The Confrontation Between General Relativity and Experiment

We review the experimental evidence for Einstein’s general relativity. A variety of high precision null experiments confirm the Einstein Equivalence Principle, which underlies the concept that gravitation is synonymous with spacetime geometry, and must be described by a metric theory. Solar system experiments that test the weak-field, post-Newtonian limit of metric theories strongly favor general relativity. Binary pulsars test gravitational-wave damping and aspects of strong-field general relativity. During the coming decades, tests of general relativity in new regimes may be possible. Laser interferometric gravitational-wave observatories on Earth and in space may provide new tests via precise measurements of the properties of gravitational waves. Future efforts using X-ray, infrared, gamma-ray and gravitational-wave astronomy may one day test general relativity in the strong-field regime near black holes and neutron stars.     

The time delay of a solar grazing photon

Moving clocks and clocks in a gravitational field slow down, not due to the effects of special relativity nor to the space-time curvature of general relativity, but due only to the principle of equivalence and the conservation of energy. However, some might argue that there has been a further “test” of the effect of gravity on time, namely, the measurement of the time-delay of a round-trip, solar-grazing radar beacon performed by Shapiro in the 1960s. In this test, Shapiro bounced a radar pulse off Mars at superior conjunction (a feat in itself for the time), and compared the measured round-trip travel time of this pulse with the expected round-trip time of a signal traveling at c for the entire trip, as determined from highly accurate planetary ephemerides. Shapiro had predicted this time-delay long before being technologically able to make such a measurement. While general relativity can be used to correctly obtain the magnitude of this delay, it is not the only explanation. As earlier with the analysis of clocks in motion and of clocks in a gravitational field, this paper derives the same result without invoking the space-time curvature of general relativity     

Plasma Motion and Kinematics in Cool and Hot Stars

The environments of both hot and cool stars are the sites of highly dynamic processes involving motion of gas and plasma in winds, flows across shocks, plasma motions in closed magnetic fields, or streams along magnetospheric accretion funnels. X-ray spectroscopy has opened new windows toward the study of these processes. Kinematics are evident in line shifts and line broadening, and also more indirectly through the analysis and interpretation of density-sensitive lines. In hot stellar winds, expanding-wind kinematics are directly seen in broadened lines although the broadening has turned out to often be smaller than anticipated, and some lines are so narrow that coronal models have been revived. Although X-ray spectra of cool stars have shown line shifts and broadening due to the kinematics of the entire corona, e.g., in binary systems, intrinsic mass motions are challenging to observe at the presently available resolution. Much indirect evidence for mass motion in magnetic coronae is nevertheless available. And finally, spectral diagnostics has also led to a new picture of X-ray production in accreting pre-main sequence stars where massive accretion flows collide with the photospheric gas, producing shocks in which gas is heated to high temperatures. We summarize evidence for the above mechanisms based on spectroscopic data from XMM-Newton and Chandra.     

无先验信息的低轨空间目标航迹起始方法

针对光学观测中缺乏预报信息的低轨道空间目标自动识别捕获问题,提出一种基于修正Hough变换的空间目标航迹起始方法,从而实现空间目标的自动识别。首先利用背景恒星运动规律对观测数据进行预处理,剔除大部分背景恒星。再利用观测数据的时序信息、定向参数进行Hough变换。本方法并没有采用传统的Hough变换对所有参数空间中的数据点进行投票,而是仅对参数空间中部分点投票,可以大幅减少运算量和存储量。试验表明:该方法能够有效地抑制背景恒星产生的虚假航迹,并快速准确地完成无先验信息的低轨道空间目标的航迹起始;可以在光学观测中实现空间目标的自动识别捕获,以及实时发现新目标,这为低轨道空间目标光学观测的无人值守和联网观测奠定了技术基础。     

Radial and nonradial oscillations in Delta Scuti stars

Mode identifications from the phase shift between the B-V color curves and the V light curves of Delta Scuti stars are discussed. For five Delta Scuti stars the frequency of highest amplitude is due to pulsation in a radial mode with one or more frequencies due to nonradial modes lying nearby. Thus there may be a resonance between the frequencies of radial and nonradial modes. This behavior has only been found in subgiant and giant stars so far, but this could easily be a selection effect. Theoretical predictions of frequencies for different 1-modes of the same overtone for main sequence, subgiant, and giant stars are needed to compare with these observations. Theoretical predictions of the strength of resonance coupling between radial and nonradial modes as a function of frequency separation are also needed.     

光速不变原理在相对论中的作用

本文首先通过速度附加定律的推导阐述了“光速不变原理”在狭义相对论中的作用,然后从公理化体系着手推导出洛仑兹变换式,从而进一步说明“光速不变原理”的作用。     

Cometary dynamics

The modern theory of cometary dynamics is based on Oort's hypothesis that the solar system is surrounded by a spherically symmetric cloud of 10¹¹ to 10¹² comets extending out to interstellar distances. Dynamical modeling and analysis of cometary motion have confirmed the ability of the Oort hypothesis to explain the observed distribution of energies for the long-period comet orbits. The motion of comets in the Oort cloud is controlled by perturbations from random passing stars, interstellar clouds, and the galactic gravitational field. Additionally, comets which enter the planetary region are perturbed by the major planets and by nongravitational forces resulting from jetting of volatiles on the surfaces of the cometary nuclei. The current Oort cloud is estimated to have a radius of 6 to 8 × 10⁴ AU, and to contain some 2 × 10¹² comets with a total mass of 7 to 8 Earth masses. Evidence has begun to accumulate for the existence of a massive inner Oort cloud extending from just beyond the orbit of Neptune to 10⁴ AU or more, with a population up to 100 times that of the outer Oort cloud. This inner cloud may serve as a reservoir to replenish the outer cloud as comets are stripped away by the various perturbers, and may also provide a more efficient source for the short-period comets. Recent suggestions of an unseen solar companion star or a tenth planet orbiting in the inner cloud and causing periodic comet showers on the Earth are likely unfounded. The formation site of the comets in the Oort cloud was likely the extended nebula accretion disc reaching from about 15 to 500 AU from the forming protosun. Comets which escape from the Oort cloud contribute to the flux of interstellar comets, though capture of interstellar comets by the solar system is extremely unlikely. The existence of Oort clouds around other main sequence stars has been suggested by the detection by the IRAS spacecraft of cool dust shells around about 10% of nearby stars.     

筒式偏心在轨分离角速度抑制方法

筒式偏心在轨分离是一类特殊的在轨分离问题,小卫星偏心安装而产生的分离力矩将导致分离角速度,进而影响小卫星的分离指向精度,甚至导致释放平台姿态失稳。而常规的姿态大角速度机动、姿态快速稳定控制方法难以在小卫星出筒前的极短时间内完成分离角速度抑制。因此,进行了卫星筒式偏心在轨分离动力学分析,基于分离角速度的产生,提出了抑制分离姿态干扰的前馈控制力矩法和角速度预偏置法。在此基础上,推导了关键控制参数的近似计算公式,给出了控制量的优化求解方法,并分析了控制干扰因素对抑制结果的影响。最后,通过仿真算例分析,对比验证了两种抑制方法的有效性,并给出了其工程应用的建议。     

弹道导弹自由段飞行中的守恒量及其应用

从精确弹道计算所用的数学模型出发，指出了弹性在自由段飞行中的两个具有明确物理意义的运动守恒量，并给出了用发射坐标系下的运动参数求这些守恒量的简洁计算公式。这两个守恒量可作为一种简单、实用的辅助评估手段用于计算结果可信性分析以及导弹发射诸元的校验。     

Power line transmission and radiation

A power line system consisting of multi-conductors above the ground is decomposed into a metal return (or balanced) and a ground return (unbalanced) circuit in terms of propagation modes. Power line radiation above the ground is sorted into two classes, transition radiation from discontinuities which takes place from a line of finite length and ?erenkov-like radiation from a ground return or unbalanced circuit due to the Earth's finite conductivity and dielectric properties. ?erenkov-like radiation is thought to be predominant and to be a main source origin of power line radiation and, therefore, is specifically discussed in this paper. The problem is reduced simply to wave propagation along an infinite wire above the ground, without taking into account line discontinuities and ionospheric effects. It is shown that a TEM-type wave of a ground return circuit at low frequencies virtually transfers to the principal TM wave of a surface waveguide at high frequencies, via a hybrid EH-type wave in an intermediate range of frequencies, i.e., ‘transition region’ with increasing frequency. Consequently, the attenuation characteristics of wave propagation, which should include radiation losses as well as ohmic losses in the wire and in the ground, possess a maximum and minimum at certain frequencies in the transition region. This is due to two reasons: one is that the Earth transfers to a dielectric from a conductor and the other is that the field concentrates more densely around the wire with increasing frequency. Experimental evidence of this phenomenon is also given.     

Spacecraft orbit determination using GPS navigation solutions

The orbit determination using the GPS navigation solutions for the KOMPSAT-1 spacecraft has been studied. The Cowell method of special perturbation theories was employed to develop a precision orbit propagation, and the perturbations due to geopotential, the gravity of the Sun and the Moon, solid Earth tides, ocean tides, the Earth's dynamic polar motion, solar radiation pressure, and atmospheric drag were modeled. Specifically, the satellite box-wing macro model was applied to minimize the drag errors at low altitude. The estimation scheme consisted of an extended Kalman filter and Bayesian least square method. To investigate the applicability of the method to the KOMPSAT-1 spacecraft, the orbit determination was accomplished using the GPS navigation solutions for the TOPEX/POSEIDON and TAOS satellites. The orbit determination results were compared with NASA POE generated by global laser tracking. The position and velocity accuracy was estimated about 16∼7 m and 0.0157∼0.0074 m·s⁻¹ RMS, respectively, for the two satellites in the presence of SA. These results verify that an orbit determination scheme using GPS navigation solutions can provide the static orbit information and reduce conspicuously the position and velocity errors of navigation solutions. It can be suggested that the sequential and batch orbit determination using the GPS navigation solutions be the most appropriate method in the KOMPSAT-1 type mission.     

The LISA Pathfinder Mission

LISA Pathfinder, formerly known as SMART-2, is the second of the European Space Agency’s Small Missions for Advance Research and Technology, and is designed to pave the way for the joint ESA/NASA Laser Interferometer Space Antenna (LISA) mission, by testing the core assumption of gravitational wave detection and general relativity: that free particles follow geodesics. The new technologies to be demonstrated in a space environment include: inertial sensors, high precision laser interferometry to free floating mirrors, and micro-Newton proportional thrusters. LISA Pathfinder will be launched on a dedicated launch vehicle in late 2011 into a low Earth orbit. By a transfer trajectory, the sciencecraft will enter its final orbit around the first Sun-Earth Lagrange point. First science results are expected approximately 3 months thereafter. Here, we give an overview of the mission including the technologies being demonstrated.     

Astrometric search for unseen stellar and sub-stellar companions to nearby stars and the possibility of their detection

Unseen companions to nearby stars are found astrometrically through perturbations in the proper motion from photographs taken with long-focus telescopes. The number of known unseen astrometric companions to nearby stars with photocentric orbits has grown by thirty percent in the last few years. Individual cases are discussed and optimum epochs given for resolution of the components. Orbital analysis of the photocentric positions on the photographic plates provides all information for accurate mass determination of the components except for m and angular separation, , of the two components which must come from another technique. There are potentially thirty low luminosity stars including some likely sub-stellar objects whose masses could be instantly found with the observations of these additional two parameters.A list of the stars known within five parsecs as of 1978 July is given and the status of unseen companions to these stars is discussed on the basis of long interval astrometric coverage.     

The contribution of observations of satellites to the determination of the Earth's gravitational potential

The history of the determination of the external gravitational potential of the Earth is sketched briefly. A discussion of the principles by which the potential may be derived from the observations of changes in the orbits of artificial satellites is followed by outlines of the principal theories and by detailed consideration of the formal differences between them that arise from differences in the ways that the orbits are described and it is shown that those formulae on which most of the numerical results depend are equivalent in the principal theories. The usual methods of treatment break down in certain special conditions and the analysis of these cases is also considered although they are not of great practical importance in the derivation of the potential; similarly a short account is given of the behaviour of a satellite having an orbital angular velocity commensurable with the spin angular velocity of the Earth. Methods by which satellites are observed are mentioned and the main numerical results on the external potential of the Earth are discussed critically. Finally the results are compared with those derived from observations of gravity on the surface of the Earth and the application of the results to problems of geodesy, of the physical state of the Earth and of the motion of the Moon are described.     

基于Hamilton-Jacobi方程的编队飞行控制

 针对椭圆参考轨道的编队飞行控制系统,基于哈密尔顿 雅克比（Hamilton-Jacobi）方程,给出一种新的生成函数半解析近似解的计算方法,同时推导了编队飞行控制系统相对运动的线性状态转移矩阵,并且验证了模型的精度。在此基础上,结合一个具体的编队飞行任务,采用线性规划算法得到了队形保持的闭环控制器,该算法得到的是脉冲形式的控制序列,因此更具有实际意义。同时还提出了一种简单的避免生成函数奇异点的方法,从而减少了生成函数间相互转换所带来的计算量。最后的非线性仿真结果验证了控制方法的有效性。     

The interaction between a magnetized plasma flow and a magnetized celestial body: A review of magnetospheric studies

Through an intensive study of the magnetospheres of the Earth, Mercury, and Jupiter, we have begun to understand how a magnetized celestial body interacts with a magnetized plasma flow. Some of the important findings are:Some of these findings may have significant implications in interpreting a variety of astrophysical processes associated with the magnetosphere of magnetic stars, pulsars and head-tail galaxies and also with transient processes, such as solar flares and flarings of particular types of variable stars, etc.     

一种用于机载设备的高精度转动型柔性铰链

 传统铰链应用于机载设备时会产生摩擦、磨损、接触面热梯度等问题,而利用材料变形产生运动的柔性铰链可避免此类缺陷,获得高性能的同时,降低维护成本。现阶段使用的交叉簧片柔性铰链无法满足某些超精密航空机载设备的定位精度要求,因此将交叉点推广到任意位置以改善性能。首先,考虑机械接口,建立了广义交叉簧片柔性铰链的刚度和轴漂模型,从而分析了各个参数与刚度及轴漂的关系,并评估了由于加工因素造成的簧片不等长给性能带来的影响,得到了具有等值刚度和较小轴漂特性的柔性铰链。然后,通过有限元仿真验证了所分析特性的有效性。最后,通过组合提出了一种更大行程的复合柔性铰链,当转角为15°时,且在垂直力作用下,轴漂小于3 μm,精度优于在国外已得到应用的蝶形铰链。     

亮星辅助下基于坐标转换的快速星图识别方法

数字天顶仪作为一种地面使用的星敏感器，主要用于高精度定位。为提高仪器的工作效率需要对星图识别的快速性进行研究。通过对恒星像点理论坐标与图像坐标的分析，构建坐标转换模型。依据星表中恒星的分布及星等筛选出视场范围内的亮星，并构建导航星表。结合导航星表完成3颗亮星的准确识别，在识别亮星的基础上解算坐标转换模型的参数。通过构建的坐标转换模型对视场范围内的恒星进行坐标转换，将转换后的星点坐标与提取的星点图像坐标进行匹配完成星图的识别，这样能够提高星图识别的快速性。实验数据表明：在保证识别星点数量的基础上，采用亮星辅助下基于坐标转换的星图识别方法使时间缩短为改进三角形星图识别算法的五分之一。     

钝缘舵高超音速湍流分离特性

给出由半圆柱前缘舵诱导的高超音速湍流分离的实验结果。实验气流Ｍａｃｈ数为７．８,单位长度Ｒｅ数为３．５×１０７ｍ－１。结果表明：钝缘舵诱导的湍流分离极不稳定,分离激波出现大尺度低频振荡,使壁面压力和热流率无量纲标准偏差在主分离线附近达最大值。Ｍａｃｈ数愈高,最大无量纲标准偏差值越大。在前缘区前缘直径是控制分离流场尺度和平均壁面压力、热流率分布的主要参数     

Hydrodynamic atmosphere models for hot luminous stars II. Method and improvements over unified models

In a paper submitted to A&A we present the first line blanketed hydrodynamic models of spherically expanding atmospheres of hot stars. This paper is complementary to the submitted paper. Here, we emphasize the advantages and the weak points of our approach and we present additional technical aspects.The models are characterised by a simultaneous solution of the equation of motion, the non-LTE populations of H and He, and radiation transfer in a line blanketed atmosphere. The entire domain from the optically thick photosphere out to the terminal velocity of the wind is treated. The radiative forces are evaluated consistently with the depth-dependent radiation field, taking into account multiple scattering by metal lines and line overlap.