Fission and reconfiguration of bilobate comets as revealed by 67P/Churyumov-Gerasimenko.

The solid, central part of a comet--its nucleus--is subject to destructive processes, which cause nuclei to split at a rate of about 0.01 per year per comet. These destructive events are due to a range of possible thermophysical effects; however, the geophysical expressions of these effects are unknown. Separately, over two-thirds of comet nuclei that have been imaged at high resolution show bilobate shapes, including the nucleus of comet 67P/Churyumov-Gerasimenko (67P), visited by the Rosetta spacecraft. Analysis of the Rosetta observations suggests that 67P's components were brought together at low speed after their separate formation. Here, we study the structure and dynamics of 67P's nucleus. We find that sublimation torques have caused the nucleus to spin up in the past to form the large cracks observed on its neck. However, the chaotic evolution of its spin state has so far forestalled its splitting, although it should eventually reach a rapid enough spin rate to do so. Once this occurs, the separated components will be unable to escape each other; they will orbit each other for a time, ultimately undergoing a low-speed merger that will result in a new bilobate configuration. The components of four other imaged bilobate nuclei have volume ratios that are consistent with a similar reconfiguration cycle, pointing to such cycles as a fundamental process in the evolution of short-period comet nuclei. It has been shown that comets were not strong contributors to the so-called late heavy bombardment about 4 billion years ago. The reconfiguration process suggested here would preferentially decimate comet nuclei during migration to the inner solar system, perhaps explaining this lack of a substantial cometary flux.

The effect of zonal harmonics on dynamical structures in the circular restricted three-body problem near the secondary body.

The circular restricted three-body model is widely used for astrodynamical studies in systems where two major bodies are present. However, this model relies on many simplifications, such as point-mass gravity and planar, circular orbits of the bodies, and limiting its accuracy. In an effort to achieve higher-fidelity results while maintaining the autonomous simplicity of the classic model, we employ zonal harmonic perturbations since they are symmetric about the z-axis, thus bearing no time-dependent terms. In this study, we focus on how these perturbations affect the dynamic environment near the secondary body in real systems. Concise, easily implementable equations for gravitational potential, particle motion, and modified Jacobi constant in the perturbed model are presented. These perturbations cause a change in the normalized mean motion, and two different formulations are addressed for assigning this new value. The shifting of collinear equilibrium points in many real systems due to J 2 of each body is reported, and we study how families of common periodic orbits-Lyapunov, vertical, and southern halo-shift and distort when J 2 , J 4 , and J 6 of the primary and J 2 of the secondary body are accounted for in the Jupiter-Europa and Saturn-Enceladus systems. It is found that these families of periodic orbits change shape, position, and energy, which can lead to dramatically different dynamical behavior in some cases. The primary focus is on moons of the outer planets, many of which have very small odd zonal harmonic terms, or no measured value at all, so while the developed equations are meant for any and all zonal harmonic terms, only even terms are considered in the simulations. Early utilization of this refined CR3BP model in mission design will result in a more smooth transition to full ephemeris model.

Passive Microfluidic device for Sub Millisecond Mixing.

We report the investigation of a novel microfluidic mixing device to achieve submillisecond mixing. The micromixer combines two fluid streams of several microliters per second into a mixing compartment integrated with two T- type premixers and 4 butterfly-shaped in-channel mixing elements. We have employed three dimensional fluidic simulations to evaluate the mixing efficiency, and have constructed physical devices utilizing conventional microfabrication techniques. The simulation indicated thorough mixing at flow rate as low as 6 µL/s. The corresponding mean residence time is 0.44 ms for 90% of the particles simulated, or 0.49 ms for 95% of the particles simulated, respectively. The mixing efficiency of the physical device was also evaluated using fluorescein dye solutions and FluoSphere-red nanoparticles suspensions. The constructed micromixers achieved thorough mixing at the same flow rate of 6 µL/s, with the mixing indices of 96% ± 1%, and 98% ± 1% for the dye and the nanoparticle, respectively. The experimental results are consistent with the simulation data. The device demonstrated promising capabilities for time resolved studies for macromolecular dynamics of biological macromolecules.