Composition of the Volatile Material in Halley's Coma from In Situ Measurements

The investigation of the volatile material in the coma of comets is a key to understanding the origin of cometary material, the physical and chemical conditions in the early solar system, the process of comet formation, and the changes that comets have undergone during the last 4.6 billion years. So far, in situ investigations of the volatile constituents have been confined to a single comet, namely P/Halley in 1986. Although, the Giotto mission gave only a few hours of data from the coma, it has yielded a surprising amount of new data and has advanced cometary science by a large step. In the present article the most important results of the measurements of the volatile material of Halley's comet are summarized and an overview of the identified molecules is given. Furthermore, a list of identified radicals and unstable molecules is presented for the first time. At least one of the radicals, namely CH₂, seems to be present as such in the cometary ice. As an outlook to the future we present a list of open questions concerning cometary volatiles and a short preview on the next generation of mass spectrometers that are being built for the International Rosetta Mission to explore the coma of Comet Wirtanen. This revised version was published online in June 2006 with corrections to the Cover Date.     

Scan-based emitter passive localization

This paper investigates the problem of estimating the position of a scanning emitter using a passive scan-based geolocation technique. This is achieved by taking advantage of the geometric constraints introduced by the uniform rotating motion of the antenna main beam as it sweeps across a number of separate receivers. A detailed analysis of the estimation process and accuracy of the emitter position is given and a number of computer simulations showing the benefits of this geolocation technique are also included     

Single ion actions: the induction of micronuclei in V79 cells exposed to individual protons.

Understanding the effects of single-particles from conventional radiation biology experiments is problematic due to the stochastics of particle tracks. This complicates the determinations of risk associated with low doses. We have developed a charged particle microbeam, which allows individually counted particles to be delivered to precise cellular locations. The system is capable of delivering a single charged particle with > 99% efficiency. Of these particles 90% are delivered with a resolution of +/- 2 micrometers and 96% with a resolution of +/- 5 micrometers. We have carried out preliminary studies in Chinese hamster V79 cells to monitor the effectiveness of low energy protons at inducing cytological damage. We have used the micronucleus assay as a measure of predominantly lethal chromosome damage. The effects of a single 3.2 MeV proton delivered individually to cells could be measured, with less than 2% of the exposed cells producing micronuclei 24 hours later. The yield of micronuclei formation was essentially linear up to the highest dose (30 particles per cell nucleus) delivered. Ultimately, the ability to target particles to different parts of the cell nucleus may start to impact on models available for chromosome aberration formation and chromosomal Organisation and mechanisms underlying genomic instability.     

Effects of carbon ions on primary cultures of mouse brain cells.

Primary mixed cultures of astrocytes and microglia were obtained from neonatal mice, and were irradiated with high-LET carbon ions. Immunohistochemical staining showed astrocytes survived more prominently than microglia. Tagged with specific antibodies, astrocytes and microglia surviving after irradiation were counted by flow cytometry. Decreases in the number of microglia and astrocytes were detected at a dose as small as 2 Gy when Day 5 cultures were irradiated with 13 keV/micrometer carbon ions. When the cultures were irradiated on Day 10, the dose-dependent decrease of microglia was more prominent for 13 keV/micrometer carbon ions than 70 keV/micrometer carbon ions. Astrocytes showed a marginal decrease at Day 10 and Day 14. We concluded that microglia are more sensitive than astrocytes to carbon ions and X-rays, and that the radiosensitivity of microglia depends on both differentiation/proliferation status and radiation quality.     

External Cue Effects on Memory for Spatial Location within a Rotated Task Field

Abstract

We propose a mathematical model of the Path Integration (PI) process. Its core assumption is that orientations of a path are summarized by circular probability distributions. We compare our model with classical, deterministic models of PI and find that, although they are indistinguishable in terms of information encoded, the probabilistic model is more parsimonious when considering navigation strategies. We show how sensory events can enrich the probability distributions memorized, resulting in a continuum of navigation strategies, from PI to stimulus-triggered response. We analyze the combination of circular probability distributions (e.g., multicue fusion), and demonstrate that, contrary to the linear case, adding orientation cues does not always increase reliability of estimates. We discuss experimental predictions entailed by our model.     

Short-Lived Radionuclides in Meteorites: Constraints on Nebular Timescales for the Production of Solids

Variations in the abundances of short-lived radionuclides such as ²⁶Al (τ_1/2 ≈ 0.74 Ma) and ⁵³Mn (τ_1/2 ≈ 3.7 Ma) in meteoritic solids may be used to infer relative formation intervals of these solids in the nebula at precisions of less than 1 Ma. In a strict chronometric interpretation of the isotopic variations, whereby criteria such as spatial and temporal isotopic homogeneity and closed system isotopic evolution are met, solid formation occurred in the nebula for at least several million years. This is longer than some theoretical and astronomical estimates for the duration of the active nebula. The evidence for live ⁴¹Ca (τ_1/2 ≈ 0.10 Ma) in meteoritic inclusions further indicates that the onset of solid formation occurred quite early, perhaps within a few hundred thousand years after the onset of the collapse of the sun's parent molecular cloud. Failure of the chronometric interpretation may arise for a variety of reasons, including but not limited to, the late, inhomogeneous injection of material from a nearby stellar source or the local production of short-lived radionuclides by an energetic particle irradiation, e. g., from T Tauri (X-wind) or galactic cosmic ray sources. Although some isotopic evidence exists that the criteria required for a strict chronometric interpretation are not met by each of the short-lived chronometers, there is no compelling reason to shorten the interval of solid formation in the nebula to less than 1 Ma. This revised version was published online in June 2006 with corrections to the Cover Date.     

Atomic Physics for Atmospheric Composition Measurements

The atomic physics relevant to the interpretation of solar spectra produced by plasmas at temperatures ≳ 105 K are discussed. Methods for determining relative abundance ratios are presented and examples provided from the Coronal Diagnostic Spectrometer on board SOHO. In particular, the Fe/Si ratio in the corona is found to be close to photospheric; the Mg/Ne ratio in the transition region is found to vary by an order of magnitude in different solar features. The Mg/Ne ratios in supergranule cell centres and the network are separated for the first time, although no significant differences are found. This revised version was published online in June 2006 with corrections to the Cover Date.     

Benefits and risks of using electrodynamic tethers to de-orbit spacecraft

By using electrodynamic drag to greatly increase the orbital decay rate, an electrodynamic space tether can remove spent or dysfunctional spacecraft from low Earth orbit (LEO) rapidly and safely. Moreover, the low mass requirements of such tether devices make them highly advantageous compared to conventional rocket-based de-orbit systems. However, a tether system is much more vulnerable to space debris impacts than a typical spacecraft and its design must be proved to be safe up to a certain confidence level before being adopted for potential applications. To assess space debris related concerns, in March 2001 a new task (Action Item 19.1) on the “Potential Benefits and Risks of Using Electrodynamic Tethers for End-of-life De-orbit of LEO Spacecraft” was defined by the Inter-Agency Space Debris Coordination Committee (IADC). Two tests were proposed to compute the fatal impact rate of meteoroids and orbital debris on space tethers in circular orbits, at different altitudes and inclinations, as a function of the tether diameter to assess the survival probability of an electrodynamic tether system during typical de-orbiting missions. IADC members from three agencies, the Italian Space Agency (ASI), the Japan Aerospace Exploration Agency (JAXA) and the US National Aeronautics and Space Administration (NASA), participated in the study and different computational approaches were specifically developed within the framework of the IADC task. This paper summarizes the content of the IADC AI 19.1 Final Report. In particular, it introduces the potential benefits and risks of using tethers in space, it describes the assumptions made in the study plan, it compares and discusses the results obtained by ASI, JAXA and NASA for the two tests proposed. Some general conclusions and recommendations are finally extrapolated from this massive and intensive piece of research.     

Gravisensing in plants and fungi.

The principle of establishing and maintaining a gravitropic set point angle depends on gravisensing and a subsequent cascade of events that result in differential elongation of the responsive structures. Since gravity acts upon masses, the gravisensing mechanisms of all biological systems must follow the same principle, namely the sensing of some force due to differential acceleration of the perceiving entity and a reference structure. This presentation will demonstrate that gravisensing can be accomplished by various means, ranging from cytoskeletal organization, mechano-elastic stress to perturbation of electric signals. However, several arguments indicate that sedimentation of either dense plastids (statoliths), the entire protoplast, or a combination of these represents the primary step in graviperception in plants. In fungi, nuclei and cytoskeletal proteins are believed to form a network capable of gravisensing but sedimenting organelles that may function as statoliths have been identified. Theoretical and practical limitations of gravisensing and detection of acceleration forces necessitate microgravity experiments to identify the primary perceptor, subsequent biochemical mechano-transduction, and biological response processes.