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排序方式: 共有83条查询结果,搜索用时 31 毫秒
31.
JK Harmon SJ Ostro LAM Benner KD Rosema RF Jurgens R Winkler DK Yeomans D Choate R Cormier JD Giorgini DL Mitchell PW Chodas R Rose D Kelley MA Slade ML Thomas 《Science (New York, N.Y.)》1997,278(5345):1921-1924
Radar observations of comet Hyakutake (C/1996 B2) made at the Goldstone Deep Space Communications Complex in California have detected echoes from the nucleus and from large grains in the inner coma. The nucleus of this bright comet was estimated to be only 2 to 3 kilometers in diameter. Models of the coma echo indicate backscatter from porous, centimeter-size grains ejected anisotropically at velocities of tens of meters per second. The radar observations suggest that a comet's activity may be a poor indicator of its size and provide evidence that large grains constitute an important component of the mass loss from a typical active comet. 相似文献
32.
通常情况下,管道涂敷商在涂敷新涂层之前,都要将管道表面处理到Sa 2.5级或者近白级,但是在施工现场却很难达到这一标准.讨论了旧涂层去除的常规方法、表面处理以及可预见的结果,并讨论了当前在表面处理方面的研究成果以及在涂层涂敷过程中二次生锈的各种影响. 相似文献
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JD Archibald 《Science (New York, N.Y.)》1996,272(5265):1150-1153
Seventeen of eighteen orders of living placental mammals are not known before 65 million years ago. The monophyly of each order is well established, but interrelations have been less certain. A superordinal grouping of up to seven extant orders plus a variety of extinct orders, all included within Ungulata ("hoofed" mammals), can be linked to Late Cretaceous mammals from the 85-million-year-old Bissekty Formation, Uzbekistan (and, less certainly, North America and Europe), thus pushing the origin of this major clade back by 20 million years. Ungulatomorphs are not closely related to primates, rodents, or rabbits. 相似文献
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Verminous encephalomyelitis due to Angiostrongylus cantonensis larvae was diagnosed in 2 foals at necropsy. The principal clinical feature was tetraparesis, although history and neurological examination revealed progressive and multifocal neurological disease. At presentation, a tentative diagnosis of parasitic larval migration involving the central nervous system (CNS), presumably due to Strongylus vulgaris, was proposed. Dissection of the spinal cord in one case resulted in recovery of intact larvae of both sexes of A. cantonensis. In both foals, histopathology of the brain and spinal cord revealed nematode sections which were consistent with A. cantonensis larvae. 相似文献
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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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Testosterone serum profile,semen characteristics and testicular biometry of Mangalarga Marchador stallions in a tropical environment 下载免费PDF全文
B Waddington JM Penitente‐Filho JGS Neves RO Pinho AY Chaya PP Maitan CO Silveira MG Neves SEF Guimarães GR de Carvalho JD Guimarães 《Reproduction in domestic animals》2017,52(2):335-343
This study was conducted to characterize the daily profile of testosterone secretion and its mean concentrations in the four seasons as well as to evaluate the semen characteristics and testicular biometry of Mangalarga Marchador stallions throughout the year in a tropical region. Three stallions were submitted to semen collections and evaluation of testicular biometry every 14 days along a year. Blood samples were collected once at the middle of each season, in a 20‐min interval during 24 hr in order to evaluate the testosterone secretion profiles among seasons. Testosterone concentrations along the day were higher at the beginning of the afternoon (from 12:00 to 15:00 hr), but a circadian secretion was not clearly observed. Mean testosterone concentrations did not differ among seasons (p > .05), but a pattern of secretion along the day showed variations with higher concentrations in the afternoon during the winter. Ejaculate volume was higher during summer; however, sperm motility decreased in summer and spring. Total sperm in ejaculate, sperm morphology and testicular biometry kept constant along the year showing no differences among the seasons. The results demonstrated that in a tropical region, reproductive aspects of stallions did not show a clearly defined seasonal variation, and months of autumn and winter were not unsuitable for reproduction of the males. 相似文献