全文获取类型
收费全文 | 1627篇 |
免费 | 198篇 |
国内免费 | 79篇 |
专业分类
航空 | 264篇 |
航天技术 | 846篇 |
综合类 | 8篇 |
航天 | 786篇 |
出版年
2023年 | 30篇 |
2022年 | 31篇 |
2021年 | 60篇 |
2020年 | 59篇 |
2019年 | 42篇 |
2018年 | 56篇 |
2017年 | 12篇 |
2016年 | 28篇 |
2015年 | 49篇 |
2014年 | 164篇 |
2013年 | 98篇 |
2012年 | 64篇 |
2011年 | 158篇 |
2010年 | 112篇 |
2009年 | 106篇 |
2008年 | 100篇 |
2007年 | 79篇 |
2006年 | 69篇 |
2005年 | 99篇 |
2004年 | 58篇 |
2003年 | 33篇 |
2002年 | 49篇 |
2001年 | 39篇 |
2000年 | 48篇 |
1999年 | 55篇 |
1998年 | 32篇 |
1997年 | 22篇 |
1996年 | 24篇 |
1995年 | 23篇 |
1994年 | 7篇 |
1993年 | 15篇 |
1992年 | 9篇 |
1991年 | 15篇 |
1990年 | 12篇 |
1989年 | 11篇 |
1988年 | 22篇 |
1987年 | 8篇 |
1986年 | 4篇 |
1984年 | 2篇 |
排序方式: 共有1904条查询结果,搜索用时 468 毫秒
251.
Astronauts are often on a voluntarily reduced energy intake during space missions, possibly caused by a metabolic or emotional stress response with involvement of the central serotonergic system (SES). We investigated 24 h urinary excretion (24 h-E) of serotonin (5-HT) and 5-hydroxyindol acidic acid as indicators of the SES in healthy males under two different normocaloric conditions: normal physical activity (NPA) and -6 degree head-down-tilt (HDT). HDT or NPA were randomly arranged with a recovery period of 6 months in between. 24 h-E of hormones varied widely among individuals. Values were higher in HDT compared to NPA. Assuming that the 24 h-E values are, beside being indicators for alterations in the number and metabolism of platelets. Also indicators of central SES, HDT condition seems to activate central SES in a higher degree compared to NPA. Therefore, changes in central SES might be involved in the mechanisms associated with space flight or microgravity, including possible maladaptations such as voluntary undernutrition. 相似文献
252.
空间微波交会雷达总体构想 总被引:1,自引:0,他引:1
结合我国载人航天工程未来发展的需要,分析了国外微波交会雷达技术的发展方向,提出了我国微波交会雷达的总体实施方案构想。 相似文献
253.
254.
随着科技的发展,空间天气对电力系统、通信导航系统和航天资产等遍布全球的技术基础设施的影响越来越深.需要加强对空间天气事件过程的理解,提升空间天气的预报能力,优化基础设施设计,从而减缓空间天气对社会造成的影响.基于这些需求,国际空间研究委员会(COSPAR)联合国际与日共存计划(ILWS)共同成立专家组,研究制定了全球2015-2025空间天气发展路线图.本文对该路线图进行介绍和解读,讨论该路线图对中国空间天气发展的启示. 相似文献
255.
Alberto Buzzoni Giuseppe Altavilla Siwei Fan Carolin Frueh Italo Foppiani Marco Micheli Jaime Nomen Noelia Sánchez-Ortiz 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2019,63(1):371-393
We report on extensive photometry and low-resolution () spectroscopy of the deep-space debris WT1190F, which impacted Earth offshore from Sri Lanka, on 2015 November 13. In spite of its likely artificial origin (as a relic of some past lunar mission), the case offered important points of discussion for its suggestive connection with the envisaged scenario for a (potentially far more dangerous) natural impactor, like an asteroid or a comet.Our observations indicate for WT1190F an absolute magnitude , with a flat dependence of reflectance on the phase angle, such as ?mag?deg?1. The detected short-timescale variability suggests that the body was likely spinning with a period twice the nominal figure of , as from the observed lightcurve. In the color domain, WT1190F closely resembled the Planck deep-space probe. This match, together with a depressed reflectance around 4000 and 8500 Å may be suggestive of a “grey” (aluminized) surface texture.The spinning pattern remained in place also along the object fiery entry in the atmosphere, a feature that may have partly shielded the body along its fireball phase perhaps leading a large fraction of its mass to survive intact, now lying underwater along a tight (?km) strip of sea, at a depth of 1500?m or less.Under the assumption of Lambertian scatter, an inferred size of ?cm is obtained for WT1190F. By accounting for non-gravitational dynamical perturbations, the Area-to-Mass ratio of the body was in the range ?m2?kg?1.Both these figures resulted compatible with the two prevailing candidates to WT1190F’s identity, namely the Athena II Trans-Lunar Injection Stage of the Lunar Prospector mission, and the ascent stage of the Apollo 10 lunar module, callsign “Snoopy”. Both candidates have been analyzed in some detail here through accurate 3D CAD design mockup modelling and BRDF reflectance rendering to derive the inherent photometric properties to be compared with the observations. 相似文献
256.
Will Furnell Abhishek Shenoy Elliot Fox Peter Hatfield 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2019,63(5):1523-1540
The Langton Ultimate Cosmic ray Intensity Detector (LUCID) is a payload onboard the satellite TechDemoSat-1, used to study the radiation environment in Low Earth Orbit (635?km). LUCID operated from 2014 to 2017, collecting over 2.1 million frames of radiation data from its five Timepix detectors on board. LUCID is one of the first uses of the Timepix detector technology in open space, with the data providing useful insight into the performance of this technology in new environments. It provides high-sensitivity imaging measurements of the mixed radiation field, with a wide dynamic range in terms of spectral response, particle type and direction. The data has been analysed using computing resources provided by GridPP, with a new machine learning algorithm that uses the Tensorflow framework. This algorithm provides a new approach to processing Medipix data, using a training set of human labelled tracks, providing greater particle classification accuracy than other algorithms. For managing the LUCID data, we have developed an online platform called Timepix Analysis Platform at School (TAPAS). This provides a swift and simple way for users to analyse data that they collect using Timepix detectors from both LUCID and other experiments. We also present some possible future uses of the LUCID data and Medipix detectors in space. 相似文献
257.
258.
Honglei Ma Feng Zhang Yu Zhu Yanhua Xiao Abrar Wazir 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2014
The purpose of the present study was to analyze and predict the changes in acceleration tolerance of human vertebra as a result of bone loss caused by long-term space flight. A human L3–L4 vertebra FEM model was constructed, in which the cancellous bone was separated, and surrounding ligaments were also taken into account. The simulation results demonstrated that bone loss has more of an effect on the acceleration tolerance in x-direction. The results serve to aid in the creation of new acceleration tolerance standards, ensuring astronauts return home safely after long-term space flight. This study shows that more attention should be focused on the bone degradation of crew members and to create new protective designs for space capsules in the future. 相似文献
259.
《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2020,65(7):1714-1722
The space debris environment is one of the major threats against payloads. Space debris orbital distribution is of great importance for space debris environment modeling. Due to perturbation factors, the Right Ascension of Ascending Node (RAAN) of space objects changes consistently, causing regular rotation of the orbit plane around Earth’s axis. Based on the investigation of the RAAN perturbation rate of concerned objects, this paper proposes a RAAN discretization method in order to present the space debris longitude-dependent distribution. Combined with two line element (TLE) data provided by the US Space Surveillance Network, the estimated value from RAAN discretization method is compared with the real case. The results suggest that using only the initial orbital data at the beginning of the time interval of interest, the RAAN discretization method is able to provide reliable longitude distribution of concerned targets in the next following period. Furthermore, spacecraft cumulative flux against space debris is calculated in this paper. The results suggest that the relevance between spacecraft RAAN setup and flux output is much smaller for LEO targets than MEO targets, which corresponds with the theory analysis. Since the nonspherical perturbation is the major factor for RAAN variation, the RAAN perturbation rate has little connection with the size of orbital objects. In other words, the RAAN discretization method introduced in this paper also applies to space debris of different size range, proposing a possible suggestion for the improvement of space debris environment engineering models. 相似文献
260.
Kohei Fujimoto Daniel J. Scheeres Johannes Herzog Thomas Schildknecht 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2014
The direct Bayesian admissible region approach is an a priori state free measurement association and initial orbit determination technique for optical tracks. In this paper, we test a hybrid approach that appends a least squares estimator to the direct Bayesian method on measurements taken at the Zimmerwald Observatory of the Astronomical Institute at the University of Bern. Over half of the association pairs agreed with conventional geometric track correlation and least squares techniques. The remaining pairs cast light on the fundamental limits of conducting tracklet association based solely on dynamical and geometrical information. 相似文献