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1.
A low frequency impedance probe designed to detect the ion-electron hybrid resonance in the ionospheric plasma is studied. Firstly, the effect of finite resistance of an ion sheath surrounding a probe is analyzed for the case of a cylindrical probe and quantitative insight into this is given. Secondly, the dissipations due to warm plasma effects which appear in the actual experiment flown aboard a space vehicle are discussed. These depend upon the dimensions of the probe system and the velocity of the system relative to the mean thermal velocity of charged particles. Analyses are then carried out for a simple planar grid model using electrostatic and hydrodynamic approximations.  相似文献   

2.
We present a study of an impacting descent probe that increases the science return of spacecraft orbiting or passing an atmosphere-less planetary bodies of the solar system, such as the Galilean moons of Jupiter. The descent probe is a carry-on small spacecraft (<100 kg), to be deployed by the mother spacecraft, that brings itself onto a collisional trajectory with the targeted planetary body in a simple manner. A possible science payload includes instruments for surface imaging, characterisation of the neutral exosphere, and magnetic field and plasma measurement near the target body down to very low-altitudes (~1 km), during the probe’s fast (~km/s) descent to the surface until impact. The science goals and the concept of operation are discussed with particular reference to Europa, including options for flying through water plumes and after-impact retrieval of very-low altitude science data. All in all, it is demonstrated how the descent probe has the potential to provide a high science return to a mission at a low extra level of complexity, engineering effort, and risk. This study builds upon earlier studies for a Callisto Descent Probe for the former Europa-Jupiter System Mission of ESA and NASA, and extends them with a detailed assessment of a descent probe designed to be an additional science payload for the NASA Europa Mission.  相似文献   

3.
Reliable measurements of the Moon's global heat flow would serve as an important diagnostic test for models of lunar thermal evolution and would also help to constrain the Moon's bulk abundance of radioactive elements and its differentiation history. The two existing measurements of lunar heat flow are unlikely to be representative of the global heat flow. For these reasons, obtaining additional heat flow measurements has been recognized as a high priority lunar science objective. In making such measurements, it is essential that the design and deployment of the heat flow probe and of the parent spacecraft do not inadvertently modify the near-surface thermal structure of the lunar regolith and thus perturb the measured heat flow. One type of spacecraft-related perturbation is the shadow cast by the spacecraft and by thermal blankets on some instruments. The thermal effects of these shadows propagate by conduction both downward and outward from the spacecraft into the lunar regolith. Shadows cast by the spacecraft superstructure move over the surface with time and only perturb the regolith temperature in the upper 0.8 m. Permanent shadows, such as from thermal blankets covering a seismometer or other instruments, can modify the temperature to greater depth. Finite element simulations using measured values of the thermal diffusivity of lunar regolith show that the limiting factor for temperature perturbations is the need to measure the annual thermal wave for 2 or more years to measure the thermal diffusivity. The error induced by permanent spacecraft thermal shadows can be kept below 8% of the annual wave amplitude at 1 m depth if the heat flow probe is deployed at least 2.5 m away from any permanent spacecraft shadow. Deploying the heat flow probe 2 m from permanent shadows permits measuring the annual thermal wave for only one year and should be considered the science floor for a heat flow experiment on the Moon. One way to meet this separation requirement would be to deploy the heat flow and seismology experiments on opposite sides of the spacecraft. This result should be incorporated in the design of future lunar geophysics spacecraft experiments. Differences in the thermal environments of the Moon and Mars result in less restrictive separation requirements for heat flow experiments on Mars.  相似文献   

4.
The purpose of this paper is to make a numerical search for natural orbits that can be used for a spacecraft to study a possible small moon of Pallas. There are many speculations about the existence of a small companion around this large asteroid, so finding and classifying orbits around this possible celestial body is an interesting problem in astrodynamics and that can be used for a spacecraft to observe this body. It is assumed that this moon has a radius that can vary from 0.125 to 1 km and that is located 750 or 500 km away from the center of Pallas. The idea is to show the effects of this parameter in the orbits around this moon. It means that the moon is much smaller than Pallas, so Keplerian orbits are not possible around it. To solve this problem, it is possible to find some special orbits that are called "Quasi Satellite Orbits" (QSO). They are orbits dominated by the gravity of Pallas, but that use the smaller perturbation from the moon to keep the spacecraft close to it. The present work searches for orbits that make the spacecraft to remain at given limits in its distance from the moon, like in the range from 3 to 50 km, the values used as an example in the present paper. This value is used because it is a good range to observe the body without getting to close to it, so reducing the risks of collisions. Each trajectory can be identified by the initial conditions of the spacecraft with respect to the moon, which means its initial position and velocity. The dynamics considers the restricted three-body problem and the influence of the solar radiation pressure, because some spacecraft may have higher values for the area-to-mass ratio, which gives a non-negligible effect in the trajectory of the spacecraft.  相似文献   

5.
Electrostatic charging has given rise to problems on several geostationary spacecraft. This has led to a rigorous electrostatic cleanliness approach in the case of the scientific geostationary satellite GEOS in order to secure correct electric field and low energy plasma measurements. The present paper outlines the relevant charging mechanism, describes a new method for the determination of the equilibrium potential, and reports on actual potential measurements. The potentials observed are very closely related to the actual plasma conditions at the geostationary orbit. It is generally possible to use the potential measurements to characterize the particle population encountered by the spacecraft.Measurements carried out over a period of 4 years are presented by way of examples. A careful analysis shows that the chosen examples are representative and reflect the conditions observed on all other days of the mission. The results lead to the overall conclusion that the equilibrium potential of GEOS in sunlight is always moderately positive and only rarely exceeds + 10 V with respect to ambient space. At no instance in the sunlit portion of the orbit does the spacecraft assume a negative potential. We find that the observed moderate positive equilibrium potential generally is a function of cold plasma density. During the night and early morning part of the orbit we can, however, identify periods where the high energy particle population dictates the equilibrium potential. The electrostatic cleanliness design of GEOS avoids negative charging also under these conditions. In eclipse, a negative potential cannot be avoided but here the electrostatic cleanliness approach chosen for GEOS prevents any differential charging and avoids potentials of several thousand volts which have appeared on other satellites. The cost, in time and effort, of the precautions employed has clearly been justified. The specially developed techniques have since been used on other satellites and the lessons learned have also been applied successfully to operational spacecraft such as METEOSAT 2.  相似文献   

6.
The theory of interactions between a probe and the surrounding plasma at rest is developed in a spherically and in a cylindrically symmetric model (probe theory). The theory is based on the Vlasov-Poisson system; a general numerical program was developed to solve this system by means of an iterative procedure. Various ambient plasma and charged particle emission properties are described by the complete set of boundary conditions for the distribution functions in the phase space. By use of this numerical method, potential and space charge density in the whole surroundings of the probe as well as the current densities of all plasma constituents are calculated self-consistently.Furthermore, the regions of the phase space with particle trajectories of the same kind can be approximated depending on the plasma properties. Then, the current densities can be estimated analytically. This approach to the problem yields self-consistent approximations and is the only stringent derivation of the thick sheath and of the thin sheath approximation of the classical Langmuir theory. These approximations are generalized with respect to the charged particle emission from the surface.The symmetric probe theory is applied to the following problems of spacecraft environment and spacecraft charging: (i) a spacecraft in the ionosphere with very negative surface potential, (ii) a spacecraft in the solar wind with strong photoelectron emission, and (iii) a spacecraft in the transition region of comet Halley with very strong secondary plasma emission.  相似文献   

7.
用脉冲星钟作航天器时间标准   总被引：3，自引：0，他引：3  
在介绍参考坐标系和时间标准的基础上,讨论了用脉冲星为航天器导航的时间标准问题。利用X射线脉冲星实现航天器自主导航,星载钟的任何误差都会直接影响航天器位置测量。脉冲星钟具有较高的长期频率稳定度,适合用作各类航天器的时间标准。重点讨论了时间标准误差对航天器定位的影响;给出了用脉冲星钟作航天器时间标准的物理实现方法。  相似文献   

8.
The most striking feature in the spatial distribution of the smallest dust grains observed at Halley's comet by the VEGA-1 spacecraft is the sharp glitch at a cometocentric distance of about 180 000 km, which approximately corresponds to the so-called cometopause inside which the contaminated solar wind plasma was rapidly cooled. We propose that this glitch was caused by the electrostatic disruption of larger composite grains which rapidly charged up as they traversed the cometopause. The clear asymmetry in the distribution between the inbound and outbound portion of the spacecraft trajectory is also consistent with the dynamical effects of grain charging although other causes are not excluded.  相似文献   

9.
We introduce a method for constructing large-scale (0.25 AU) interplanetary magnetic field lines using only solar wind velocity from well-separated appropriately located spacecraft. The technique is based on labeling the field lines at each spacecraft with their coronal connection longitudes calculated in the EQRH (extrapolated quasi-radial hypervelocity) approximation (Nolte and Roelof, 1973). Even though the EQRH approximation is most applicable to quasi-steady solar wind, we propose that it should also be satisfactorily accurate for moderately evolving conditions. For strongly evolving conditions (e.g., flare-associated plasma) we propose a straightforward correction based on the inferred coronal longitudinal velocity profile. To illustrate the multispacecraft EQRH technique, we perform a calculation in which the interplanetary field lines in a model evolving solar wind disturbance are deduced from model observations at separated spacecraft. Since the expected agreement is found, we use data from Pioneers 8 and 9 and Vela to construct field lines for an unusually quiet period (April 26–30, 1969) and for a flare-associated disturbance accompanied by a Forbush decrease (March 23–25, 1969). The deduced field lines (even though strongly distorted by the disturbance), order the onsets of the Forbush decrease at the separated spacecraft, and the interplanetary plasma and field structures correspond to equatorial structures apparent in H synoptic charts of chromospheric magnetic features.  相似文献   

10.
In recent spacecraft observations, coherent microscale structures such as electrostatic solitary waves are observed in various regions of the magnetosphere. The Geotail spacecraft observation has shown that these solitary waves are associated with high energy non-thermal electrons flowing along the magnetic field. The solitary structures are generated as a result of a long time evolution of coherent nonlinear trapping of electrons as found in bump-on-tail, bi-stream and Buneman instabilities. It is noted that these solitary waves can be generated at distant regions far away from the spacecraft locations, because these trapped electrons, or electron holes, are drifting much faster than the local thermal plasmas. Some of the solitary waves are accompanied by perpendicular electric fields indicating that two-or three-dimensional potential structures are passing by the spacecraft. Depending on the local plasma parameters, these multi-dimensional solitary structures couple with perpendicular modes such as electrostatic whistler modes and lower-hybrid modes. In a long time evolution, these perpendicular modes are dissipated via self-organization of small solitary potentials, leading to formation of one-dimensional potential troughs as observed in the deep magnetotail. The above dissipative small-scale processes are reproduced in particle simulations, and they can be used for diagnostics of electron dynamics from spacecraft observation of multi-dimensional solitary waves in various regions of the magnetosphere. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

11.
Space experiments to test the Equivalence Principle (EP) are affected by a systematic radiometer effect having the same signature as the target signal. In [PhRvD 63 (2001) 101101(R)] we have investigated this effect for the three proposed experiments currently under study by space agencies: μSCOPE, STEP and GG, setting the requirements to be met—on temperature gradients at the level of the test masses—for each experiment to reach its goal. We have now re-examined the radiometer effect in the case of μSCOPE and carried out a quantitative comparative analysis, on this issue, with the proposed heliocentric LISA mission for the detection of gravity waves. We find that, even assuming that the μSCOPE spacecraft and payload be built to meet all the challenging requirements of LISA, temperature gradients along its test masses would still make the radiometer effect larger than the target signal of an EP violation because of flying in the low geocentric orbit required for EP testing. We find no way to separate with certainty the radiometer systematic disturbance from the signal. μSCOPE is designed to fly a second accelerometer whose test masses have the same composition, in order to separate out systematic effects which—not being composition dependent like the signal—must be detected by both accelerometers. We point out that this accelerometer is in fact insensitive to the radiometer effect, just as it is to an EP violation signal, and therefore even having it onboard will not allow this disturbance to be separated out. μSCOPE is under construction and it is scheduled to fly in 2004. If it will detect a signal to the expected level, it will be impossible to establish with certainty whether it is due to the well known classical radiometer effect or else to a violation of the equivalence principle—which would invalidate General Relativity. The option to increase the rotation speed of the spacecraft (now set at about 10−3 Hz) so as to average out the temperature gradients which generate the radiometer effect, is allowed in the GG design, not in that of STEP and μSCOPE.  相似文献   

12.
应用X射线源的航天器姿态测量   总被引：1，自引：0，他引：1  
在介绍X射线源和X射线探测器、准直器的基础上,重点描述了应用X射线源测量航天器姿态参数的原理和方法。对X射线星扫描仪的结构、扫描观测X射线源方法和矩形准直器响应函数等做了介绍,较详细地论述了应用单准直器X射线星扫描仪和差分准直器X射线星扫描仪确定航天器姿态参数的测量模型,并进行了二者间的比较和分析。简单讨论了X射线星扫描仪测量建模和应用测量模型拟合航天器姿态参数的算法问题。最后,对采用编码孔径系统确定航天器姿态参数的方法做了概括性介绍。  相似文献   

13.
We show that, when a natural satellite like Titan is invisible (e.g., due to an opaque atmosphere) its planetary orbit and its mass can be determined by tracking a spacecraft in close flybys. This is an important problem in the Cassini mission to the Saturnian system, which will be greatly improved by a good astrometric model for all its main components; in particular, an accuracy of a few hundred meters for the orbit of Titan is necessary to allow a measurement of its moment of inertia. The orbit of the spacecraft is the union of elliptical arcs, joined by short hyperbolic transitions: a problem of singular perturbation theory, whose solution leads to a matching condition between the inner hyperbolic orbit and the elliptical orbital elements. Since the inner elements are given in terms of the relative position and velocity of the spacecraft, accurate Doppler measurements in both regions can provide a satisfactory determination of Titan's position and velocity, hence of its Keplerian elements. The errors in this determination are discussed on the basis of the expected Allan deviation of the Doppler method; it is found that the driving errors are those in the elliptical arcs; the fractional errors in Titan's orbital elements are expected to be 10–7. It is also possible to measure the mass of the satellite; however, when the eccentricity e of the flybys is large, the mass and a scaling transformation are highly correlated and the fractional error in the mass is expected to be e times worse.  相似文献   

14.
The Cassini spacecraft will arrive at Saturn in 2004 carrying the Huygens probe. The beginning of the Cassini tour at Saturn has been redesigned to achieve a different relative orbiter/probe geometry in order to compensate for the probe relay receiver design flaw that was discovered during tests in February 2000. This paper presents a numerical simulation of the Huygens atmospheric entry and descent trajectory and the Cassini flyby trajectory during the probe mission. A variety of parameters that are crucial for the probe system and its scientific payload have been calculated and analyzed together with an assessment of their uncertainties. Furthermore the orbiter/probe relay link was simulated in order to assess any potential data loss on the basis of an analytical model of the actual Huygens receiver onboard the Cassini spacecraft. The redesigned Cassini/Huygens mission satisfies all science and engineering requirements and assures the best possible radio link for the entire nominal mission duration.  相似文献   

15.
The American Lunar Reconnaissance Orbiter spacecraft acquired high-resolution images of the landing sites of the Apollo manned spaceships and the Luna automatic space probes. In the images taken with the LROC Narrow-Angle Camera, the traces of anthropogenic influence on the lunar surface are seen in these places. However, such traces are not always noticeable sufficiently well, since they are masked by inhomogeneities in the brightness of the examined surface region caused by its topographic features and albedo variations. To increase the potential of identifying the disturbances of the initial structure of the lunar surface, the data should be analyzed with so-called phase-ratio imaging. Its essence is that the ratio of two coinciding images of the same surface region obtained at different phase angles is calculated. This method was applied to the analysis of the landing site of the Soviet Luna-17 space probe that transported the Lunokhod-1 rover to the lunar surface. The structural disturbance caused by the impact of jet flows from the probe’s engines and the tracks of the Lunokhod-1 wheels, which are faintly discernible in the usual images, has been detected.  相似文献   

16.
A double probe instrument with a probe separation of 42 m was used aboard the geostationary spacecraft GEOS-2 to measure the ambient electric field. The probes contained built-in pre-amplifiers and were current biased in order to clamp their surface potential near to the local plasma potential while the spacecraft surfaces floated at a more positive potential. Depending on the plasma density, the potential difference ranges between 1 V (dense plasma, Ne > 10cm−3) and more than 10 V (tenuous plasma, Ne < 3 cm−3). In the case of the more tenuous plasma, photoelectrons emitted from the probes tend to be picked up by the more positive potential of the wire booms. The net current of this electron flow depends on the spin orientation of the wire booms with respect to the Sun. It occurrs in the data as a spurious signal and was already earlier found to be a function of the spacecraft potential. With data from quiet magnetospheric conditions (Kp ⩽ 2) the long-term variation of the spurious signal was investigated. It turned out to be constant during the initial 3 y of the mission. Afterwards, as from 1981, it became smaller and did so for the following 2 y. In 1983, the end of the 5-y period of available data, it had vanished. The cause of the drop is suggested to be a consequence of the partial removal of the conductive surface layer of the wire booms by sputter effects. Expressions were derived that yield more accurate correction of the data. A valuable by-product of this work is the statistical relation of spacecraft potential and total electron flux.  相似文献   

17.
We have examined a sizeable set of flux transfer event (FTE) signatures observed simultaneously by the ISEE 1 and ISEE 2 spacecraft when they were separated by large distances, of order a few thousand kilometres. In this paper we describe events in which simultaneous signatures were observed when the spacecraft were on opposite sides of the magnetopause. We call such events “two-regime” events. Sometimes clear events at one spacecraft are associated with weak perturbations at the other which do not satisfy the criteria used hitherto for FTEs. The data presented here are consistent with the interpretation that the perturbations are caused by a single reconnected flux tube which causes a disturbance on both sides of the magnetopause.  相似文献   

18.
The paper analyzes an experiment in an orbiting laboratory to determine the gravitational constantG. A massive sphere, according to a suggestion of L. S. Wilk, is to have three tunnels drilled through it along mutually perpendicular diameters. The sphere either floats in the orbiting laboratory, with its center held fixed by means of external jets issuing from the spacecraft, or is tethered to the spacecraft. In either case it is free to rotate; in the second case this freedom would be achieved by a system of gimbals.Each tunnel contains a small test object, which is held on the tunnel's axis by means of a suspension system, perhaps electrostatic, and held at rest relative to the sphere by slowly rotating the latter by means of inertia reaction wheels, governed by a servomechanism. Fundamentally, one balances the gravitational forces on the test objects by centrifugal force, determines the latter by measuring the components of angular velocity, and calculatesG from the resulting balance. It is better to use three tunnels than one because their use minimizes the effects of the Earth's gravity-gradient.Many other measurements and corrections are required. The latter arise from Earth gravity-gradient, aerodynamic drag (with the tethered sphere), gravitational forces produced by the spacecraft itself, and the force reductions produced by the empty space in all three tunnels. After the consideration of these effects there is a presentation and discussion of the equations required to reduce the observations to obtainG. There then follow the extra equations, not needed in the reduction, that are required for a computer simulation to investigate the possible extraction of a test object and to aid in designing the servomechanisms.In Appendix B, I have devised another version of the experiment, in which the sphere is kept intact, but has short thin hollow vestigial tunnels attached to the outside of the sphere, along perpendicular diameters. These external tunnels would contain the test objects and the suspension systems. The servomechanisms would then have to prevent collision of a test object with the sphere, as well as extraction. This second method could allow for some inhomogeneities in the sphere, would require no accurate drilling, and would make the suspension systems more accessible for construction and adjustment.This paper was prepared under the sponsorship of the National Aeronautics and Space Administration through NASA Contract NAS 9-8328.  相似文献   

19.
High rate sampling detectors measuring the potential difference between the main body and boom antennas of interplanetary spacecraft have been shown to be efficient means to measure the voltage pulses induced by nano dust impacts on the spacecraft body itself (see Meyer-Vernet et al. in Sol. Phys. 256:463, 2009). However, rough estimates of the free charge liberated in post impact expanding plasma cloud indicate that the cloud’s own internal electrostatic field is too weak to account for measured pulses as the ones from the TDS instrument on the STEREO spacecraft frequently exceeding 0.1 V/m. In this paper we argue that the detected pulses are not a direct measure of the potential structure of the plasma cloud, but are rather the consequence of a transitional interruption of the photoelectron return current towards the portion of the antenna located within the expanding cloud.  相似文献   

20.
It is proposed to convert the thermal motion of a plasma into electrical power: energetic electrons collected by a plate dissipate their energy into a load, and are re-injected into the medium by means of an electron source. This concept may find applications in the magnetospheres of the outer planets, but present knowledge does not allow one to assess whether the energy fluxes are sufficient for practical applications. It is therefore neccessary to perform in situ preliminary investigations with electron emitters. It is pointed out that electron sources can be simultaneously used for additional tasks: spacecraft potential clamping, plasma diagnostics and detection of electromagnetic and electrostatic waves.  相似文献   

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