バルーズ ロナルド

In May of 2011, NASA selected the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) asteroid sample return mission as the third mission in the New Frontiers program. The other two New Frontiers missions are New Horizons, which explored Pluto during a flyby in July 2015 and is on its way for a flyby of Kuiper Belt object 2014 MU69 on January 1, 2019, and Juno, an orbiting mission that is studying the origin, evolution, and internal structure of Jupiter. The spacecraft departed for near-Earth asteroid (101955) Bennu aboard an United Launch Alliance Atlas V 411 evolved expendable launch vehicle at 7:05 p.m. EDT on September 8, 2016, on a seven-year journey to return samples from Bennu. The spacecraft is on an outbound-cruise trajectory that will result in a rendezvous with Bennu in November 2018. The science instruments on the spacecraft will survey Bennu to measure its physical, geological, and chemical properties, and the team will use these data to select a site on the surface to collect at least 60 g of asteroid regolith. The team will also analyze the remote-sensing data to perform a detailed study of the sample site for context, assess Bennu's resource potential, refine estimates of its impact probability with Earth, and provide ground-truth data for the extensive astronomical data set collected on this asteroid. The spacecraft will leave Bennu in 2021 and return the sample to the Utah Test and Training Range (UTTR) on September 24, 2023.

As the target of the proposed Asteroid Impact & Deflection Assessment (AIDA) mission, the near-Earth binary asteroid 65803 Didymos represents a special class of binary asteroids, those whose primaries are at risk of rotational disruption. To gain a better understanding of these binary systems and to support the AIDA mission, this paper investigates the creep stability of the Didymos primary by representing it as a cohesionless self-gravitating granular aggregate subject to rotational acceleration. To achieve this goal, a soft-sphere discrete element model (SSDEM) capable of simulating granular systems in quasi-static states is implemented and a quasi-static spin-up procedure is carried out. We devise three critical spin limits for the simulated aggregates to indicate their critical states triggered by reshaping and surface shedding, internal structural deformation, and shear failure, respectively. The failure condition and mode, and shear strength of an aggregate can all be inferred from the three critical spin limits. The effects of arrangement and size distribution of constituent particles, bulk density, spin-up path, and interparticle friction are numerically explored. The results show that the shear strength of a spinning self-gravitating aggregate depends strongly on both its internal configuration and material parameters, while its failure mode and mechanism are mainly affected by its internal configuration. Additionally, this study provides some constraints on the possible physical properties of the Didymos primary based on observational data and proposes a plausible formation mechanism for this binary system. With a bulk density consistent with observational uncertainty and close to the maximum density allowed for the asteroid, the Didymos primary in certain configurations can remain geo-statically stable without requiring cohesion. (C) 2017 Elsevier Inc. All rights reserved.

We study the B ring's complex optical depth structure. The source of this structure may be the complex dynamics of the Keplerian shear and the self-gravity of the ring particles. The outcome of these dynamic effects depends sensitively on the collisional and physical properties of the particles. Two mechanisms can emerge that dominate the macroscopic physical structure of the ring: self-gravity wakes and viscous overstability. Here we study the interplay between these two mechanisms by using our recently developed particle collision method that allows us to better model the inter-particle contact physics. We find that for a constant ring surface density and particle internal density, particles with rough surfaces tend to produce axisymmetric ring features associated with the viscous overstability, while particles with smoother surfaces produce self-gravity wakes.

Many asteroids are likely rubble-piles that are a collection of smaller objects held together by gravity and possibly cohesion. These asteroids are seismically shaken by impacts, which leads to excitation of their constituent particles. As a result it has been suggested that their surfaces and sub-surface interiors may be governed by a size sorting mechanism known as the Brazil Nut Effect. We study the behavior of a model asteroid that is a spherical, self-gravitating aggregate with a binary size-distribution of particles under the action of applied seismic shaking. We find that above a seismic threshold, larger particles rise to the surface when friction is present, in agreement with previous studies that focussed on cylindrical and rectangular box configurations. Unlike previous works we also find that size sorting takes place even with zero friction, though the presence of friction does aid the sorting process above the seismic threshold. Additionally we find that while strong size sorting can take place near the surface, the innermost regions remain unsorted under even the most vigorous shaking. (C) 2016 Elsevier Inc. All rights reserved.

Our knowledge of the strengths of small bodies in the Solar System is limited by our poor understanding of their internal structures, and this, in turn, clouds our understanding of the formation and evolution of these bodies. Observations of the rotational states of asteroids whose diameters are larger than a few hundreds of meters have revealed that they are dominated by gravity and that most are unlikely to be monoliths; however, there is a wide range of plausible internal structures. Numerical and analytical studies of shape and spin limits of gravitational aggregates and their collisional evolution show a strong dependence on shear strength. In order to study this effect, we carry out a systematic exploration of the dependence of collision outcomes on dissipation and friction parameters of the material components making up the bodies. We simulate the catastrophic disruption (leading to the largest remnant retaining 50% of the original mass) of km-size asteroids modeled as gravitational aggregates using pkdgrav, a cosmology N-body code adapted to collisional problems and recently enhanced with a new soft-sphere collision algorithm that includes more realistic contact forces. We find that for a range of three different materials, higher friction and dissipation values increase the catastrophic disruption threshold by about half a magnitude. Furthermore, we find that pre-impact rotation systematically increases mass loss on average, regardless of the target's internal configuration. Our results have important implications for the efficiency of planet formation via planetesimal growth, and also more generally to estimate the impact energy threshold for catastrophic disruption, as this generally has only been evaluated for non-spinning bodies without detailed consideration of material properties. (C) 2014 Published by Elsevier Ltd.

Asteroid (99942) Apophis' close approach in 2029 will be one of the most significant small-body encounter events in the near future and offers a good opportunity for in situ exploration to determine the asteroid's surface properties and measure any tidal effects that might alter its regolith configuration. Resurfacing mechanics has become a new focus for asteroid researchers due to its important implications for interpreting surface observations, including space weathering effects. This paper provides a prediction for the tidal effects during the 2029 encounter, with an emphasis on whether surface refreshing due to regolith movement will occur. The potential shape modification of the object due to the tidal encounter is first confirmed to be negligibly small with systematic simulations, thus only the external perturbations are taken into account for this work (despite this, seismic shaking induced by shifting blocks might still play a weak role and we will look into this mechanism in future work). A two-stage approach is developed to model the responses of asteroid surface particles (the regolith) based on the soft-sphere implementation of the parallel N-body gravity tree code pkdgrav. A full-body model of Apophis is sent past the Earth on the predicted trajectory to generate the data of all forces acting at a target point on the surface. A sandpile constructed in the local frame is then used to approximate the regolith materials; all the forces the sandpile feels during the encounter are imposed as external perturbations to mimic the regolith's behavior in the full scenario. The local mechanical environment on the asteroid surface is represented in detail, leading to an estimation of the change in global surface environment due to the encounter. Typical patterns of perturbation are presented that depend on the asteroid orientation and sense of rotation at perigee. We find that catastrophic avalanches of regolith materials may not occur during the 2029 encounter due to the small level of tidal perturbation, although slight landslides might still be triggered in positions where a sandpile's structure is weak. Simulations are performed at different locations on Apophis' surface and with different body- and spin-axis orientations; the results show that the small-scale avalanches are widely distributed and manifest independently of the asteroid orientation and the sandpile location. We also include simulation results of much closer encounters of the Apophis with Earth than what is predicted to occur in 2029, showing that much more drastic resurfacing takes place in these cases. (C) 2014 Elsevier Inc. All rights reserved.

Out of the handful of asteroids that have been imaged, some have distributions of blocks that are not easily explained. In this paper, we investigate the possibility that seismic shaking leads to the size sorting of particles in asteroids. In particular, we focus on the so-called Brazil nut effect (BNE) that separates large particles from small ones under vibrations. We study the BNE over a wide range of parameters by using the N-body code PKDGRAV, and find that the effect is largely insensitive to the coefficients of restitution, but sensitive to friction constants and oscillation speeds. Agreeing with the previous results, we find that convection drives the BNE, where the intruder rises to the top of the particle bed. For the wide-cylinder case, we also observe a 'whale' effect, where the intruder follows the convective current and does not stay at the surface. We show that the non-dimensional critical conditions for the BNE agree well with previous studies. We also show that the BNE is scalable for low-gravity environments and that the rise speed of an intruder is proportional to the square root of the gravitational acceleration. Finally, we apply the critical conditions to observed asteroids, and find that the critical oscillation speeds are comparable to the seismic oscillation speeds that are expected from non-destructive impacts.

We carry out a systematic exploration of the effect of pre-impact rotation on<br /> the outcomes of low-speed collisions between planetesimals modeled as<br /> gravitational aggregates. We use pkdgrav, a cosmology code adapted to<br /> collisional problems and recently enhanced with a new soft-sphere collision<br /> algorithm that includes more realistic contact forces. A rotating body has<br /> lower effective surface gravity than a non-rotating one and therefore might<br /> suffer more mass loss as the result of a collision. What is less well<br /> understood, however, is whether rotation systematically increases mass loss on<br /> average regardless of the impact trajectory. This has important implications<br /> for the efficiency of planet formation via planetesimal growth, and also more<br /> generally for the determination of the impact energy threshold for catastrophic<br /> disruption (leading to the largest remnant retaining 50% of the original mass),<br /> as this has generally only been evaluated for non-spinning bodies. We find that<br /> for most collision scenarios, rotation lowers the threshold energy for<br /> catastrophic dispersal. For head-on collisions, we develop a semi-analytic<br /> description of the change in the threshold description as a function of the<br /> target&#039;s pre-impact rotation rate, and find that these results are consistent<br /> with the &quot;universal law&quot; of catastrophic disruption developed by Leinhardt &amp;<br /> Stewart. Using this approach, we introduce re-scaled catastrophic disruption<br /> variables that take into account the interacting mass fraction of the target<br /> and the projectile in order to translate oblique impacts into equivalent<br /> head-on collisions.

We present the results of a multi-component synthetic spectral analysis of the archival far-ultraviolet spectra of the hot components of several AMCVn double degenerate interacting binaries with known distances from trigonometric parallaxes. Our analysis was carried out using the code BINSYN, which takes into account the donor companion star, the shock front which forms at the disk edge, and the FUV and NUV energy distribution. We fixed the distance of each system at its parallax-derived value and adopted appropriate values of orbital inclination and white dwarf (WD) mass. We find that the accretion-heated "DO/DB" WDs are contributing significantly to the FUV flux in five of the systems (ES Ceti, CR Boo, V803 Cen, HP Lib, GP Com). In three of the systems, GP Com, ES Ceti, and CR Boo, the WD dominates the FUV/NUV flux. We present model-derived accretion rates which agree with the low end of the range of accretion rates derived earlier from blackbody fits over the entire spectral energy distribution. We find that the WD in ES Ceti is very likely not a direct impact accretor but has a small disk. The WD in ES Ceti has T-eff similar to 40,000 +/- 10,000 K. This is far cooler than the previous estimate of Espaillat et al.. We find that the WD in GP Com has T-eff = 14,800 +/- 500 K, which is hotter than the previously estimated temperature of 11,000 K. We present a comparison between our empirical results and current theoretical predictions for these systems.

We present the results of a multi-component synthetic spectral analysis of<br /> the archival far ultraviolet spectra of several key nova-like variables<br /> including members of the SW Sex, RW Tri, UX UMa and VY Scl subclasses: KR Aur,<br /> RW Tri, V825 Her, V795 Her, BP Lyn, V425 Cas and HL Aqr. Accretion rates as<br /> well as the possible flux contribution of the accreting white dwarf are<br /> included in our analysis. Except for RW Tri which has a reliable trigonometric<br /> parallax, we computed the distances to the nova-like systems using the method<br /> of Knigge (2006). Our analysis of seven archival IUE spectra of RW Tri at its<br /> parallax distance of 341 pc consistently indicates a low mass (0.4Msun) white<br /> dwarf and an average accretion rate, 6.3 E-9Msun/yr. For KR Aur, we estimate<br /> that the white dwarf has Teff=29,000K, log g = 8.4 and contributes 18% of the<br /> FUV flux while an accretion disk with accretion rate of 3 E-10Msun/yr at an<br /> inclination of 41 degrees, contributes the remainder. We find that an accretion<br /> disk dominates the far UV spectrum of V425 Cas but a white dwarf contributes<br /> non-negligibly with approximately 18% of the FUV flux. For the two high state<br /> nova-likes, HL Aqr and V825 Her, their accretion disks totally dominate with 1<br /> E-9Msun/yr and 3 E-9Msun/yr, respectively. For BP Lyn we find an accretion rate<br /> of 1 E-8Msun/yr while for V795 Her, we find an accretion rate of 1 E-10Msun/yr.<br /> We discuss the implications of our results for the evolutionary status of<br /> nova-like variables.

We obtained Hubble STIS spectra of three nova-like variables: V751 Cygni,<br /> V380 Oph, and - the only confirmed nova-like variable known to be below the<br /> period gap - BK Lyn. In all three systems, the spectra were taken during high<br /> optical brightness state, and a luminous accretion disk dominates their far<br /> ultraviolet (FUV) light. We assessed a lower limit of the distances by applying<br /> the infrared photometric method of \citet{Knigge2006}. Within the limitations<br /> imposed by the poorly known system parameters (such as the inclination, white<br /> dwarf mass, and the applicability of steady state accretion disks) we obtained<br /> satisfactory fits to BK Lyn using optically thick accretion disk models with an<br /> accretion rate of $\dot{M} = 1\times10^{-9} M_{\odot}$ yr$^{-1}$ for a white<br /> dwarf mass of $M_{wd} = 1.2 M_{\odot}$ and $\dot{M} = 1 \times 10^{-8}<br /> M_{\odot}$ yr$^{-1}$ for $M_{wd} = 0.4 M_{\odot}$. However, for the VY Scl-type<br /> nova-like variable V751 Cygni and for the SW Sex star V380 Oph, we are unable<br /> to obtain satisfactory synthetic spectral fits to the high state FUV spectra<br /> using optically thick steady state accretion disk models. The lack of FUV<br /> spectra information down to the Lyman limit hinders the extraction of<br /> information about the accreting white dwarf during the high states of these<br /> nova-like systems.

We present accretion rates for selected samples of nova-like variables having IUE archival spectra and distances uniformly determined using an infrared method by Knigge. A comparison with accretion rates derived independently with a multiparametric optimization modeling approach by Puebla et al. is carried out. The accretion rates of SW Sextantis nova-like systems are compared with the accretion rates of non-SW Sextantis systems in the Puebla et al. sample and in our sample, which was selected in the orbital period range of three to four and a half hours, with all systems having distances using the method of Knigge. Based upon the two independent modeling approaches, we find no significant difference between the accretion rates of SW Sextantis systems and non-SW Sextantis nova-like systems insofar as optically thick disk models are appropriate. We find little evidence to suggest that the SW Sex stars have higher accretion rates than other nova-like cataclysmic variables (CVs) above the period gap within the same range of orbital periods.