Diversity of Comets: Formation Zones and Dynamical Paths

The past dozen years have produced a new paradigm with regard to the source regions of comets in the early solar system. It is now widely recognized that the likely source of the Jupiter-family short-period comets (those with Tisserand parameters, T > 2 and periods, P, generally < 20 years) is the Kuiper belt in the ecliptic plane beyond Neptune. In contrast, the source of the Halley-type and long-period comets (those with T < 2 and P > 20 years) appears to be the Oort cloud. However, the comets in the Oort cloud almost certainly originated elsewhere, since accretion is very inefficient at such large heliocentric distances. New dynamical studies now suggest that the source of the Oort cloud comets is the entire giant planets region from Jupiter to Neptune, rather than primarily the Uranus-Neptune region, as previously thought. Some fraction of the Oort cloud population may even be asteroidal bodies formed inside the orbit of Jupiter. These comets and asteroids underwent a complex dynamical random walk among the giant planets before they were ejected to distant orbits in the Oort cloud, with possible interesting consequences for their thermal and collisional histories. Observational evidence for diversity in cometary compositions is limited, at best. This revised version was published online in June 2006 with corrections to the Cover Date.     

Periodic relative orbits of two spacecraft subject to differential gravity and electrostatic forcing

Coulomb forces between charged close-flying satellites can be used for formation control, and constant electric potentials enable static equilibria solutions. In this work, open-loop time-varying potential functions, which produce periodic, two-craft, Coulomb formation motions are demonstrated for the first time. This is done in the rotating Hill-Frame, with linearized gravity, and craft position components assumed in the form of simple harmonic oscillators. Substitution of the oscillatory functions into the dynamics, further constrains these functions, and yields necessary potential histories, to produce the periodic flow. The assumed position functions, however, are not arbitrary, since the dynamical model restricts what oscillatory trajectories are allowed. Specifically, a Hill-Frame integral of motion is derived, and this is used to show certain candidate periodic functions to be inadmissible. The system dynamics are then linearized to expose stability properties of the solutions, and it is established that asymptotic stability is impossible for all orbit families. Finally, the degree of instability in the assumed motions, over free parameter ranges, is determined numerically via the Floquet multipliers of the associated full-cycle state-transition matrices.     

Human Mission from Planet Earth: finding a rationale for exploration of the Moon and Mars

This paper advances scenarios for an evolutionary approach to the establishment of a Human Mission from Planet Earth involving exploration and utilization of the Moon and Mars. Of critical importance are the concepts of robotic–human and Moon–Mars synergies. The paper discusses the rationale variables that have an influence on types of space activities pursued and thus the policy that leads to particular scenarios. In this regard, the technological, scientific, and sociopolitical/socioeconomic impacts and consequences related to a Human Mission from Planet Earth are examined.     

Hybrid optimization for multiple-impulse reconfiguration trajectories of satellite formation flying

A hybrid optimization method is developed for fuel-optimal reconfigurations of a group of satellites flying in formation. The genetic algorithm performs a global search to find two-impulse trajectories, and primer vector analysis finds multiple-impulsive local optimal trajectories with the two-impulse trajectories as initial guesses. Hybrid optimization finds globally optimal trajectories for formation reconfigurations, including formation resizing, reassignment and reorientation maneuvers. Multiple-impulse trajectories reduce the fuel consumption from the two-impulse trajectories by up to 4.4% for those maneuvers. In real missions, satellites can follow two-impulse trajectories to gain the advantage of a smaller number of impulses, with the cost of slightly more propellant. The qualitative characteristics of the optimal trajectories are analyzed from the number of optimal trajectories found by hybrid optimization.     

Formation of Giant Planets– An Attempt in Matching Observational Constraints

We present models of giant planet formation, taking into account migration and disk viscous evolution. We show that migration can significantly reduce the formation timescale bringing it in good agreement with typical observed disk lifetimes. We then present a model that produces a planet whose current location, core mass and total mass are comparable with the one of Jupiter. For this model, we calculate the enrichments in volatiles and compare them with the one measured by the Galileo probe. We show that our models can reproduce both the measured atmosphere enrichments and the constraints derived by Guillot et al. (2004), if we assume the accretion of planetesimals with ices/rocks ratio equal to 4, and that a substantial amount of CO₂ was present in vapor phase in the solar nebula, in agreement with ISM measurements.     

Dynamics and Composition of Rings

Planetary rings are found around all four giant planets of our solar system. These collisional and highly flattened disks exhibit a whole wealth of physical processes involving dust grains up to meter-sized boulders. These processes, together with ring composition, can help understand better the formation and evolution of proto-satellite and proto-planetary disks in the early solar system. The present chapter reviews some fundamental aspects of ring dynamics and composition. The forthcoming exploration of the Saturn system by the Cassini mission will bring both high resolution and time-dependent information on Saturn’s rings.     

On a non-primordial origin of the cosmic background radiation and pregalactic density fluctuations

Dusty pregalactic Population III objects may provide a mechanism for an effective thermalization of the star radiation. They may generate the observed microwave background and so the high cosmic photon entropy. Assuming a tepid universe a smaller primordial entropy contribution results in reasonable mass scales and amplification factors of pregalactic density fluctuations.     

Cosmology and GUTs: The matter of the universe

To illustrate the interaction of Grand Unified Theories (GUTs), supersymmetry (SUSY), and cosmology, a worked example is carried out. This example is the dark matter problem, or “What is the dominant matter of the Universe?” It is shown that if GUTs are assumed then the primordial perturbations are probably adiabatic, if inflation is assumed then Ω = 1 and GUTs first name is probably SUSY. If Ω = 1, big bang nucleosynthesis tells us that the bulk of the matter is non-baryonic. SUSY-GUTs gives us some possible candidate inos to which massive neutrinos, axion or planetary mass black holes can be added. These candidates can be classified hot (or warm) or cold types of dark matter. It is shown that hot gives Ω = 1 and naturally gives large scale structure but does not give small scale structure or galaxy formation times, whereas cold gives small scale structure and formation times but cannot easily yield Ω = 1. It is concluded that either a hybrid of both hot and cold or non-random phases for the perturbations may be needed.     

Hi content in cluster and supercluster galaxies

In this lecture I review part of the published and unpublished observational evidence showing that cluster spiral galaxies are hydrogen deficient. Always using observational data, it is argued that the deficiency is due to the interaction between the galaxies and the intergalactic medium. Furthermore it is shown that the stellar population of deficient galaxies has also been affected and a brief discussion is given of the mechanisms at work.     

The structure and effects of cooling flows in clusters of galaxies

Rapidly cooling gas is commonly found near the centres of clusters of galaxies. The structure of the resulting gas flows is reviewed. Total gas cooling rates of several hundred M yr⁻¹ have been observed in a number of cases. Thermal instability and the ultimate fate of the cooled gas are discussed. The cooled gas could easily have formed a massive central galaxy.