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Abstract In this paper we propose a spatial ontology for reasoning about holes, rigid objects and a string, taking a classical puzzle as a motivating example. In this ontology the domain is composed of spatial regions whereby a theory about holes is defined over a mereotopological basis. Within this theory we define a data structure, named chain, that facilitates a clear and efficient representation of the puzzle states and its solution. 相似文献
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Over the last 3 years, a team at JPL has worked to design a new concept for a small, low cost lander applicable to a variety of in-situ lunar exploration activities. This concept, named Lunette, originated as a design which would exploit potential excess capacity of EELV launches by being compatible with the EELV Secondary Payload Adapter (ESPA). The original Lunette mission concept would have allowed up to six low cost landers to be delivered to a targeted region of the moon, with landings separated by a few km, allowing establishment of a regional network with a single, shared launch. The original concept faced limits in the extent of regional distribution of landing sites since all six landers were dependent on a single solid rocket braking motor. In the last year the Lunette team has focused on a modification of the original ESPA-based concept to a design that would allow launch of multiple individual landers (each with its own braking stage) on a single launch vehicle, where each lander would be capable of independent targeting and landing. With such an implementation, the entire lunar surface could be accessed for establishment of network nodes that could enable high priority geophysical measurements on a scale not seen since Apollo. The present paper discusses the current state of the design of the Lunette geophysical network lander, as well as describing mission design, science operations, and an innovative design solution allowing the lander to take critical data continuously, even over the lunar night, without the need for radioisotope power systems. 相似文献
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Cullers DK 《Acta Astronautica》1986,13(1):31-37
Highly monochromatic signals, such as TV carriers, can be detected sensitively by using a narrow filter (b < or = 1 Hz) and performing power accumulation on its output. If instead a low-duty-cycle pulsed signal of the same total energy is present, the sensitivity of a square law device, followed by a thresholding operation (to eliminate most samples containing only noise), followed by the algorithm to be described, is greater by about 7 dB in typical cases. This is particularly interesting to SETI because such a pulsed signal is exactly what is sent by a rotating beacon with a directional antenna. Such a pulsed signal is, therefore, a good candidate for an extraterrestrial beacon. Software for detecting this signal type is now ready for field testing with the NASA Multichannel Spectrum Analyzer (MCSA). 相似文献
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When a micro-debris or a micrometeoroid impacts a spacecraft surface, a large number of secondary particles, called ejecta, are produced. These particles can contribute to a modification of the debris environment: either locally by the occurrence of secondary impacts on the components of complex and large space structures, or at great distance by the formation of a population of small orbital debris. This paper describes firstly, the ejecta overall production, and secondly, the lifetime and the orbital evolution of the particles. Finally the repartition of ejecta in LEO is computed. Some results describing the population as a function of size and altitude are presented. 相似文献
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全失重液体晃动的固有频率 总被引:1,自引:0,他引:1
本文研究失重条件下液滴,带核液体和球腔内液体晃动的固有频率,对于几何球对称系统,本文给出了其解析解;而对于非球对系系统,则采用边界元方法计算得到晃动频率的数值解。 相似文献
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The aim of this paper is to study, from a mission analysis point of view, the performance of a hybrid propulsion concept for a two-dimensional transfer towards a planet of the Solar System. The propulsion system is obtained by combining a chemical thruster, used for the phases of Earth escape and/or target planet capture, with an electric sail, which provides a continuous thrust during the heliocentric transfer. Two possible mission scenarios are investigated: in the first case the sailcraft reaches the target planet with zero hyperbolic excess velocity, thus performing a classical rendezvous mission in a heliocentric framework. In the second mission scenario, a given final hyperbolic excess velocity relative to the planet is tolerated in order to decrease the mission flight time. The amount of final hyperbolic excess velocity is used as a simulation parameter for a tradeoff study in which the minimum flight time is related to the total velocity variation required by the chemical thruster to accomplish the mission, that is, for Earth escape and planetary capture. 相似文献
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火星稀薄的大气使得探测器再入火星时难以获得足够的减速阻力,为此,文章提出利用磁阻尼增加阻力的概念。高速再入的探测器与火星大气剧烈摩擦产生等离子体,利用探测器上的磁偶极场将等离子体捕获,同时在火星大气中形成一个“磁泡”区域并跟随探测器。由“磁泡”束缚的等离子体与来流的中性气体发生碰撞获得动量,又通过磁场作用将动量传递给探测器,从而使探测器获得一定的减速阻力。在磁阻尼的作用下,在同样的开伞高度探测器速度可降至更低,而低速开伞又可提高阻力伞打开的可靠性。因此,磁阻尼不仅可加快降低再入速度,而且还有助于提高探测器着陆火星的安全性。 相似文献
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为了防止空间结构与机构产品中的非金属材料对航天器造成污染,要在产品装配前去除其中的可挥发性物质,需对产品进行真空烘烤清洁处理,因此研制了一套结构与机构防污染的真空烘烤设备。该套设备主要包括真空容器、热沉、真空抽气系统、氮系统、去污染系统、电气控制系统等。文章对该设备的系统组成、主要性能指标及使用功能进行了详细的说明。 相似文献
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If Europa is to be of primary exobiological interest, namely, as a habitat for extant life, it is obvious that (a) a hydrosphere must prevail beneath the cryosphere for a long time, (b) internal energy sources must be present in a sufficient state of activity, and (c) a reasonable technical means must be available for assessing if indeed life does exist in the hypothesized hydrosphere. This discussion focuses on the last point, namely, technological issues, because the trend of the compounding evidence about Europa indicates that the first two points are likely to be true. First, we present a consideration of time-of-flight mass spectroscopy conducted in situ on the cryosphere surface of Europa during a first landed robotic mission. We assert that this is a reasonable technical means not only for exploring the composition of the cryosphere itself, but also for locating any biomolecular indicators of extant life brought to the surface through cryosphere activity. Secondly, this work also addresses practical issues inherent in any kind of instrumental interrogation of a surface whose properties are governed by radiation chemistry. This includes advocating the construction of a Europan surface simulator to familiarize instrumental system developers with the spacecraft- and instrument-scale conditions under which such an interrogation would take place on Europa. Such a simulator is mandatory in certification of the functional utility of a flight instrument. 相似文献
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月面巡视器的任务层路径规划 总被引:2,自引:1,他引:1
使用巡视器对月球表面进行巡视探测是一种高效率、低成本的月球探测方法。路径规划作为巡视器的一项重要技术,通常把它作为导航系统的一部分,只考虑地形通过性的问题。实际上除了地形通过性,还有很多因素对路径选择起到决定性的作用。针对月面巡视器,在大范围区域综合考虑地形、能源、热控、通信等全局因素,给出了一种新的路径规划方法——实时贪婪(Realtime Greedy,RG)算法。运用该算法得到了任务层路径,为巡视器的导航系统提供路标点,并为巡视器的动作安排提供了依据。 相似文献
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A mathematically well-posed technique is suggested to obtain first-order necessary conditions of local optimality for the problems of optimization to be solved in a pulse formulation for flight trajectories of a spacecraft with a high-thrust jet engine (HTJE) in an arbitrary gravitational field in vacuum. The technique is based on the Lagrange principle of derestriction for conditional extremum problems in a function space. It allows one to formalize an algorithm of change from the problems of optimization to a boundary-value problem for a system of ordinary differential equations in the case of any optimization problem for which the pulse formulation makes sense. In this work, such a change is made for the case of optimizing the flight trajectories of a spacecraft with a HTJE when terminal and intermediate conditions (like equalities, inequalities, and the terminal functional of minimization) are taken in a general form. As an example of the application of the suggested technique, we consider in this work, within the framework of a bounded circular three-point problem in pulse formulation, the problem of constructing the flight trajectories of a spacecraft with a HTJE through one or several libration points (including the case of going through all libration points) of the Earth–Moon system. The spacecraft is launched from a circular orbit of an Earth's artificial satellite and, upon passing through a point (or points) of libration, returns to the initial orbit. The expenditure of mass (characteristic velocity) is minimized at a restricted time of transfer. 相似文献
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For long enough tethers, the coupling of the attitude and orbital dynamics may show non-negligible effects in the orbital motion of a tethered satellite about a central body. In the case of fast rotating tethers the attitude remains constant, on average, up to second order effects. Besides, for a tether rotating in a plane parallel to the equatorial plane of the central body, the attitude–orbit coupling effect is formally equal to the perturbation of the Keplerian motion produced by the oblateness of the central body and, therefore, may have a stabilizing effect in the orbital dynamics. In the case of a tethered satellite in a low lunar orbit, it is demonstrated that feasible tether lengths can help in modifying the actual map of lunar frozen orbits. 相似文献
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Papagiannis MD 《Journal of the British Interplanetary Society》1992,45(6):227-230
The availability of liquid water is the most important factor that makes a planet habitable, because water is a very effective polar molecule and hence an excellent solvent and facilitator for the complex chemistry of life. Its presence presupposes a planet with a significant mass that guarantees the presence of a substantial atmosphere, and a reasonable spinning rate to avoid overheating. It also implies that the planet is at moderate distances from its central star, a range that is called the Ecosphere or the Habitable Zone. Since the evolution of life to high intelligence seems to take billions of years, it requires also that the central star must be neither too massive, that will produce a lot of lethal UV radiation and will have too short a life-span to allow life to evolve, nor of very small mass which will be producing too feeble a radiation to sustain life. The detection of free Oxygen in the atmosphere of a planet is a very strong evidence for the presence of life, because Oxygen is highly reactive and would rapidly disappear by combining with other elements, unless it is continuously replenished by life as the by-product of the process of photosynthesis that builds food for life (sugars) from CO2 and H2O. 相似文献
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In 1994–1995 Lavochkin Association (Russia) together with the other enterprises in accordance with technical requirements of the Russian Space agency, developed a new Russian communication satellite of a small class that will operate in both the geostationary (GSO) and high-elliptical (HEO) orbits. This satellite may be injected into operational orbits using a SOYUZ-2 launch vehicle (LV) and a FREGAT upper stage (US) from Plesetsk and Baykonur space launch sites (SLS).The main reason for creating such a satellite was to decrease the cost of the support and development of the Russian communication geostationary satellites group.Russian satellites Horizont, Express, Ekran and Gals, which operate in GSO, are the basis of the space segment for communications, radio and TV broadcasting. All of these satellites are injected into GSO by the PROTON LV. PROTON is a launch vehicle of a heavy class. The use of a middle class LV instead of a heavy class will allow to reduce considerably the launch cost. The change of a heavy class LV to a LV of middle class determined one economic reason for this project. Besides, the opportunity to launch S/C into GSO from Russian Plesetsk SLS increases the independence of Russia in the domain of space communications, despite the presence of the contract with Kazachstan about the rent of Baykonur SLS. Finally, use of small satellites with a rather small number of transponders is more effective than the use of big satellites. It will allow also to increase a satellite group (by the launch of additional satellites) precisely in accordance to the development of the ground segment. 相似文献