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151.
Meteoroids and orbital debris pose a serious damage threat to all spacecraft. The effects of a meteoroid/orbital debris (M/OD) impact depend on a variety of factors, including where the M/OD impact occurs, the size, composition, and speed of the impacting object, and the function of the impacted spacecraft system. These effects can be minimal, can degrade a functional spacecraft component, or can compromise spacecraft functionality, even to the point of mission loss or loss of life. To minimize the damage threat from the meteoroid/orbital debris environment, it is often necessary to install protective shielding around critical spacecraft systems. If a system cannot be shielded, operational constraints may need to be imposed to reduce the damage threat. This paper presents an overview of the research and development activities performed since the late 1950s with an aim of increasing the level of protection afforded satellites and spacecraft operating in the M/OD environment and ultimately mitigating the mechanical and structural effects of an M/OD impact. 相似文献
152.
Tewell J. Robert Tobey William H. 《IEEE transactions on aerospace and electronic systems》1966,(3):346-352
One purpose of the GEMINI program is to study the problem of and establish techniques associated with acquisition, tracking, and photography of terrestrial objects from the orbiting spacecraft. This paper describes a ground-based simulation which was performed to obtain a realistic understanding of this problem and to provide the astronauts with preflight experience. The results presented evaluate the test subjects' performance in terms of tracking-rate errors, position errors, and fuel consumption for various orbital altitudes and target offsets. 相似文献
153.
Cosmogenic argon isotopes are produced in feldspars via nuclear reactions between cosmic rays and Ca and K atoms within the lattice. These cosmogenic isotopes can be used as proxies for K and Ca, much like nuclear reactor-derived 39Ar and 37Ar are used as proxies for K and Ca, respectively, in 40Ar/39Ar geochronology. If Ca and K are uniformly distributed, then the ratio of radiogenic 40Ar (40Ar?) to cosmogenic 38Ar or 36Ar (38Arcos or 36Arcos) is proportional to the difference between the radioisotopic and exposure ages, as well as the K/Ca ratio of the degassing phase. Thus cosmogenic, radiogenic, and trapped Ar isotopes, all of which can be measured remotely and are stable over geologic time, are sufficient to generate an isochron-like diagram from which the isotopic composition of the trapped component may be inferred. Such data also provide a means to assess the extent to which the system has remained closed with respect to 40Ar?, thereby mitigating otherwise unquantifiable uncertainties that complicate the conventional K–Ar dating method. 相似文献
154.
Mavroides William G. Dennett Lancer G. Dorr Leon S. 《IEEE transactions on aerospace and electronic systems》1966,(3):323-331
A height-finding technique utilizing the relative phase between a series of point sources of a traveling-wave array is described. The point sources in the focal region of a torus antenna are used to control the phase of the antenna elevation pattern in space. Signals received from a given beam angle will arrive at each terminal of the traveling-wave feed with a different phase. By comparing this phase with a reference phase, the angular direction of an arriving plane wave can be measured with considerable accuracy. Thus a radar system with a single antenna and feed structure can be employed to yield instantaneous height coverage along with the usual range data. 相似文献
155.
David Auslander Joshua Cermenska Gregory Dalton Mauricio de la Pena C. K. H. Dharan William Donokowski Robert Duck Jonghak Kim David Pankow Alec Plauche Mustapha Rahmani Stephen Sulack Tien Fak Tan Paul Turin Tyler Williams 《Space Science Reviews》2008,141(1-4):185-211
The five “Time History of Events and Macroscale Interactions during Substorms” (THEMIS) micro-satellites launched on a common carrier by a Delta II, 7925 heavy, on February 17, 2007. This is the fifth launch in the NASA MeDIum class EXplorer (MIDEX) program. In the mission proposal the decision was made to have the University of California Berkeley Space Sciences Laboratory (UCB-SSL) mechanical engineering staff provide all of the spacecraft appendages, in order to meet the short development schedule, and to insure compatibility. This paper describes the systems engineering, design, development, testing, and on-orbit deployment of these boom systems that include: the 1 and 2 meter carbon fiber composite magnetometer booms, the 40 and 50 m tip to tip orthogonal spin-plane wire boom pairs, and the 6.3 m dipole stiff axial booms. 相似文献
156.
A major difficulty that has long hindered studies of organic-walled Precambrian microbes in petrographic thin sections is the accurate documentation of their three-dimensional morphology. To address this need, we here demonstrate the use of confocal laser scanning microscopy. This technique, both non-intrusive and non-destructive, can provide data by which to objectively characterize, in situ and at submicron-scale resolution, the cellular and organismal morphology of permineralized (petrified) microorganisms. Application of this technique can provide information in three dimensions about the morphology, taphonomy, and fidelity of preservation of such fossils at a spatial resolution unavailable by any other means. 相似文献
157.
Microbial life on Earth uses a wide range of chemical and energetic resources from diverse habitats. An outcome of this microbial diversity is an extensive and varied list of metabolic byproducts. We review key points of Earth-based microbial metabolism that are useful to the astrophysical search for biosignature gases on exoplanets, including a list of primary and secondary metabolism gas byproducts. Beyond the canonical, unique-to-life biosignature gases on Earth (O(2), O(3), and N(2)O), the list of metabolic byproducts includes gases that might be associated with biosignature gases in appropriate exoplanetary environments. This review aims to serve as a starting point for future astrophysical biosignature gas research. 相似文献
158.
Abstract Modern ecological niches are teeming with an astonishing diversity of microbial life in biofilms closely associated with mineral surfaces, which highlights the remarkable success of microorganisms in conquering the challenges and capitalizing on the benefits presented by the mineral-water interface. Biofilm formation capability likely evolved on early Earth because biofilms provide crucial cell survival functions. The potential toxicity of mineral surfaces toward cells and the complexities of the mineral-water-cell interface in determining the toxicity mechanisms, however, have not been fully appreciated. Here, we report a previously unrecognized role for extracellular polymeric substances (EPS), which form biofilms in shielding cells against the toxicity of mineral surfaces. Using colony plating and LIVE/DEAD staining methods in oxide suspensions versus oxide-free controls, we found greater viability of wild-type, EPS-producing strains of Pseudomonas aeruginosa PAO1 compared to their isogenic knockout mutant with defective biofilm-producing capacity. Oxide toxicity was specific to its surface charge and particle size. High resolution transmission electron microscopy (HRTEM) images and assays for highly reactive oxygen species (hROS) on mineral surfaces suggested that EPS shield via both physical and chemical mechanisms. Intriguingly, qualitative as well as quantitative measures of EPS production showed that toxic minerals induced EPS production in bacteria. By determining the specific toxicity mechanisms, we provide insight into the potential impact of mineral surfaces in promoting increased complexity of cell surfaces, including EPS and biofilm formation, on early Earth. Key Words: Mineral toxicity-Bacteria-EPS evolution-Biofilms-Cytotoxicity-Silica-Anatase-Alumina. Astrobiology 12, 785-798. 相似文献
159.
Redox chemistry is central to life on Earth. It is well known that life uses redox chemistry to capture energy from environmental chemical energy gradients. Here, we propose that a second use of redox chemistry, related to building biomass from environmental carbon, is equally important to life. We apply a method based on chemical structure to evaluate the redox range of different groups of terrestrial biochemicals, and find that they are consistently of intermediate redox range. We hypothesize the common intermediate range is related to the chemical space required for the selection of a consistent set of metabolites. We apply a computational method to show that the redox range of the chemical space shows the same restricted redox range as the biochemicals that are selected from that space. By contrast, the carbon from which life is composed is available in the environment only as fully oxidized or reduced species. We therefore argue that redox chemistry is essential to life for assembling biochemicals for biomass building. This biomass-building reason for life to require redox chemistry is in addition (and in contrast) to life's use of redox chemistry to capture energy. Life's use of redox chemistry for biomass capture will generate chemical by-products-that is, biosignature gases-that are not in redox equilibrium with life's environment. These potential biosignature gases may differ from energy-capture redox biosignatures. 相似文献
160.
JW Schopf JD Farmer IS Foster AB Kudryavtsev VA Gallardo C Espinoza 《Astrobiology》2012,12(7):619-633
Abstract Orbital and in situ analyses establish that aerially extensive deposits of evaporitic sulfates, including gypsum, are present on the surface of Mars. Although comparable gypsiferous sediments on Earth have been largely ignored by paleontologists, we here report the finding of diverse fossil microscopic organisms permineralized in bottom-nucleated gypsums of seven deposits: two from the Permian (~260?Ma) of New Mexico, USA; one from the Miocene (~6?Ma) of Italy; and four from Recent lacustrine and saltern deposits of Australia, Mexico, and Peru. In addition to presenting the first report of the widespread occurrence of microscopic fossils in bottom-nucleated primary gypsum, we show the striking morphological similarity of the majority of the benthic filamentous fossils of these units to the microorganisms of a modern sulfuretum biocoenose. Based on such similarity, in morphology as well as habitat, these findings suggest that anaerobic sulfur-metabolizing microbial assemblages have changed relatively little over hundreds of millions of years. Their discovery as fossilized components of the seven gypsiferous units reported suggests that primary bottom-nucleated gypsum represents a promising target in the search for evidence of past life on Mars. Key Words: Confocal laser scanning microscopy-Gypsum fossils-Mars sample return missions-Raman spectroscopy-Sample Analysis at Mars (SAM) instrument-Sulfuretum. Astrobiology 12, 619-633. 相似文献