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排序方式: 共有71条查询结果,搜索用时 15 毫秒
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Estimating the mass of asteroid 253 mathilde from tracking data during the NEAR flyby 总被引:1,自引:0,他引:1
DK Yeomans J Barriot DW Dunham RW Farquhar JD Giorgini CE Helfrich AS Konopliv JV McAdams JK Miller WM Owen DJ Scheeres SP Synnott BG Williams 《Science (New York, N.Y.)》1997,278(5346):2106-2109
The terminal navigation of the Near Earth Asteroid Rendezvous (NEAR) spacecraft during its close flyby of asteroid 253 Mathilde involved coordinated efforts to determine the heliocentric orbits of the spacecraft and Mathilde and then to determine the relative trajectory of the spacecraft with respect to Mathilde. The gravitational perturbation of Mathilde on the passing spacecraft was apparent in the spacecraft tracking data. As a result of the accurate targeting achieved, these data could be used to determine Mathilde's mass as 1.033 (+/- 0.044) x 10(20) grams. Coupled with a volume estimate provided by the NEAR imaging team, this mass suggests a low bulk density for Mathilde of 1.3 grams per cubic centimeter. 相似文献
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The confidential enquiry into perioperative equine fatalities (CEPEF): mortality results of Phases 1 and 2 总被引:3,自引:0,他引:3
GM Johnston JK Eastment JLN Wood & PM Taylor 《Veterinary anaesthesia and analgesia》2002,29(4):159-170
Objectives To document the equine perioperative mortality rate and to highlight any factor associated with an increased risk of death up to 7 days after anaesthesia. Study design A prospective observational epidemiological multicentre study. Methods Data were recorded from all equidae undergoing general anaesthesia in 62 clinics. Power calculations indicated that 45 000 cases were required to detect the significance of important variables. Details of each horse, operation, anaesthetic agents and clinic personnel were recorded. Outcome at 7 days was recorded as: alive, put to sleep (PTS) or dead. Data were analysed by a standard multilevel logistic regression approach, considering the effects of clustering at the level of clinic. Results Data were collected from 41 824 cases over 6 years. A total of 39 025 (93.3%) were alive on day 7 and 785 were dead giving an overall death rate of 1.9% (95% CI: 1.8–2.0) and 2014 (4.8%) were PTS. About 5846 horses undergoing emergency abdominal surgeries (‘colics’) were excluded from subsequent analyses. A total of 35 107 ‘noncolic’ horses were alive at 7 days and 328 dead giving a death rate for noncolics of 0.9% (95% CI: 0.8–1.0). Five hundred and forty‐three (1.5%) noncolic horses classified PTS were excluded from further analyses. There were 109 (33%) deaths from cardiac arrest or post‐operative cardiovascular collapse, with 107 (32%) from fractures and myopathies. Fracture repair, out of hours surgery, and age below 1 month was associated with increased risk of dying whereas the use of acepromazine and intravenous anaesthetic agent maintenance of anaesthesia was associated with reduced risk. Conclusions A number of potential contributors to the high risk of anaesthetic‐related mortality have been identified. Further investigation of the underlying mechanism for their apparent harmful effects and development of alternative techniques is merited. 相似文献
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Several outbreaks of anaemia, jaundice, abortion and mortality in cattle in New South Wales were attributed to the intracellular parasite, Theileria buffeli . Disease occurred predominantly in periparturient animals that had been moved from inland to coastal areas. Diagnosis was made via exclusion of other causes of haemolytic anaemia and observation of the parasite in blood smears. Treatments included both registered and non-registered products. There is a possibility of a new strain of Theileria sp. in Australia and the possible vectors encountered in NSW are discussed. 相似文献
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JK Gimzewski C Joachim RR Schlittler V Langlais H Tang I Johannsen 《Science (New York, N.Y.)》1998,281(5376):531-533
Experimental visualization and verification of a single-molecule rotor operating within a supramolecular bearing is reported. Using a scanning tunneling microscope, single molecules were observed to exist in one of two spatially defined states laterally separated by 0.26 nanometers. One was identified as a rotating state and the other as an immobilized state. Calculations of the energy barrier for rotation of these two states show that it is below the thermal energy at room temperature for the rotating state and above it for the immobilized state. 相似文献
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Design of a surface alloy catalyst for steam reforming 总被引:1,自引:0,他引:1
F Besenbacher I Chorkendorff BS Clausen B Hammer AM Molenbroek JK Norskov I Stensgaard 《Science (New York, N.Y.)》1998,279(5358):1913-1915
Detailed studies of elementary chemical processes on well-characterized single crystal surfaces have contributed substantially to the understanding of heterogeneous catalysis. Insight into the structure of surface alloys combined with an understanding of the relation between the surface composition and reactivity is shown to lead directly to new ideas for catalyst design. The feasibility of such an approach is illustrated by the synthesis, characterization, and tests of a high-surface area gold-nickel catalyst for steam reforming. 相似文献
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Is pH increasing in the noncalcareous topsoils of France under agricultural management? A statistical framework to overcome the limitations of a soil test database
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N. P. A. Saby C. Swiderski B. Lemercier C. Walter B. P. Louis P. Eveillard D. Arrouays 《Soil Use and Management》2017,33(3):460-470
Soil pH is one of the most common and important measurements used to assess soil quality and manage soil fertility. Soil acidification is a slow process that can have large consequences. Therefore, it is important to detect soil changes early. Using a French soil test database, we show that the soil pH increased in 36% of arable soils monitored from 1996 to 2010, which suggests that soils became less acidic across the country. Conversely, decreases in pH were rarely detected. To perform this assessment, we used a statistical framework to analyse 488 717 soil test results from the French national soil test database. We excluded calcareous soils due to their relatively stable pH. Two periods were analysed: 1996 to 2000 (period 1) and 2006 to 2010 (period 2). Then, we performed nonparametric Mann–Whitney U tests following a resampling step to account for the noncontrolled selection of spatial and temporal localization in the observations. Our statistical framework smoothed the effects of some limitations of the database related to the variable number of samples collected during both periods and uncertainties in sampling locations. Overall, this study demonstrates the potential of using this type of database and a relevant statistical framework to detect long‐term changes in soil properties. 相似文献
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