Multiple spacecraft rendezvous maneuvers by differential drag and low thrust engines

A novel two-phase hybrid controller is proposed to optimize propellant consumption during multiple spacecraft rendezvous maneuvers in Low Earth Orbit. This controller exploits generated differentials in aerodynamic drag on each involved chaser spacecraft to effect a propellant-free trajectory near to the target spacecraft during the first phase of the maneuver, and then uses a fuel optimal control strategy via continuous low-thrust engines to effect a precision dock during the second phase. In particular, by varying the imparted aerodynamic drag force on each of the chaser spacecraft, relative differential accelerations are generated between each chaser and the target spacecraft along two of the three translational degrees of freedom. In order to generate this required differential, each chaser spacecraft is assumed to include a system of rotating flat panels. Additionally, each chaser spacecraft is assumed to have continuous low-thrust capability along the three translational degrees of freedom and full-axis attitude control. Sample simulations are presented to support the validity and robustness of the proposed hybrid controller to variations in the atmospheric density along with different spacecraft masses and ballistic coefficients. Furthermore, the proposed hybrid controller is validated against a complete nonlinear orbital model to include relative navigation errors typical of carrier-phase differential GPS (CDGPS). Limitations of the proposed controller appear relative to the target spacecraft’s orbit eccentricity and a general characterization of the atmospheric density. Bounds on these variables are included to provide a framework within which the proposed hybrid controller can effect an extremely low propellant rendezvous of multiple chaser spacecraft to a desired target spacecraft.     

Fractal and Multifractal Properties of Active Regions as Flare Precursors: A Case Study Based on SOHO/MDI and SDO/HMI Observations

Several studies indicate that fractal and multifractal parameters inferred from solar photospheric magnetic field measurements may help assessing the eruptive potential of Active Regions (ARs) and also predicting their flare activity. We further investigate this topic, by exploring the sensitivity of some parameters already used in the literature on data and methods employed for their estimation. In particular, we measured the generalized fractal dimensions D ₀ and D ₈, and the multifractal parameters C _div and D _div, on the time series of photospheric magnetograms of the flaring AR NOAA 11158 obtained with the SOHO/MDI and SDO/HMI. The observations by the latter instrument are characterized by a higher spatial and temporal resolution, as well as higher flux sensitivity, than the ones obtained from SOHO/MDI, which were widely employed in earlier studies. We found that the average and peak values of complexity parameters measured on the two data sets agree within measurement uncertainties. The temporal evolution of the parameters measured on the two data sets show rather similar trends, but the ones derived from the SOHO/MDI observations show larger and spurious variations over time than those deduced from analysis of the corresponding SDO/HMI data. We also found a larger sensitivity of these measurements to characteristics of the data analyzed than reported by earlier studies. In particular, analysis of the higher resolution and higher cadence SDO/HMI data allows us also to detect slight variations of the complexity indicators that cannot be derived from the analysis of the SOHO/MDI data. These variations occur right after the major events in the analyzed AR. They may be the signature of photospheric effects of coronal magnetic field re-arrangement.     

The current dynamics inside the Strait of Bonifacio: Impact of the wind effect in a little coastal strait

The dynamics of Bonifacio strait (south of Corsica) are investigated in a comprehensive study at long term from two set of data. First, by moored ADCP put in middle of the strait during six weeks (4 November 2004 to 7 January 2005), at a depth of 40 m and at few Kilometers from coast. Second, by velocity profiles obtained with a 314.4 kHz Acoustic Doppler Current Profiler (ADCP) during seven cruises (Cyrce) covering entirety strait. From moored ADCP data, a study is made on time correlation between wind stress and current. Shipboard ADCP data permits to obtain a general view in totality of strait and mostly to observe spatial variability. This study shows that the circulation is governed both by wind stress and by general circulation of Mediterranean Sea.     

The rheology of crystal-bearing basaltic magmas from Stromboli and Etna

Our ability to forecast volcanic hazards at active volcanoes stems from our knowledge of parameters affecting eruption dynamics. Persistent activity displayed by basaltic volcanic systems, such as Etna and Stromboli, is governed mainly by the textural evolution and rheological properties of eruptive products. Here, we investigate the high temperature (1131 °C < T < 1187 °C) rheological behavior and textural evolution of remelted natural basaltic magmas from Stromboli and Etna volcanic systems upon cooling and crystallization at atmospheric conditions. The pure liquid and subliquidus isothermal viscosities are investigated using a concentric cylinder apparatus by varying strain rate applied to the investigated materials. Detailed textural analysis has been performed in order to evaluate the effect of crystal distribution on the rheological evolution of magmas. At subliquidus conditions, the mineralogical assemblage is dominated by the presence of spinel and plagioclase with the occurrence of stable clinopyroxene only at the lowest temperatures (1157 and 1131 °C for the Stromboli and the Etna samples, respectively). The overall crystal fraction (?) varies between 0.06 and 0.27. In the high T regime the viscous response to applied stress and strain rate is strongly affected by the presence of even small amounts of crystals. Large discrepancies between measured and predicted viscosities obtained using the Einstein-Roscoe (ER) equation are also found at low crystal fractions. We find here that, at the investigated conditions, the physical effect caused by the presence of elongated crystals is orders of magnitude higher than that predicted by existing models which only account for spherical particles. It also appears that a weak shear thinning behavior occurs at the lowest temperatures investigated. Crystal shape and, with it, the strain-rate dependence of the rheological properties appear to play primary roles in influencing the transport properties of these basaltic magmas.     

Response surfaces of vulnerability to climate change: the Colorado River Basin,the High Plains,and California

We quantify the vulnerability of water supply to shortage for the Colorado River Basin and basins of the High Plains and California and assess the sensitivity of their water supply system to future changes in the statistical variability of supply and demand. We do so for current conditions and future socio-economic scenarios within a probabilistic framework that incorporates the inherent uncertainties in the drivers of vulnerability. Our analysis indicates that the most sensitive basins to both current and future variability of demand and supply are the Central California and the San Joaquin-Tulare basins. Large sensitivity is also found for the Kansas basin of the High Plains. Within the Colorado River Basin, the Lower Colorado and Gila were found to be the most vulnerable and sensitive sub-basins. By accounting for future uncertainty within the above probabilistic framework, this study unveils and isolates the individual responses of a given basin to changes in the statistical properties of demand and supply and offers a valuable tool for the identification of policy strategies and adaptation measures.