Energetic Hydrogen and Oxygen Atoms Observed on the Nightside of Mars

We present measurements of energetic hydrogen and oxygen atoms (ENAs) on the nightside of Mars detected by the neutral particle detector (NPD) of ASPERA-3 on Mars Express. We focus on the observations for which the field-of-view of NPD was directed at the nightside of Mars or at the region around the limb, thus monitoring the flow of ENAs towards the nightside of the planet. We derive energy spectra and total fluxes, and have compiled maps of hydrogen ENA outflow. The hydrogen ENA intensities reach 10⁵ cm⁻² sr⁻¹ s⁻¹, but no oxygen ENA signals above the detection threshold of 10⁴ cm⁻² sr⁻¹ s⁻¹ are observed. These intensities are considerably lower than most theoretical predictions. We explain the discrepancy as due to an overestimation of the charge-exchange processes in the models for which too high an exospheric density was assumed. Recent UV limb emission measurements (Galli et al., this issue) point to a hydrogen exobase density of 10¹⁰ m⁻³ and a very hot hydrogen component, whereas the models were based on a hydrogen exobase density of 10¹² m⁻³ and a temperature of 200 K predicted by Krasnopolsky and Gladstone (1996). Finally, we estimate the global atmospheric loss rate of hydrogen and oxygen due to the production of ENAs.     

Heliotropic dust rings for Earth climate engineering

This paper examines the concept of a Sun-pointing elliptical Earth ring comprised of dust grains to offset global warming. A new family of non-Keplerian periodic orbits, under the effects of solar radiation pressure and the Earth’s J₂ oblateness perturbation, is used to increase the lifetime of the passive cloud of particles and, thus, increase the efficiency of this geoengineering strategy. An analytical model is used to predict the orbit evolution of the dust ring due to solar-radiation pressure and the J₂ effect. The attenuation of the solar radiation can then be calculated from the ring model. In comparison to circular orbits, eccentric orbits yield a more stable environment for small grain sizes and therefore achieve higher efficiencies when the orbit decay of the material is considered. Moreover, the novel orbital dynamics experienced by high area-to-mass ratio objects, influenced by solar radiation pressure and the J₂ effect, ensure the ring will maintain a permanent heliotropic shape, with dust spending the largest portion of time on the Sun facing side of the orbit. It is envisaged that small dust grains can be released from a circular generator orbit with an initial impulse to enter an eccentric orbit with Sun-facing apogee. Finally, a lowest estimate of 1 × 10¹² kg of material is computed as the total mass required to offset the effects of global warming.     

Simulation of the flow around a stationary circular cylinder taking into account laminar-turbulent transition

We consider a problem of a stationary incompressible viscous fluid flow around a flat circular cylinder. In the vicinity of the critical Reynolds number Re _cr a stepwise drop of the cylinder drag takes place, which is called the drag crisis.     

Gravity and perceptual stability during translational head movement on earth and in microgravity

We measured the amount of visual movement judged consistent with translational head movement under normal and microgravity conditions. Subjects wore a virtual reality helmet in which the ratio of the movement of the world to the movement of the head (visual gain) was variable. Using the method of adjustment under normal gravity 10 subjects adjusted the visual gain until the visual world appeared stable during head movements that were either parallel or orthogonal to gravity. Using the method of constant stimuli under normal gravity, seven subjects moved their heads and judged whether the virtual world appeared to move “with” or “against” their movement for several visual gains. One subject repeated the constant stimuli judgements in microgravity during parabolic flight. The accuracy of judgements appeared unaffected by the direction or absence of gravity. Only the variability appeared affected by the absence of gravity. These results are discussed in relation to discomfort during head movements in microgravity.     

一种新型多普勒噪声抑制技术对BepiColombo任务无线电科学实验的性能提升

在欧洲空间局和日本宇宙开发机构联合开展的BepiColombo水星任务中,将开展水星轨道器无线电科学实验,包括估计水星的引力场及其旋转状态,并对广义相对论进行验证。目前地面系统和星上设备的主流配置可以在无线电科学实验中建立X/X、X/Ka和Ka/Ka多个频段的链路,测速精度可达3 um/s（1 000 s积分）,测距精度为20 cm。提出了基于时延机械噪声对消技术提高无线电科学实验性能的方案。时延机械噪声对消技术需要处理在两个测站不同时刻的测量数据,一个测站实施双向多普勒测距,对另一个单收测站的要求较为严格,该测站需要具有较好的对流层条件。这种方法能够显著降低Ka频段双向链路的主要测量噪声,包括由对流层和天线机械系统震动引起的噪声。我们给出了端到端的仿真性能,并估计了在使用时延机械噪声对消技术前提下的水星引力场和旋转状态。考虑使用NASA位于美国本土戈尔德斯敦的DSS-25天线或欧空局位于阿根廷马拉圭的DSA-3天线作为双程测量站,并考虑使用位于智力的APEX天文观测天线作为单收站。分析结果表明在最好的噪声条件下,使用DSA-3天线作为双程测量站时,时延机械噪声对消技术可将待估计的全局和局部参数的估计精度提升一倍。对于无线电科学实验的目标,这一可能的性能提升对行星地质物理学很有意义,它将有益于研究水星内部的结构。     

New maximum entropy spectrum using uncertain eigenstructureconstraints

A number of modern spectral estimators are shown to have a common generic formulation. These include minimum variance, MUSIC, and maximum entropy. A new maximum entropy spectral estimator is derived using constraints on the modal powers or the expected-square projections of the data onto the eigenvectors of the data covariance matrix. The formulation incorporates uncertainty in the modal power constraints and the signal-versus-noise subspace separation. The resulting estimators have forms which incorporate all other modern estimators, including maximum entropy and minimum norm. The new estimators allow further development when a priori information is used in the constraints. Comparison of one version of the estimator with the minimum norm verifies the greater probability of resolution of the minimum norm but indicates in some instances the value of the incorporated uncertainties. Another version uses complex constraints and reduces to conventional maximum entropy or minimum norm under certain conditions     

Effect of Observation Errors on Satellite Attitude Estimation

A study of the effect of observation errors on the best least squares estimate of satellite attitude based on two sets of direction measurements, where one set contains two independent measurements which are corrupted by zero-mean normally distributed additive errors, is summarized. Using Monte-Carlo techniques, the statistics of the estimated rotation of the satellite, from a known reference, were obtained. It was found that one of the parameterizations of the rotation, namely, the vector z, is best suited for estimation, since ?z is normally distributed with zero mean and its standard deviation is independent of the two direction measurements.     

Ground based ISS payload microgravity disturbance assessments

In order to verify that the International Space Station (ISS) payload facility racks do not disturb the microgravity environment of neighboring facility racks and that the facility science operations are not compromised, a testing and analytical verification process must be followed. Currently no facility racks have taken this process from start to finish. The authors are participants in implementing this process for the NASA Glenn Research Center (GRC) Fluids and Combustion Facility (FCF). To address the testing part of the verification process, the Microgravity Emissions Laboratory (MEL) was developed at GRC. The MEL is a 6 degree of freedom inertial measurement system capable of characterizing inertial response forces (emissions) of components, sub-rack payloads, or rack-level payloads down to 10(-7) g's. The inertial force output data, generated from the steady state or transient operations of the test articles, are utilized in analytical simulations to predict the on-orbit vibratory environment at specific science or rack interface locations. Once the facility payload rack and disturbers are properly modeled an assessment can be made as to whether required microgravity levels are achieved. The modeling is utilized to develop microgravity predictions which lead to the development of microgravity sensitive ISS experiment operations once on-orbit. The on-orbit measurements will be verified by use of the NASA GRC Space Acceleration Measurement System (SAMS). The major topics to be addressed in this paper are: (1) Microgravity Requirements, (2) Microgravity Disturbers, (3) MEL Testing, (4) Disturbance Control, (5) Microgravity Control Process, and (6) On-Orbit Predictions and Verification.