The rotation of Ganymede and Callisto

<p>The rotation rates of Ganymede and Callisto, the two largest satellites of Jupiter, are on average equal to their orbital mean motion but cannot be constant as a result of the varying gravitational torque exerted by Jupiter on the satellites. For a Keplerian orbit, the period of the torque and of the rotation variations is equal to the orbital period. Gravitational interaction with the other Galilean satellites and the Sun induces deviations from a purely Keplerian orbital motion, leading to changes in the gravitational torque of Jupiter on the satellites with respect to the mean Keplerian orbital motion and therefore to additional rotation variations. Here we discuss small variations from the average rotation on different time scales and assess the potential of using rotation as a probe of the interior structure.</p> <p>The ESA JUICE (JUpiter ICy moons Explorer) mission will measure the rotation and tides of Ganymede and Callisto in the early 30s, and will in particular very accurately determine those quantities for Ganymede during the orbital phase of the spacecraft around that satellite starting in 2032. We report on different theoretical aspects of the rotation for realistic models of the interior of the satellites, include tidal deformations and take into account the low-degree gravity field and topography of Ganymede and Callisto. We assess the advantages of a joint use of rotation and tides to constrain the satellite's interior structure, in particular its ice shell and ocean.</p>

A gravitational torque-compensated 2-DOF planar robotic arm design and its active control

Serial robot manipulators have their servo motors with reduction gears on the link joints. When it comes to hyper-redundant robots, this kind of joint actuation mechanism cannot be implemented since this makes hyper-redundant robots too heavy. Instead, cable driven mechanisms are preferred. However, the positioning accuracy is negatively affected by the cables. This paper addresses the positioning accuracy problem of cable driven hyper-redundant robots by employing a 2-DOF robotic arm whose modules are counter-balanced. While the actuators connected to the base actively do most of the work using cables and springs, light and compact actuators connected to the links produce precise motion. The method will result in compact, light and precise hyper-redundant robotic arms. The above-mentioned procedure governed by a control software including a 2D simulator developed is experimentally proved to be a feasible method to compensate the gravitational torque successfully.

A backward-spinning star with two coplanar planets

It is widely assumed that a star and its protoplanetary disk are initially aligned, with the stellar equator parallel to the disk plane. When observations reveal a misalignment between stellar rotation and the orbital motion of a planet, the usual interpretation is that the initial alignment was upset by gravitational perturbations that took place after planet formation. Most of the previously known misalignments involve isolated hot Jupiters, for which planet–planet scattering or secular effects from a wider-orbiting planet are the leading explanations. In theory, star/disk misalignments can result from turbulence during star formation or the gravitational torque of a wide-orbiting companion star, but no definite examples of this scenario are known. An ideal example would combine a coplanar system of multiple planets—ruling out planet–planet scattering or other disruptive postformation events—with a backward-rotating star, a condition that is easier to obtain from a primordial misalignment than from postformation perturbations. There are two previously known examples of a misaligned star in a coplanar multiplanet system, but in neither case has a suitable companion star been identified, nor is the stellar rotation known to be retrograde. Here, we show that the star K2-290 A is tilted by 124○±6○ compared with the orbits of both of its known planets and has a wide-orbiting stellar companion that is capable of having tilted the protoplanetary disk. The system provides the clearest demonstration that stars and protoplanetary disks can become grossly misaligned due to the gravitational torque from a neighboring star.

Investigating the “inverted attitude” problem in a spacecraft stabilisation system featuring non-linear sensors

The paper shows that gravitational torque may cause an emergency “inverted attitude” mode in a relay spacecraft stabilisation system due to the fact that a field-of-view limit type sensor may be non-linear. We performed structural partitioning of the phase cylinder into regions with various trajectory types for the case of a simplified planar motion model. We demonstrate the synthesis routine for finding the boundaries separating the region marking the system transitioning into the regular attitude mode from the region signifying it entering the emergency mode. We derived analytical conditions under which emergency modes arise, which can serve as the basis for synthesising algorithms to prevent these modes from occurring.

Simultaneous gravity and gripping force compensation mechanism for lightweight hand-arm robot with low-reduction reducer

SummaryIn this paper, we propose a novel mechanism to compensate for gravity and the gripping force in a hand-arm robot. This mechanism compensates for the gravitational torque produced by an object gripped by the hand-arm robot. The gripping force required for the robot hand to prevent the object from dropping is also simultaneously compensated for. This mechanism requires only one actuator placed on the shoulder part of the robot. Therefore, this mechanism can reduce the torque requirement of joint actuators and lower the weight of the robot. The gear ratio of the reduction gears in each robot joint can then also be reduced. These advantages are critical for future robots that perform tasks in unstructured environments and collaborate with humans. We carried out experiments with a 6-DoF robot arm having a 1-DoF gripper to demonstrate the effectiveness of the proposed mechanism.

Vestibular and corticospinal control of human body orientation in the gravitational field

Body orientation with respect to the direction of gravity changes when we lean forward from upright standing. We tested the hypothesis that during upright standing, the nervous system specifies the referent body orientation that defines spatial thresholds for activation of multiple muscles across the body. To intentionally lean the body forward, the system is postulated to transfer balance and stability to the leaned position by monotonically tilting the referent orientation, thus increasing the activation thresholds of ankle extensors and decreasing their activity. Consequently, the unbalanced gravitational torque would start to lean the body forward. With restretching, ankle extensors would be reactivated and generate increasing electromyographic (EMG) activity until the enhanced gravitational torque would be balanced at a new posture. As predicted, vestibular influences on motoneurons of ankle extensors evaluated by galvanic vestibular stimulation were smaller in the leaned compared with the upright position, despite higher tonic EMG activity. Defacilitation of vestibular influences was also observed during forward leaning when the EMG levels in the upright and leaned position were equalized by compensating the gravitational torque with a load. The vestibular system is involved in the active control of body orientation without directly specifying the motor outcome. Corticospinal influences originating from the primary motor cortex evaluated by transcranial magnetic stimulation remained similar at the two body postures. Thus, in contrast to the vestibular system, the corticospinal system maintains a similar descending facilitation of motoneurons of leg muscles at different body orientations. The study advances the understanding of how body orientation is controlled.NEW & NOTEWORTHY The brain changes the referent body orientation with respect to gravity to lean the body forward. Physiologically, this is achieved by shifts in spatial thresholds for activation of ankle muscles, which involves the vestibular system. Results advance the understanding of how the brain controls body orientation in the gravitational field. The study also extends previous evidence of empirical control of motor function, i.e., without the reliance on model-based computations and direct specification of motor outcome.