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学术报告

【学术报告3场】7.7—7.11, Prof. Daniel J. Scheeres

发布时间:2017-07-05 10:41 来源:未知 责任编辑:qla
报告题目:The Strength of Rubble Pile Asteroids: Evidence and Implications (Planetary Science topic), which I plan it as a report to the university undergraduates and graduates
 
报告人:Prof. Daniel J. Scheeres
 
时间:7月7日(周五)上午10点
地点: 仙II 311
 
摘要:
Rubble pile asteroids have been frequently characterized as being cohesionless, self-gravitating aggregates of boulders and grains. This picture has motivated many geophysical theories for how these bodies evolve in the solar system, and what assumptions can be made concerning their evolution and exploration. However, recent astronomical observations of asteroid 1950DA and active asteroid P/2013 R3 have provided evidence that this picture is incomplete, and indeed that rubble pile asteroids can exhibit a degree of cohesion between their components. The net effect of even small amounts of cohesion has significant implications for the evolution of small asteroids in the main belt and in the NEA population, and will be reviewed. A recent theory by Sanchez and Scheeres provides one hypothesis for how such cohesive strength could exist in gravitationally attracting aggregates — through weak van der Waals attractive forces between the finest grains in a rubble pile. This theory has several additional implications for the asteroid environment that have specific predictions that can be probed with both ground-based observations and with spacecraft rendezvous missions such as Hayabusa II and OSIRIS-REx.
 
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报告题目:New relative equilibria and their implications in the Full 3-Body Problem (Celestial Mechanics topic)
 
报告人:Prof. Daniel J. Scheeres
 
时间:7月10日(周一),上午10点
地点:天文楼202会议室
 
摘要:
Celestial Mechanics systems have two fundamental conservation principles that enable their deeper analysis: conservation of momentum and conservation of (mechanical) energy. Of the two, conservation of momentum provides the most constraints on a general system, with three translational symmetries (which can be trivially removed) and three rotational symmetries. If no external force acts on the system, these quantities are always conserved independent of the internal interactions of the system. Conservation of energy instead involves assumptions on both the lack of exogenous forces and on the nature of internal interactions within the system. For this reason energy is often not conserved for "real" systems that involve internal interactions, such as tidal deformations or impacts, even though they may conserve their total momentum. Thus mechanical energy generally decays through dissipation until the system has found a local or global minimum energy configuration that corresponds to its constant level of angular momentum.
This observation motivates a fundamental question for celestial mechanics:
 
What is the minimum energy configuration of a N-body system with a fixed level of angular momentum?
 
In this talk we show that this is an ill-defined question for traditional point-mass celestial mechanics systems. If instead the system and problem are formulated accounting for finite density distributions this question becomes well posed and provides new light on celestial mechanics systems.  We show that this question naturally leads to a granular mechanics extension of usual celestial mechanics questions such as relative equilibria and stability. Applying this theory, we identify new relative equilibria for the finite-density 3-body problem and identify their stability and existence as a function of system angular momentum.  
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报告题目: The Mechanics of Asteroid Exploration (Astrodynamics topic)
 
报告人: Prof. Daniel J. Scheeres
 
时间:7月11日(周二),上午10点
地点:天文楼202会议室
 
摘要:
The robotic exploration of asteroids is an important human endeavor motivated by significant themes: the scientific study of solar system formation and evolution, the exploration of the solar system, and the protection of society against future hazardous asteroid impactors. This endeavor involves significant challenges across a range of technical issues associated with fundamental dynamics and control: development of new models for mathematically describing the asteroid environment, developing new understanding of the basic mechanics of asteroids, describing and predicting orbital motion in these highly perturbed environments, and developing novel methods of guidance and control for asteroid orbiters. This talk will introduce the technical challenges the asteroid environment poses, review progress which has occurred over the last two decades, and indicate where research is still progressing. It will also give a preview of upcoming asteroid exploration missions and their scientific and technical significance.