meteorite analogs of the three asteroids. Based on our Bayes classifier we find the following analogs and probabilities: Juno is likely H chondrite (89%), Iris is likely LL chondrite (97.5%), and Phocaea is likely H chondrite (98.6%). While Phocaea has the highest probability of being an H chondrite, it is dynamically unlikely to deliver material to near-Earth space. While Juno has spectral properties similar to H chondrites, its family is unlikely to produce sizeable H-chondrite-type near-Earth objects (NEOs). If Juno is the primary source of H chondrite meteorites, it suggests that an additional source is needed to explain the H-chondrite-type NEOs.

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 similar to 10 km. We find Kleopatra to be a bi-lobate contact binary of overall dimensions 276 x 94 x 78 km +/- 15% and equivalent diameter D-eq = 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 (lambda, beta) = (74 degrees + 20 degrees) 5 degrees. 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 degrees at the similar to 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. (C) 2018 Elsevier Inc. All rights reserved.

Introduction: In the three years between 2016 and 2018 three small Jupiter-family comets (JFCs)—45P/ Honda-Mrkos-Pajdušáková (HMP), 41P/TuttleGiacobini-Kresák (TGK), and 46P/Wirtanen—will pass within 0.2 au of Earth, affording a unique opportunity to study the inner coma of these objects at unprescedented resolution. After the 2018 apparition of 46P we expect no similar opportunity to observe JFCs at close range for several decades. Studying the inner comae of these JFCs reveals cometary surface activity patterns, compositional uniformity, and the rotation states (whether rotating uniformly or tumbling). Ground-based telescopic measurements provide information about gas physics processes and interaction between the solid, central nucleus surface and the inner coma. We will present the results of a coordinated multiwavelength observational campaign focused on the inner coma of comet HMP with a 0.08 au in February 2017, close enough to observe structures in the near nucleus region before they merge with the larger coma. From these high resolution observations short lived molecular species can be detected [1] and, from radial distribution models, used to constrain the photochemical evolution of short lived or multiple source molecular species [2]. The observation campaign includes data from a global network of amateur observers, as well as a course at the University of Arizona on handson observational comet astronomy. Our observational campaign includes temporal monitoring of comet volatiles using narrow band imaging (including CN, C2, C3, NH) [3], as well as radio measurements of OH production [4] and planetary radar measurements of shape, rotation, and dust distrubtion [5]. Narrowband temporal monitoring data will be obtained over the course of 60+ nights of observations from telescope facilities of Steward Observatory at the University of Arizona as well as the Vatican Advanced Technology Telescope. Radar and radio measurements will be obtained from the Green Bank Radio Telescope and Arecibo Observatory. In addition to presenting narrowband imaging results from HMP and preliminary OH and radar results from this comet, we will present preliminary narrowband imaging of comet TGK. References: [1] Samarasinha, N. H. et al. (2011) ApJ Lett, 734, L3. [2] Harris, W. M. et al. (1997) Science, 277, 676-681. [3] Samarasinha N., et. al. (2015) Plan. Space Sci., 118, 127. [4] Schloerb F. P., De Vries C. H., Lovell A. J., Irvine W. M., Senay M., and Wooten H. A. (1999) Earth, Moon, and Planets, 78, 45-51. [5] Harmon J. K. et al. (2004) Comets II, 265279. [6] Howell et al. (2014) AAS/DPS meeting 46, abstract #209.24. [7] Lovell A. J. and Howell E. S. (2009) AAS/DPS meeting 41, abstract #15.06. Acknowledgements: This program is supported by NASA Solar System Observations Program grant NNX16AG70G, Steward Observatory at the University of Arizona, and the Vatican Observatory.

We present our shape model of Amor class near-Earth asteroid (1627) Ivar. During Ivar's apparition in 2013, in which it had a minimum distance of 0.32 AU, we obtained both CCD photometry and radar observations. The radar data consist of echo power spectra and delay-Doppler imaging with 300 m resolution and were obtained using the Arecibo Observatory's 2380 MHz radar. Lightcurve data were gathered using the 0.35 m telescope at the Palmer Divide Station. Using these data, we constructed a detailed shape model of Ivar that reveals more surface detail than earlier models. Ivar was found to have a rotational period of 4.7951689 +/- 0.0000026 hours with a pole located within 6 of ecliptic longitude and latitude 336 degrees and 37 degrees respectively. Ivar is an elongated asteroid with maximum extensions along the three body fixed coordinates being 15.15 x 6.25 x 5.66 km +/- 10%. The results of surface slope analysis suggest that Ivar is covered with a loose regolith. Ivar appears to reside in, or near, an optimum state with respect to its shape, spin, and bulk density, such that dynamic topography, surface slopes, and erosion rates on the body are near minimum levels and is therefore dynamically stable. (C) 2016 Published by Elsevier Inc.

P. Abell, D. Mazanek, D. Reeves, P. Chodas, M. Gates, L. Johnson, and R. Ticker 1 Astromaterials Research and Exploration Science Division, NASA Johnson Space Center, Houston, TX 2 Systems Analysis and Concepts Directorate, NASA Langley Research Center, Hampton, VA 3 Center for Near-Earth Object Studies, Jet Propulsion Laboratory, Pasadena, CA 4 Human Exploration and Operations Mission Directorate, NASA Headquarters, Washington, DC 5 Planetary Defense Coordination O ce, NASA Headquarters, Washington DC

Using the S-band radar at Arecibo Observatory, we observed thirteen X/M-class asteroids; nine were previously undetected and four were re-observed, bringing the total number of Tholen X/M-class asteroids observed with radar to 29. Of these 29M-class asteroids, 13 are also W-class, defined as M-class objects that also display a 3-mu m absorption feature which is often interpreted as the signature of hydrated minerals (Jones, T.D., Lebofsky, LA., Lewis, J.S., Marley, M.S. [1990]. Icarus 88, 172-192; Rivkin, A.S., Howell, E.S., Britt, D.T., Lebofsky, LA., Nolan, M.C., Branston, D.D. [1995]. Icarus 117,90-100; Rivkin, A.S., Howell, E.S., Lebofsky, LA., Clark, B.E., Britt, D.T. [2000]. Icarus 145, 351-368).Consistent with our previous work (Shepard, M.K. et al. [2008]. Icarus 195, 184-205; Shepard, M.K., Harris, A.W., Taylor, P.A., Clark, B.E., Ockert-Bell, M., Nolan, M.C., Howell, E.S., Magri, C., Giorgini, J.D., Benner, L.A.M. [2011]. Icarus 215, 547-551), we find that 38% of our sample (11 of 29) have radar albedos consistent with metal-dominated compositions. With the exception of 83 Beatrix and 572 Rebekka, the remaining objects have radar albedos significantly higher than the mean S-or C-class asteroid (Magri, C., Nolan, M.C., Ostro, Sj., Giorgini, J.D. [2007]. Icarus 186, 126-151).Seven of the eleven high-radar-albedo asteroids, or 64%, also display a 3-mu m absorption feature (Wclass) which is thought to be inconsistent with the formation of a metal dominated asteroid. We suggest that the hydration absorption could be a secondary feature caused by low-velocity collisions with hydrated asteroids, such as Cl or CM analogs, and subsequent implantation of the hydrated minerals into the upper regolith. There is recent evidence for this process on Vesta (Reddy, V. et al. [2012]. Icarus 221, 544-559; McCord, T.B. et al. [2012]. Nature 491, 83-86; Prettyman, T.H. et al. [2012]. Science 338, 242246; Denevi, B.W. et al. [2012]. Science 338, 246-249).Eleven members of our sample show bifurcated radar echoes at some rotation phases; eight of these are high radar albedo targets. One interpretation of a bifurcated echo is a contact binary, like 216 Kleopatra, and several of our sample are contact binary candidates. However, evidence for other targets indicates they are not contact binaries. Instead, we hypothesize that these asteroids may have large-scale variations in surface bulk density, i.e. isolated patches of metal-rich and silicate-rich regions at the near-surface, possibly the result of collisions between metal and silicate-rich asteroids. (C) 2014 Elsevier Inc. All rights reserved.

On 2011 June 23, stellar occultations by both Pluto (this work) and Charon (future analysis) were observed from numerous ground stations as well as the Stratospheric Observatory for Infrared Astronomy (SOFIA). This first airborne occultation observation since 1995 with the Kuiper Airborne Observatory resulted in the best occultation chords recorded for the event, in three visible wavelength bands. The data obtained from SOFIA are combined with chords obtained from the ground at the IRTF, the U.S. Naval Observatory Flagstaff Station, and Leeward Community College to give the detailed state of the Pluto–Charon system at the time of the event with a focus on Pluto's atmosphere. The data show a return to the distinct upper and lower atmospheric regions with a knee or kink in the light curve separating them as was observed in 1988, rather than the smoothly transitioning bowl-shaped light curves of recent years. The upper atmosphere is analyzed by fitting a model to all of the light curves, resulting in a half-light radius of 1288 ± 1 km. The lower atmosphere is analyzed using two different methods to provide results under the differing assumptions of particulate haze and a strong thermal gradient as causes for the lower atmospheric diminution of flux. These results are compared with those from past occultations to provide a picture of Pluto's evolving atmosphere. Regardless of which lower atmospheric structure is assumed, results indicate that this part of the atmosphere evolves on short timescales with results changing the light curve structures between 1988 and 2006, and then reverting these changes in 2011 though at significantly higher pressures. Throughout these changes, the upper atmosphere remains remarkably stable in structure, again except for the overall pressure changes. No evidence of onset of atmospheric collapse predicted by frost migration models is seen, and the atmosphere appears to be remaining at a stable pressure level, suggesting it should persist at this full level through New Horizon's flyby in 2015.