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排序方式: 共有409条查询结果,搜索用时 46 毫秒
331.
Searching for biosignatures using electron paramagnetic resonance (EPR) analysis of manganese oxides 总被引:1,自引:0,他引:1
Kim SS Bargar JR Nealson KH Flood BE Kirschvink JL Raub TD Tebo BM Villalobos M 《Astrobiology》2011,11(8):775-786
Manganese oxide (Mn oxide) minerals from bacterial sources produce electron paramagnetic resonance (EPR) spectral signatures that are mostly distinct from those of synthetic simulants and abiogenic mineral Mn oxides. Biogenic Mn oxides exhibit only narrow EPR spectral linewidths (~500 G), whereas abiogenic Mn oxides produce spectral linewidths that are 2-6 times broader and range from 1200 to 3000 G. This distinction is consistent with X-ray structural observations that biogenic Mn oxides have abundant layer site vacancies and edge terminations and are mostly of single ionic species [i.e., Mn(IV)], all of which favor narrow EPR linewidths. In contrast, abiogenic Mn oxides have fewer lattice vacancies, larger particle sizes, and mixed ionic species [Mn(III) and Mn(IV)], which lead to the broader linewidths. These properties could be utilized in the search for extraterrestrial physicochemical biosignatures, for example, on Mars missions that include a miniature version of an EPR spectrometer. 相似文献
332.
333.
We investigate how ideas from the International Environmental Agreement (IEA) literature can be applied to the problem of space debris mitigation. Space debris pollution is similar to other international environmental problems in that there is a potential for a “tragedy of the commons” effect: individual nations bear all the cost of their mitigation measures but share only a fraction of the benefit. As a consequence, nations have a tendency to underinvest in mitigation. Coalitions of nations, brought together by IEAs, have the potential to lessen the tragedy of the commons effect by pooling the costs and benefits of mitigation. This work brings together two recent modeling advances: (i) a game theoretic model for studying the potential gains from IEA cooperation between nations with asymmetric costs and benefits, (ii) an orbital debris model that gives the societal cost that specific actions, such as failing to deorbit an inactive spacecraft, have on the environment. We combine these two models with empirical launch-share data for a “proof of concept” of an IEA for a single mitigation measure—deorbiting spacecraft at the end of operational lifetime. Simulations of empirically derived and theoretical launch distributions among nations suggest the possibility that voluntary coalitions can provide significant deorbiting gains relative to nations acting in the absence of an IEA agreement. 相似文献
334.
The observational and theoretical arguments for spatial fragmentation of the bulk of the thermal and non-thermal components of solar flare plasma are summarised. Observational aspects considered include XUV filling factors, EUV centre to limb variations, andH impact polarisation. Theoretical points addressed are the high flare inductance and beam/return current closure at the acceleration site.A high degree of beam/plasma filamentation implies strong transverse temperature gradients so that cross-field conduction must be included in energy transport modelling. Preliminary results are described for a simple two-component model. 相似文献
335.
There is still little theory on the strategic use of space. Some have proposed naval theorist A.T. Mahan as a guide but this viewpoint argues that, while the analogy with sea power makes sense, it is the naval theories of Sir Julian Corbett that are most apt. Parallels are drawn between the latter's emphasis on attacking commerce as the best means of crippling a seafaring state and the implications of the current US dependence on trade in information. Ways for the USA to mitigate its vulnerability in this area are suggested. 相似文献
336.
John H. McElroy 《Space Policy》2001,17(4):257-260
US government proposals to cut the scientific budget of the ISS are placed within the historical context of the US space program. The author divides this into three phases—early days to the end of Apollo, the Shuttle era, and the Space Station era—and shows that all of these have suffered from decisions to reduce scientific capabilities. Even without cuts, it is unlikely that the Station can produce scientific outcomes that are commensurate with the investment that has taken place and it is imperative that policy makers learn from the lost opportunities of the two earlier phases if greater disappointment is to be avoided. 相似文献
337.
Space Science Reviews - 相似文献
338.
John W Wilson John Tweed Steve A Walker Francis A Cucinotta Ram K Tripathi Steve Blattnig Christopher J Mertens 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2005,35(2):185-193
A new version of the HZETRN code capable of simulating HZE ions with either laboratory or space boundary conditions is under development. The computational model consists of combinations of physical perturbation expansions based on the scales of atomic interaction, multiple scattering, and nuclear reactive processes with use of asymptotic/Neumann expansions with non-perturbative corrections. The code contains energy loss with straggling, nuclear attenuation, nuclear fragmentation with energy dispersion and downshifts, and off-axis dispersion with multiple scattering under preparation. The present benchmark is for a broad directed beam for 1 A GeV iron ion beams with 2 A MeV width and four targets of polyethylene, polymethyl metachrylate, aluminum, and lead of varying thickness from 5 to 30 g/cm2. The benchmark quantities will be dose, track averaged LET, dose averaged LET, fraction of iron ion remaining, and fragment energy spectra after 23 g/cm2 of polymethyl metachrylate. 相似文献
339.
Current thinking about the Moon as a destination has revitalized interest in lunar astronomical observatories. Once seen by a large scientific community as a highly enabling site, the dramatic improvement in capabilities for free-space observatories prompts reevaluation of this interest. Whereas the lunar surface offers huge performance advantages for astronomy over terrestrial sites, free-space locales such as Earth orbit or Lagrange points offer performance that is superior to what could be achieved on the Moon. While astronomy from the Moon may be cost-effective once infrastructure is there, it is in many respects no longer clearly enabling compared with free space. 相似文献
340.
Monitoring Changes in Continental Water Storage with GRACE 总被引:1,自引:0,他引:1
The Gravity Recovery and Climate Experiment, GRACE , will enable the recovery of monthly estimates of changes in water storage,
on land and in the ocean, averaged over arbitrary regions having length scales of a few hundred km and larger. These data
will allow the examination of changes in the distribution of water in the ocean, in snow and ice on polar ice sheets, and
in continental water and snow storage. Extracting changes in water storage from the GRACE dataset requires the use of averaging
kernels which can isolate a particular region. To estimate the accuracy to which continental water storage changes in a few
representative regions may be recovered, we construct a synthetic GRACE dataset from global, gridded models of surface-mass
variability. We find that regional changes in water storage can be recovered with rms error less than 1 cm of equivalent water
thickness, for regions having areas of 4 × 105 km2 and larger. Signals in smaller regions may also be recovered; however, interpretations of such results require a careful
consideration of model resolution, as well as the nature of the averaging kernel.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献