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91.
92.
MESSENGER: Exploring Mercury’s Magnetosphere   总被引:1,自引:0,他引:1  
The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission to Mercury offers our first opportunity to explore this planet’s miniature magnetosphere since the brief flybys of Mariner 10. Mercury’s magnetosphere is unique in many respects. The magnetosphere of Mercury is among the smallest in the solar system; its magnetic field typically stands off the solar wind only ∼1000 to 2000 km above the surface. For this reason there are no closed drift paths for energetic particles and, hence, no radiation belts. Magnetic reconnection at the dayside magnetopause may erode the subsolar magnetosphere, allowing solar wind ions to impact directly the regolith. Inductive currents in Mercury’s interior may act to modify the solar wind interaction by resisting changes due to solar wind pressure variations. Indeed, observations of these induction effects may be an important source of information on the state of Mercury’s interior. In addition, Mercury’s magnetosphere is the only one with its defining magnetic flux tubes rooted beneath the solid surface as opposed to an atmosphere with a conductive ionospheric layer. This lack of an ionosphere is probably the underlying reason for the brevity of the very intense, but short-lived, ∼1–2 min, substorm-like energetic particle events observed by Mariner 10 during its first traversal of Mercury’s magnetic tail. Because of Mercury’s proximity to the sun, 0.3–0.5 AU, this magnetosphere experiences the most extreme driving forces in the solar system. All of these factors are expected to produce complicated interactions involving the exchange and recycling of neutrals and ions among the solar wind, magnetosphere, and regolith. The electrodynamics of Mercury’s magnetosphere are expected to be equally complex, with strong forcing by the solar wind, magnetic reconnection, and pick-up of planetary ions all playing roles in the generation of field-aligned electric currents. However, these field-aligned currents do not close in an ionosphere, but in some other manner. In addition to the insights into magnetospheric physics offered by study of the solar wind–Mercury system, quantitative specification of the “external” magnetic field generated by magnetospheric currents is necessary for accurate determination of the strength and multi-polar decomposition of Mercury’s intrinsic magnetic field. MESSENGER’s highly capable instrumentation and broad orbital coverage will greatly advance our understanding of both the origin of Mercury’s magnetic field and the acceleration of charged particles in small magnetospheres. In this article, we review what is known about Mercury’s magnetosphere and describe the MESSENGER science team’s strategy for obtaining answers to the outstanding science questions surrounding the interaction of the solar wind with Mercury and its small, but dynamic, magnetosphere.  相似文献   
93.
The techniques of recursive estimation and Kalman filtering are applied to the problem of estimation of space vehicle orbits and trajectories using only measurements of the Doppler shifts in radio signals transmitted by or reflected from the target. Two approaches are described. One is a global fit technique in which the parameters of an assumed trajectory model are estimated. The second is a pointwise mapping technique in which the trajectory estimate is extrapolated as the target tracking data is received. The paper provides a tutorial illustration of the direct application of recursive optimization techniques to a class of engineering problems.  相似文献   
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J.P.B. Vreeburg   《Acta Astronautica》2001,48(5-12):479-483
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95.
High-frequency-link power conversion and distribution based on a resonant inverter has been recently proposed. The design of several topologies is reviewed and a simple approximate design procedure is developed for the phase-controlled parallel-loaded resonant inverter. This design procedure seeks to ensure the expected benefits of resonant conversion and is verified by data from a laboratory 2.5 kVA, 20-kHz converter. A simple phasor analysis is introduced as a useful approximation for design purposes. Experimental results show that under transient conditions such as load short-circuit, a reversal of the expected commutation sequence is possible. This should be accounted for in the design of the power circuit, or prevented by the design of the controller  相似文献   
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For the foreseeable future, the search for evidence of past life in rocks acquired from other planets will be constrained by the amount of sample available and by the fidelity of preservation of any fossils present. What amount of rock is needed to establish the existence of past life? To address this question, we studied a minute amount of rock collected from cherty dolomites of the Proterozoic Buxa Formation in the metamorphically altered tectonically active northeastern Himalaya. In particular, we investigated 2 small petrographic thin sections-one from each of 2 bedded chert horizons exposed in the Ranjit River stratigraphic section northwest of Rishi, Sikkim, India-that together comprise an area of approximately 5 cm(2) (about the size of a US postage stamp) and have a total rock weight of approximately 0.1 g. Optical microscopy, confocal laser scanning microscopy, and Raman spectroscopy and imagery demonstrate that each of the thin sections contains a rich assemblage of 3-dimensionally permineralized organic-walled microfossils. This study, the first report of Proterozoic microfossils in units of the Ranjit tectonic window, demonstrates that firm evidence of early life can be adduced from even a minuscule amount of fossil-bearing ancient rock.  相似文献   
99.
Electromechanical Actuation Technology for the All-Electric Aircraft   总被引:1,自引:0,他引:1  
Electromechanical actuation is a critical element that must be developed and verified to make the all-electric aircraft a viable concept. For several years the Flight Control Division of the Air Force Wright Aeronautical Laboratories has sponsored activities to demonstrate the credibility of electromechanical actuation systems (EMAS) for primary flight control actuation functions. The foundation for these EMAS activities and several electromechanical actuation development programs are described here. One involves the design, fabrication, and laboratory test of a rotary, hingeline electromechanical actuator. Another involves the development and flight test demonstration of a linear electromechanical actuator for controlling an aileron of a C-141 aircraft. A third involves the design and development of a linear electromechanical actuator for missiles having severe performance, temperature, and volumetric requirements. In addition, a brief summary of the results from two aircraft actuation trade studies compare the baseline (conventional) hydraulic flight control system with an all-electric airplane concept including quantitative comparisons of weight, reliability and maintainability, and life cycle costs.  相似文献   
100.
The case of data fusion of sensors dissimilar in their measurement/tracking errors is considered. It is shown that the fused track performance is similar whether the sensor data are fused at the track level or at the measurement level. The case of a cluster of targets, resolved by one sensor but not the other, is also considered. Under certain conditions the fused track may perform worse than the worst of the sensors. A remedy to this problem through modifications of the association algorithm is presented  相似文献   
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