全文获取类型
收费全文 | 62篇 |
免费 | 1篇 |
专业分类
航空 | 31篇 |
航天技术 | 11篇 |
综合类 | 2篇 |
航天 | 19篇 |
出版年
2019年 | 1篇 |
2018年 | 1篇 |
2015年 | 1篇 |
2014年 | 4篇 |
2012年 | 1篇 |
2011年 | 2篇 |
2010年 | 2篇 |
2009年 | 4篇 |
2008年 | 8篇 |
2007年 | 6篇 |
2006年 | 2篇 |
2005年 | 3篇 |
2004年 | 2篇 |
2003年 | 4篇 |
2000年 | 4篇 |
1998年 | 1篇 |
1996年 | 2篇 |
1995年 | 2篇 |
1992年 | 1篇 |
1990年 | 2篇 |
1989年 | 3篇 |
1987年 | 1篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1968年 | 1篇 |
排序方式: 共有63条查询结果,搜索用时 296 毫秒
51.
A.L. Dobbin A.D. Aylward 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,42(9):1576-1585
The processes leading to enhancements in mid latitude nitric oxide (NO) densities following geomagnetic storms have been investigated using the University College London (UCL) Coupled Middle Atmosphere and Thermosphere (CMAT) general circulation model. A comparison of calculated storm time and quiet time NO densities at 110 km altitude reveals the presence of aurorally produced NO at both high and mid latitudes for several days after subsidence of activity. At 150 km, the NO enhancements are shorter lived and remain for up to approximately 2 days after the storm. By separating the contribution of chemical production and loss, horizontal and vertical advection, and molecular and eddy diffusion in the calculation of NO densities, we show that at 150 km altitude, horizontal transport must be taken into consideration if post-storm mid latitude enhancements are to be reproduced. Chemical production of NO at high latitudes continues for up to 2 days after subsidence of a storm at altitudes of around 150 km. We show that equatorward winds at this altitude are sufficiently strong to transport the aurorally produced NO to mid latitudes. Vertical diffusion transports NO from altitudes of 150 km and above, to lower altitudes where it is longer lived. At 110 km altitude, chemical, diffusive and advective terms must all be included in the calculation of NO density in order to simulate realistic mid latitude enhancements. We propose that it is the combined effects of increased chemical production, downward diffusion from altitudes of 150 km and above, and transport by winds that lead to increases in mid latitude NO density at altitudes of around 110 km. This is the first detailed study of the causes of post-storm mid latitude NO enhancements to use a three-dimensional general circulation model. 相似文献
52.
John F. Cavanaugh James C. Smith Xiaoli Sun Arlin E. Bartels Luis Ramos-Izquierdo Danny J. Krebs Jan F. McGarry Raymond Trunzo Anne Marie Novo-Gradac Jamie L. Britt Jerry Karsh Richard B. Katz Alan T. Lukemire Richard Szymkiewicz Daniel L. Berry Joseph P. Swinski Gregory A. Neumann Maria T. Zuber David E. Smith 《Space Science Reviews》2007,131(1-4):451-479
The Mercury Laser Altimeter (MLA) is one of the payload science instruments on the MErcury Surface, Space ENvironment, GEochemistry,
and Ranging (MESSENGER) mission, which launched on August 3, 2004. The altimeter will measure the round-trip time of flight
of transmitted laser pulses reflected from the surface of the planet that, in combination with the spacecraft orbit position
and pointing data, gives a high-precision measurement of surface topography referenced to Mercury’s center of mass. MLA will
sample the planet’s surface to within a 1-m range error when the line-of-sight range to Mercury is less than 1,200 km under
spacecraft nadir pointing or the slant range is less than 800 km. The altimeter measurements will be used to determine the
planet’s forced physical librations by tracking the motion of large-scale topographic features as a function of time. MLA’s
laser pulse energy monitor and the echo pulse energy estimate will provide an active measurement of the surface reflectivity
at 1,064 nm. This paper describes the instrument design, prelaunch testing, calibration, and results of postlaunch testing. 相似文献
53.
G. Randall Gladstone S. Alan Stern Kurt D. Retherford Ronald K. Black David C. Slater Michael W. Davis Maarten H. Versteeg Kristian B. Persson Joel W. Parker David E. Kaufmann Anthony F. Egan Thomas K. Greathouse Paul D. Feldman Dana Hurley Wayne R. Pryor Amanda R. Hendrix 《Space Science Reviews》2010,150(1-4):161-181
54.
Alan M. Levine Hale Bradt Edward H. Morgan Ron Remillard The MIT/GSFC ASM Team 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2006,38(12):2970-2975
Selected results from the Rossi X-ray Timing Explorer All-Sky Monitor are presented to illustrate the phenomenology of the light curves. The sensitivity to periodic intensity variations is indicated by the folded light curve of AM Her. The gray line between transient and persistent sources is emphasized. Light curves of a range of systems comprising black holes or neutron stars and low and high mass companion stars show that the behavior of these systems is often, but not always, characteristic. 相似文献
55.
56.
Alan Burgess G 《航空制造技术》2004,(Z1):5-11
The use of fine powders in thermal spray can lead to many advantages. These advantages include denser coatings, coatings with increased wear resistance, coatings with smoother surface finish, coatings that can be applied to internal surfaces, less expensive coatings. The use of fine powders also has an disadvantage that th ey can have poor flow characteristics. The paper will discuss a feeder that is able to feed fine powders to overcome this difficulty and the coating equipment, both axial plasma and HVOF systems, used to deposit these materials to produce smooth dense coatings. 相似文献
57.
Alan F. M. Moorwood 《Space Science Reviews》1996,77(3-4):303-366
Bursts of massive star formation can temporarily dominate the luminosity of galaxies spanning a wide range of morphological types. This review is concerned primarily with such events in the central 1 kpc region of spiral galaxies which result from bar driven inflows of gas triggered by interactions or mergers. Most of the stellar radiant luminosity of such bursts is absorbed by dust and re-emitted in the far-infrared and is accompanied by radio and X-ray emission from supernova remnants which can also act collectively to drive galaxy scale outflows. Both evolutionary stellar models and estimates of the gas depletion times are consistent with typical burst durations of 107–8 yr. Spatially-resolved studies of nearby starburst galaxies reveal that the activity is distributed over many individual star forming complexes within rings and other structures organized by interactions between bars and the disc over a range of scales. More distant and extreme examples associated with mergers of massive spirals have luminosities > 1013
L
and molecular gas masses > 1010
M
implying star formation rates > 1000 M
yr–1 which can only be sustained for 107 yr. In the most luminous merging systems, however, the relative importance of starburst and AGN activity and their possible evolutionary connection is still a hotly debated issue. Also controversial are suggestions that starbursts in addition to a black hole are required to account for the properties of AGNs or that starbursts alone may be sufficient under certain conditions. In a wider context, starbursts must clearly have played an important role in galaxy formation and evolution at earlier times. Recent detections of high redshift galaxies show that star formation was underway at z 4 but do not support a continuing increase of the strong evolution in the co-moving star formation density seen out to z l. Primeval starburst pre-cursors of spheroidal systems also remain elusive. The most distant candidates are radio galaxies and quasars at z = 4–5 and a possible population of objects responsible for an isotropic sub-mm wave background tentatively claimed to have been detected by the COBE satellite. 相似文献
58.
Alan Steinberg 《Space Policy》2011,27(4):240-246
Is the government responsive to public opinion of space policy? In 1995, Stimson et al. demonstrated that changes in domestic public policy were in response to changes in public opinion. Ten years later, Jacobs and Page demonstrated that foreign policy was not responsive to public opinion, and instead responds to the opinion of business leaders. This research builds off these seminal works to explore who influences space policy. Findings suggest that the public supports the idea of space exploration, while also feeling that spending on space exploration is “too high.” Therefore, the government appears to be giving the people exactly what they want in regards to NASA’s budget – more money each year – but at the same time a smaller percentage of the federal budget. 相似文献
59.
Exploring Regional Variation in Spatial Language Using Spatially Stratified Web-Sampled Route Direction Documents 总被引:1,自引:0,他引:1
Sen Xu Alexander Klippel Alan M. MacEachren Prasenjit Mitra 《Spatial Cognition & Computation》2014,14(4):255-283
Spatial language, such as route directions, can be analyzed to shed light on how humans communicate and conceptualize spatial knowledge. This article details a computational linguistic approach using route directions to study regional variations in spatial language. We developed a web-sourcing approach to collect human-generated route direction documents on a geographical scale. Specifically, we built the Spatially strAtified Route Direction (SARD) Corpus through automated scraping, classifying, and georeferencing of route directions. Based on semantic categories of cardinal and relative direction terms, the analysis of the SARD Corpus reveals significant differences and patterns on both national (United States, United Kingdom, and Australia) and regional (contiguous U.S. states) levels. Combining computational linguistics and georeferencing approaches offers the potential for extending classic spatial linguistic studies. 相似文献
60.
Tarter JC Backus PR Mancinelli RL Aurnou JM Backman DE Basri GS Boss AP Clarke A Deming D Doyle LR Feigelson ED Freund F Grinspoon DH Haberle RM Hauck SA Heath MJ Henry TJ Hollingsworth JL Joshi MM Kilston S Liu MC Meikle E Reid IN Rothschild LJ Scalo J Segura A Tang CM Tiedje JM Turnbull MC Walkowicz LM Weber AL Young RE 《Astrobiology》2007,7(1):30-65
Stable, hydrogen-burning, M dwarf stars make up about 75% of all stars in the Galaxy. They are extremely long-lived, and because they are much smaller in mass than the Sun (between 0.5 and 0.08 M(Sun)), their temperature and stellar luminosity are low and peaked in the red. We have re-examined what is known at present about the potential for a terrestrial planet forming within, or migrating into, the classic liquid-surface-water habitable zone close to an M dwarf star. Observations of protoplanetary disks suggest that planet-building materials are common around M dwarfs, but N-body simulations differ in their estimations of the likelihood of potentially habitable, wet planets that reside within their habitable zones, which are only about one-fifth to 1/50th of the width of that for a G star. Particularly in light of the claimed detection of the planets with masses as small as 5.5 and 7.5 M(Earth) orbiting M stars, there seems no reason to exclude the possibility of terrestrial planets. Tidally locked synchronous rotation within the narrow habitable zone does not necessarily lead to atmospheric collapse, and active stellar flaring may not be as much of an evolutionarily disadvantageous factor as has previously been supposed. We conclude that M dwarf stars may indeed be viable hosts for planets on which the origin and evolution of life can occur. A number of planetary processes such as cessation of geothermal activity or thermal and nonthermal atmospheric loss processes may limit the duration of planetary habitability to periods far shorter than the extreme lifetime of the M dwarf star. Nevertheless, it makes sense to include M dwarf stars in programs that seek to find habitable worlds and evidence of life. This paper presents the summary conclusions of an interdisciplinary workshop (http://mstars.seti.org) sponsored by the NASA Astrobiology Institute and convened at the SETI Institute. 相似文献