Terrestrial Gravitational Field Research Articles

МОЛЕКУЛЯРНЫЕ МЕХАНИЗМЫ ГРАВИЧУВСТВИТЕЛЬНОСТИ: ОТ КЛЕТОК К ВНЕКЛЕТОЧНОМУ МАТРИКСУ

Gravity deprivation is one of the major risk factors in spaceflight. Medical and biological studies show that as a result of prolonged exposure to weightlessness, astronauts and cosmonauts experience unfavorable changes in many physiological systems (loss of bone mass, muscle atrophy, cardiovascular insufficiency, sensorimotor coordination disorders, etc.). Physiological and pathophysiological mechanisms of response to microgravity exposure at different levels of organization (molecular, cellular, tissue, organismal) and subsequent adaptation are actively studied. Under the conditions of the constantly acting terrestrial gravitational field, multicellular organisms have formed a multicomponent mechanoreceptor apparatus including cellular (nucleo- and cytoskeletal skeleton) and extracellular (connective tissue matrix) mechanosensitive elements. The coordination of these compartments is carried out by specialized protein complexes, forming a mechanoreceptor unit. When the Earth's gravity is removed, this unit will be the basis for adaptation processes, acting as a kind of complex gravity receptor. Ground-based simulations are particularly important to elucidate the mechanisms that provide the response to microgravity, as flight limitations do not allow large-scale studies in space. In this review, we analyze the current understanding of the molecular mechanisms of gravity sensiing and the cellular response to microgravity under both real and simulated microgravity environments and discuss the directions for further research in this area.

Positron accumulation in the GBAR experiment

We present a description of the GBAR positron (e+) trapping apparatus, which consists of a three stage Buffer Gas Trap (BGT) followed by a High Field Penning Trap (HFT), and discuss its performance. The overall goal of the GBAR experiment is to measure the acceleration of the neutral antihydrogen (H¯) atom in the terrestrial gravitational field by neutralising a positive antihydrogen ion (H¯+), which has been cooled to a low temperature, and observing the subsequent H¯ annihilation following free fall. To produce one H¯+ ion, about 1010 positrons, efficiently converted into positronium (Ps), together with about 107 antiprotons (p¯), are required. The positrons, produced from an electron linac-based system, are accumulated first in the BGT whereafter they are stacked in the ultra-high vacuum HFT, where we have been able to trap 1.4(2) × 109 positrons in 1100 s.

Human movements do not look the same in a tilted world: Gravitational constraints influence the perception of biological motion.

We investigated whether gravitational constraints influence the interaction of visual, proprioceptive and vestibular cues for Biological Motion Perception (BMP). Participants were asked to distinguish between plausible and random point-light movements, while passively placed in either an upright or a tilted body orientation. Manipulating the body orientation with respect to gravity leads to different gravitational signals transmitted by the visual, proprioceptive, and vestibular systems. Participants were overall faster in distinguishing plausible point-light movements than random movements. Critically, response times for biologically plausible point-light movements - but not for random movements - were significantly prolonged in the tilted body orientation. Our results suggest that BMP depends not only on the spatial-temporal cues embedded in point-light movements but also rely on the congruency between current gravitational signals detected by the sensory systems and our previous knowledge of terrestrial gravity. STATEMENT OF RELEVANCE: As humankind is preparing for a new space age, understanding how gravity influences behaviour and cognition has never been more pressing. All living organisms have evolved to survive in a terrestrial gravitational field. Although we cannot consciously feel gravity, it has an impact in our life: it affects how we move and interact with the external environment. The sensory signals from the vestibular system are continuously combined with visual and proprioceptive cues to help us in maintaining a stable representation of the world. Here we placed participants in a tilted body orientation and were able to determine that a conflict between prior gravitational knowledge and what was actively sensed about gravity affected human Biological Movement Perception. Humans suffer changes in perception under non-terrestrial gravity conditions that may potentially compromise performance during space exploration.

The Accuracy of the Navigational Accelerometer with a Nonlinear Metrological Model in Operating Conditions

Abstract A mathematical model of the error of the navigational accelerometer caused by the nonlinearity of its metrological model, taking into account the influence of vibration, was developed. The method of experimental estimation of the vibration error based on the developed model was proposed. The main idea of the method is to evaluate parameters of the developed model during static tests in the terrestrial gravitational field and to calculate error according to the specific vibration characteristics – the amplitude in the case of harmonic vibration profile or the frequency band and the power spectral density in the case of random vibration. The effectiveness of the proposed method has been tested using three types of navigation accelerometers in comparison with the results of classical dynamic testing in various vibration conditions (harmonic, white noise, etc.).

Comparative estimation of the accuracy of methods of autonomous navigation of small spacecraft in formation flying

The results of comparative estimation of the accuracy of autonomous navigation of small spacecraft in formation flying are presented. To carry out the research, the zenith method and the method of navigation by orbital references were chosen. These methods are based on measurements of the angular position of the Earth and an orbital reference point relative to navigational stars. Assumptions concerning the central terrestrial gravitational field and the normality of errors of the on-board navigation measurements with known constant variability were introduced in the studies. The studies were carried out using the theory of analytical estimation of the accuracy of spacecraft autonomous navigation methods. The use of this theory makes it possible to obtain the covariance error matrix of the required vector of navigation parameters and to estimate the potential (maximum achievable) characteristics of the accuracy of the navigation methods used. A dimensionless navigation error coefficient was chosen as an indicator of the accuracy of small spacecraft navigation method. The coefficient is associated with the elements of the main diagonal of the covariance matrix, it characterizes the precision properties of the method, is integrated by nature and does not depend on the volume and accuracy of the results of navigation measurements. The criterion of expediency of applying the method of determining the parameters of motion of the spacecraft center of mass is based on the comparison of navigation error rates. The presented results allow us to make reasonable choice of the method of autonomous navigation and of the composition of the onboard control of small spacecraft in formation flying.

Identification of the vibration error of the navigation accelerometer during its static tests

This article describes the vibration error of the navigation accelerometer caused by the nonlinearity of its metrological model.Vibration error is the regular steady-state error in the time-average output of accelerometer during vibratory disturbances. It is defined that error varies with the amplitude and frequency of the vibrations and consists of additive and multiplicative components. The additive components are only dependent on the vibration value and determined by even nonlinear coefficients. The multiplicative components are also dependent on measured acceleration and determined by the asymmetric of sensitivity, odd nonlinear coefficients, and cross-sensitivity. To express these dependencies, an appropriate mathematical model was developed.The method of experimental estimation of the vibration error based on the developed model was proposed. The main idea of the method is to evaluate parameters of the developed model during static tests in the terrestrial gravitational field and to calculate error according to the specific vibration characteristics – the amplitude in the case of harmonic vibration profile or the frequency band and the power spectral density in the case of random vibration.The effectiveness of the proposed method has been tested by using three types of navigation accelerometers in comparison with the results of classical dynamic testing in various vibration conditions (harmonic, white noise, etc.). The mismatch between the calculated values of the vibration error and the reference results did not exceed 35%, which confirms the correctness of reasoning regarding the type of proposed functional dependence.

Computational analyses of perturbative effects on geostationary satellites: Case SGDC

This work aims to study the main perturbative effects on SGDC - Geostationary Satellite of Defense and Communication, result of the partnership between Telebrás and Brazilian Ministry of Defense. The name “SGDC” corresponds to the group of satellites that will be launched with the purpose of transmitting broadband Internet to the less favored zones of Brazil and to intermediate communication between the military sectors. Through numerical simulations, analyses are carried out to determine the influence of the perturbations of the terrestrial gravitational field, the solar radiation pressure and the lunissolar gravitational force, besides checking the combination of these perturbations, evaluating which of the effects presents predominance.

The GBAR antimatter gravity experiment

The GBAR project (Gravitational Behaviour of Anti hydrogen at Rest) at CERN, aims to measure the free fall acceleration of ultracold neutral anti hydrogen atoms in the terrestrial gravitational field. The experiment consists preparing anti hydrogen ions (one antiproton and two positrons) and sympathetically cooling them with Be (+) ions to less than 10 mu K. The ultracold ions will then be photo-ionized just above threshold, and the free fall time over a known distance measured. We will describe the project, the accuracy that can be reached by standard techniques, and discuss a possible improvement to reduce the vertical velocity spread.

Quantum limit on time measurement in a gravitational field

Good clocks are of importance both to fundamental physics and for applications in astronomy, metrology and global positioning systems. In a recent technological breakthrough, researchers at NIST have been able to achieve a stability of one part in 1018 using an ytterbium clock. This naturally raises the question of whether there are fundamental limits to time keeping. In this article we point out that gravity and quantum mechanics set a fundamental limit on the fractional frequency uncertainty of clocks. This limit comes from a combination of the uncertainty relation, the gravitational redshift and the relativistic time dilation effect. For example, a single ion aluminium clock in a terrestrial gravitational field cannot achieve a fractional frequency uncertainty better than one part in 1022. This fundamental limit explores the interaction between gravity and quantum mechanics on a laboratory scale.

Numerical-analytic modeling of perturbed oscillatory motions of the Earth pole

An improved numerical-analytic model of multifrequency oscillatory motion of Earth’s pole with temporal variations in the geopotential coefficients taken into account is considered. The model is a natural improvement of the earlier developed basic model of the pole oscillations (the Chandler and annual components) by using the methods of celestial mechanics and the data of the terrestrial gravitational field observations. This model allows one to improve the accuracy of forecast of Earth’s pole motion trajectory in the periods of significant anomalies (irregular deviations). The fundamental aspects of Earth’s pole oscillation process are investigated, which allows qualitatively explaining the observed irregular effects in the oscillatory process. The results of numerical modeling of Earth’s pole coordinate oscillations are compared with the observation and measurement data of the International Earth Rotation and Reference Systems Service (IERS).

The Gbar project, or how does antimatter fall?

The Einstein classical Weak Equivalence Principle states that the trajectory of a particle is independent of its composition and internal structure when it is only submitted to gravitational forces. This fundamental principle has never been directly tested with antimatter. However, theoretical models such as supergravity may contain components inducing repulsive gravity, thus violating this principle. The GBAR project (Gravitational Behaviour of Antihydrogen at Rest) proposes to measure the free fall acceleration of ultracold neutral antihydrogen atoms in the terrestrial gravitational field. The experiment consists in preparing antihydrogen ions (one antiproton and two positrons) and sympathetically cool them with Be+ ions to a few 10 μ K. The ultracold ions will then be photoionized just above threshold, and the free-fall time over a known distance measured. In this work, the GBAR project is described as well as possible improvements that use quantum reflection of antihydrogen on surfaces to use quantum methods of measurements.

Probing strongly coupled chameleons with slow neutrons

We consider different methods to probe the chameleon scalar field with slow neutrons. Chameleons modify the potential of bouncing neutrons over a flat mirror in the terrestrial gravitational field. This induces a shift in the energy levels of the neutrons which could be detected in current experiments like GRANIT. Chameleons between parallel plates have a field profile which is bubblelike and which would modify the phase of neutrons in interferometric experiments. We show that this new method of detection is competitive with the bouncing neutron one, hopefully providing an efficient probe of chameleons when strongly coupled to matter.

Trap with ultracold neutrons as a detector of dark matter with long-range forces

The possibility of using a trap with ultracold neutrons (UCNs) as a detector of dark matter particles with long-range forces is considered. The main advantage of this method is the possibility of detecting recoil energies ∼10−7 eV. The limitations on the parameters of the interaction potential in the form \(\phi (r) = a\frac{{e^{ - r/b} }} {r}\) for dark matter particles and neutrons at different values of the dark matter density on the Earth are represented. It is shown that the suggestion about the long-range character of the interaction between dark matter particles leads to a significant increase in the elastic scattering cross section at low energies. As a consequence, dark matter can be captured and accumulated by the terrestrial gravitational field. The first experimental limitations on the existence of long-range dark matter on the Earth are presented.

Strongly Coupled Chameleons and the Neutronic Quantum Bouncer

We consider the potential detection of chameleons using bouncing ultracold neutrons. We show that the presence of a chameleon field over a planar plate would alter the energy levels of ultracold neutrons in the terrestrial gravitational field. When chameleons are strongly coupled to nuclear matter, β≳10(8), we find that the shift in energy levels would be detectable with the forthcoming GRANIT experiment, where a sensitivity of the order of 1% of a peV is expected. We also find that an extremely large coupling β≳10(11) would lead to new bound states at a distance of order 2 μm, which is already ruled out by previous Grenoble experiments. The resulting bound, β≲10(11), is already 3 orders of magnitude better than the upper bound, β≲10(14), from precision tests of atomic spectra.

The sodium tail of the Moon

During the few days centered about new Moon, the lunar surface is optically hidden from Earth-based observers. However, the Moon still offers an observable: an extended sodium tail. The lunar sodium tail is the escaping “hot” component of a coma-like exosphere of sodium generated by photon-stimulated desorption, solar wind sputtering and meteoroid impact. Neutral sodium atoms escaping lunar gravity experience solar radiation pressure that drives them into the anti-solar direction forming a comet-like tail. During new Moon time, the geometry of the Sun, Moon and Earth is such that the anti-sunward sodium flux is perturbed by the terrestrial gravitational field resulting in its focusing into a dense core that extends beyond the Earth. An all-sky camera situated at the El Leoncito Observatory (CASLEO) in Argentina has been successfully imaging this tail through a sodium filter at each lunation since April 2006. This paper reports on the results of the brightness of the lunar sodium tail spanning 31 lunations between April 2006 and September 2008. Brightness variability trends are compared with both sporadic and shower meteor activity, solar wind proton energy flux and solar near ultra violet (NUV) patterns for possible correlations. Results suggest minimal variability in the brightness of the observed lunar sodium tail, generally uncorrelated with any single source, yet consistent with a multi-year period of minimal solar activity and non-intense meteoric fluxes.

Grounding the figure: Surface attachment influences figure-ground organization

We investigated whether the lower region effect on figure-ground organization (Vecera, Vogel, and Woodman, 2002) would generalize to contextual depth planes in vertical orientations, as is predicted by a theoretical analysis based on the ecological statistics of edges arising from objects that are attached to surfaces of support. Observers viewed left/right ambiguous figure-ground displays that occluded middle sections of four types of contextual inducers: two types of attached, receding, vertical planes (walls) that used linear perspective and/or texture gradients to induce perceived depth and two types of similar trapezoidal control figures that used either uniform color or random texture to reduce or eliminate perceived depth. The results showed a reliable bias toward seeing as "figure" the side of the figure-ground display that was attached to the receding depth plane, but no such bias for the corresponding side in either of the control conditions. The results are interpreted as being consistent with the attachment hypothesis that the lower region cue to figure-ground organization results from ecological biases in edge interpretation that arise when objects are attached to supporting surfaces in the terrestrial gravitational field.

How to reach a few percent level in determining the Lense–Thirring effect?

In this paper we explore the possibility of suitably combining the nodes Ω of the existing geodetic LAGEOS, LAGEOS II and Ajisai laser-ranged satellites and of the radar altimeter Jason–1 satellite in order to increase the accuracy in testing the general relativistic gravitomagnetic Lense–Thirring secular effect in the gravitational field of the Earth. The proposal of introducing Ajisai and Jason–1 in such a combination comes from the expected benefits which could be obtained in reducing the aliasing secular impact of the classical part of the terrestrial gravitational field. According to the recently released EIGEN-CG01C combined GRACE + CHAMP + terrestrial gravimetry/altimetry Earth gravity model, the impact of the static part of the mismodelled even zonal harmonics of geopotential, which represent the major source of systematic error, amounts to 1.6%, at 1−σ level. It is better than the error which could be obtained with a two-node LAGEOS-LAGEOS II only combination (6% at 1−σ). Moreover, the proposed combination would be insensitive also to the secular variations of the low-degree even zonal harmonics, contrary to the LAGEOS-LAGEOS II only combination. Such variations could be a serious limiting factor over observational time spans many years long. The price to be paid for this improvements of the systematic error of gravitational origin is represented by the non-conservative forces introduced along with the new orbital elements. However, they would induce periodic perturbations, contrary to the gravitational noise. A major concern would be the assessment of the impact of the non-conservative accelerations on the Jason–1 node. According to the present-day force models, the mismodelling in the non-conservative forces would, at worst, induce an aliasing periodic signal with an amplitude of 4% of the Lense-Thirring effect over a time span of 2 years. However, an observational time span of just some years could safely be adopted in order to fit and remove the residual long–period non-gravitational signals affecting Jason’s node, which, in the case of the direct solar radiation pressure, have a main periodicity of approximately 120 days. Of course, the possibility of getting time series of the Jason’s node some years long should be demonstrated in reality.

NUMERICAL ANALYSIS OF NATURAL CONVECTION IN AN ENCLOSURE WITH ROTATIONALLY PRODUCED ARTIFICIAL GRAVITY

Buoyancy-induced flow and heat transfer in an artificial gravitational field produced by rotation is examined. A comparison is made between such flows and those produced in a stationary enclosure exposed to a terrestrial gravitational field. The computational results show that the structure of the buoyancy-induced flow fields is different for the two cases. Natural convection in an artificial gravity field is examined in terms of the ratio of artificial gravity buoyancy forces to viscous forces. Unlike a stationary enclosure in a terrestrial gravitational field, in an artificial gravity field there is no bifurcation in the Nusselt number behavior.

Investigation of quantum neutron states in the terrestrial gravitational field above a mirror

Investigation of quantum neutron states in the terrestrial gravitational field above a mirror

Issledovanie kvantovykh sostoyanii neitronov v gravitatsionnom pole Zemli nad zerkalom

Issledovanie kvantovykh sostoyanii neitronov v gravitatsionnom pole Zemli nad zerkalom