Kinematics of Saturn's spokes

Voyager 2 images of Saturn's rings have been analyzed for spoke activity. More than 80 and 40 different spokes have been measured at the morning and at the evening ansa, respectively. Higher rate of spoke formation has been found at 145° ± 15° SLS and at 305° ± 15° SLS which persisted for at least 3 Saturn revolutions. Higher spoke activity (formation and growth in width) by more than a factor 3 has been observed over the nightside hemisphere of Saturn than over the dayside hemisphere. The age distribution (i.e. time from radial formation until observation, assuming Keplerian shear) of the leading (old) edges of spokes has its maximum at ～ 9,000 s and ～ 6,000 s for spokes observed at the morning ansa and at the evening ansa, respectively. The highest spoke age observed is ～ 20,000 s. The age distribution of the trailing (young) edges of spokes peaks at < 2,000 s at both ansae but has its mean at ～ 4,500 s and ～ 3,500 s, respectively. On the average the observed spokes grew in width for ～ 4,500 s at the morning ansa and for ～ 2,500 s at the evening ansa. The maximum time of growth in width was ～ 12,000 s.     

Thermal model and thermo-mechanical stresses in cometary nuclei

Spherically symmetric radial temperature profiles of cometary nuclei have been determined numerically (and for simplified models analytically) in dependence on the orbital position of the periodic comet Halley. These temperature fields in the nucleus are connected with thermal stress fields which have been calculated with the assumption of elastic properties of cometary matter. The remarkable result is the possible existence of stresses, strong enough to cause internal cracking of the nucleus and break-ups of the cometary surface. This may be essential understanding normal cometary activity as well as outbursts and splits.     

Solution numérique du problème de Képler

A numerical solution of the Kepler problem. Requiring a six-decimal accuracy, the Kepler problem can be solved with four iterations at most, even for an eccentricity close to 1, provided that a new initial approximation be introduced in this last case.     

The Hanle effect applied to magnetic field diagnostics

The Hanle effect is the modification by the local magnetic field of the linear polarization parameters of a spectral line. A brief summary of the theory is presented and it is shown how the complete magnetic vector field can be recovered from the interpretation of the Hanle effect in spectral lines polarized by resonance scattering.Proceedings of the Conference Solar Physics from Space, held at the Swiss Federal Institute of Technology Zurich (ETHZ), 11–14 November 1980.     

X-Ray images of the supernova remnant puppis A

Space Science Reviews -     

EXOSAT observations of the bright X-ray source GX9+1

We present the results of the spectral and timing analysis of an observation of GX9+1/4U1758-205 performed with the Medium Energy Experiment aboard EXOSAT. During our observation the source flux varied irregularly in time scales from minutes to hours. No periodic emission in the period range from 16 msec to 2000 sec was found with an upper limit of around 1% (3 ) for the pulsed fraction. The hardness ratio shows a correlated change with the flux intensity (Sco X-1 behaviour). The spectrum could be fitted by a double component model, a black body component (kT=1.16–1.26 keV) together with a thermal bremsstrahlung law (kT=13–15keV). The black-body temperature-black-body flux relation follows a Stefan Boltzmann law with R_BB=15.3 km^*D/10 kpc. No iron line was detected. The upper limit for the line equivalent width of a 6.7 keV iron emission line is 40 eV (1). The X-ray spectral behaviour of GX9+1 indicates, that this source belongs to the class of Low-Mass X-ray Binaries (LMXB).     

Correction to: Mission to Planet Earth: The First Two Billion Years

Space Science Reviews - The Ionospheric Connection Explorer (ICON) mission makes measurements in near-Earth space that provide knowledge of the state of the ionosphere. From the vantage of...     

Dust Evolution in Protoplanetary Discs and the Formation of Planetesimals

After 25 years of laboratory research on protoplanetary dust agglomeration, a consistent picture of the various processes that involve colliding dust aggregates has emerged. Besides sticking, bouncing and fragmentation, other effects, like, e.g., erosion or mass transfer, have now been extensively studied. Coagulation simulations consistently show that \(\upmu\mbox{m}\)-sized dust grains can grow to mm- to cm-sized aggregates before they encounter the bouncing barrier, whereas sub-\(\upmu\mbox{m}\)-sized water-ice particles can directly grow to planetesimal sizes. For siliceous materials, other processes have to be responsible for turning the dust aggregates into planetesimals. In this article, these processes are discussed, the physical properties of the emerging dusty or icy planetesimals are presented and compared to empirical evidence from within and without the Solar System. In conclusion, the formation of planetesimals by a gravitational collapse of dust “pebbles” seems the most likely.     

Simulation-based evaluation of a cold atom interferometry gradiometer concept for gravity field recovery

The prospects of future satellite gravimetry missions to sustain a continuous and improved observation of the gravitational field have stimulated studies of new concepts of space inertial sensors with potentially improved precision and stability. This is in particular the case for cold-atom interferometry (CAI) gradiometry which is the object of this paper. The performance of a specific CAI gradiometer design is studied here in terms of quality of the recovered gravity field through a closed-loop numerical simulation of the measurement and processing workflow. First we show that mapping the time-variable field on a monthly basis would require a noise level below $5\text{mE}/\sqrt{\text{Hz}}$. The mission scenarios are therefore focused on the static field, like GOCE. Second, the stringent requirement on the angular velocity of a one-arm gradiometer, which must not exceed ${10}^{-6}$?rad/s, leads to two possible modes of operation of the CAI gradiometer: the nadir and the quasi-inertial mode. In the nadir mode, which corresponds to the usual Earth-pointing satellite attitude, only the gradient $V_{\mathit{yy}}$, along the cross-track direction, is measured. In the quasi-inertial mode, the satellite attitude is approximately constant in the inertial reference frame and the 3 diagonal gradients $V_{\mathit{xx}}\text{,}{V}_{\mathit{yy}}$ and $V_{\mathit{zz}}$ are measured. Both modes are successively simulated for a 239?km altitude orbit and the error on the recovered gravity models eventually compared to GOCE solutions. We conclude that for the specific CAI gradiometer design assumed in this paper, only the quasi-inertial mode scenario would be able to significantly outperform GOCE results at the cost of technically challenging requirements on the orbit and attitude control.