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121.
C A Mitchell 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1992,12(1):219-225
Growth, development, and orientation of higher plants is altered by physical disturbances such as shaking, touching, or vibration. Plant growth responses to thigmic (contact rubbing) forces are almost always negative, whereas growth responses to periodic seismic (shaking) or vibric (vibrational) disturbances may be positive or negative, depending on intensity and duration of force, and prevailing environmental conditions. Seedlings are most sensitive to mechanical stress when grown in darkness or under the low-light conditions typically available in plant flight hardware. Brief exposure to physical perturbation causes immediate growth inhibition of dark-grown seedlings followed by gradual recovery of growth rate beginning 10-12 minutes later. For mild vibration, growth rate may overshoot that of undisturbed control plants within an hour of a stress episode, whereas for thigmic stress recovery may remain incomplete for 24 hours or longer. Lack of physical stimulation by gravity should make plants even more responsive to random physical perturbation. Threshold growth response of seedlings to vibrational parameters needs to be determined under real spaceflight conditions. 相似文献
122.
Phase shift for the ramp-comparator pulsewidth modulator (PWM) between a continuous input control signal and the signal frequency component of the output is analyzed. It is shown that transport lag is not implicit to these PWM systems in general but rather depends upon the existence of explicit delays between the time at which a switching decision is made and the time at which that decision is executed. 相似文献
123.
Ray Williamson 《Space Policy》2001,17(1):433
This paper examines the debate within the USA over how to meet the perceived competition from the successful European Ariane launcher and the loss of US market share for space launch services that it represented. In particular, it explores the origins of the 1983 Reagan Administration policy to turn over expendable launch vehicle production and operation to private industry. The Administration's other decisions to: (1) use the Space Shuttle to fly all government payloads, and (2) allow NASA to market Space Shuttle services commercially, conflicted with this commercialization policy. These policies effectively caused US industry to delay entry into the international competition for launch services until after the loss of the Space Shuttle Challenger in January 1986. 相似文献
124.
The following report presents the main conclusions of the sixth and seventh EOPOLE workshops, held in Hydra, Greece, 3–4 May and Leiden, the Netherlands, 3–5 July 2000, respectively. The objectives of the first workshop were to evaluate different approaches to pricing policy and to assess how new developments in Earth observation and information technology are having an impact on Earth observation data pricing policy. Those of the second were to assess the constraints that legal frameworks impose on Earth observation and to explore ways in which they can be used to its advantage. 相似文献
125.
126.
Deborah Scherrer Morris Cohen Todd Hoeksema Umran Inan Ray Mitchell Philip Scherrer 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008
The International Heliophysical Year (IHY) aims to advance our understanding of the fundamental processes that govern the Sun, Earth, and heliosphere. The IHY Education and Outreach Program is dedicated to inspiring the next generation of space and Earth scientists as well as spreading the knowledge, beauty, and relevance of our solar system to the people of the world. In our Space Weather Monitor project we deploy a global network of sensors to high schools and universities to provide quantitative diagnostics of solar-induced ionospheric disturbances, thunderstorm intensity, and magnetospheric activity. We bring real scientific instruments and data in a cost-effective way to students throughout the world. Instruments meet the objectives of being sensitive enough to produce research-quality data, yet inexpensive enough for placement in high schools and universities. The instruments and data have been shown to be appropriate to, and usable by, high school age and early university students. Data contributed to the Stanford data center is openly shared and partnerships between groups in different nations develop naturally. Students and teachers have direct access to scientific expertise. 相似文献
127.
Sara E. Mitchell Emilie Drobnes M. Sol Colina-Trujillo Jacob Noel-Storr 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008
Parents and families have the greatest influence on children’s attitudes towards education and career choices. If students’ attitudes towards science, particularly the physical sciences, are not influenced positively by parental/familial attitudes, efforts to improve the quality of content and teaching of these subjects in school may be futile. Research shows that parental involvement increases student achievement outcomes, and family-oriented programs have a direct impact on student performance. 相似文献
128.
Mark A. Skinner Ray W. Russell Tom Kelecy Steve Gregory Richard J. Rudy Daryl L. Kim Kirk Crawford 《Acta Astronautica》2014
There exists a population of defunct satellites in the geo-stationary arc that potentially pose a hazard to current and future operational satellites. These drifting, non-station-kept objects have a variety of ages and sizes, and many are trapped in libration orbits around the Earth?s two gravitational potential wells (the non-spherical nature of the Earth gives rise to two geo-potential wells or “stable points” that affect objects in geostationary and geosynchronous orbits), whereas others were boosted to higher altitudes into so-called “graveyard” orbits. 相似文献
129.
Monika Sinha Mira Dey Subharthi Ray Jishnu Dey 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2006,38(12):2915-2916
We suggest that superbursts from some low mass X-ray binaries may be due to breaking and re-formation of diquark pairs, on the surface of realistic strange stars. Diquarks are expected to break up due to the explosion and shock of the thermonuclear process. After a prolonged accretion when almost all pairs get broken, the subsequent production of copious diquarks may produce sufficient energy to produce the superbursts. 相似文献
130.
Ulug B. Ahalt S.C. Mitchell R.A. 《IEEE transactions on aerospace and electronic systems》1997,33(4):1199-1211
We examine various model-based automatic target recognition (MBATR) classifiers to investigate the utility of model-catalog compression realized via signal-vector quantization (VQ) and feature extraction. We specifically investigate the impact of various compression rates and common automatic target recognition (ATR) scenario variations such as noise and occlusion through simulations on high-range resolution (HRR) radar and synthetic aperture radar (SAR) data. For this data, we show that significant computational savings are possible for modest decreases in classification performance. 相似文献