Gravity field determination of a Comet Nucleus: Rosetta at P/Wirtanen

One of the prime objectives of the Rosetta Radio Science Investigations (RSI) experiment is the determination of the mass, the bulk density and the low degree and order gravity of the nucleus of comet P/Wirtanen, the target object of the international Rosetta mission. The RSI experiment will use the spacecraft's radio carrier frequencies at X-band (8.4 GHz) and S-band (2.3 GHz) in order to measure slight changes of the orbit velocity via the classical Doppler effect induced by the gravity attraction of the comet nucleus. Based on an estimate of the background Doppler noise, it is expected that a mass determination (assuming a representative radius of 700 m and a bulk density of 500 kg/m^3) at an accuracy of 0.1% can be achieved if the spacecraft's orbit is iteratively reduced below 7 km altitude. The gravity field of degree and order two can be detected for reasonable tracking times below 5 km altitude. The major competing forces acting on the spacecraft are the radiation pressure and the gas mass flux from cometary activity. While the radiation pressure may be predicted, it is recommended to begin a gravity mapping campaign well before the onset of outgassing activity (>3.25 AU heliocentric distance). Radial acceleration by water outgassing is larger by orders of magnitude than the accelerations from the low degree and order gravity field and will mask the contributions from the gravity field

Driving forces for interface kinetics and phase field models

AbstractPhase field models for applications in physics and materials science are typically written in variational form starting from a free energy functional, and sharp interface descriptions for moving boundary problems can be formulated similarly. Here we discuss why and under which circumstances this postulate for deriving the equations of motion is justified, and what are limitations for specific cases. We investigate this in particular for alloys, systems with elastic, viscoelastic and plastic effects, mainly based on analytical and numerical investigations in one dimension. We find that the naturally guessed equations of motion, as derived via partial functional derivatives from a free energy, are usually reasonable, only for materials with plastic effects this assumption is more delicate due to the presence of internal variables

Accelerated energy-minimization in the quasicontinuum method with application to nanopillar compression

The focus of this contribution is on a novel, improved technique for energy minimization in atomic simulations and its adaption to a variationally consistent formulation of the quasicontinuum (QC) method. The optimization algorithm called FIRE for Fast Inertial Relaxation Engine can be understood as a modification of the Steepest Descent (SD) method, which improves SD by accelerating the system in the direction of the force, making the minimization more aggressive. The performance of FIRE is assessed in the example of nanopillar compression with respect to efficiency and stability against competitive optimization methods

Uniaxial and Biaxial Response of Anisotropic Polypropylene

The response of uniaxial and biaxial anisotropic polypropylene is discussed. A 3D elasto-viscoplastic constitutive model is developed to account for material anisotropy. The famous Hill’s anisotropic yield criterion is combined with the Eyring relation and implemented in a finite element framework to model the response of the polymer during uniaxial loading. An associated viscoplastic flow rule that describes the magnitude and the direction of the viscoplastic flow is incorporated in the model to simulate complex loading conditions. The model quantitatively captures the yield stresses for uniaxial deformation at a given anisotropic state and material orientation. In addition, the results of simulations demonstrate that the constitutive relations qualitatively describe the material deformation during biaxial loading for both isotropic and anisotropic cases

Elastic and plastic effects on solid-state transformations: A phase field study

We discuss a model of diffusion limited growth in solid-state transformations, which are strongly influenced by elastic effects. Density differences and structural transformations provoke stresses at interfaces, which affect the phase equilibrium conditions. We study the growth of a stable phase from a metastable solid in a channel geometry, and perform phase field simulations. Extensions to plastic models are discussed

Mass and density determination of 140 Siwa and 4979 Otawara as expected from the Rosetta flybys.

During its interplanetary cruise to comet P/Wirtanen, the Rosetta spacecraft will encounter the asteroids 4979 Otawara and 140 Siwa on 11 July 2006 and 24 July 2008, respectively. The objective of the Rosetta Radio Science Investigations (RSI) experiment at these flybys is a determination of the asteroid's mass and bulk density by analyzing the radio tracking data (Doppler and range) received from Rosetta before, during and after closest approach. The spacecraft's flyby trajectory will be gravitationally deflected by an amount proportional to the mass of the asteroid for a given flyby distance and velocity. An analysis of the Doppler noise sources indicates that the mass can be determined to an accuracy of 1% for 140 Siwa. The corresponding bulk density show be accurate to 20% . Unfortunately, a detectable trajectory perturbation seems to be hopeless for Otawara because of its small size and the large nominal flyby distance

Mass and density determinations of 140 Siwa and 4979 Otawara as expected from the Rosetta flybys

During its interplanetary cruise to comet P/Wirtanen, the Rosetta spacecraft will encounter the asteroids 4979 Otawara and 140 Siwa on 11 July 2006 and 24 July 2008, respectively. The objective of the Rosetta Radio Science Investigations (RSI) experiment at these flybys is a determination of the asteroid's mass and bulk density by analyzing the radio tracking data (Doppler and range) received from Rosetta before, during and after closest approach. The spacecraft's flyby trajectory will be gravitationally deflected by an amount proportional to the mass of the asteroid for a given flyby distance and velocity. An analysis of the Doppler noise sources indicates that the mass can be determined to an accuracy of 1% for 140 Siwa. The corresponding bulk density show be accurate to 20% . Unfortunately, a detectable trajectory perturbation seems to be hopeless for Otawara because of its small size and the large nominal flyby distance