全文获取类型
收费全文 | 606篇 |
免费 | 86篇 |
国内免费 | 155篇 |
专业分类
航空 | 307篇 |
航天技术 | 241篇 |
综合类 | 37篇 |
航天 | 262篇 |
出版年
2024年 | 4篇 |
2023年 | 14篇 |
2022年 | 12篇 |
2021年 | 25篇 |
2020年 | 35篇 |
2019年 | 26篇 |
2018年 | 27篇 |
2017年 | 22篇 |
2016年 | 29篇 |
2015年 | 31篇 |
2014年 | 46篇 |
2013年 | 36篇 |
2012年 | 50篇 |
2011年 | 34篇 |
2010年 | 35篇 |
2009年 | 32篇 |
2008年 | 42篇 |
2007年 | 48篇 |
2006年 | 76篇 |
2005年 | 38篇 |
2004年 | 14篇 |
2003年 | 28篇 |
2002年 | 18篇 |
2001年 | 5篇 |
2000年 | 13篇 |
1999年 | 30篇 |
1998年 | 15篇 |
1997年 | 6篇 |
1996年 | 11篇 |
1995年 | 21篇 |
1994年 | 14篇 |
1993年 | 3篇 |
1991年 | 4篇 |
1990年 | 2篇 |
1984年 | 1篇 |
排序方式: 共有847条查询结果,搜索用时 46 毫秒
721.
2009年6至7月华南地区电离层TEC扰动研究 总被引:1,自引:0,他引:1
电离层TEC(Total Electron Content)扰动与多种扰动源相关联.2009年6至7月期间存在地震和日全食的扰动源.利用广州地区GPS监测网在2009年6至7月连续监测到的TEC数据,通过采用前15天数据的滑动窗口对数据进行处理,从时间序列和空间分布两方面分析了华南地区电离层TEC扰动特征.结果显示,2009年6至7月华南地区电离层TEC扰动和该时期发生的地震以及日全食事件可能有关联;2009年7月多个地震发生引起的电离层扰动特征为,震前出现的是正异常,发震当天或震后有可能是正异常,也有可能是负异常;7月22日日全食当天TEC扰动为正异常,推测该正异常是地磁活动、地震活动及日全食综合效应的结果. 相似文献
722.
R.A. Nymmik 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
Based on the author’s experience in ISO TC20/SC14 Working Group 4, this paper discusses the common problems encountered when developing a standard for solar energetic particle (SEP) fluxes. The problem involving the reliability of the distribution function describing the SEP events and the interpolation of this function into the region of not-yet-observed large events are discussed. The problems with describing the fluences of SEPs over a wide range of energy in the form of energetic spectra are analyzed. Requirements for SEP flux models are formulated. The reliability of some SEP flux models is determined by comparing their predictions with the experimental data. 相似文献
723.
724.
针对混合推进航天器编队日心悬浮轨道保持控制问题进行了研究.首先推导出在日心悬浮轨道附近的航天器编队相对运动方程,考虑到航天器间距离变化值较小且航天器间距离与航天器到太阳的距离的比值为小量,将其在悬浮轨道附近线性化.基于该线性化方程,设计了一种LQR编队控制方式,该控制方式可通过调节太阳帆的姿态及航天器间库仑力的大小对编队构型进行改变或保持,具有响应速度快和控制简单的特点.最后对控制律进行数值仿真,表明该控制方法能实现编队. 相似文献
725.
针对刚柔耦合的圆形薄膜UltraFlex太阳翼结构动力学建模与分析困难、微重力下薄膜运动复杂和展开精度要求高的问题,搭建了UltraFlex展开动力学数值分析模型,分析了扭簧和绳索驱动下UltraFlex的有序展开动力学特性。基于绝对坐标方法建立包含柔性附件和柔性薄膜的UltraFlex动力学模型,采用两步检测算法处理薄膜间的复杂接触碰撞问题,利用扭簧逐步释放扭矩的方法驱动结构有序展开,通过控制绳索释放速率的方法完成移动箱板的转动规划和限位跟踪,提高展开位置精度。将该展开驱动策略运用到NASA实际样机尺寸的UltraFlex分析模型中,仿真结果表明该展开策略能够使得UltraFlex结构高精度、有序、稳定地展开;绳索始终处于张紧状态,最大拉力为62.5N;薄膜展开过程复杂,重复出现张紧、回弹的现象,最终趋于稳定。 相似文献
726.
M. I. Desai G. M. Mason R. E. Gold S. M. Krimigis C. M. S. Cohen R. A. Mewaldt J. E. Mazur J. R. Dwyer 《Space Science Reviews》2007,130(1-4):243-253
Using high-resolution mass spectrometers on board the Advanced Composition Explorer (ACE), we surveyed the event-averaged
∼0.1–60 MeV/nuc heavy ion elemental composition in 64 large solar energetic particle (LSEP) events of cycle 23. Our results
show the following: (1) The Fe/O ratio decreases with increasing energy up to ∼10 MeV/nuc in ∼92% of the events and up to
∼60 MeV/nuc in ∼64% of the events. (2) The rare isotope 3He is greatly enhanced over the corona or the solar wind values in 46% of the events. (3) The heavy ion abundances are not
systematically organized by the ion’s M/Q ratio when compared with the solar wind values. (4) Heavy ion abundances from C–Fe exhibit systematic M/Q-dependent enhancements that are remarkably similar to those seen in 3He-rich SEP events and CME-driven interplanetary (IP) shock events. Taken together, these results confirm the role of shocks
in energizing particles up to ∼60 MeV/nuc in the majority of large SEP events of cycle 23, but also show that the seed population
is not dominated by ions originating from the ambient corona or the thermal solar wind, as previously believed. Rather, it
appears that the source material for CME-associated large SEP events originates predominantly from a suprathermal population
with a heavy ion enrichment pattern that is organized according to the ion’s mass-per-charge ratio. These new results indicate
that current LSEP models must include the routine production of this dynamic suprathermal seed population as a critical pre-cursor
to the CME shock acceleration process. 相似文献
727.
Philippe L. Lamy Imre Toth Björn J. R. Davidsson Olivier Groussin Pedro Gutiérrez Laurent Jorda Mikko Kaasalainen Stephen C. Lowry 《Space Science Reviews》2007,128(1-4):23-66
In 2003, comet 67P/Churyumov–Gerasimenko was selected as the new target of the Rosetta mission as the most suitable alternative
to the original target, comet 46P/Wirtanen, on the basis of orbital considerations even though very little was known about
the physical properties of its nucleus. In a matter of a few years and based on highly focused observational campaigns as
well as thorough theoretical investigations, a detailed portrait of this nucleus has been established that will serve as a
baseline for planning the Rosetta operations and observations. In this review article, we present a novel method to determine
the size and shape of a cometary nucleus: several visible light curves were inverted to produce a size–scale free three–dimensional
shape, the size scaling being imposed by a thermal light curve. The procedure converges to two solutions which are only marginally
different. The nucleus of comet 67P/Churyumov–Gerasimenko emerges as an irregular body with an effective radius (that of the
sphere having the same volume) = 1.72 km and moderate axial ratios a/b = 1.26 and a/c = 1.5 to 1.6. The overall dimensions
measured along the principal axis for the two solutions are 4.49–4.75 km, 3.54–3.77 km and 2.94–2.92 km. The nucleus is found
to be in principal axis rotation with a period = 12.4–12.7 h. Merging all observational constraints allow us to specify two
regions for the direction of the rotational axis of the nucleus: RA = 220°+50°
−30° and Dec = −70° ± 10° (retrograde rotation) or RA = 40°+50°
-30° and Dec = +70°± 10° (prograde), the better convergence of the various determinations presently favoring the first solution. The phase function,
although constrained by only two data points, exhibits a strong opposition effect rather similar to that of comet 9P/Tempel
1. The definition of the disk–integrated albedo of an irregular body having a strong opposition effect raises problems, and
the various alternatives led to a R-band geometric albedo in the range 0.045–0.060, consistent with our present knowledge of cometary nuclei. The active fraction
is low, not exceeding ~ 7% at perihelion, and is probably limited to one or two active regions subjected to a strong seasonal
effect, a picture coherent with the asymmetric behaviour of the coma. Our slightly downward revision of the size of the nucleus
of comet 67P/Churyumov-Gerasimenko resulting from the present analysis (with the correlative increase of the albedo compared
to the originally assumed value of 0.04), and our best estimate of the bulk density of 370 kg m−3, lead to a mass of ~ 8 × 1012 kg which should ease the landing of Philae and insure the overall success of the Rosetta mission. 相似文献
728.
Karl-Heinz Glassmeier Hermann Boehnhardt Detlef Koschny Ekkehard Kührt Ingo Richter 《Space Science Reviews》2007,128(1-4):1-21
The ROSETTA Mission, the Planetary Cornerstone Mission in the European Space Agency’s long-term programme Horizon 2000, will
rendezvous in 2014 with comet 67P/Churyumov-Gerasimenko close to its aphelion and will study the physical and chemical properties
of the nucleus, the evolution of the coma during the comet’s approach to the Sun, and the development of the interaction region
of the solar wind and the comet, for more than one year until it reaches perihelion. In addition to the investigations performed
by the scientific instruments on board the orbiter, the ROSETTA lander PHILAE will be deployed onto the surface of the nucleus.
On its way to comet 67P/Churyumov-Gerasimenko, ROSETTA will fly by and study the two asteroids 2867 Steins and 21 Lutetia. 相似文献
729.
730.