共查询到20条相似文献，搜索用时 26 毫秒
1.
O'leary B 《Science (New York, N.Y.)》1972,175(4026):1108-1112
Calculations show that several occultations of stars by the large satellites of the outer planets, Pluto, and the large asteroids could be observed each decade with existing equipment at Earth-based telescopes. A systematic program of occultation predictions and observations is urged in order to improve our knowledge about the atmospheres, sizes, shapes, topography, and positions of these poorly understood bodies, in support of forthcoming spacecraft missions to the outer solar system. 相似文献
2.
Space missions currently on the drawing boards are expected to gather data at rates exceeding the transmission capabilities of today's telemetry systems by many orders of magnitude. Even on current missions, onboard data compression techniques are being implemented to compensate for lack of transmission speed. But while data compression can minimize the loss of data, it is no substitute for transmitting all of the data through a faster communications link. The transmission problem will soon reach crisis proportions and will affect astronomical, Earth resources, geophysical, meteorological, planetary and other space science missions. To overcome this communications bottleneck, the authors advocate the implementation of telemetry systems based on near-infrared laser transmission techniques. The fiber-optics communications industry has developed most of the basic components required for signal transmission in this wavelength band, which should make such a system affordable on scales relevant to the cost of anticipated space science missions. 相似文献
3.
Kerr RA 《Science (New York, N.Y.)》2000,290(5498):1879-1880
The latest images from the Red Planet, a sampling of which is shown on page 1927, are suggesting that water ponded across its equatorial region eons ago, just when life might have been emerging. Although the authors offer more than one interpretation, the one they prefer has the sediments laid down beneath broad lakes and shallow seas at a relatively clement time in the planet's history. The geologic implications of the pictures plus supportive signs from earlier missions mean that these possible lake sediments will be prime candidates for NASA missions seeking signs of past life on Mars. 相似文献
4.
《Science (New York, N.Y.)》1982,216(4552):1274
The briefing "GAO points up military use of shuttle" (News and Comment, 14 May, p. 717) states that Defense Department missions will occupy 49 percent of the space shuttle's flights over the next 3 years. The percentage is correctly applied to total missions during the next 13 years. 相似文献
5.
Comstock GM Fleischer RL Giard WR Hart HR Nichols GE Price PB 《Science (New York, N.Y.)》1971,172(3979):154-157
Counts of tracks from heavy cosmic-ray nuclei in helmets from Apollo missions 8 and 12 show variations caused by solar modulation of the galactic cosmic-ray flux. Specific estimates of the biological damage to certain nonreplaceable cells by track-forming particles during these space missions indicate that the fraction of deactivated cells could range from a lower limit of 3 x 10(-7) to an upper limit of 1.4 x 10(-4). 相似文献
6.
张智敏 《沈阳农业大学学报》1988,(4)
本文介绍了国内外植物园建设的现状,指出植物园建设方向及任务的多极性特点和不同类型植物园的具体任务及应具有的特色,强调了确定建设方向及任务时应坚持的原则,提出植物园展览区的划分方法以及提出对我国大学植物园建设的几点看法。 相似文献
7.
谈非图书馆学专业人员如何做好图书馆工作 总被引：2，自引：0，他引：2
张家生 《农业图书情报学刊》2007,19(9):74-75
介绍了目前高校图书馆工作人员构成情况,着重阐述了非图书馆学专业人员的地位、作用以及如何做好图书馆工作的方法策略。 相似文献
8.
McDonald FB 《Science (New York, N.Y.)》1987,235(4790):751-754
The current state and future directions of the U.S. space-science program are assessed in the wake of the Challenger accident, the Gramm-Rudman-Hollings budget reduction act, and the report of the National Commission on Space. A renewed emphasis on moderate-scale, quickresponse missions will be of special importance for scientific progress and will compensate in part for the postponement of most major space missions. Satellites and manned space stations in Earth orbit, along with unmanned planetary missions, will continue to be the dominant elements in the space program. Future progress and the continuation of U.S. leadership depend on the vitality of U.S. space research. 相似文献
9.
Tolansky S 《Science (New York, N.Y.)》1972,176(4035):671-673
Glass objects in the fines from the Apollo 11 and Apollo 12 missions are shown, by two-beam reflection interferometry, to have been subject to shock at temperatures below the melting or softening point of the glass. Possible causes for the glass fragmentation are discussed. 相似文献
10.
Morse JG 《Science (New York, N.Y.)》1963,139(3560):1175-1180
Radioisotope-fueled generators are classed by (i) application (space or terrestrial), (ii) fuel (alpha-emitter or fission product, short or long half-life), and (iii) converter type (thermoelectric or thermionic). The environment determines the design of the heat sink mechanism. The type of fuel is chosen according to the application and the availability and cost of the fuel. The space systems are suitable for low-powered (less than 1 kw), long-lived, earth-orbit missions, making storage batteries unnecessary. They are independent of radiation belts and solar transients and are suitable for lunar missions, where power is needed during the long lunar night; for probes into the dense atmosphere of Venus; and in general, for missions away from the sun. 相似文献
11.
水稻叶片SPAD空间分布与氮素营养及种植密度的关系 总被引：1，自引：0，他引：1
通过大田试验,研究在4个施氮水平和3个种植密度下,水稻孕穗和抽穗两个时期水稻不同叶位叶片SPAD值的空间分布特征,并对叶片氮素营养和种植密度与不同叶位的SPAD值之间的相关性进行分析。结果表明,随着施氮量的增加,孕穗期SPAD值表现为顶3叶>顶2叶>顶4叶>顶1叶;抽穗期SPAD值表现为顶1叶>顶2叶>顶3叶>顶4叶。不同种植密度对水稻各叶位的SPAD值差异不显著。相关分析发现,孕穗期不同叶位SPAD值与施氮量的相关性顺序是顶4叶>顶3叶>顶2叶;抽穗期不同叶位SPAD值与施氮量的相关性顺序是顶1叶>顶3叶>顶2叶>顶4叶。 相似文献
12.
Astronomers believe that the baryon (stellar) halo of the Milky Way retains a fossil imprint of how it was formed. But a vast literature shows that the struggle to interpret the observations within a consistent framework continues. The evidence indicates that the halo has built up through a process of accretion and merging over billions of years, which is still going on at a low level. Future satellite missions to derive three-dimensional space motions and heavy element (metal) abundances for a billion stars will disentangle the existing web and elucidate how galaxies like our own came into existence. 相似文献
13.
Colin L 《Science (New York, N.Y.)》1979,205(4401):44-46
This report is an introduction to the accompanying collection of early results from the Pioneer Venus orbiter and multiprobe missions, which encountered Venus on 4 December and 9 December 1978, respectively. Initial results for the multiprobe mission and for the first 30 days of the orbiter mission were reported in the 23 February issue of Science. Additional mission features and updated mission parameters based on refined tracking data and trajectory computations are presented here. New scientific results for both missions are given in the subsequent reports which cover the first 130 days (or orbits) of the nominal 243-day orbiter mission. 相似文献
14.
Lawler A 《Science (New York, N.Y.)》2000,290(5495):1270-1271
An unusual coalition of scientists, activists, and politicians is pressuring NASA to rethink a September decision to put a 2004 mission to Pluto on hold because of budget constraints. The growing clamor is shaking up the planetary science community, which is also preparing for a mission at mid-decade to Europa, a moon of Jupiter. The biggest impact may be felt at the Jet Propulsion Laboratory in Pasadena, California, which could face serious competition for the first time in decades on contracts to build planetary missions. 相似文献
15.
Neugebauer M 《Science (New York, N.Y.)》1983,219(4584):443-449
Mariner Mark II is a concept for the next generatioon of deep-space missions. The project would provide limited, focused sets of Voyager- and Galileo-quality planetary observations at a fraction of the cost of the Voyager and Galileo projects. This article discusses Mariner Mark II's cost goals, scientific objectives, and mission requirements. Strategies for limiting costs include the use of a reconfigurable spacecraft, a multimission ground-support system, and selected new technologies. 相似文献
16.
研究了碧峰峡自然保护区多年生草本植物玉簪花序水平上雌性适合度成分变化.玉簪每花胚珠数由基部向顶部逐渐降低.在自然条件下,每果结籽率由基部向顶部增加,每果种子数以中部最高,基部最低.每果单粒种子重量由基部向顶部降低,以基部最高.而每果种子总重不依赖于部位而变化.每果种子数与每果单粒种子均重存在权衡关系. 相似文献
17.
18.
Colin L 《Science (New York, N.Y.)》1979,203(4382):743-745
This report is an introduction to the accompanying collection of initial results from the successful Pioneer Venus orbiter and mutiprobe missions that encountered Venus on 4 December and 9 December 1978, respectively. The mission features are briefly described and furnish data accumulated over the first 30 days of the mission. 相似文献
19.
The relationship between the codon usage bias, gene expression level and the AUG context (from -20 to +6 positions relative to the initiator AUG codon) was examined in 541unigene sequences of rice. A significant correlation for CAI values (codon adaptation index) was observed at five nucleotide positions (-19, -18, -9, -4, +5), eight (-19, -18,-14, -9, -6, -4, -1, +5) for CPP (codon preference parameter), and seven (-18, -16, -15,-9, -7, -1, + 6) for mRNA abundance in the flanking sequence of the initiator AUG codon respectively, but a significantly positive correlation for both CAI and CPP at two positions (-4 and +5), indicating that both those positions are evolutionally under the natural selection constraint at the translational level. By site-directed mutagenesis at seven specific positions (-18, -16, -15, -9, -7, -1 and + 6) for allergenic protein that had the highest mRNA abundance in this study, its expression level decreased dramatically 63.3 and 72.5% respectively, indicating the importance of those 7 positions for gene expression. A highly positive correlation (r= 0.625, P＜ 0.01) between AUGCAI and GC content in the flanking sequence of the initiator AUG codon showed a more effective higher GC content on translation initiation efficiency. The strong preference for G or C at those 8 positions (-6, -5, -3, -2, -1, +4, +5 and +6) in the AUG context suggested that an important factor in modulation of the translation efficiency, as well as synonymous codon usage bias, particularly in highly expressed genes. 相似文献
20.
汪家耀 《安徽农业大学学报》2022,31(4):55-61
党的十九届四中全会明确提出，要加强制度理论研究和宣传教育，坚定制度自信。这为综合性人文社会科学学术期刊提出了新的办刊使命。综合性人文社会科学学术期刊要紧紧抓住中国特色社会主义制度的伟大实践与探索，以更大的政治勇气与学术担当，充分发挥期刊的"综合性"这一显著特征，更好引领制度理论研究、展示制度建设与发展新成果，努力成为坚定制度自信的强大思想理论阵地，这也是制度自信视域下综合性人文社会科学学术期刊的学术自觉。 相似文献