Ceres: Its Origin, Evolution and Structure and Dawn��s Potential Contribution

Ceres appears likely to be differentiated and to have experienced planetary evolution processes. This conclusion is based on current geophysical observations and thermodynamic modeling of Ceres?? evolution. This makes Ceres similar to a small planet, and in fact it is thought to represent a class of objects from which the inner planets formed. Verification of Ceres?? state and understanding of the many steps in achieving it remains a major goal. The Dawn spacecraft and its instrument package are on a mission to observe Ceres from orbit. Observations and potential results are suggested here, based on number of science questions.     

Local fracture of the burning solid propellant

The problem of fracture mechanics for the solid propellant deteriorated by a crack-shaped cavity, the surface of which burns is considered. It is assumed that the propellant reagents are uniformly distributed in the solid phase while the combustion products are gaseous. The sufficient condition for the “combustion -fracture” stability regime is obtained.     

Medical survey of European astronauts during Mir missions

This paper reviews the medical operations performed on six European astronauts during seven space missions on board the space station Mir. These missions took place between November 1988 and August 1999, and their duration ranged from 14 days to 189 days. Steps of pre-flight medical selection and flight certification are presented. Countermeasures program used during the flight, as well as rehabilitation program following short and long-duration missions are described. Also reviewed are medical problems encountered during the flight, post-flight physiological changes such as orthostatic intolerance, exercise capacity, blood composition, muscle atrophy, bone density, and radiation exposure.     

Pareto-optimal solution of a two-objective problem in choosing conditions of monoimpeller milling

A problem of optimization of cutting parameters with respect to the criteria of maximum efficiency and maximum tool resistance is formulated. To solve the problem, a method is developed and applied for determining the Pareto-optimal values of machining conditions in milling the present-day GTE monoimpeller blade passages with a circular milling cutter.     

A mission template for exploration and damage mitigation of potential hazard of Near Earth Asteroids

The Apophis Exploratory and Mitigation Platform (AEMP) concept was developed as a prototype mission to explore and potentially deflect the Near Earth Asteroid (NEA) 99942 Apophis. Deflection of the asteroid from the potential 2036 impact will be achieved using a gravity tractor technique, while a permanent deflection, eliminating future threats, will be imparted using a novel albedo manipulation technique. This mission will serve as an archetypal template for future missions to small NEAs and could be adapted to mitigate the threat of collision with other potential Earth-crossing objects.     

The Solar System d/h Ratio: Observations and Theories

The measured D/H ratios in interstellar environments and in the solar system are reviewed. The two extreme D/H ratios in solar system water - (720±120)×10⁻⁶ in clay minerals and (88±11)×10⁻⁶ in chondrules, both from LL3 chondritic meteorites - are interpreted as the result of a progressive isotopic exchange in the solar nebula between deuterium-rich interstellar water and protosolar H₂. According to a turbulent model describing the evolution of the nebula (Drouart et al., 1999), water in the solar system cannot be a product of thermal (neutral) reactions occurring in the solar nebula. Taking 720×10⁻⁶ as a face value for the isotopic composition of the interstellar water that predates the formation of the solar nebula, numerical simulations show that the water D/H ratio decreases via an isotopic exchange with H₂. During the course of this process, a D/H gradient was established in the nebula. This gradient was smoothed with time and the isotopic homogenization of the solar nebula was completed in 10⁶ years, reaching a D/H ratio of 88×10⁻⁶. In this model, cometary water should have also suffered a partial isotopic re-equilibration with H₂. The isotopic heterogeneity observed in chondrites result from the turbulent mixing of grains, condensed at different epochs and locations in the solar nebula. Recent isotopic determinations of water ice in cold interstellar clouds are in agreement with these chondritic data and their interpretation (Texeira et al., 1999). This revised version was published online in June 2006 with corrections to the Cover Date.     

Optimum Acquisition of the Idling First-Order Phase-Locked Loop

Acquisition of the idling first-order phase-locked loop (PLL) is investigated. An analytical solution for the Fokker-Planck equation is found if the noise term is neglected. Furthermore, the modulation signal that optimizes the acquisition is obtained by use of phase-plane techniques and Pontryagin's minimum principle.