ARISTOTELES: A European approach for an Earth gravity field recovery mission

Under contract of the European Space Agency a system study for a spaceborne gravity field recovery mission was performed, covering as a secondary mission objective geodetic point positioning in the cm range as well. It was demonstrated that under the given programmatic constraints including dual launch and a very tight development schedule, a six months gravity field mission in a 200 km near polar, dawn-dusk orbit is adequate to determine gravity anomalies to better than 5 mgal with a spatial resolution of 100 x 100 km half wavelength. This will enable scientists to determine improved spherical harmonic coefficients of the Earth gravity field equation to the order and degree of 180 or better

Device and method for frictionally testing materials for ignitability

Test apparatus for determining ignition characteristics of various metal in oxidizer environments simulating operating conditions for materials is invented. The test apparatus has a chamber through which the oxidizing agent flows, and means for mounting a stationary test sample therein, a powered, rotating shaft in the chamber rigidly mounts a second test sample. The shaft is axially movable to bring the samples into frictional engagement and heated to the ignition point. Instrumentation connected to the apparatus provides for observation of temperatures, pressures, loads on and speeds of the rotating shaft, and torques whereby components of stressed oxygen systems can be selected which will avoid accidental fires under working conditions

Collisions of inhomogeneous pre-planetesimals

In the framework of the coagulation scenario, kilometre-sized planetesimals form by subsequent collisions of pre-planetesimals of sizes from centimetre to hundreds of metres. Pre-planetesimals are fluffy, porous dust aggregates, which are inhomogeneous owing to their collisional history. Planetesimal growth can be prevented by catastrophic disruption in pre-planetesimal collisions above the destruction velocity threshold. We develop an inhomogeneity model based on the density distribution of dust aggregates, which is assumed to be a Gaussian distribution with a well-defined standard deviation. As a second input parameter, we consider the typical size of an inhomogeneous clump. These input parameters are easily accessible by laboratory experiments. For the simulation of the dust aggregates, we utilise a smoothed particle hydrodynamics (SPH) code with extensions for modelling porous solid bodies. The porosity model was previously calibrated for the simulation of silica dust, which commonly serves as an analogue for pre-planetesimal material. The inhomogeneity is imposed as an initial condition on the SPH particle distribution. We carry out collisions of centimetre-sized dust aggregates of intermediate porosity. We vary the standard deviation of the inhomogeneous distribution at fixed typical clump size. The collision outcome is categorised according to the four-population model. We show that inhomogeneous pre-planetesimals are more prone to destruction than homogeneous aggregates. Even slight inhomogeneities can lower the threshold for catastrophic disruption. For a fixed collision velocity, the sizes of the fragments decrease with increasing inhomogeneity. Pre-planetesimals with an active collisional history tend to be weaker. This is a possible obstacle to collisional growth and needs to be taken into account in future studies of the coagulation scenario.Comment: 12 pages, 9 figures, 4 table

How primordial is the structure of comet 67P/C-G? Combined collisional and dynamical models suggest a late formation

There is an active debate about whether the properties of comets as observed today are primordial or, alternatively, if they are a result of collisional evolution or other processes. We investigate the effects of collisions on a comet with a structure like 67P/C-G. We develop scaling laws for the critical specific impact energies required for a significant shape alteration. These are then used in simulations of the combined dynamical and collisional evolution of comets in order to study the survival probability of a primordially formed object with a shape like 67P/C-G. The effects of impacts on comet 67P/C-G are studied using a SPH shock physics code. The resulting critical specific impact energy defines a minimal projectile size which is used to compute the number of shape-changing collisions in a set of dynamical simulations. These simulations follow the dispersion of the trans-Neptunian disk during the giant planet instability, the formation of a scattered disk, and produce 87 objects that penetrate into the inner solar system with orbits consistent with the observed JFC population. The collisional evolution before the giant planet instability is not considered here. Hence, our study is conservative in its estimation of the number of collisions. We find that in any scenario considered here, comet 67P/C-G would have experienced a significant number of shape-changing collisions, if it formed primordially. This is also the case for generic bi-lobe shapes. Our study also shows that impact heating is very localized and that collisionally processed bodies can still have a high porosity. Our study indicates that the observed bi-lobe structure of comet 67P/C-G may not be primordial, but might have originated in a rather recent event, possibly within the last 1 Gy. This may be the case for any kilometer-sized two-component cometary nuclei.Comment: Astronomy & Astrophysics, accepted pending minor revision

Reconnection in Marginally Collisionless Accretion Disk Coronae

We point out that a conventional construction placed upon observations of accreting black holes, in which their nonthermal X-ray spectra are produced by inverse comptonization in a coronal plasma, suggests that the plasma is marginally collisionless. Recent developments in plasma physics indicate that fast reconnection takes place only in collisionless plasmas. As has recently been suggested for the Sun's corona, such marginal states may result from a combination of energy balance and the requirements of fast magnetic reconnection.Comment: Revised in response to referee. Accepted ApJ. 11 pp., no figures. Uses aastex 5.0

Stochastic Acceleration of Low Energy Electrons in Cold Plasmas

We investigate the possibility of stochastic acceleration of background low-energy electrons by turbulent plasma waves. We consider the resonant interaction of the charged particles with all branches of the transverse plasma waves propagating parallel to a uniform magnetic field. Numerical results and asymptotic analytic solutions valid at non-relativistic and ultra-relativistic energies are obtained for the acceleration and scattering times of electrons. These times have a strong dependence on plasma parameter alpha = Omega_pe / Omega_e (the ratio of electron plasma frequency to electron gyrofrequency) and on the spectral index of plasma turbulence. It is shown that particles with energies above certain critical value may interact with higher frequency electromagnetic plasma waves and this interaction is allowed only in plasmas with alpha < 1. We show that for non-relativistic and semi-relativistic electrons in low-alpha plasmas the ratio of the acceleration time to the scattering time can be less than unity for a wide range of energies. From this we conclude that the transport equation derived for cosmic rays which requires this ratio to be much larger than one is not applicable at these energies. An approximate "critical" value of particle energy above which the dynamics of charged particles may be described by this transport equation is determined as a function of plasma parameters. We propose new transport equation for the opposite limit (energies less than this critical value) when the acceleration rate is much faster than the pitch angle scattering rate. This equation is needed to describe the electron dynamics in plasmas with alpha <= 0.1.Comment: 22 pages, 13 figures, Latex, submitted to Astrophysical Journa