大功率离子推力器和流动控制单元验证现状 及其2000小时寿命试验

为了发展基于电推进的大功率空间运输系统,需要开发和验证功率达数十千瓦的电推进系统,深空任务电推进系统优化的比冲要求高达10⁵s。凯尔迪什研究中心（KeRC）正在开发这样的电推进部件。本文概述了 35kW离子推力器 IT-500及其流动单元FCU-500的验证现状。作为其验证的一部分,完成了IT-500 和 FCU-500的2000h寿命试验。其中,离子推力器大部分验证条件是：输入功率17.8kW,使用了40kg氙,2018h寿命试验。本文介绍了磁场和离子光学以及石墨格栅开发现状。     

Absolute and Convective Instabilities of Circularly Polarized Alfvén Waves

We discuss the recent progress in studying the absolute and convective instabilities of circularly polarized Alfvén waves (pump waves) propagating along an ambient magnetic field in the approximation of ideal magnetohydrodynamics (MHD). We present analytical results obtained for pump waves with small dimensionless amplitude a, and compare them with numerical results valid for arbitrary a. The type of instability, absolute or convective, depends on the velocity U of the reference frame where the pump wave is observed with respect to the rest plasma. One of the main results of our analysis is that the instability is absolute when U _l < U < U _r and convective otherwise. We study the dependences of U _l and U _r on a and the ratio of the sound speed to the Alfvén speed b. We also present the results of calculation of the increment of the absolute instability on U for different values of a and b. When the instability is convective (U < U _l or U > U _r) we consider the signalling problem, and show that spatially amplifying waves exist only when the signalling frequency is in two symmetric frequency bands. Then, we write down the analytical expressions determining the boundaries of these frequency bands and discuss how they agree with numerically calculated values. We also present the dependences of the maximum spatial amplification rate on U calculated both analytically and numerically. The implication of the obtained results on the interpretation of observational data from space missions is discussed. In particular, it is shown that circularly polarized Alfvén waves propagating in the solar wind are convectively unstable in a reference frame of any realistic spacecraft.     

The Swift Ultra-Violet/Optical Telescope

The Ultra-Violet/Optical Telescope (UVOT) is one of three instruments flying aboard the Swift Gamma-ray Observatory. It is designed to capture the early (∼1 min) UV and optical photons from the afterglow of gamma-ray bursts in the 170–600 nm band as well as long term observations of these afterglows. This is accomplished through the use of UV and optical broadband filters and grisms. The UVOT has a modified Ritchey–Chrétien design with micro-channel plate intensified charged-coupled device detectors that record the arrival time of individual photons and provide sub-arcsecond positioning of sources. We discuss some of the science to be pursued by the UVOT and the overall design of the instrument.     

Progress in multilateral Earth observation cooperation: CEOS, IGOS and the ad hoc Group on Earth Observations

The Committee on Earth Observation Satellites (CEOS) coordinates international civil space-borne missions designed to observe and study planet Earth. With over 100 Earth observation satellites expected to be launched during the next 10 years, it is clear that collaborative opportunities have not been fully maximized. In 2003 CEOS has been focusing on articulating a more comprehensive satellite data utilization approach and in following up on its significant involvement in the World Summit on Sustainable Development. The CEOS Chair also serves as Co-Chair of the Integrated Global Observing Strategy (IGOS) Partnership, which seeks to reduce observation gaps and unnecessary overlaps and to harmonize and integrate common interests of space-based and in situ systems. IGOS focused in 2003 on development of a number of themes, including Carbon Cycle, Water Cycle and GeoHazards. The IGOS Ocean Theme is now in its implementation phase. NOAA, while chairing CEOS and co-chairing IGOS, has also been actively involved in organizing and hosting a ministerial-level Earth Observation Summit with a follow-on Group on Earth Observations (GEO) charged with developing the framework for a comprehensive global Earth observation system(s). All these activities demonstrate the commitment to developing more coherent and sustained Earth observation strategies for the good of the planet.     

Planetary resources and astroecology. Planetary microcosm models of asteroid and meteorite interiors: electrolyte solutions and microbial growth--implications for space populations and panspermia

Planetary microcosms were constructed using extracts from meteorites that simulate solutions in the pores of carbonaceous chondrites. The microcosms were found to support the growth of complex algal and microbial populations. Such astroecology experiments demonstrate how a diverse ecosystem could exist in fluids within asteroids, and in meteorites that land on aqueous planets. The microcosm solutions were obtained by extracting nutrient electrolytes under natural conditions from powders of the Allende (CV3) and Murchison (CM2) meteorites at low (0.02 g/ml) and high (10.0 g/ml) solid/solution ratios. The latter solutions contain > 3 mol/L electrolytes and about 10 g/L organics, that simulate natural fluids in asteroids during aqueous alteration and in the pores of meteorites, which can help prebiotic synthesis and the survival of early microorganisms. These solutions and wet solids were in fact found to support complex self-sustaining microbial communities with populations of 4 x 10(5) algae and 6 x 10(6) bacteria and fungi for long periods (> 8 months). The results show that planetary microcosms based on meteorites can: assay the fertilities of planetary materials; identify space bioresources; target astrobiology exploration; and model past and future space-based ecosystems. The results show that bioresources in the carbonaceous asteroids can sustain a biomass of 10(18) kg, comprising 10(32) microorganisms and a human population of 10(14). The results also suggest that protoplanetary nebulae can support and disperse microorganisms and can be therefore effective environments for natural and directed panspermia.     

Long-term manipulations of intact microbial mat communities in a greenhouse collaboratory: simulating earth's present and past field environments

Photosynthetic microbial mat communities were obtained from marine hypersaline saltern ponds, maintained in a greenhouse facility, and examined for the effects of salinity variations. Because these microbial mats are considered to be useful analogs of ancient marine communities, they offer insights about evolutionary events during the >3 billion year time interval wherein mats co-evolved with Earth's lithosphere and atmosphere. Although photosynthetic mats can be highly dynamic and exhibit extremely high activity, the mats in the present study have been maintained for >1 year with relatively minor changes. The major groups of microorganisms, as assayed using microscopic, genetic, and biomarker methodologies, are essentially the same as those in the original field samples. Field and greenhouse mats were similar with respect to rates of exchange of oxygen and dissolved inorganic carbon across the mat-water interface, both during the day and at night. Field and greenhouse mats exhibited similar rates of efflux of methane and hydrogen. Manipulations of salinity in the water overlying the mats produced changes in the community that strongly resemble those observed in the field. A collaboratory testbed and an array of automated features are being developed to support remote scientific experimentation with the assistance of intelligent software agents. This facility will permit teams of investigators the opportunity to explore ancient environmental conditions that are rare or absent today but that might have influenced the early evolution of these photosynthetic ecosystems.     

Methods to obtain the principal parameters of simple attitude control systems for small satellites

The present article deals with methods to determine the dynamical characteristics of satellites with passive and semipassive attitude control systems (ACS). On the basis of determined dynamical characteristics the principal parameters of ACS are obtained. The ACS interacting with the geomagnetic are considered here. It's reputed the ACS of such kind are wide used and simple one. Analytical relationships of these characteristics with the basic attitude control system parameters are presented, and the most important peculiarities of such systems that were found while carrying out satellite dynamics analysis are discussed. The results of system parameters optimization with certain criteria are also given where possible. The results presented may be applied at the preliminary design phase of satellite development.     

Hydrothermal systems in small ocean planets

We examine means for driving hydrothermal activity in extraterrestrial oceans on planets and satellites of less than one Earth mass, with implications for sustaining a low level of biological activity over geological timescales. Assuming ocean planets have olivine-dominated lithospheres, a model for cooling-induced thermal cracking shows how variation in planet size and internal thermal energy may drive variation in the dominant type of hydrothermal system-for example, high or low temperature system or chemically driven system. As radiogenic heating diminishes over time, progressive exposure of new rock continues to the current epoch. Where fluid-rock interactions propagate slowly into a deep brittle layer, thermal energy from serpentinization may be the primary cause of hydrothermal activity in small ocean planets. We show that the time-varying hydrostatic head of a tidally forced ice shell may drive hydrothermal fluid flow through the seafloor, which can generate moderate but potentially important heat through viscous interaction with the matrix of porous seafloor rock. Considering all presently known potential ocean planets-Mars, a number of icy satellites, Pluto, and other trans-neptunian objects-and applying Earth-like material properties and cooling rates, we find depths of circulation are more than an order of magnitude greater than in Earth. In Europa and Enceladus, tidal flexing may drive hydrothermal circulation and, in Europa, may generate heat on the same order as present-day radiogenic heat flux at Earth's surface. In all objects, progressive serpentinization generates heat on a globally averaged basis at a fraction of a percent of present-day radiogenic heating and hydrogen is produced at rates between 10(9) and 10(10) molecules cm(2) s(1).     

Spaceflight-relevant types of ionizing radiation and cortical bone: Potential LET effect?

Extended exposure to microgravity conditions results in significant bone loss. Coupled with radiation exposure, this phenomenon may place astronauts at a greater risk for mission-critical fractures. In a previous study, we identified a profound and prolonged loss of trabecular bone (29–39%) in mice following exposure to an acute, 2 Gy dose of radiation simulating both solar and cosmic sources. However, because skeletal strength depends on trabecular and cortical bone, accurate assessment of strength requires analysis of both bone compartments. The objective of the present study was to examine various properties of cortical bone in mice following exposure to multiple types of spaceflight-relevant radiation. Nine-week old, female C57BL/6 mice were sacrificed 110 days after exposure to a single, whole body, 2 Gy dose of gamma, proton, carbon, or iron radiation. Femora were evaluated with biomechanical testing, microcomputed tomography, quantitative histomorphometry, percent mineral content, and micro-hardness analysis. Compared to non-irradiated controls, there were significant differences compared to carbon or iron radiation for only fracture force, medullary area and mineral content. A greater differential effect based on linear energy transfer (LET) level may be present: high-LET (carbon or iron) particle irradiation was associated with a decline in structural properties (maximum force, fracture force, medullary area, and cortical porosity) and mineral composition compared to low-LET radiation (gamma and proton). Bone loss following irradiation appears to be largely specific to trabecular bone and may indicate unique biological microenvironments and microdosimetry conditions. However, the limited time points examined and non-haversian skeletal structure of the mice employed highlight the need for further investigation.     

Survival of methanogens during desiccation: implications for life on Mars

The relatively recent discoveries that liquid water likely existed on the surface of past Mars and that methane currently exists in the martian atmosphere have fueled the possibility of extant or extinct life on Mars. One possible explanation for the existence of the methane would be the presence of methanogens in the subsurface. Methanogens are microorganisms in the domain Archaea that can metabolize molecular hydrogen as an energy source and carbon dioxide as a carbon source and produce methane. One factor of importance is the arid nature of Mars, at least at the surface. If one is to assume that life exists below the surface, then based on the only example of life that we know, liquid water must be present. Realistically, however, that liquid water may be seasonal just as it is at some locations on our home planet. Here we report on research designed to determine how long certain species of methanogens can survive desiccation on a Mars soil simulant, JSC Mars-1. Methanogenic cells were grown on JSC Mars-1, transferred to a desiccator within a Coy anaerobic environmental chamber, and maintained there for varying time periods. Following removal from the desiccator and rehydration, gas chromatographic measurements of methane indicated survival for varying time periods. Methanosarcina barkeri survived desiccation for 10 days, while Methanobacterium formicicum and Methanothermobacter wolfeii were able to survive for 25 days.