排序方式： 共有36条查询结果，搜索用时 15 毫秒
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Michael J.S. Belton Karen J. Meech Steven Chesley Jana Pittichová Brian Carcich Michal Drahus Alan Harris Stephen Gillam Joseph Veverka Nicholas Mastrodemos William Owen Michael F. A’Hearn S. Bagnulo J. Bai L. Barrera Fabienne Bastien James M. Bauer J. Bedient B.C. Bhatt Hermann Boehnhardt H. Zhao 《Icarus》2011,213(1):345-368
The evolution of the spin rate of Comet 9P/Tempel 1 through two perihelion passages (in 2000 and 2005) is determined from 1922 Earth-based observations taken over a period of 13 year as part of a World-Wide observing campaign and from 2888 observations taken over a period of 50 days from the Deep Impact spacecraft. We determine the following sidereal spin rates (periods): 209.023 ± 0.025°/dy (41.335 ± 0.005 h) prior to the 2000 perihelion passage, 210.448 ± 0.016°/dy (41.055 ± 0.003 h) for the interval between the 2000 and 2005 perihelion passages, 211.856 ± 0.030°/dy (40.783 ± 0.006 h) from Deep Impact photometry just prior to the 2005 perihelion passage, and 211.625 ± 0.012°/dy (40.827 ± 0.002 h) in the interval 2006–2010 following the 2005 perihelion passage. The period decreased by 16.8 ± 0.3 min during the 2000 passage and by 13.7 ± 0.2 min during the 2005 passage suggesting a secular decrease in the net torque. The change in spin rate is asymmetric with respect to perihelion with the maximum net torque being applied on approach to perihelion. The Deep Impact data alone show that the spin rate was increasing at a rate of 0.024 ± 0.003°/dy/dy at JD2453530.60510 (i.e., 25.134 dy before impact), which provides independent confirmation of the change seen in the Earth-based observations.The rotational phase of the nucleus at times before and after each perihelion and at the Deep Impact encounter is estimated based on the Thomas et al. (Thomas et al. [2007]. Icarus 187, 4–15) pole and longitude system. The possibility of a 180° error in the rotational phase is assessed and found to be significant. Analytical and physical modeling of the behavior of the spin rate through of each perihelion is presented and used as a basis to predict the rotational state of the nucleus at the time of the nominal (i.e., prior to February 2010) Stardust-NExT encounter on 2011 February 14 at 20:42.We find that a net torque in the range of 0.3–2.5 × 107 kg m2 s?2 acts on the nucleus during perihelion passage. The spin rate initially slows down on approach to perihelion and then passes through a minimum. It then accelerates rapidly as it passes through perihelion eventually reaching a maximum post-perihelion. It then decreases to a stable value as the nucleus moves away from the Sun. We find that the pole direction is unlikely to precess by more than ～1° per perihelion passage. The trend of the period with time and the fact that the modeled peak torque occurs before perihelion are in agreement with published accounts of trends in water production rate and suggests that widespread H2O out-gassing from the surface is largely responsible for the observed spin-up. 相似文献
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Bastien Caplain Dominique Astruc Vincent Regard Frédéric Y. Moulin 《Comptes Rendus Geoscience》2011,343(7):471-477
Wave flume experiments have been performed to investigate a sandy cliff recession under monochromatic wave forcing. We varied the wave climate through the wave energy flux F and the surf similarity parameter ξ. The various processes of the cliff erosion cycle are depicted. The sea bed evolution mostly depends on the surf similarity parameter ξ. Steep planar (ξ > 0.7), gentle planar (0.5 < ξ < 0.7) and bared (ξ < 0.5) profiles are observed. We observed different bar dynamics, including steady and unsteady self-sustained oscillating states. Then we analyze the role of the eroded material on the cliff recession rate. We show that the cliff recession rate increases with the wave energy flux. Moreover, for a given wave energy flux, it is larger for a gentle planar profile than for a bared profile. However it is similar for both a bared profile and a steep planar profile. The cliff recession rate is not a monotonic function of the cliff height as the type of bottom profile influences the wave energy at the cliff. 相似文献
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Internal parts of the Alps have undergone widespread extensional deformation in the course of their Neogene exhumation history. Palaeostress inversion methods are used to map the prevailing stress fields and their evolution through time. Here we present new data from 100 sites with a total of about 2000 faults/striae couples, covering a large portion of the inner north‐western Alps. Palaeostress tensors are mostly extensional, although one‐third of them are transcurrent. The dominant direction of minimum horizontal stress axes (σ3) is in an orogen‐parallel (N30° to N70°) orientation around the bend of the north‐west alpine arc. A comparison between this older (Neogene, post‐metamorphic) stress field with the current stress and strain field determined from seismotectonics and geodesy indicates a change in deformation mode from early orogen‐parallel extrusion to a late and ongoing orogen‐perpendicular spreading. 相似文献
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Detlef P. van Vuuren Marcel T.J. Kok Bastien Girod Paul L. Lucas Bert de Vries 《Global Environmental Change》2012,22(4):884-895
Over the last 10 years a large set of global environmental assessment studies has been published that include scenario projections. Comparison of these studies shows that there is actually a limited set of scenario families that form the basis of many scenarios used in different environmental assessments. Mapping these scenario families allow a simpler comparison across the different assessments. The fact that many assessments can be positioned within these families gives some confidence of their relevance. It is also noticeable that several recent assessments have been focusing on more focussed policy-scenarios as variant to a single baseline. This is partly a response to a different stage in the policy-making process. While there are clear advantages of both categories of scenarios (explorative scenarios and baseline/policy variants), it might be important to test the robustness of scenario outcomes of the policy-scenario approach against the different storyline scenario-families identified in this paper. 相似文献
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The risk of both severe and frequent injury associated with active pursuits in mountain areas is acknowledged. This phenomenon is all the most relevant to investigate in France that there is a growing enthusiasm for outdoor sports and recreation. This paper focuses on the social-demographic and sporting profiles of accident victims. Our intent is to better understand who is specifically at risk, while practicing mountain recreation, in the French mountains. Such identification doubtlessly constitutes an important asset in order to enhance prevention. Three critical activities are notably tackled, among some others: hiking, a widespread activity that exposes many practitioners to hazardous settings; mountaineering and ski touring, supposedly entailing the most severe injuries. In order to align the information collected, then provide estimations based on the reasoned crossing of these secondary data, we consulted and summarised all the databases that we knew of on the French territory: rescue team intervention reports, aggregated statistics from the sports ministry, sporting federation accident declarations, avalanche experts statistics, mountain guides union accident/incident reports, healthcare facilities admissions, etc. Only the sources considered to be the most reliable and representative were used, owing to the number of cases processed, the exhaustive nature of data collection, or the level of information detail available. Our method also included consultation of all the parties playing an active role in prevention, training and mountain rescue, thus ensuring a participative, co-constructive approach to the study with the people that have the best knowledge of victims’ profiles. Around fifty such parties were interviewed on the methods implemented, the benefits and limits of current data, and the main expectations in terms of accidentology progress, thus further enhancing the credibility of the study proposed in this paper. Our results show that the parent population profiles (in terms of age, gender, place of residence, etc.), weighted by the level of exposure (frequency of participation, level of engagement, etc.) appears to have an impact on the accident victim profiles. There is certainly a very marked gender bias and a serious age impact to be taken into account, as well as the influence of the level of experience, which counter certain preconceived ideas and which, at the very least, go against instinct. However, aside from a probable over-exposure of experienced or expert participants, much remains to be explained concerning the accident mechanisms and circumstances involved. At this stage, we regret the analytic deficiencies induced by a lack of knowledge concerning the parent populations in many cases, which makes interpretation difficult. 相似文献
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