Effect of enclosure shape on natural convection velocities in microgravity

A numerical analysis was performed to compare natural convection velocities in two-dimensional enclosures of various shape. The following shapes were investigated: circle, square, horizontal and upright 2 × 1 aspect ratio rectangles, horizontal and upright half-circles, diamond (square oriented with diagonal vertical) and triangle (equilateral and horizontal base). In all cases, the length scale in the various dimensionless parameters, such as Rayleigh number, is defined as the diameter of the equal area circle. Natural convection velocities were calculated for Rayleigh numbers of 100 and 500 with the temperature difference taken to be across (a) the maximum horizontal dimension, (b) the median horizontal line (line through centroid) and (c) the horizontal distance such that the temperature gradient is the same for shapes of equal area. A Rayleigh number of 1000 is within the “low Rayleigh number” range for agreement with first order theory for circular enclosures. A Rayleigh number of 5000 is slightly out of this range. For the class of shapes including the square, upright half-circle and upright rectangle, the computed velocities were found to agree very closely with that of the equal area circle when the temperature difference is taken to be across the maximum horizontal dimension [condition (a)]. The velocities for the horizontal rectangle and half circle were found to be approximately one-half that of the equal area circle for the same condition. Better overall agreement among all shapes was obtained by setting the temperature difference across a distance such that the temperature gradients were equal for shapes of equal area.     

Time variations of cosmic-ray helium isotopes with BESS-Polar I

The Balloon-borne Experiment with a Superconducting Spectrometer (BESS) is configured with a solenoidal superconducting magnet and a suite of precision particle detectors, including time-of-flight hodoscopes based on plastic scintillators, a silica-aerogel Cherenkov detector, and a high resolution tracking system with a central jet-type drift chamber. The charges of incident particles are determined from energy losses in the scintillators. Their magnetic rigidities (momentum/charge) are measured by reconstructing each particle trajectory in the magnetic field, and their velocities are obtained by using the time-of-flight system. Together, these measurements can accurately identify helium isotopes among the incoming cosmic-ray helium nuclei up to energies in the GeV per nucleon region. The BESS-Polar I instrument flew for 8.5 days over Antarctica from December 13th to December 21st, 2004. Its long-duration flight and large geometric acceptance allow the time variations of isotopic fluxes to be studied for the first time. The time variations of helium isotope fluxes are presented here for rigidities from 1.2 to 2.5 GV and results are compared to previously reported proton data and neutron monitor data.     

Biological investigations aboard the biosatellite Cosmos-1129

Experiments on insects, higher plants and lower fungi were carried out aboard the biological satellite Cosmos-1129, in Earth orbit, from 25 September to 14 October 1979. The main objective of these experiments was to gain more profound knowledge of the effect of weightlessness on living organisms and to study the mechanisms by which these various organisms with different life cycles can adjust and develop in weightlessness. Experiments on insects ($\text{Drosophila melanogaster}$) were made with a view towards understanding gravitational preference in flies, the life cycle of which took place on board the biosatellite under conditions of artificial gravity. Experiments on higher plants ($\text{Zea mays, Arabidopsis taliana, Lycopersicum esculentum}$) and lower fungi ($\text{Physarum polycephalum}$) were performed.     

How rare is complex life in the Milky Way?

An integrated Earth system model was applied to calculate the number of habitable Earth-analog planets that are likely to have developed primitive (unicellular) and complex (multicellular) life in extrasolar planetary systems. The model is based on the global carbon cycle mediated by life and driven by increasing stellar luminosity and plate tectonics. We assumed that the hypothetical primitive and complex life forms differed in their temperature limits and CO(2) tolerances. Though complex life would be more vulnerable to environmental stress, its presence would amplify weathering processes on a terrestrial planet. The model allowed us to calculate the average number of Earth-analog planets that may harbor such life by using the formation rate of Earth-like planets in the Milky Way as well as the size of a habitable zone that could support primitive and complex life forms. The number of planets predicted to bear complex life was found to be approximately 2 orders of magnitude lower than the number predicted for primitive life forms. Our model predicted a maximum abundance of such planets around 1.8 Ga ago and allowed us to calculate the average distance between potentially habitable planets in the Milky Way. If the model predictions are accurate, the future missions DARWIN (up to a probability of 65%) and TPF (up to 20%) are likely to detect at least one planet with a biosphere composed of complex life.     

Costas arrays with small number of cross-coincidences

A simple method is described for obtaining two or more Costas arrays with a maximum of one mutual bit or cross-coincidence, assuming that the vertical components of their relative shifts are appropriately bounded. The result can be useful for designing multiple Costas waveforms with small cross-ambiguities, if the potential Doppler shifts are restricted to a small portion of the waveform bandwidth