Order of magnitude analysis of convective effects in dendritic solidification

The different types of convective phenomena which may occur during the dendritic solidification of metallic alloys are discussed from an order of magnitude analysis. Bulk thermal convection and/or interdendritic solutal convection have to be considered according to the values of the experimental data. Scaling laws for the solute boundary layer resulting from bulk thermal convection have already been derived. It is shown here that the interdendritic flow depends on a solutal Grashof number Gr based on the horizontal density gradient and a characteristic length L_s which is of the order of the liquid channels width. For Gr < 1, which is generally verified in practical cases, the interdendritic flow velocity U_r is proportional to the Grashof number. This a priori law compares favorably with the results of horizontal solidification experiments where the mean interdendritic flow velocity has been estimated from the resulting measured macrosegregation. In these experiments, as well as for most horizontal dendritic solidifications of metallic alloys at 1 g, the ratio $\text{U}{}_{\mathrm{r}}\text{R}$ (R is the growth rate) is of order one. In order to cancel the interdendritic flow effects, this ratio has to be lowered by one order of magnitude. According to our analysis, this can be obtained by performing the experiments either at a slightly reduced g level (～10^?1 g), or at 1 g in a vertical stable configuration with a sufficiently low residual horizontal thermal gradient.     

In situ measurements to determine if Phobos is layered

The complex system of linear features on Phobos—the inner moon of Mars—found by the Viking Orbiters in 1976, can be classified morphologically into three types according to their appearance as well as their geometrical distribution on the surface. One kind of grooves appears to form arcs of small circles normal to the Phobos-Mars direction. We propose that these grooves are the surface manifestation of layering within Phobos[4–6] which could have been formed when Phobos was a part of a much larger and geologically active parent planet.Here we suggest some measurements to observe whether Phobos has a layered structure or not. The suggestion contains remote sensing measurements (particularly determination of the magnetic field by magnetometer and by electron reflection method) by a spacecraft orbiting around Mars as well as surface measurements (active seismic reflection experiment) by a landing module on Phobos.     

The evolution of a serviceable EURECA

Within the European space platform program the EURECA is being established as a ground-based platform for short microgravity missions. The development towards a serviceable platform for longer, scientific missions is described. The plan of an advanced space-based platform for increasing payload demands is outlined. The platform design and the adaptation to scientific missions and servicing operations are investigated. The cost-effective utilization of the different platform types using new operational concepts is analyzed in parametric life cycle cost calculations for different payloads and mission scenarios.     

Study of radiation conditions onboard the International space station by means of the Liulin-5 dosimeter

For estimating radiation risk in space flights it is necessary to determine radiation dose obtained by critical organs of a human body. For this purpose the experiments with human body models are carried out onboard spacecraft. These models represent phantoms equipped with passive and active radiation detectors which measure dose distributions at places of location of critical organs. The dosimetric Liulin-5 telescope is manufactured with using three silicon detectors for studying radiation conditions in the spherical tissue-equivalent phantom on the Russian segment of the International space station (ISS). The purpose of the experiment with Liulin-5 instrument is to study dynamics of the dose rate and particle flux in the phantom, as well as variations of radiation conditions on the ISS over long time intervals depending on a phase of the solar activity cycle, orbital parameters, and presence of solar energetic particles. The Liulin-5 dosimeter measures simultaneously the dose rate and fluxes of charged particles at three depths in the radial channel of the phantom, as well as the linear energy transfer. The paper presents the results of measurements of dose rate and particle fluxes caused by various radiation field components on the ISS during the period from June 2007 till December 2009.     

Comparative assessment of human–Mars-mission technologies and architectures

We compare a variety of mission scenarios to assess the strengths and weaknesses of options for Mars exploration. The mission design space is modeled along two dimensions: trajectory architectures and propulsion system technologies. We examine direct, semi-direct, stop-over, semi-cycler, and cycler architectures, and we include electric propulsion, nuclear thermal rockets, methane and oxygen production on Mars, Mars water excavation, aerocapture, and reusable propulsion systems in our technology assessment. The mission sensitivity to crew size, vehicle masses, and crew travel time is also examined. Many different combinations of technologies and architectures are applied to the same Mars mission to determine which combinations provide the greatest potential reduction in the injected mass to LEO. We approximate the technology readiness level of a mission to rank development risk, but omit development cost and time calculations in our assessment. It is found that Earth–Mars semi-cyclers and cyclers require the least injected mass to LEO of any architecture and that the discovery of accessible water on Mars has the most dramatic effect on the evolution of Mars exploration.     

Near-infrared detection of potential evidence for microscopic organisms on Europa

The possibility of an ocean within the icy shell of Jupiter's moon Europa has established that world as a primary candidate in the search for extraterrestrial life within our Solar System. This paper evaluates the potential to detect evidence for microbial life by comparing laboratory studies of terrestrial microorganisms with measurements from the Galileo Near Infrared Imaging Spectrometer (NIMS). If the interior of Europa at one time harbored life, some evidence may remain in the surface materials. Examination of laboratory spectra of terrestrial extremophiles measured at cryogenic temperatures reveals distorted, asymmetric nearinfrared absorption features due to water of hydration. The band centers, widths, and shapes of these features closely match those observed in the Europa spectra. These features are strongest in reddish-brown, disrupted terrains such as linea and chaos regions. Narrow spectral features due to amide bonds in the microbe proteins provide a means of constraining the abundances of such materials using the NIMS data. The NIMS data of disrupted terrains exhibit distorted, asymmetric near-infrared absorption features consistent with the presence of water ice, sulfuric acid octahydrate, hydrated salts, and possibly as much as 0.2 mg cm(-3) of carbonaceous material that could be of biological origin. However, inherent noise in the observations and limitations of spectral sampling must be taken into account when discussing these findings.     

A mission template for exploration and damage mitigation of potential hazard of Near Earth Asteroids

The Apophis Exploratory and Mitigation Platform (AEMP) concept was developed as a prototype mission to explore and potentially deflect the Near Earth Asteroid (NEA) 99942 Apophis. Deflection of the asteroid from the potential 2036 impact will be achieved using a gravity tractor technique, while a permanent deflection, eliminating future threats, will be imparted using a novel albedo manipulation technique. This mission will serve as an archetypal template for future missions to small NEAs and could be adapted to mitigate the threat of collision with other potential Earth-crossing objects.