8,027 research outputs found
Potential Advantages of Conducting Short Duration Visits to the Martian Surface
Recent NASA concepts for human missions to Mars, including the Evolvable Mars Campaign and Design Reference Architecture 5.0, have focused on the conduct of missions with long duration stays on the Martian surface. The decision to focus on long duration missions (typically to a single site) is driven by a desire to increase the perceived sustainability of the human Mars campaign, predicated on the assumption that sustainability is best achieved by maximizing the level of activity on the surface, providing for continuous growth in operations, and promoting pioneering of Mars. However, executing a series of long duration missions to a single site is not the only option for human exploration of Mars that has been proposed. Other architectures have been evaluated that focus on missions with short duration surface stays, with each mission visiting a separate site on the surface. This type of architecture is less efficient in that elements are not typically reused from one mission to the next but requires a far less complex surface architecture. There are potentially valid arguments to be made that a short duration, multiple site approach could result in different types of advantages when compared to the long duration, single site approach to Mars exploration, particularly for initial human missions to Mars. These arguments revolve around four areas: Achieved Value, Risk Mitigation, Developmental Affordability, and Operational Affordability & Flexibility. The question of Achieved Value relates to the prioritization of goals for Martian exploration. As discussed, goals related to pioneering and expanding human presence are often referenced as justifications for the long duration approach. However, there are other competing goals, including science and exploration. While there is not a clear consensus among planetary scientists, many have argued that the value of being able to visit multiple sites could outweigh the value of continually visiting a single site. Risk Mitigation is a major concern for initial human missions to Mars. There are a number of hazards related to operating on the Martian surface that are not well characterized. It may be desirable to conduct a series of short duration missions to better understand the nature of these risks prior to committing to a long duration mission. Developmental Affordability relates to the ability of NASA and its partners to develop and deploy the proposed architecture. Any human missions to Mars will be among the most complex endeavors ever undertaken. The capabilities that must be developed to enable any human Mars missions are extremely challenging. The total design, development, test, and evaluation (DDT&E) budget required to develop just the essential capabilities alone will be substantial. If additional surface capabilities are required to support long duration surface stays, the development effort could be unaffordable. Operational Affordability & Flexibility relates to the continued costs to execute the Mars campaign. Long duration missions, even with some amount of in-situ resource utilization, require a significant level of resupply for every mission. This requires additional launches and in-space transportation assets, increasing the operational complexity and total operational cost. This paper will explore each of the four potential advantages of short duration missions in detail. The authors will present comparisons between proposed long duration and short duration architectures through an evaluation of relevant performance, cost, and risk metrics
Highly collimated microquasar jets as efficient cosmic-ray sources
Supernova remnants are believed to be the main sites where Galactic cosmic
rays originate. This scenario, however, fails to explain some of the features
observed in the cosmic-ray spectrum. Microquasars have been proposed as
additional candidates, because their non-thermal emission indicates the
existence of efficient particle acceleration mechanisms in their jets. A
promising scenario envisages the production of relativistic neutrons in the
jets, that decay outside the system injecting relativistic protons to the
surroundings. The first investigations of this scenario suggest that
microquasars might be fairly alternative cosmic-ray sources.
We aim at assessing the role played by the degree of collimation of the jet
on the cosmic-ray energetics in the neutron-carrier scenario, as well as the
properties of the emission region. Our goals are to explain the Galactic
component of the observed proton cosmic-ray spectrum at energies higher than
GeV and to relate the mentioned jet properties with the power and
spectral index of the produced cosmic rays. We find that collimated jets, with
compact acceleration regions close to the jet base, are very efficient sources
that could deliver a fraction of up to of their relativistic proton
luminosity into cosmic rays. Collimation is the most significant feature
regarding efficiency; a well collimated jet might be orders of
magnitude more efficient than a poorly collimated one. The main feature of the
presented mechanism is the production of a spectrum with a steeper spectral
index ( at energies up to TeV) than in the supernova
scenario, and closer to what is observed. The predictions of our model may be
used to infer the total contribution of the population of Galactic microquasars
to the cosmic ray population, and therefore to quantitatively assess their
significance as cosmic-ray sources.Comment: 11 pages, 14 figure
Photo-desorption of H2O:CO:NH3 circumstellar ice analogs: Gas-phase enrichment
We study the photo-desorption occurring in HO:CO:NH ice mixtures
irradiated with monochromatic (550 and 900 eV) and broad band (250--1250 eV)
soft X-rays generated at the National Synchrotron Radiation Research Center
(Hsinchu, Taiwan). We detect many masses photo-desorbing, from atomic hydrogen
(m/z = 1) to complex species with m/z = 69 (e.g., CHNO, CHO,
CHN), supporting the enrichment of the gas phase.
At low number of absorbed photons, substrate-mediated exciton-promoted
desorption dominates the photo-desorption yield inducing the release of weakly
bound (to the surface of the ice) species; as the number of weakly bound
species declines, the photo-desorption yield decrease about one order of
magnitude, until porosity effects, reducing the surface/volume ratio, produce a
further drop of the yield.
We derive an upper limit to the CO photo-desorption yield, that in our
experiments varies from 1.4 to 0.007 molecule photon in the range ~absorbed photons cm. We apply these findings to a
protoplanetary disk model irradiated by a central T~Tauri star
Runaway evaporation for optically dressed atoms
Forced evaporative cooling in a far-off-resonance optical dipole trap is
proved to be an efficient method to produce fermionic- or bosonic-degenerated
gases. However in most of the experiences, the reduction of the potential
height occurs with a diminution of the collision elastic rate. Taking advantage
of a long-living excited state, like in two-electron atoms, I propose a new
scheme, based on an optical knife, where the forced evaporation can be driven
independently of the trap confinement. In this context, the runaway regime
might be achieved leading to a substantial improvement of the cooling
efficiency. The comparison with the different methods for forced evaporation is
discussed in the presence or not of three-body recombination losses
Photocatalytic activity of nitrogen-doped and undoped titanium dioxide sputtered thin films
In the present work titanium dioxide (TiO2) thin films were grown by d.c. reactive magnetron sputtering process, systematically varying the Ar/O2 ratio in the gas mixture, in order to study the influence of the oxygen partial pressure on the crystallographic structure and photocatalytic activity of the TiO2 thin films. After the sputtering process the TiO2 coatings were nitrided in a microwave (f= 2.45 GHz) Electron Cyclotron Resonance (ECR) plasma discharge in pure nitrogen, to compare the photocatalytic activity of undoped and nitrogen-doped TiO2 thin films. The crystal structure of the TiO2 grown samples was studied by x-ray diffraction (XRD) and the presence of the anatase phase in these films were corroborated by Raman spectroscopy. On the other hand, X-ray photoelectron spectroscopy (XPS) measurements carried out in the nitrogen-doped TiO2 samples, showed that the nitrogen was incorporated to the films with an average concentration of 18 at% of N. The UV-Vis optical spectroscopy allowed calculating the band gap. A narrowing of the optical band gap from 3.2 eV for the undoped films to 2.5 eV for the N-doped films was observed. Photocatalytic activity tests were done using a methylene blue (MB) dye solution. The irradiation of the films in the MB dye solution was carried out with an emission lamp in the UV and in the visible range for undoped and N-doped TiO2 films, respectively. The results showed that the N-doped TiO2 films had a higher photocalytic activity in the visible range, reaching a greater MB degradation in comparison with undoped samples, which were subjected to a higher energy radiation.Fil: Franco Arias, Lina Maria. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad del Valle; ColombiaFil: Zambrano, G.. Universidad del Valle; ColombiaFil: Gómez, M. E.. Universidad del Valle; ColombiaFil: Camps, E.. Instituto Nacional de Investigaciones Nucleares; MéxicoFil: Escobar Alarcón, L.. Instituto Nacional de Investigaciones Nucleares; Méxic
Quantum open systems and turbulence
We show that the problem of non conservation of energy found in the
spontaneous localization model developed by Ghirardi, Rimini and Weber is very
similar to the inconsistency between the stochastic models for turbulence and
the Navier-Stokes equation. This sort of analogy may be useful in the
development of both areas.Comment: to appear in Physical Review
Experimental analysis of drainage and water storage of litter layers
International audienceMany hydrological studies of forested ecosystems focus on the study of the forest canopy and have partitioned gross precipitation into throughfall and stemflow. However, the presence of forest litter can alter the quantities of water available for soil infiltration and runoff. Little information exists regarding the value of storage and drainage parameters for litter layers. Vegetation parameters of this kind are required in physically-based and lumped conceptual models to quatify the availabilty and distribution of water. Using a rainfall simulator and laboratory conditions two main objectives were investigated using layers of recently seneced poplar leaves, fresh grass or woodchips: 1) Effect of rain intensity on storage. With this respect we found that: maximum storage (Cmax), defined as the detention of water immediately before rainfall cessation, increased with rainfall intensity. The magnitude of the increment was up to 0.5 mm kg?1 m?2 between the lowest (9.8 mm h?1) and highest (70.9 mm h?1) rainfall intensities for poplar leaves. Minimum storage (Cmin), defined as the detention of water after drainage ceased, was not influenced by rainfall intensity. Repeated wetting-draining cycles or layer thickness have no effect on Cmax or Cmin. 2) The evaluation of drainage coefficient for the Rutter model. This model was found accurate to predict storage and drainage in the case of poplar leaves, was less accurate for fresh grass and resulted in overestimations for woodchips. Additionally, the effect of an underlaying soil matrix on lateral movement of water and storage of poplar leaves was studied. Results indicated that the soil matrix have no effect on Cmax or Cmin of the litter layer. Lateral movement of water in the poplar layer was observed at intermediate rainfall intensities (30.2 and 40.4 mm h?1), but not a the lowest or highest rates
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