Stability of Surface Motion on a Rotating Ellipsoid

The dynamical environment on the surface of a rotating, massive ellipsoid is studied, with applications to surface motion on an asteroid. The analysis is performed using a combination of classical dynamics and geometrical analysis. Due to the small sizes of most asteroids, their shapes tend to differ from the classical spheroids found for the planets. The tri-axial ellipsoid model provides a non-trivial approximation of the gravitational potential of an asteroid and is amenable to analytical computation. Using this model, we study some properties of motion on the surface of an asteroid. We find all the equilibrium points on the surface of a rotating ellipsoid and we show that the stability of these points is intimately tied to the conditions for a Jacobi or MacLaurin ellipsoid of equilibria. Using geometrical analysis we can define global constraints on motion as a function of shape, rotation rate, and density, we find that some asteroids should have accumulation of material at their ends, while others should have accumulation of surface material at their poles. This study has implications for motion of a rover on an asteroid, and for the distribution of natural material on asteroids, and for a spacecraft hovering over an asteroid.     

Evolution of the photospheric magnetic field and coronal null points before solar flares

Based on a topological model for the magnetic field of a solar active region (AR), we suggest a criterion for the existence of magnetic null points on the separators in the corona. With the problem of predicting solar flares in mind, we have revealed a model parameter whose decrease means that the AR evolves toward a major eruptive flare. We analyze the magnetic field evolution for AR 9077 within two days before the Bastille Day flare on July 14, 2000. The coronal conditions are shown to have become more favorable for magnetic reconnection, which led to a 3B/X5.7 eruptive flare.     

Vacuum Effects in a Conformally Flat Brane-Cosmology and Radion Stabilization

Vacuum quantum effects in a conformally coupled scalar field on the background of a conformally flat geometry of brane worlds are studied. Based on the corresponding results for a Minkowski space-time in the general case of mixed boundary conditions on the branes, formulas are derived for the vacuum averaged energy-momentum tensor and for the vacuum forces acting on the boundaries. The important special case of AdS background is examined and applications to the Randall-Sundrum model are discussed. The possible stabilization of the radion by vacuum forces is demonstrated.     

Discrepancy of masses in double galaxies

The estimations of integral masses of double galaxies are obtained taking as a basis recent observational data on 150 pairs of galaxies with measured radial velocities for both components. It is shown that the mean orbital mass-to-luminosity ratio essentially depends on a morphological type of double galaxies and on a kind of interaction between pair components. Minimal values of mass-to-luminosity ratio take place for double galaxies having linear features of interaction, namely: bridges, tails; as well for pairs with compact and Markarian components, too. The empirical relation between mass-to-luminosity ratio and linear separation of pair components do not indicate on an existence of a hidden stellar coronae around double galaxies.     

Application of a massively parallel computer to the N-body problem

Parallel processor computers represent a new technology that has recently become available for astronomical applications. We have implemented an N-body code on a TMC Connection Machine CM-2 in order to investigate the advantages of a massively parallel computer over serial machines, including conventional supercomputers. For collisionless problems following N stars, a direct integration code scales as O(N²) on serial machines and on the CM-2 as O(log(N)) for small N and O(N log(N)) for large N. The CM-2 outperforms workstations for N>50 and supercomputers for N>4000.     

Sea-surface wave growth under extraterrestrial atmospheres: Preliminary wind tunnel experiments with application to Mars and Titan

We describe for the first time the generation and measurement of capillary waves in a water surface in a wind tunnel running with air at pressures of 15-1000 mbar. These experiments suggest a stronger dependence of wave generation on atmospheric density than the simple proportionality that might be expected from energy transfer arguments. Additionally, airflow over a nonaqueous fluid (kerosene) was found to produce waves of higher amplitude than for water under the same conditions. These preliminary results may indicate different efficiencies of wave generation on other planets, for which empirical terrestrial relations therefore do not apply, and thus may have a bearing on the lack of strong shoreline features on Mars and the possibility of specular glints from hydrocarbon lakes on Titan.     

Electric potential on solid spheres in a plasma

We derive the general expression for the potential on a solid sphere immersed in a plasma, showing the dependence of the potential on the radiusa of the sphere ands of the plasma sheath that develops around the sphere. In the limit where the radiusa is much larger than the sheath thicknesss-a we recover from this expression the well-known result for the potential on an infinite wall in contact with a plasma. On the other extreme wheres is much larger thana, we get the result derived by Spitzer (1941) for the potential on spherical grains in the interstellar plasma. Since the surface of the sphere forms a sink for the charged particles, there is a net drift of the plasma towards the surface. The effect of this drift on the potential is examined. Finally, for very small metallic spheres, an effect leading to a revision of the potential is discussed. This effect consists in a lowering of the potential barrier for the electrons due to the image force. The various effects limiting the potential on spheres are discussed.     

Astrobiology—What Can We Do on the Moon?

The Moon does not seem to be a place for a biologist. However, it offers the possibility of unravelling a better understanding of the conditions for habitability on the Earth and the conditions for life on the early Earth. It will be a place where much of the life sciences technologies required to establish a permanent human presence in space can be tested to complete reliability. Specifically, a long-term life sciences laboratory on the Moon can be used to investigate three areas of science that are currently poorly understood: (1) the linearity or non-linearity of the effects of different magnitudes of space environmental stresses on organisms, particularly gravity; (2) the effects of cumulative environmental effects both in individual organisms and across generations, (3) the synergistic effects of different space environmental parameters on organisms. The close proximity and scientific importance of the Moon makes it a useful permanent location and staging post for the human expansion into space.     

Lagrange’s planetary equations for the motion of electrostatically charged spacecraft

A spacecraft that generates an electrostatic charge on its surface in a planetary magnetic field will be subject to a perturbative Lorentz force. Active modulation of the surface charge can take advantage of this electromagnetic perturbation to modify or to do work on the spacecraft’s orbit. Lagrange’s planetary equations are derived using the Lorentz force as the perturbation on a Keplerian orbit, incorporating orbital inclination and true anomaly for the first time for an electrostatically charged vehicle. The planetary equations reveal that orbital inclination is a second-order effect on the perturbation, explaining results found in earlier studies through numerical integration. All of the orbital elements are coupled, but the coupling notably does not depend on the magnitude of the electrostatic charge or on the strength of the magnetic field. Analytical expressions that characterize this coupling are tested with a propellantless escape example at Jupiter. A closed-form solution exists that constrains the set of equatorial orbits for which planetary escape is possible, and a sufficient condition is identified for escape from inclined orbits. The analytical solutions agree with results from the numerically integrated equations of motion to within a fraction of a percent.     

Can magnetic fields be generated in the icy mantles of Uranus and Neptune?

It has been shown by us previously that a hydromagnetic dynamo can operate in the core of Uranus but probably not on Neptune. A similar analysis is made for the “icy” liquid mantles of both planets. It is concluded that pressure ionization and the associated increased conductivity of water is probably not enough to satisfy the necessary conditions for a dynamo on Uranus and that it is marginal for Neptune. On the other hand the expected presence of metallic water in a thick layer around the core of Neptune makes the operation of a dynamo on this planet plausible. A similar layer on Uranus might be too thin to play the same role. It appears that if a magnetic field is indeed present on Uranus it is probably generated in the core of the planet, while on Neptune it is more likely operating in the icy mantle.     

Frame transformation of spherical-harmonic coefficients of differential particle intensity

We present a general second-order-correct frame transformation on spherical-harmonic coefficients of differential particle intensity. The transformation, valid for relativistic particles as well, provides a clear view of the Compton-Getting effect. It shows explicitly how each transformed harmonic coefficient depends on a subset of the original harmonic coefficients. The general expression for the first-order Compton-Getting vector anisotropy is derived and interpreted. In addition, we show how the new transformation allows one to simplify a current procedure for determining the directional intensity in a comoving frame. This involves the directional particle data measured on a spacecraft.     

Spherically symmetric cosmological perturbations in the de Donder gauge

Cosmological perturbations on a F-R-W background are considered in a modified de Donder gauge. To guarantee the energy-momentum conservation for the perturbations in the de Donder gauge a compatibility condition is obtained. Finally we present the basic equations for the propagation of spherically symmetric perturbations. These equations are the basis for investigating the influence of cosmological epoches on the growing of density contrasts.     

The economic impact of climate change on Kenyan crop agriculture: A Ricardian approach

This paper measures the economic impact of climate on crops in Kenya. We use cross-sectional data on climate, hydrological, soil and household level data for a sample of 816 households. We estimate a seasonal Ricardian model to assess the impact of climate on net crop revenue per acre. The results show that climate affects crop productivity. There is a non-linear relationship between temperature and revenue on one hand and between precipitation and revenue on the other. Estimated marginal impacts suggest that global warming is harmful for crop productivity. Predictions from global circulation models confirm that global warming will have a substantial impact on net crop revenue in Kenya. The results also show that the temperature component of global warming is much more important than precipitation. Findings call for monitoring of climate change and dissemination of information to farmers to encourage adaptations to climate change. Improved management and conservation of available water resources, water harvesting and recycling of wastewater could generate water for irrigation purposes especially in the arid and semi-arid areas.     

Rate of H2 formation on amorphous grains

The rate of formation of molecular hydrogen from hydrogen atoms adsorbed on grains is analyzed, assuming that the grains are single crystals, polycrystalline or amorphous. On polycrystalline grains, and on graphite platelets, this rate could be orders of magnitude lower than on single crystal grains. The same is true for amorphous grains because there, at low temperatures, only atoms absorbed on neighboring sites can form molecules. Suitable formulae are derived and compared with the classical results for single crystal grains. Quantitative results are given for crystalline and amorphous ice, but with small changes these should also be valid for other solids. The rates for amorphous grains can approximate, within a factor of 10 or so, those for crystalline grains if the density of H atoms is high and the density of H₂ molecules is low and only when the temperature of the grains satisfies a relation which for ice and graphite leads to a value in the proximity of 15–17 K. This maximum rate occurs only a degree or so above the temperature at which the grains are totally covered by an H₂ layer and the reaction ceases. Furthermore, for a constant number density of grains, the rates on amorphous grains are second order while those on crystalline grains are first order. Both these circumstances predict amorphous grains to lead to H₂ clouds with irregular and sharply delineated features in contrast to more uniform clouds formed on crystalline grains.     

Exploration of hydrothermal targets on Mars

Based on various lines of geologic, geomorphic, topographic, geophysical, spectral, and elemental evidence, we conclude that hydrothermal environments have certainly existed on Mars and are likely to still exist. Here, we present candidate targets of endogenic- and exogenic-driven hydrothermal environments on Mars based on a set of selection criteria and suggest strategies for the detection of such targets. This includes a re-evaluation of potential targets using both existing and yet-to-be-released remote information provided by the instruments onboard the Mars orbiters and rovers. We also provide terrestrial analogs for possible martian hydrothermal environments to highlight the implications of these targets for potential martian life. This compilation and synthesis of data from martian localities indicating hydrothermal activity is timely and a first step towards prioritizing candidate targets for further investigation, which will likely add more targets to this list. Future in situ exploration will have to focus on the most promising of the hydrothermal targets and investigate them utilizing a novel integrated multi-tier, multi-agent reconnaissance mission architecture.     

Uninhabited habitats on Mars

Investigations of Mars as a potential location for life often make the assumption that where there are habitats, they will contain organisms. However, the observation of the ubiquitous distribution of life in habitable environments on the Earth does not imply the presence of life in martian habitats. Although uninhabited habitats are extremely rare on the Earth, a lack of a productive photosynthetic biosphere on Mars to generate organic carbon and oxygen, thus providing a rapidly available redox couple for energy acquisition by life and/or a lack of connectivity between habitats potentially increases the scope and abundance of uninhabited habitats for much of the geological history of the planet. Uninhabited habitats could have existed on Mars from the Noachian to the present-day in impact hydrothermal systems, megaflood systems, lacustrine environments, transient melted permafrost, gullies and local regions of volcanic activity; and there may be evidence for them in martian meteorites. Uninhabited habitats would provide control habitats to investigate the role of biology in planetary-scale geochemical processes on the Earth and they would provide new constraints on the habitability of Mars. Future robotic craft and samples returned from Mars will be able to directly show if uninhabited habitats exist or existed on Mars.     

Lanai Meteorite Crater Apparently Myth

Abstract A survey by air and on the ground revealed no depression at the place supposedly called Ka-imu-hoku, Hawaiian for “The Star Oven”, on the island of Lanai. It had been reported as a “pit in the sand” or “the place where a meteor fell”. Reasons are given for believing the name was based on native observation of a nineteenth century fireball.     

Near-field effects of Cherenkov radiation induced by ultra high energy cosmic neutrinos

The radio approach based on the Askaryan effect for detecting the ultra-high energy cosmic neutrinos has become a mature experimental technique. So far the existing calculations of the Cherenkov radiation associated with the Askaryan effect has been mostly based on the far-field approximation, whose validity maybe challenged when the detector is close to the event. In this paper we present an alternative approach to calculate the Cherenkov pulse by a numerical code based on the finite difference time-domain (FDTD) method. This approach has the advantage of providing the solution everywhere in space, contrary to other methods that rely on the far-field approximation. We also present a one-dimensional theoretical model for the shower with analytical solution, which helps to elucidate our nonzero-width simulation results. We show that for a shower with symmetric longitudinal development, the resulting near-field waveform would be asymmetric in time. In addition, we demonstrate that for a shower elongated by the LPM (Landau-Pomeranchuk-Migdal) effect and thus with a multi-peak structure, a bipolar, asymmetric waveform is still preserved in the near-field regime irrespective of the specific variations of the multi-peak structure, which makes it a generic, distinctive feature. This should provide an important characteristic signature for the identification of ultra-high energy cosmogenic neutrinos.     

Proposal for Creating a Device for Deorbiting Low-Earth-Orbit CubeSats

The article discusses proposals for creating a standard unit based on CubeSat technology, an independent autonomous aerobraking device based on inflatable balloons or a solar sail cone section. The proposed devices have a simple start-up system and are an additional component in the CubeSat assembly for deorbiting from low-Earth orbits.