全文获取类型
收费全文 | 4871篇 |
免费 | 14篇 |
国内免费 | 19篇 |
专业分类
航空 | 2477篇 |
航天技术 | 1677篇 |
综合类 | 17篇 |
航天 | 733篇 |
出版年
2021年 | 30篇 |
2018年 | 66篇 |
2017年 | 34篇 |
2016年 | 39篇 |
2014年 | 89篇 |
2013年 | 117篇 |
2012年 | 99篇 |
2011年 | 171篇 |
2010年 | 128篇 |
2009年 | 182篇 |
2008年 | 243篇 |
2007年 | 124篇 |
2006年 | 96篇 |
2005年 | 126篇 |
2004年 | 125篇 |
2003年 | 153篇 |
2002年 | 83篇 |
2001年 | 142篇 |
2000年 | 79篇 |
1999年 | 118篇 |
1998年 | 149篇 |
1997年 | 102篇 |
1996年 | 143篇 |
1995年 | 180篇 |
1994年 | 145篇 |
1993年 | 107篇 |
1992年 | 150篇 |
1991年 | 69篇 |
1990年 | 49篇 |
1989年 | 118篇 |
1988年 | 52篇 |
1987年 | 45篇 |
1986年 | 50篇 |
1985年 | 153篇 |
1984年 | 131篇 |
1983年 | 97篇 |
1982年 | 130篇 |
1981年 | 141篇 |
1980年 | 48篇 |
1979年 | 61篇 |
1978年 | 55篇 |
1977年 | 40篇 |
1976年 | 37篇 |
1975年 | 54篇 |
1974年 | 41篇 |
1973年 | 26篇 |
1972年 | 40篇 |
1970年 | 30篇 |
1969年 | 35篇 |
1967年 | 30篇 |
排序方式: 共有4904条查询结果,搜索用时 187 毫秒
131.
Michael J. S. Belton Karen J. Meech Michael F. A’Hearn Olivier Groussin Lucy Mcfadden Carey Lisse Yanga R. Fernández Jana PittichovÁ Henry Hsieh Jochen Kissel Kenneth Klaasen Philippe Lamy Dina Prialnik Jessica Sunshine Peter Thomas Imre Toth 《Space Science Reviews》2005,117(1-2):137-160
In 1998, Comet 9P/Tempel 1 was chosen as the target of the Deep Impact mission (A’Hearn, M. F., Belton, M. J. S., and Delamere, A., Space Sci. Rev., 2005) even though very little was known about its physical properties. Efforts were immediately begun to improve this situation
by the Deep Impact Science Team leading to the founding of a worldwide observing campaign (Meech et al., Space Sci. Rev., 2005a). This campaign has already produced a great deal of information on the global properties of the comet’s nucleus
(summarized in Table I) that is vital to the planning and the assessment of the chances of success at the impact and encounter.
Since the mission was begun the successful encounters of the Deep Space 1 spacecraft at Comet 19P/Borrelly and the Stardust spacecraft at Comet 81P/Wild 2 have occurred yielding new information on the state of the nuclei of these two comets. This
information, together with earlier results on the nucleus of comet 1P/Halley from the European Space Agency’s Giotto, the Soviet Vega mission, and various ground-based observational and theoretical studies, is used as a basis for conjectures on the morphological,
geological, mechanical, and compositional properties of the surface and subsurface that Deep Impact may find at 9P/Tempel 1. We adopt the following working values (circa December 2004) for the nucleus parameters of prime importance to Deep Impact as follows: mean effective radius = 3.25± 0.2 km, shape – irregular triaxial ellipsoid with a/b = 3.2± 0.4 and overall dimensions of ∼14.4 × 4.4 × 4.4 km, principal axis rotation with period = 41.85± 0.1 hr, pole directions
(RA, Dec, J2000) = 46± 10, 73± 10 deg (Pole 1) or 287± 14, 16.5± 10 deg (Pole 2) (the two poles are photometrically, but not
geometrically, equivalent), Kron-Cousins (V-R) color = 0.56± 0.02, V-band geometric albedo = 0.04± 0.01, R-band geometric
albedo = 0.05± 0.01, R-band H(1,1,0) = 14.441± 0.067, and mass ∼7×1013 kg assuming a bulk density of 500 kg m−3. As these are working values, {i.e.}, based on preliminary analyses, it is expected that adjustments to their values may be made before encounter
as improved estimates become available through further analysis of the large database being made available by the Deep Impact observing campaign. Given the parameters listed above the impact will occur in an environment where the local gravity is
estimated at 0.027–0.04 cm s−2 and the escape velocity between 1.4 and 2 m s−1. For both of the rotation poles found here, the Deep Impact spacecraft on approach to encounter will find the rotation axis close to the plane of the sky (aspect angles 82.2 and 69.7
deg. for pole 1 and 2, respectively). However, until the rotation period estimate is substantially improved, it will remain
uncertain whether the impactor will collide with the broadside or the ends of the nucleus. 相似文献
132.
We have performed a joint survey of anisotropic ≳40 keV electron events from August 1997 to September 2000 using the matched
detectors on the Ulysses (ULS)/HI-SCALE and the ACE/EPAM instruments. A computer algorithm selected events with strong, statistically significant
pitch-angle anisotropies. Electron pitch-angle distributions at ACE (∼1 AU) are often ‘beams’ that are strongly collimated
along the local interplanetary magnetic field (IMF). These flare-associated impulsive injections can display rapid rise times
(∼15 min) and slower decays, or more irregular intensity histories. At ULS, the electron intensities are lower and the time
histories smoother, but strong anisotropies are still observable, indicating direct, nearly field-aligned propagation outward
from the Sun. We focus on four event periods, selected from the survey, during times when the angle between the footpoints
of the IMF lines intersecting ACE and ULS is small. These events span three full years and cover a wide range of distances
and heliographic latitudes. We found one reasonably good association between impulsive electron events at ACE and ULS, and
two events with small field-aligned gradients.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
133.
A simple and fast zero tracking algorithm for adaptive arrays with large look direction errors is presented and investigated. Basically, the algorithm is based on adjusting the complex zeroes of a power inversion array in a time-multiplexed manner to track all the sources in the environment. To preserve the desired signal which is supposed to be closest to the look direction of 0°, the algorithm removes the zero with the shortest distance to ej0 so that the directional response consists of only nulls steered at the jammers. When compared with the least mean square (LMS) algorithm employing zeroth and first-order look direction constraints, the new algorithm has about the same implementation complexity, is considerably faster, and possesses a much better signal-to-noise ratio (SNR) performance when the look direction is erroneous 相似文献
134.
The magnetotail and substorms 总被引:5,自引:0,他引:5
The tail plays a very active and important role in substorms. Magnetic flux eroded from the dayside magnetosphere is stored here. As more and more flux is transported to the magnetotail and stored, the boundary of the tail flares more, the field strength in the tail increases, and the currents strengthen and move closer to the Earth. Further, the plasma sheet thins and the magnetic flux crossing the neutral sheet lessens. At the onset of the expansion phase, the stored magnetic flux is returned from the tail and energy is deposited in the magnetosphere and ionosphere. During the expansion phase of isolated substorms, the flaring angle and the lobe field strength decrease, the plasma sheet thickens and more magnetic flux crosses the neutral sheet.In this review, we discuss the experimental evidence for these processes and present a phenomenological or qualitative model of the substorm sequence. In this model, the flux transport is driven by the merging of the magnetospheric and interplanetary magnetic fields. During the growth phase of substorms the merging rate on the dayside magnetosphere exceeds the reconnection rate in the neutral sheet. In order to remove the oversupply of magnetic flux in the tail, a neutral point forms in the near earth portion of the tail. If the new reconnection rate exceeds the dayside merging rate, then an isolated substorm results. However, a situation can occur in which dayside merging and tail reconnection are in equilibrium. The observed polar cap electric field and its correlation with the interplanetary magnetic field is found to be in accord with open magnetospheric models. 相似文献
135.
Geiss J. Bühler F. Cerutti H. Eberhardt P. Filleux Ch. Meister J. Signer P. 《Space Science Reviews》2004,110(3-4):307-335
Space Science Reviews - The Apollo Solar Wind Composition (SWC) experiment was designed to measure elemental and isotopic abundances of the light noble gases in the solar wind, and to investigate... 相似文献
136.
Haines K. Hipkin R. Beggan C. Bingley R. Hernandez F. Holt J. Baker T. Bingham R.J. 《Space Science Reviews》2003,108(1-2):205-216
Accurate local geoids derived from in situ gravity data will be valuable in the validation of GOCE results. In addition it will be a challenge to use GOCE data in an
optimal way, in combination with in situ gravity, to produce better local geoid solutions. This paper discusses the derivation of a new geoid over the NW European
shelf, and its comparison with both tide gauge and altimetric sea level data, and with data from ocean models. It is hoped
that over the next few years local geoid methods such as these can be extended to cover larger areas and to incorporate both
in situ and satellite measured gravity data.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
137.
Present-Day Sea Level Change: Observations and Causes 总被引:3,自引:0,他引:3
Cazenave A. Cabanes C. Dominh K. Gennero M.C. Le Provost C. 《Space Science Reviews》2003,108(1-2):131-144
We investigate climate-related processes causing variations of the global mean sea level on interannual to decadal time scale.
We focus on thermal expansion of the oceans and continental water mass balance. We show that during the 1990s where global
mean sea level change has been measured by Topex/Poseidon satellite altimetry, thermal expansion is the dominant contribution
to the observed 2.5 mm/yr sea level rise. For the past decades, exchange of water between continental reservoirs and oceans
had a small, but not totally negligible contribution (about 0.2 mm/yr) to sea level rise. For the last four decades, thermal
contribution is estimated to about 0.5 mm/yr, with a possible accelerated rate of thermosteric rise during the 1990s. Topex/Poseidon
shows an increase in mean sea level of 2.5 mm/yr over the last decade, a value about two times larger than reported by historical
tide gauges. This would suggest that there has been significant acceleration of sea level rise in the recent past, possibly
related to ocean warming.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
138.
Jurewicz A.J.G. Burnett D.S. Wiens R.C. Friedmann T.A. Hays C.C. Hohlfelder R.J. Nishiizumi K. Stone J.A. Woolum D.S. Becker R. Butterworth A.L. Campbell A.J. Ebihara M. Franchi I.A. Heber V. Hohenberg C.M. Humayun M. McKeegan K.D. McNamara K. Meshik A. Pepin R.O. Schlutter D. Wieler R. 《Space Science Reviews》2003,105(3-4):535-560
Genesis (NASA Discovery Mission #5) is a sample return mission. Collectors comprised of ultra-high purity materials will be
exposed to the solar wind and then returned to Earth for laboratory analysis. There is a suite of fifteen types of ultra-pure
materials distributed among several locations. Most of the materials are mounted on deployable panels (‘collector arrays’),
with some as targets in the focal spot of an electrostatic mirror (the ‘concentrator’). Other materials are strategically
placed on the spacecraft as additional targets of opportunity to maximize the area for solar-wind collection.
Most of the collection area consists of hexagonal collectors in the arrays; approximately half are silicon, the rest are for
solar-wind components not retained and/or not easily measured in silicon. There are a variety of materials both in collector
arrays and elsewhere targeted for the analyses of specific solar-wind components.
Engineering and science factors drove the selection process. Engineering required testing of physical properties such as the
ability to withstand shaking on launch and thermal cycling during deployment. Science constraints included bulk purity, surface
and interface cleanliness, retentiveness with respect to individual solar-wind components, and availability.
A detailed report of material parameters planned as a resource for choosing materials for study will be published on a Genesis
website, and will be updated as additional information is obtained. Some material is already linked to the Genesis plasma
data website (genesis.lanl.gov). Genesis should provide a reservoir of materials for allocation to the scientific community
throughout the 21st Century.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
139.
Mitchell Donald G. C:son Brandt Pontus Roelof Edmond C. Hamilton Douglas C. Retterer Kyle C. Mende Steven 《Space Science Reviews》2003,109(1-4):63-75
The magnetospheric O+ population in the 52–180 keV range during storms is investigated through the analysis of energetic neutral atom (ENA) images. The images are obtained from the high energy neutral atom (HENA) imager onboard the IMAGE satellite. At each substorm onset following the commencement of a geomagnetic storm the oxygen ENA display ~30 min intense bursts. Only very weak corresponding features in the 60–119 keV hydrogen ENA can be occasionally seen. The dominating fraction of the oxygen ENA emissions are produced when O+ ions mirror/precipitate at low altitudes, where the number density of the neutral atmosphere is high. During the storm we observed several bursts of oxygen ENA, but it is still not clear how much the O+ content of the ring current increases during the storm main phase. Our observations suggest that the responsible injection mechanism is mass-dependent and scatters the pitch angles. This leads us to favor a non-adiabatic mechanism proposed by (Delcourt, 2002). 相似文献
140.
Israel G. Cabane M. Brun J-F. Niemann H. Way S. Riedler W. Steller M. Raulin F. Coscia D. 《Space Science Reviews》2002,104(1-4):433-468
ACP's main objective is the chemical analysis of the aerosols in Titan's atmosphere. For this purpose, it will sample the
aerosols during descent and prepare the collected matter (by evaporation, pyrolysis and gas products transfer) for analysis
by the Huygens Gas Chromatograph Mass Spectrometer (GCMS). A sampling system is required for sampling the aerosols in the
135'32 km and 22'17 km altitude regions of Titan's atmosphere. A pump unit is used to force the gas flow through a filter.
In its sampling position, the filter front face extends a few mm beyond the inlet tube. The oven is a pyrolysis furnace where
a heating element can heat the filter and hence the sampled aerosols to 250 °C or 600 °C. The oven contains the filter, which
has a thimble-like shape (height 28 mm). For transferring effluent gas and pyrolysis products to GCMS, the carrier gas is
a labeled nitrogen 15N2, to avoid unwanted secondary reactions with Titan's atmospheric nitrogen.
Aeraulic tests under cold temperature conditions were conducted by using a cold gas test system developed by ONERA. The objective
of the test was to demonstrate the functional ability of the instrument during the descent of the probe and to understand
its thermal behavior, that is to test the performance of all its components, pump unit and mechanisms.
In order to validate ACP's scientific performance, pyrolysis tests were conducted at LISA on solid phase material synthesized
from experimental simulation. The chromatogram obtained by GCMS analysis shows many organic compounds. Some GC peaks appear
clearly from the total mass spectra, with specific ions well identified thanks to the very high sensitivity of the mass spectrometer.
The program selected for calibrating the flight model is directly linked to the GCMS calibration plan. In order not to pollute
the two flight models with products of solid samples such as tholins, we excluded any direct pyrolysis tests through the ACP
oven during the first phase of the calibration. Post probe descent simulation of flight results are planned, using the much
representative GCMS and ACP spare models.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献