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1.
Optimal deflection of NEOs en route of collision with the Earth 总被引:1,自引:0,他引:1
Recently, a method for the n-body analysis of the velocity change required to deflect a hazardous near-Earth object (NEO) was presented by Carusi et al. [Carusi, A., Valsecchi, G.B., D'Abramo, G., Boattini A., 2002. Icarus 159, 417-422]. We extent this method in order to optimize the velocity change vector instead of its along-track magnitude. From an application of both methods to a fictitious NEO we find Carusi's parallel approach to be reasonable for phases of unperturbed two-body motion. But, for orbit phases inhering third-body perturbations, i.e., for planetary close approaches or prior to a collision, the results obtained from the new method show the radial component of deflection impulse to play a major role. We show that a fivefold greater efficiency can be achieved by a deflection impulse being non-parallel to orbital velocity. The new method is applied to two possible 99942 Apophis impact trajectories in order to provide constraints for future Apophis deflection mission analysis. 相似文献
2.
Heinz Isliker 《Solar physics》1992,141(2):325-334
Solar flares have a fragmented structure. Dynamical systems theory, for instance in its form of dimensional analysis, can analyze such structures. It answers the question whether the underlying process is deterministic or stochastic. If the process is deterministic, it provides a measure of how complicated the process is (the fractal dimension). In order to be reliable, the analysis has to be combined with the investigation of stationarity.We apply this method to ms-spikes, observed in the decimetric range, which are possibly a manifestation of flare fragmentation. We compare the system-theoretical properties - such as stationarity, stochasticity or deterministic behaviour - of the ms-spikes to the properties of several classes of suggested scenarios. This permits us to discuss different scenarios from a general point of view and to derive general properties of the source.Paper presented at the 4th CESRA Workshop in Ouranopolis (Greece) 1991. 相似文献
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We present archival Rossi X-ray Timing Explorer ( RXTE ) and simultaneous Advanced Satellite for Cosmology and Astrophysics ( ASCA ) data of the eclipsing low mass X-ray binary (LMXB) X 1822−371. Our spectral analysis shows that a variety of simple models can fit the spectra relatively well. Of these models, we explore two in detail through phase-resolved fits. These two models represent the case of a very optically thick and a very optically thin corona. While systematic residuals remain at high energies, the overall spectral shape is well approximated. The same two basic models are fitted to the X-ray light curve, which shows sinusoidal modulations interpreted as absorption by an opaque disc rim of varying height. The geometry we infer from these fits is consistent with previous studies: the disc rim reaches out to the tidal truncation radius, while the radius of the corona (approximated as spherical) is very close to the circularization radius. Timing analysis of the RXTE data shows a time-lag from hard to soft consistent with the coronal size inferred from the fits. Neither the spectra nor the light curve fits allow us to rule out either model, leaving a key ingredient of the X 1822−371 puzzle unsolved. Furthermore, while previous studies were consistent with the central object being a 1.4 M⊙ neutron star, which has been adopted as the best guess scenario for this system, our light curve fits show that a white dwarf or black hole primary can work just as well. Based on previously published estimates of the orbital evolution of X 1822−371, however, we suggest that this system contains either a neutron star or a low mass (≲2.5 M⊙ ) black hole and is in a transitional state of duration shortward of 107 yr. 相似文献
5.
Geobarometry for Peridotites: Experiments in Simple and Natural Systems from 6 to 10 GPa 总被引:3,自引:0,他引:3
Experiments with peridotite minerals in simple (MgO–Al2O3–SiO2,CaO–MgO–SiO2 and CaO–MgO–Al2O3–SiO2)and natural systems were conducted at 1300–1500°Cand 6–10 GPa using a multi-anvil apparatus. The experimentsin simple systems demonstrated consistency with previous lowerpressure experiments in belt and piston–cylinder set-ups.The analysis of spatial variations in pyroxene compositionswithin experimental samples was used to demonstrate that pressureand temperature variations within the samples were less than0·4 GPa and 50°C. Olivine capsules were used in natural-systemexperiments with two mineral mixtures: SC1 (olivine + high-Alorthopyroxene + high-Al clinopyroxene + spinel) and J4 (olivine+ low-Al orthopyroxene + low-Al clinopyroxene + garnet). Theexperiments produced olivine + orthopyroxene + garnet ±clinopyroxene assemblages, occasionally with magnesite and carbonate-richmelt. Equilibrium compositions were derived by the analysisof grain rims and evaluation of mineral zoning. They were comparedwith our previous experiments with the same starting mixturesat 2·8–6·0 GPa and the results from simplesystems. The compositions of minerals from experiments withnatural mixtures show smooth pressure and temperature dependencesup to a pressure of 8 GPa. The experiments at 9 and 10 GPa producedandradite-rich garnets and pyroxene compositions deviating fromthe trends defined by the lower pressure experiments (e.g. higherAl in orthopyroxene and Ca in clinopyroxene). This discrepancyis attributed to a higher degree of oxidation in the high-pressureexperiments and an orthopyroxene–high-P clinopyroxenephase transition at 9 GPa. Based on new and previous resultsin simple and natural systems, a new version of the Al-in-orthopyroxenebarometer is presented. The new barometer adequately reproducesexperimental pressures up to 8 GPa. KEY WORDS: garnet; mineral equilibrium; multi-anvil apparatus; orthopyroxene; geobarometry 相似文献
6.
Gerhard Ludwig 《Mineralium Deposita》1966,1(3):193-200
Zusammenfassung Mikroskopische Anschliffe feinkörniger Aufbereitungsprodukte können nur dann quantitativ ausgewertet werden, wenn ihre Schliffqualität so hoch ist, daß auch feinste Partien sicher zu bestimmen sind. Daneben muß gewährleistet sein, daß die an der Schliffoberfläche erfaßten Körner die tatsächliche Verteilung der Mineralien der Probe repräsentieren. Diese Forderung kann nur durch zweckentsprechende Präparation, die dem eigentlichen Schleif- und Polierprozeß vorausgeht, erfüllt werden. Als günstigste Lösung erscheint es, das Korngemisch exakt als Einkornschicht zu präparieren und anschließend wie Stückproben anzuschleifen. Die Schliffe können dann nach den gleichen statistischen Gesetzmäßigkeiten ausgewertet werden, wie sie bei praktisch zweidimensionalen Dünnschliffen oder bei Streupräparaten nichtopaker Korngemische vorliegen.
Mitteilung Nr. 141 aus dem Forschungsinstitut für Aufbereitung der Deutschen Akademie der Wissenschaften zu Berlin, Freiberg (Sachsen). Direktor: Prof. Dr.-Ing. habil. H. Kirchberg. 相似文献
Quantitative examination of fine-grained ore dressing products depends on good quality of the sections and on whether very small particles and areas of grains can be determined. Besides, it must absolutely be guaranteed that the grains at the section surface represent the actual distribution of the minerals in the sample. This condition can only be realized by suitable grain mounting prior to grinding and polishing. The best method for this purpose appears to be the preparation of a mono-grain layer. The polished sections can then be examined according to the same statistical laws as two-dimensional thin sections or immersion-liquid slides of non-opaque mixtures of grains.
Mitteilung Nr. 141 aus dem Forschungsinstitut für Aufbereitung der Deutschen Akademie der Wissenschaften zu Berlin, Freiberg (Sachsen). Direktor: Prof. Dr.-Ing. habil. H. Kirchberg. 相似文献
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Microbial Biomass Dynamics Along a Trophic Gradient at the Atlantic Barrier Reef off Belize (Central America) 总被引:2,自引:0,他引:2
Abstract. Recent findings indicate that heterotrophic bacteria and not phytoplankton are the most numerous biomass components even in the euphotic zone of oligotrophic, open oceans. In this study it was hypothesized that the microbial biomass components change within a few hundred meters as oligotrophic water flows across the reef and becomes enriched with nutrients. Along a trophic gradient, four stations at the Atlantic Barrier Reef off Belize (Central America) were sampled for microbial biomass components. Phytoplankton biomass (measured as chlorophyll a) ranged from the most oligotrophic station (St. 1) to the most eutrophic station (St. 4) from 6.9–415.5 μg CI"' (assuming a C:chl a ratio of 30): heterotrophic bacterial biomass increased 4-fold (from 10.1–46.4μg C 1-1 ), heterotrophic nanoflagellate (HNAN) biomass increased from 4.6-19ug C 1-1 , and cyanobacteria from 0.9-4.5 μg C-1-1 . Production estimates derived from seawater cultures revealed a 5-fold increase in bacterial production from the oligotrophic station (3.7 ug C 1-1 d-1 ) to the eutrophic St. 4 (17.8ug C-1-d1-1 )- Cyanobacterial production rose from 1.1–3.5ug C-1–d-1 and HNAN production from 0.65-1.13 μg C-1-1 -d-1 . While cyanobacteria contributed between 13 and 20% to the autotrophic plankton component in the oligotrophic waters, their contribution dropped to about 1 % at the eutrophic stations. 相似文献
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Diatoms are major primary producers of microbial biomass in the Antarctica. They are found in the water and sea ice. The distribution, abundance of the ice diatoms and their relation to the environmental factors inside and outside the ice have been studied for its special role in the Antarctic Ocean ecology. In this paper we describe the abundance, distribution and composition of diatom assemblages in 相似文献