Barrel Instability in Binary Asteroids

Most close-in planetary satellites are in synchronous rotation, which is usually the stable end-point of tidal despinning. Saturn’s moon Hyperion is a notable exception by having a chaotic rotation. Hyperion’s dynamical state is a consequence of its high eccentricity and its highly prolate shape. As many binary asteroids also have elongated secondaries, chaotic rotation is expected for moons in eccentric binaries, and a minority of asteroidal secondaries may be in that state. The question of secondary rotation is also important for the action of the binary Yarkovsky–O’Keefe–Radzievskii–Paddack (BYORP) effect, which can quickly evolve orbits of synchronous (but not nonsynchronous) secondaries. Here we report results of a large set of short numerical simulations which indicate that, apart from synchronous and classic chaotic rotation, close-in irregularly shaped asteroidal secondaries can occupy an additional, intermediate rotational state. In this “barrel instability” the secondary slowly rolls along its long axis, while the longest axis is staying largely aligned with the primary–secondary line. This behavior may be more difficult to detect through lightcurves than a fully chaotic rotation, but would likewise shut down BYORP. We show that the binary’s eccentricity, separation measured in secondary’s radii and the secondary’s shape are all important for determining whether the system settles in synchronous rotation, chaotic tumbling, or barrel instability. We compare our results for synthetic asteroids with known binary pairs to determine which of these behaviors may be present in the near-Earth asteroid binary population.

Research on the Critical Technique of Synchronous Rotation Construction With Large Angle for T-shape Curve Rigid Frame Bridge

The superstructure rotation method (SRM) can optimize bridge construction in terms of reducing impacts on traffic, safety, and overall budget. This paper focuses on the key scientific problems of synchronous rotation construction with large angles and takes two T-shaped rigid frame bridges of the Wuyi Expressway over the Chengdu-Kunming Railway as the engineering background. Construction technologies of this project are introduced, including the installation process of spherical hinges, the design of the traction system, and the precision control method. Finite element models of the full-bridge were established to obtain the lateral unbalanced moment of the structure and the test method of the longitudinal unbalanced moment is shown in detail. Furthermore, the key parameters of SRM are analyzed and calculated, including the derivation and modification of the existing static friction coefficient calculation formula of the spherical hinge. Based on the measured data, the accuracy of different friction coefficient prediction formulas is evaluated.

Effects of tidal heating in Proxima Centauri b's thermal evolution

<p>The recent discovery of a terrestrial planet orbiting Proxima Centauri, our closest neighbor (an M5.5V star of 0.1 M<sub>Sun</sub> mass and only 1.3 pc distance to the Sun), offers an excellent planet laboratory to study the most important theories of planet evolution and composition. The planet (Proxima b) is orbiting the star in its habitable zone at a separation of only ~0.05 AU and an orbital period of ~11 days, and most recent studies suggest a non-zero eccentricity of about 0.17. With a mass of >=1.2 M<sub>Earth</sub>, Proxima b is expected to have a rocky composition, which might resemble the Earth. It is therefore an excellent target to characterize terrestrial planets similar to Earth, avoiding the inherent biases of only studying the terrestrial planets of the solar system.</p> <p>Due to its close orbit and expected eccentricity, Proxima b most likely suffers from severe tidal heating, which can have an extreme incidence in the planet's habitability and the survival of an atmosphere. In this work, we perform a comprehensive analysis of the incidence that different distribution patterns of tidal heating can have on Proxima b's interior and thermal evolution. To accomplish this goal, we consider various possible geometries of the planet, from the simplest case, homogeneous distribution of the generated heat, to the more complicated cases, with an inhomogeneous distribution pattern that depends on the planet's interior structure (a stratified sphere, an incompressible homogeneous planet, or a two-layer structure with a differentiated core). The different models considered alter how tidal heat is distributed throughout the planet's interior, which can highly affect its overall thermal evolution.</p> <p>Furthermore, due to its proximity to the central star, Proxima b may as well be in synchronous rotation with Proxima Centauri. This can cause an extreme surface temperature variation between the hemisphere that permanently faces the star and the opposite hemisphere. In this work, the effect that synchronous rotation may have on Proxima b's interior is also thoroughly investigated.</p>

Control of oscillations in two-rotor cyberphysical vibration units with time-varying observer

In this paper the control of oscillations in the two-rotor vibration unit is studied. It is assumed that the velocity of the oscillation of the platform cannot be accurately measured. The time-varying observer is proposed to restore it. In order to guarantee stability of the frequency and amplitude of oscillations of the vibrating parts of a two-rotor vibration unit special control algorithms based on speed-gradient methodology. Simulation results confirm stability of the synchronous rotation modes of the unbalanced rotors of the vibration unit.

"Barrel Instability" for Elongated Secondaries in Binary Asteroids

<p>Most close-in planetary satellites are in synchronous rotation, which is usully the stable end-point of tidal despinning. Saturn's moon Hyperion is a notable exception by having a chaotic rotation. Hyperion's dynamical state is a consequence of its high eccentricity and its highly prolate shape (Wisdom et al. 1984). As many binary asteroids also have elongated secondaries, chaotic rotation is expected for moons in eccentric binaries (Cuk & Nesvorny 2010), and a minority of asteroidal secondaries may be in that state (Pravec et al. 2016). The question of the secondary's rotation is importrant for the action of the BYORP effect, which can quickly evolve orbits of synchrnous (but not non-synchronous) secondaries (Cuk & Burns 2005). Here we report preliminary numerical simulations which indicate that in binary systems with a large secondary and significant spin-orbit coupling a different kind of non-synchronous rotation may arise. In this "barrel instability" the secondary slowly rolls along its long axis, while the longest diameter is staying largelly aligned with the primary-secondary line. This behavior  may be more difficult to detect through lightcurves than a fully chaotic rotation, but would likewise shut down BYORP. Unlike fully chaotic rotation, barrel instability can happen even at low eccentricties. In our presentation we will discuss our theoretical results and their implications for the evolution of binary asteroids, such as the Didymos-Dimorphos pair.</p>

Spin: Ubiquitous, Fundamental, Purposeful: Its Complementary Interactions with Gravity

In a prior article we explained how axial spin of celestial bodies interact with mutual gravitation in the phenomenon of synchronous rotation of our moon and the major moons of the gas giants. We also showed how the same complementary interactions could explain both the nonsynchronous (regular orbits) and the negative rotations in Venus, Uranus and Pluto, as well as in the peripheral small satellites of the gas giants. This paper expands on that theme and identifies the various other areas in which these two fundamental and ubiquitous forces cooperate to bring about many other phenomena in the larger universe. Prominent among these phenomena is the role played by the mother bodies’ axial rotation in determining the direction of the orbital motion of their satellite bodies. The other effects include the appearance and maintenance of the spherical shape of large celestial bodies, generation of magnetism in planets, their respective roles in the formation of solar nebulae and proto-planetary disks, and the flattened profile of spiral and elliptical galaxies. Another important finding reported in this paper is the close relationship that exists between the size of planets, as well as the stars, with their axial rotation speeds. This increase of axial rotation speed of celestial bodies in direct proportion to the mass of those bodies, we believe serves to counteract the inward thrust of gravity, in exact proportions and thus help maintain the roughly spherical contour of those bodies. This finding even extends to spiral galaxies, where the axial rotation speed seems to be positively related to the size of the galaxy. This phenomenon and others suggest that spin is a fundamental and purposeful property of matter. Thus, in this paper we stress the important contributions made by the collaborative interactions between the ubiquitous gravity and spin in matter at the level of the fundamental particles, as well as in large celestial bodies, including the largest units in the universe, the galaxies.

Tidal evolution of eccentric binaries driven by convective turbulent viscosity

ABSTRACT Tidal dissipation due to convective turbulent viscosity shapes the evolution of a variety of astrophysical binaries. For example, this type of dissipation determines the rate of orbital circularization in a binary with a post-main-sequence star that is evolving toward a common envelope phase. Viscous dissipation can also influence binaries with solar-type stars, or stars with a close-in giant planet. In general, the effective viscosity in a convective stellar envelope depends on the tidal forcing frequency ωtide; when ωtide is larger than the turnover frequency of convective eddies, the viscosity is reduced. Previous works have focused on binaries in nearly circular orbits. However, for eccentric orbits, the tidal potential has many forcing frequencies. In this paper, we develop a formalism for computing tidal dissipation that captures the effect of frequency-dependent turbulent viscosity and is valid for arbitrary binary eccentricities. We also present an alternative simpler formulation that is suitable for very high eccentricities. We apply our formalisms to a giant branch (GB) star model and a solar-type star model. We find that a range of pseudo-synchronous rotation rates are possible for both stellar models, and the pseudo-synchronous rate can differ from the prediction of the commonly used weak tidal friction theory by up to a factor of a few. We also find that tidal decay and circularization due to turbulent viscosity can be a few orders of magnitude faster than predicted by weak tidal friction in GB stars on eccentric, small pericentre orbits, but is suppressed by a few orders of magnitude in solar-type stars due to viscosity reduction.

Stability of synchronous regimes in unbalanced rotors on elastic base

The stationary dynamics of an unbalanced rotor (vibrator) on a movable base (linear oscillator) under excitation by a driving torque is studied with focusing on the stability of 1:1 stationary regimes of rotation and oscillation. This problem was well studied previously in the first approximation, dealing, in fact, with averaged regimes, mainly in the framework of asymptotic procedures. We use an efficient analytical procedure, proposed in our previous works for another problem, which sequentially separates the averaged regimes and deviations from them. Describing in the first approximation the known features of the synchronous stationary regimes under consideration, this approach in the second approximation results in the analytical solution for nonuniform rotation whose exactness is confirmed in the numerical simulation. The solution enables us to reveal possibility of parametric instability for oscillations of the rotor angular velocity and to describe two possible mechanisms of this instability. It is shown that the known condition of stability of stationary synchronous rotation-oscillation regimes is only necessary but not sufficient criterion, and two additional necessary conditions of stability are obtained and confirmed by the numerical simulation.