다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

Discrete particle simulation of particulate systems: A review of major applications and findings

During its approach to asteroid (101955) Bennu, NASA’s Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) spacecraft surveyed Bennu’s immediate environment, photometric properties, and rotation state. Discovery of a dusty environment, a natural satellite, or unexpected asteroid characteristics would have had consequences for the mission’s safety and observation strategy. Here we show that spacecraft observations during this period were highly sensitive to satellites (sub-meter scale) but reveal none, although later navigational images indicate that further investigation is needed. We constrain average dust production in September 2018 from Bennu’s surface to an upper limit of 150 g s–1 averaged over 34 min. Bennu’s disk-integrated photometric phase function validates measurements from the pre-encounter astronomical campaign. We demonstrate that Bennu’s rotation rate is accelerating continuously at 3.63 ± 0.52 × 10–6 degrees day–2, likely due to the Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect, with evolutionary implications.

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The operational environment and rotational acceleration of asteroid (101955) Bennu from OSIRIS-REx observations

Computer Methods in Applied Mechanics and Engineering

Dynamic fracture mechanics

A ball bouncing elastically upon a vertically vibrated platform is one of the simplest examples of a chaotic system. If dissipation is introduced at each bounce through a coefficient of restitution, the motion is no longer chaotic; the trajectories exhibit locking solutions that result in periodic behavior. Here we investigate the dynamics of a bouncing ball influenced by air damping. We consider the effects of both static air and air moving with the platform, and show that there is an exact mapping between them. In either case, the system has a rather complex dynamical behavior including truly chaotic trajectories. Our results highlight the importance of air effects for fine particulate systems.

Chaotic dynamics of an air-damped bouncing ball.

Cohesive regolith on fast rotating asteroids

Motivated by discovery of the superconductivity in strontium titanate (SrTiO3) doped with a smali number of electron carriers, we have carried out quantum-chemical Hartree-Fock calculations to study La-doped SrTiO3. The crystalline lattice distortion due to the impurity presence is obtained for both cubic and tetragonal structures. The atomie movements in a region surrounding the defects show considerable differences for two crystallographic phases. La-induced local energy levels are found in the gap between the upper valence band and the conduction band. Two different States of the system, namely singlet and triplet states, are considered. These states oceur depending on how the extra electrons fili in the local energy levels. The observed electron transfer from the local energy levels to the conduction band is discussed in the light of the available experimental data on the superconductivity studies.

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Computational study of structural and electronic properties of superconducting La-doped SrTiO3

When the OSIRIS-REx spacecraft pressed its sample collection mechanism into the surface of Bennu, it provided a direct test of the poorly understood near-subsurface physical properties of rubble-pile asteroids, which consist of rock fragments at rest in microgravity. Here, we find that the forces measured by the spacecraft are best modeled as a granular bed with near-zero cohesion that is half as dense as the bulk asteroid. The low gravity of a small rubble-pile asteroid such as Bennu effectively weakens its near subsurface by not compressing the upper layers, thereby minimizing the influence of interparticle cohesion on surface geology. The underdensity and weak near subsurface should be global properties of Bennu and not localized to the contact point.

/pdf/near-zero-cohesion-and-loose-packing-of-bennus-near-2e9e8h5b.pdf

Near-zero cohesion and loose packing of Bennu’s near subsurface revealed by spacecraft contact

Recent observations and theory have indicated that rubble pile asteroids may have a small, but finite, level of tensile strength, allowing them to spin above their spin deformation limit as defined in Holsapple (Icarus 205:430–442, 2010). In Sanchez and Scheeres (Meteorit Planet Sci 49:788–811, 2014), a theory for how such strength could be present in rubble pile asteroids was presented, relying on weak van der Waals forces between fine particulate material in asteroid regolith and in their interiors. The implications of this theory are evaluated and related to the surface strength of regolith and global strength of a rubble pile body. Proposed techniques to measure the strength of regolith using cratering theory are reviewed, as are constraints placed on the global strength of rubble pile asteroids from astronomical observations. Specific examples applied to the Hayabusa2 cratering experiment at its target asteroid are given.

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Paul Sánchez

Papers

Chaotic dynamics of an air-damped bouncing ball.

Cohesive regolith on fast rotating asteroids

Computational study of structural and electronic properties of superconducting La-doped SrTiO3

Near-zero cohesion and loose packing of Bennu’s near subsurface revealed by spacecraft contact

Implications of cohesive strength in asteroid interiors and surfaces and its measurement