Testing General Relativity with Atomic Clocks

We discuss perspectives for new tests of general relativity which are based on recent technological developments as well as new ideas. We focus our attention on tests performed with atomic clocks and do not repeat arguments present in the other contributions to the present issue (Space Sci. Rev. 2009, This Issue). In particular, we present the scientific motivations of the space projects ACES (Salomon et al. in CR Acad. Sci. IV-2:1313, 2001) and SAGAS (Wolf et al. in Exp. Astron. 23:651, 2009).     

International cooperation - how to proceed

As part of a project exploring ways that international cooperation can maximize the benefits of space activities in the new political climate of the 1990s, the American Institute of Aeronautics and Astronautics (AIAA) has been organizing expert workshops on the subject. The latest of these, held in December 1994, focused on five areas: global space systems services; human and robotic exploration; space transportation; solar power to Earth; and international peacekeeping. It is only in the last few years that recommendations on the latter topic could realistically have been put forward and, as a prelude to our presentation of the Executive Summary of the report produced by AIAA, two of the 60-plus participants drawn from 15 countries, Robert Fuhrmann and Jürgen Wild, discuss the implications of this new found openness over international security and analyse the background to its genesis.     

Seismological studies of the sun and other stars

Observations of a large number of different oscillation frequencies in the Sun provide an opportunity for detailed testing of the theory of stellar structure and evolution. At present highly significant discrepancies remain between observed and computed frequencies, and so our models of the solar interior have to be modified. With further improvements in the observations it might become possible to make a direct empirical determination of the density structure throughout the Sun.Similar oscillations have so far not been detected in other stars, but attempts to do so are under way. Theoretical estimates indicate that amplitudes somewhat greater than for the Sun might be expected for early F stars on the main sequence, and that the amplitude increases rapidly with decreasing gravity. Observation of such oscillations would enable investigations of the structure of these stars, and would in addition provide valuable information about the excitation mechanism of the oscillations.     

Bump masses and radii of BL Herculis variables

Bump masses and radii are derived for 18 BL Her stars from the observed bump phase and the accurately known fundamental period. The mean mass M/M = 0.60 ± 0.09 agrees precisely with predictions from standard stellar evolution theory and gives a new test of the theoretical models. The derived radius of V553 Centauri is in good agreement with the radius recently determined by an independent modified Baade-Wesselink method by Balona. Finally, a preliminary discussion of possible continuations of the BL Her bump progression is given.     

Evidence of change after 2001 in the seasonal behaviour of the mesopause region from airglow data at El Leoncito

Airglow intensities and rotational temperatures of the OH(6-2) and O₂b(0-1) bands acquired at El Leoncito (32°S, 69°W) provide good annual coverage in 1998–2002, 2006, and 2007, with between 192 and 311 nights of observation per year. These data can therefore be used to derive the seasonal variations during each of these seven years, in airglow brightness and temperatures at altitudes of 87 and 95 km. From 1998 to 2001, seasonal variations are similar enough so that they can be well represented by a mean climatology, for each parameter. On the other hand, these climatologies do not agree with what is usually observed at other sites, maybe due to the particular orographic conditions at El Leoncito. With respect to the last three fully documented years (2002, 2006, and 2007), the similarity from year to year deteriorates, and there are greater differences in the seasonal behaviour, more or less in all the parameters. The differences include, e.g., maxima occurring earlier or later than “normal”, by one or two months. All this may suggest the build-up of a new regime of intraseasonal variability, with a possible relationship to corresponding changes in wave activity.     

Quantification of spatial structure of human proximal tibial bone biopsies using 3D measures of complexity

Changes in trabecular bone composition during development of osteoporosis are used as a model for bone loss in microgravity conditions during a space flight. Symbolic dynamics and measures of complexity are proposed and applied to assess quantitatively the structural composition of bone tissue from 3D data sets of human tibia bone biopsies acquired by a micro-CT scanner. In order to justify the newly proposed approach, the measures of complexity of the bone architecture were compared with the results of traditional 2D bone histomorphometry. The proposed technique is able to quantify the structural loss of the bone tissue and may help to diagnose and to monitor changes in bone structure of patients on Earth as well as of the space-flying personnel.     

Application of an evolution strategy in planetary ephemeris modeling

Classical planetary ephemeris construction comprises three major steps which are to be performed iteratively: numerical integration of coupled equations of motion of a multi-body system (propagator step), reduction of observations (reduction step), and optimization of model parameters (adjustment step). In future, this approach may become challenged by further refinements in force modeling (e.g. inclusion of much more significant minor bodies than in the past), an ever-growing number of planetary observations (e.g. the vast amount of spacecraft tracking data), and big data issues in general. In order to circumvent the need for both the inversion of normal equation matrices and the determination of partial derivatives, and to prepare the ephemeris for applications apart from stand-alone solar-system planetary orbit calculations, here we propose an alternative ephemeris construction method. The main idea is to solve it as an optimization problem by straightforward direct evaluation of the whole set of mathematical formulas, rather than to solve it as an inverse problem with all its tacit mathematical assumptions and potential numerical difficulties. The usual gradient search is replaced by a stochastic search, namely an evolution strategy, the latter of which is perfect for the exploitation of parallel computing capabilities. Furthermore, this new approach allows for multi-criteria optimization and time-varying optima. These issues will become important in future once ephemeris construction is just one part of even larger optimization problems, e.g. the combined and consistent determination of a generalized physical state (orbit, size, shape, rotation, gravity, $\dots$) of celestial bodies (planets, satellites, asteroids, or comets), and/or if one seeks near real-time solutions. Here, we outline the general idea and exemplarily optimize high-correlated asteroidal ring model parameters (total mass and heliocentric radius), and individual asteroid masses, based on simulated observations.     

Effects of lunar and mars dust simulants on HaCaT keratinocytes and CHO-K1 fibroblasts

Exposure to lunar dust during Apollo missions resulted in occasional reports of ocular, respiratory and dermal irritations which showed that lunar dust has a risk potential for human health. This is caused by its high reactivity as well as its small size, leading to a wide distribution also inside habitats. Hence, detailed information regarding effects of extraterrestrial lunar dusts on human health is required to best support future missions to moon, mars or other destinations. In this study, we used several methods to assess the specific effects of extraterrestrial dusts onto mammalian skin by exposing HaCaT keratinocytes and CHO-K1 fibroblasts to dusts simulating lunar or mars soils. These particular cell types were chosen because the skin protects the human body from potentially harmful substances and because a well orchestrated program ensures proper wound healing. Keratinocytes and fibroblasts were exposed to the dusts for different durations of time and their effects on morphology and viability of the cells were determined. Cytotoxicity was measured using the MTT assay and by monitoring culture impedance, while phalloidin staining of the actin cytoskeleton was performed to address structural integrity of the cells which was also investigated by propidium iodide intake. It was found that the effects of the two types of dust simulants on the different features of both cell lines varied to a considerable extent. Moreover, proliferation of HaCaT keratinocytes, as analyzed by Ki67 labeling, was suppressed in sub-confluent cultures exposed to lunar dust simulant. Furthermore, experimental evidence is provided for a delay in regeneration of keratinocyte monolayers from scratch-wounding when exposed to lunar dust simulant. The obtained results will facilitate further investigations of dust exposure during wound healing and will ease risk assessment studies e.g., for lunar lander approaches. The investigations will help to determine safety measures to be taken during extraterrestrial expeditions in order to minimize risks to human health associated with exposure of human skin to dust contaminants.