全文获取类型
收费全文 | 8479篇 |
免费 | 18篇 |
国内免费 | 22篇 |
专业分类
航空 | 3769篇 |
航天技术 | 2852篇 |
综合类 | 31篇 |
航天 | 1867篇 |
出版年
2021年 | 88篇 |
2019年 | 50篇 |
2018年 | 246篇 |
2017年 | 172篇 |
2016年 | 154篇 |
2015年 | 64篇 |
2014年 | 218篇 |
2013年 | 271篇 |
2012年 | 264篇 |
2011年 | 408篇 |
2010年 | 304篇 |
2009年 | 431篇 |
2008年 | 455篇 |
2007年 | 305篇 |
2006年 | 188篇 |
2005年 | 243篇 |
2004年 | 219篇 |
2003年 | 258篇 |
2002年 | 184篇 |
2001年 | 270篇 |
2000年 | 142篇 |
1999年 | 181篇 |
1998年 | 220篇 |
1997年 | 136篇 |
1996年 | 188篇 |
1995年 | 243篇 |
1994年 | 222篇 |
1993年 | 130篇 |
1992年 | 170篇 |
1991年 | 54篇 |
1990年 | 61篇 |
1989年 | 159篇 |
1988年 | 65篇 |
1987年 | 66篇 |
1986年 | 75篇 |
1985年 | 221篇 |
1984年 | 177篇 |
1983年 | 135篇 |
1982年 | 141篇 |
1981年 | 247篇 |
1980年 | 65篇 |
1979年 | 55篇 |
1978年 | 58篇 |
1977年 | 51篇 |
1975年 | 58篇 |
1974年 | 46篇 |
1973年 | 37篇 |
1972年 | 46篇 |
1971年 | 44篇 |
1970年 | 42篇 |
排序方式: 共有8519条查询结果,搜索用时 15 毫秒
991.
Solar wind turbulence during the solar cycle deduced from Galileo coronal radio-sounding experiments
A.I. Efimov L.N. Samoznaev M.K. Bird I.V. Chashei D. Plettemeier 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008
Seven coronal radio-sounding campaigns were carried out during the active lifetime of the Galileo spacecraft in the years 1994–2002. The observational data analyzed in the present work are S-band frequency fluctuation measurements recorded during the solar conjunctions at different phases of solar activity cycle #23, specifically: periods near solar maximum (three conjunctions), near solar minimum (three conjunctions) and during the ascending phase (one conjunction). These data are all applicable to low heliographic latitudes, i.e. to the slow solar wind. The rms frequency fluctuation and power-law index of the frequency fluctuation temporal spectra are determined as a function of heliocentric distance. The turbulence power spectrum tends to be flatter inside ca. 20 solar radii during all phases of the solar cycle. This coincides with a transition in the flow from the inner acceleration region to the outer region of constant velocity. The radial falloff rate and absolute level of the rms frequency fluctuation are essentially invariant over the solar cycle. 相似文献
992.
We describe a numerical integral-projection method used by the authors for the approximate solution of systems of interrelated two-dimensional linear boundary-value problems in mechanics of composite shell systems. The method is based on discretization in each shell substructure of a two-dimensional problem along one of coordinates using a projection-grid variant of the Galerkin-Petrov method and its subsequent transformation to a system of ordinary differential equations; by integration and introduction of sought functions as unknown derivatives, the system is reduced to a system of integral equations being solved by the method of mechanical quadratures. The method is characterized by the fact that its application requires no additional conditions of conformity with discretization parameters of substructures being mated. 相似文献
993.
In preparation for the flyby of the Rosetta spacecraft, the state of the knowledge of 21 Lutetia and 2867 Steins obtained
by ground based observations is reported. Preliminary determination of shape and/or size, rotational properties and taxonomic
types are presented for both asteroids. The dusty environment of the targets has been studied in order to help in defining
safe fly-by trajectories without the risk of damaging collisions. The expected science return from the asteroid fly-by is
also discussed. 相似文献
994.
G D Badhwar V Dudkin T Doke W Atwell 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1998,22(4):485-494
The second flight of the International Microgravity Laboratory (IML-2) on Space Shuttle flight STS-65 provided a unique opportunity for the intercomparison of a wide variety of radiation measurement techniques. Although this was not a coordinated or planned campaign, by sheer chance, a number of space radiation experiments from several countries were flown on this mission. There were active radiation measuring instruments from Japan and US, and passive detectors from US, Russia, Japan, and Germany. These detectors were distributed throughout the Space Shuttle volume: payload bay, middeck, flight deck, and Spacelab. STS-65 was launched on July 8, 1994, in a 28.45 degrees x 306 km orbit for a duration of 14 d 17 hr and 55 min. The crew doses varied from 0.935 mGy to 1.235 mGy. A factor of two variation was observed between various passive detectors mounted inside the habitable Shuttle volume. There is reasonable agreement between the galactic cosmic ray dose, dose equivalent and LET spectra measured by the tissue equivalent proportional counter flown in the payload bay with model calculations. There are significant differences in the measurements of LET spectra measured by different groups. The neutron spectrum in the 1-20 MeV region was measured. Using fluence-dose conversion factors, the neutron dose and dose equivalent rates were 11 +/- 2.7 microGy/day and 95 +/- 23.5 microSv/day respectively. The average east-west asymmetry of trapped proton (>3OMeV) and (>60 MeV) dose rate was 3.3 and 1.9 respectively. 相似文献
995.
Barrett A. Rabideau G. Estlin T. Chien S. 《Aerospace and Electronic Systems Magazine, IEEE》2007,22(2):27-33
Eight evaluation metrics are used to compare and contrast three coordination schemes for a system that continuously plans to control collections of rovers (or spacecraft) using collective mission goals instead of goals or command sequences for each spacecraft. These schemes use a central coordinator to either: 1) micromanage rovers one activity at a time; 2) assign mission goals to rovers; or 3) arbitrate mission goal auctions among rovers. A self-commanding collection of rovers would autonomously coordinate itself to satisfy high-level science and engineering goals in a changing partially understood environment - making the operation of tens or even a hundred spacecraft feasible 相似文献
996.
997.
A.H. Manson C.E. Meek M. Massebeuf J.L. Fellous W.G. Elford R.A. Vincent R.L. Craig R.G. Roper S. Avery B.B. Balsley G.J. Fraser M.J. Smith R.R. Clark S. Kato T. Tsuda A. Ebel 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1985,5(7):135-144
During the last decade a large number of radars (~12) have been developed, which have produced substantial quantities of tidally-corrected mean winds data. The distribution of the radars is not global, but many areas are well covered: the Americas with Poker Flat (65°N), Saskatoon (52°N), Durham (43°N), Atlanta (34°N), Puerto Rico (18°N); Europe with Kiruna (68°), Garchy (47°N) and Monpazier (44°N); and Oceania with Christchurch (44°S), Adelaide (35°S), Townsville (20°S), and Kyoto (35°N). Zonal and meridional wind height-time cross-sections from km (MF/Meteor Radar) to ~110 km have been prepared for the last 5–6 years. They are compared with cross-sections from CIRA-72 for zonal winds, and Groves (1969) for meridional winds.It is shown that while CIRA-72 is still a useful model for many purposes, significant differences exist between it and the new radar data. The latter demonstrate important seasonal, latitudinal, longitudinal and hemispheric variations. The new meridional cross-sections are of great value. The common features with Groves (1969) are the equatorward cells in summer near 85 km; however their strength (~10 ms?1) and size are less. Systematic and somewhat different variations emerge at higher (?52°N) and middle (35–44°) latitudes. 相似文献
998.
Y.B. Acharya A. Jayaraman B.H. Subbaraya 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1985,5(1):65-68
A balloon borne multichannel photometer for measurement of atmospheric scattering in the near ultraviolet and the visible wavelength regions has been developed at the Physical Research Laboratory, Ahmedabad for study of the size distribution and number density of aerosols at tropospheric and lower stratospheric altitudes. The instrumentation involves tracking the sun in elevation and scanning in azimuth. The payload was recently flown on a 100 kg. balloon from the Hyderabad Balloon Facility on 18 April 1984. The balloon reached a float altitude of 35 km and good quality data has been obtained from an altitude of 6 km upto float altitude. Data analysis is still in progress. The present paper details the instrument design and presents a few illustrations of the instrument performance from this flight. 相似文献
999.
L. McDonald L.K. Harra-Murnion J.L. Culhane R.A. Schwartz 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1997,20(12):2327-2331
Most solar flare observations show that intense hard X-ray bursts come from large flares that have a large GOES classification (large peak 1 – 8 Å flux). This correlation, known as the “Big Flare Syndrome”, suggests that more intense flares tend to have harder spectra. We have observed 7 flares that are exceptions to this. These flares have small GOES classifications ranging from B1.4 to C5.5 and peak hard X-ray count rates similar to those often observed from M class flares. This paper examines the cause of this anomoly using the Yohkoh Soft X-Ray Telescope, Hard X-Ray Telescope, and Bragg Crystal Spectrometer. Two hypotheses are proposed for the exceptions: (1) flares with multiple magnetic loops and common footpoints, producing multiple hard X-ray emission regions and low density thermal plasma distributed over a large volume, and (2) high densities in the magnetic loops restricting the propagation of the non-thermal electrons in the loop after magnetic reconnection has occurred and suppressing chromospheric evaporation. Two of the flares support the first hypothesis. The other flares either have data missing or are too small to be properly analysed by the Yohkoh instruments. 相似文献
1000.
O A Gusev A I Golubev 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2001,28(4):613-615
We report the results from two years of laboratory observations of the tide-associated rhythms of activity of White Sea intertidal invertebrates, Mya arenaria (Bivalvia) and Gammarus finmarchicus (Amphipoda). The tidal associated activity of these invertebrates could not be estimate as a clear circatidal clock. Gammarus activity could be phase shifted by a 0.5 h exposure to turbulent water twice a day for 2-3 days. Mya's rhythm could be changed by a single drainage of aquariums lasting about 15 min. This kind of timing system may be a relatively primitive evolution feature. 相似文献