Microlesions: theory and reality.

Efforts to assess radiation risk in space have been complicated by the considerable unknowns regarding the biological effects of the heavy ion component (HZE particles) of the cosmic rays. The attention has focused primarily on the assignation of a quality factor (Q) which would take into account the greater effectiveness of heavy ions vis-a-vis other forms of ionizing radiation. If however, as the so-called "Microlesion Theory" allows, the passage of HZE particles through living tissue produces unique biological damage, the traditional use of Q becomes meaningless. Therefore, it is critical to determine if microlesions, in fact, do exist. While the concept does not necessarily require detectable morphological damage, "tunnel-lesions" or holes in ocular tissues have been cited as evidence of microlesions. These data, however, are open to reinterpretation. On-going light, scanning and transmission electron microscopic studies of the corneas, lenses and retinas of rat eyes exposed to 450 MeV/amu 56Fe ions thus far have not revealed tunnel-lesion damage. The morphological effects of the heavy ions have been found to be qualitatively similar to the changes following other kinds of ionizing radiation.     

The influence of radiation quality on the formation of DNA breaks.

We have aimed to present a comprehensive review of our understanding to date of the formation of DNA strand breaks induced by high LET radiation. We have discussed data obtained from DNA in solution as well as from the formation and "repair" of strand breaks in cell DNA. There is good agreement, qualitatively, between these two systems. Results were evaluated for two parameters: (1) effectivity per particle, the cross section (sigma) in micrometers 2/particle; and (2) the strand break induction frequency as number of breaks per Gy per unit DNA (bp or dalton). A series of biological effects curves (one for each Z-number) is obtained in effectivity versus LET plots. The relationships between induction frequencies of single-strand breaks, or double-strand breaks, or the residual "irrepairable" breaks and LET-values have been evaluated and discussed for a wide spectrum of heavy ions, both for DNA in solution and for DNA in the cell. For radiation induced total breaks in cell DNA, the RBE is less than one, while the RBE for the induction of DSBs can be greater than one in the 100-200 keV/micrometers range. The level of irrepairable strand breaks is highest in this same LET range and may reach 25 percent of the initial break yield. The data presented cover results obtained for helium to uranium particles, covering a particle incident energy range of about 2 to 900 MeV/u with a corresponding LET range of near 16 to 16000 keV/micrometers.     

Hypervelocity acceleration techniques: A review of exisiting capabilities and prospects for future developments

During the last few decades various techniques have made it possible to accelerate microparticles (10⁻⁶ – 10⁻¹⁵ gr) up to tens of km/sec and macroparticles (1 gr or so) up to 10 km/sec, thus furthering our understanding of many impact related phenomena occurring on the surfaces of celestial bodies.

This review will deal with existing techniques for the acceleration of hypervelocity projectiles. The performance of electrostatic accelerators, electromagnetic rail guns and related systems, plasma drag accelerators, light gas guns and explosive accelerating techniques is reviewed, and the capabilities and limitations of each type are briefly discussed. An attempt is made to assess the future promise of existing techniques and the realism of some current suggestions.     

Current collection through the transport of electrons across magnetic field lines

Collection of electrons by a long conducting cylinder in a flowing plasma is studied by means of numerical simulations. The plasma flow simulates the relative motion between a spacecraft and plasma. The sheath structures and the levels of electron current collections for the cases with and without an ambient magnetic field ( ) are studied. It is found that for the flow perpendicular to the magnetic field, the current is considerably enhanced depending on the relative drift velocity. In the case of a non-zero magnetic field perpendicular to the cylinder axis, the potential structure is a two-dimensional double layer with dimensions L L_|, where L and L_| are the dimensions perpendicular and parallel to , respectively. L is found to be the current limiting radius given by the Parker-Murphy model. For the flow along , the electron current is found to be smaller than that for the flow perpendicular to . This is explained in terms of the potential structures.     

The chemical profiles of polar ice on Earth and Mars: What we read from the first; what we could read from the second

An analogy is drawn between the current knowledge on terrestrial snow and ice-cap chemistry and the possible composition of snowfall and ice caps of Mars. Terrestrial snowfall reflects the composition of the Earth's atmosphere. Snow cover further interacts with the atmosphere and is the recipient of aerosol and particulate fall-out. The snow is transformed to firn and ice and the chemical signatures become locked into the perennial ice sheets. The chemical profiles of ice thus constitute environmental records of the Earth's past. By considering the present knowledge on the hydrologie cycle of Mars and the chemistry of the atmosphere, a simple analogous model for the chemical profile of the North polar ice cap is proposed. Three major constituents of the ice are discussed: water ice, dust, and occluded air bubbles. The seasonal fluctuations and interannual variability of these components are examined as possible chemical signatures for the dating of ice, elucidating hydrologie processes, and recording long-term climatic change. The model of the north polar cap in summer consists of water-ice fine-dust layers (30–200 m thick) sandwiched between annual dust layers of variable size distribution and thickness (< 1m– > 66 m). The water ice is subjected to metamorphism and grain growth. The interpretation of the physico-chemical profile could lead to increased knowledge on the recent climatic past (1,000–2,000 years), hydrologic reservoirs, and seasonal cycles in the atmospheric dynamics of the planet.     

Lunar habitat concept employing the space shuttle external tank

The space shuttle external tank, which consists of a liquid oxygen tank, an intertank structure, and a liquid hydrogen tank, is an expendable structure used for approximately 8.5 min during each launch. A concept for outfitting the liquid oxygen tank-intertank unit for a 12-person lunar habitat is described. The concept utilizes existing structures and openings for both man and equipment access without compromising the structural integrity of the tank. Living quarters, instrumentation, environmental control and life support, thermal control, and propulsion systems are installed at Space Station Freedom. The unmanned habitat is then transported to low lunar orbit and autonomously soft landed on the lunar surface. Design studies indicate that this concept is feasible by the year 2000 with concurrent development of a space transfer vehicle and manned cargo lander for crew changeover and resupply.     

Study of the variability of blazars gamma-ray emission

The γ-ray emission of blazar jets shows a pronounced variability and this feature provides limits to the size and to the speed of the emitting region. We study the γ-ray variability of bright blazars using data from the first 18 months of activity of the Large Area Telescope on the Fermi Gamma-Ray Space Telescope. From the daily light-curves of the blazars characterized by a remarkable activity, we firstly determine the minimum variability time-scale, giving an upper limit for the size of the emitting region of the sources, assumed to be spheroidal blobs in relativistic motion. These regions must be smaller than ∼10⁻³ parsec. Another interesting time-scale is the duration of the outbursts. We conclude that they cannot correspond to radiation produced by a single blob moving relativistically along the jet, but they are either the signature of emission from a standing shock extracting energy from a modulated jet, or the superposition of a number of flares occurring on a shorter time-scale. We also derive lower limits on the bulk Lorentz factor needed to make the emitting region transparent for gamma-rays interacting through photon–photon collisions.     

A technique of shaping the barrel-type parts

In this paper, a new technique for shaping the thin-walled barrel-type parts is presented.     

Precise line-of-sight modelling for angles-only relative navigation

This work presents a precise analytical model to reconstruct the line-of-sight vector to a target satellite over time, as required by angles-only relative navigation systems for application to rendezvous missions. The model includes the effects of the geopotential, featuring: the analytical propagation in the mean relative orbital elements (up to second-order expansion), the analytical two-way osculating/mean orbital elements’ conversion (second-order in $J_2$ and up to a given degree and order of the geopotential), and a second-order mapping from the perturbed osculating elements’ set to the local orbital frame. Performances are assessed against the line-of-sight reconstructed out of the precise GPS-based positioning products of the PRISMA mission. The line-of-sight modelled over a far-range one day long scenario can be fitted against the true one presenting residuals of the order of ten arc-seconds, which is below the typical sensor noise at far-range.     

Two magnetars: SGR 1627–41 and 1E 1547–5408

We describe the results obtained with Target of Opportunity observations of the galactic sources SGR 1627–41 and 1E 1547–5408. These two transients show several similarities supporting the interpretation of Anomalous X-ray Pulsars and Soft Gamma-ray Repeaters as a single class of strongly magnetized neutron stars.