Structure and fragmentation of meteoroids

This paper is a review of the present knowledge on the structure of meteoroids.A summary of the evidence concerning the common occurrence of fragmentation among both photographic and radio meteors is given first. Then, an attempt is made to examine all the present observational, theoretical and laboratory data on the luminous and ionizing efficiencies of meteors, with the aim of establishing a mass scale. This allows the computation of the bulk density of meteoroids, which, on the average, turns out to be about 0.3 g/cm³.The paramount importance of progressive fragmentation, the behavior of abrupt-beginning meteors and the low density of nearly all meteoroids (even of those of relatively large sizes) support a porous and fragile structure for most of these particles. In turn, the crumbly structure and the cometary origin confirm Whipple's theory of comets and meteor production. A critical analysis of recent papers proposing different conclusions shows that the new theories always arrive at results which do not agree with well-established observational data.     

Meteors and the abundance of interplanetary matter

Estimates of the spatial density of interplanetary dust are derived from meteor, accretion and zodiacial cloud observations. When the most recent data are considered it is found that there is no longer any serious discrepancy between the extrapolated meteor values and those from the other sources and a density distribution is obtained which extends from meteoroids capable of producing the brightest optical meteors to particles approaching the limiting size beyond which they are removed from the solar system by solar radiation pressure. Impacts on rocket and satellite vehicles lead to much higher estimates of spatial densities and it is concluded that they originate from particles in geocentric orbits belonging to a dust cloud encompassing the earth. The evidence tends to support the view that these particles are captured from the interplanetary dust cloud rather than being produced, as suggested by Whipple, through the impact of meteorites on the moon.Some suggestions are made for the direction of future rocket and satellite investigations.Contribution to the COPERS symposium on The Interplanetary medium, held in Paris on June 19, 1962.     

Meteoroid orbits

Numerically-speaking, the orbits of meteoroids dominate our knowledge of the orbital parameters of Earth-crossing small bodies: the meteoroid orbit database outstrips the numbers of observed Earth-crossing asteroids and comets by over two orders of magnitude. Whilst it is often imagined that small meteoroids are predominantly derived from comets through stream formation, and thus must have comet-like orbits, in fact the majority of observed meteoroid orbits are more similar to those of Apollo and Aten asteroids, with small, low-inclination orbits. In all about 69 000 meteoroid orbits are available from the IAU Meteor Data Center in Lund, Sweden, having been measured in various optical and radar observation programs based in the U.S.A., Canada, the former Soviet Union, Somalia, the Czech Republic, Japan, and Australia. Depending upon the detection method used, the original meteoroids producing the observed meteoric phenomena range in size from 100 m to 10 cm. Here the raw orbital, radiant and speed distributions are presented for the major surveys, a common format being used so that they may be intercompared such that general conclusions may be drawn, and the differences between the survey results identified. These data, collected over the past several decades, provide an important source of information on the origin and evolution of the small bodies in the solar system.     

Inversion of Meteor Rayleigh Waves on Earth and Modeling of Air Coupled Rayleigh Waves on Mars

Meteor impacts and/or meteor events generate body and surface seismic waves on the surface of a planet. When meteoroids burst in the atmosphere, they generate shock waves that subsequently convert into acoustic waves in the atmosphere and seismic waves in the ground. This effect can be modeled as the amplitude of Rayleigh and other Spheroidal modes excitation, due to atmospheric/ground coupling effects.First, an inversion of the seismic source of Chelyabinsk superbolide is performed. We develop an approach in order to model a line source in the atmosphere, corresponding to the consecutive generation of shock waves by the interaction with the atmosphere. The model is based on the known trajectory. We calculate the synthetic seismograms of Rayleigh waves associated with the event by the summation of normal modes of a model of the solid part and the atmosphere of the planet. Through an inversion technique based on singular value decomposition, we perform a full Rayleigh wave inversion and we provide solutions for the moment magnitude.SEIS will likely detect seismic waves generated by impacts and the later might be further located by remote sensing differential processing. In the case of Mars, we use the same method to obtain waveforms associated with impacts on the planetary surface or in low altitudes in the Martian atmosphere. We show that the contribution of the fundamental spheroidal solid mode is dominating the waveforms, compared to that of the first two overtones. We perform an amplitude comparison and we show that small impactors (diameter of 0.5 to 2 m), can be detected by the SEIS VBB seismometer of InSight mission, even in short epicentral distances, in the higher frequencies of the Rayleigh waves. We perform an analysis based on impact rate estimations and we calculate the number of detectable events of 1 meter diameter meteor impacts to be 6.7 to 13.4 per 1 Martian year for a \(Q=500\).     

The observational data base on the motion and evolution of comets and asteroids

Current observational data base on the motion of comets and asteroids is reviewed. Particular attention is paid to the absolute and relative abundances of different dynamical types of objects, and to the time intervals between their first and last observations. The latter quantity, ranging from two days to two milliennia for individual objects, is the dominant measure of the accuracy of the orbit determination. Distribution of the tracking times of comets (distinguished by dynamical age: new, long-period, Halley type, Jupiter family) and asteroids (distinguished by stability: Apollos, Amors, main-belt asteroids, outer librators, outer unstable objects) are reconstructed. The peculiar shapes of individual distributions can be explained by the complex mechanisms of discoveries, rediscoveries, orbit computations, follow-up observations and backward identifications. A comparison is also made with the dynamical data base on meteoroids, as regards the accuracy of their orbits.The cumulative tracking times (170000 yr for all 7600 objects with known orbits taken together) are compared with the lifetimes and occurrence rates of different events of evolutionary significance. Only in the case of short-period comets the evolution is rapid enough to render observable a variety of important changes, ranging from drastic transformations of orbits to disruption or total outgassing. For asteroids, only minor cratering collisions which do not result in detectable changes of their orbits are covered by the whole observational history.Expected future improvements of observing and data-handling techniques are outlined. With these in view, the size and character of the data to become available by the end of this century are predicted. Dynamical types of objects, which are currently known in only one or a few examples, are pointed out. Apparently, other types of rare occurrence and short survival time still escape detection. A list of easiest targets of short-duration spacecraft missions is presented.The deficiencies of current statistics due to observational selection; the broad variety of regimes of motion occupied by widely differing proportional representations of the known objects; and demands for suitable targets of future spacecraft missions make it highly desirable to maintain the present rapid rate of augmentation of the data base for the years to come.Recent passages of two comets — 1983d IRAS-Araki-Alcock and 1983e Sugano-Saigusa-Fujikawa — near the Earth indicate that both the collision rate given in Table VIII and the contribution of long-period comets to it may have been slightly underestimated. The appropriate adjustment of the log-t values by less than — 0.10 has no effect of the general conclusions, however.The success of the orbiting observatory IRAS in detecting faint interplanetary objects lends better promises for the increase of the number of known objects (in particular comets) than anticipated in Section 6 and estimated in Table IX. Obviously, the outcome will largely depend on the implementation, time coverage and degree of exploitation of similar projects in the near future.     

Aeronomy of Extra-Solar Giant Planets

The intense stellar UV radiation field incident upon extra-solar giant planets causes profound changes to their upper atmospheres. Upper atmospheric temperatures can be tens of thousands of kelvins, causing thermal dissociation of H₂ to H. The stellar ionizing flux converts H to H⁺. The high temperatures also drive large escape rates of H, but for all but the planets with the smallest orbits, this flux is not large enough to affect planet evolution. The escape rate is large enough to drag off heavier atoms such as C and O. For very small orbits, when the hill sphere is inside the atmosphere, escape is unfettered and can affect planet evolution.     

Heating Diagnostics with MHD Waves

The heating of the solar atmosphere is a fundamental problem of modern solar and astrophysics. A review of the seismological aspects of magnetohydrodynamic (MHD) waves with an emphasis on standing longitudinal waves in the context of coronal heating is presented. Efforts made recently may be split into two categories: forward modelling and data inversion. Forward modelling can be applied to predict the observational footprints of various heating scenarios. A new diagnostic method based on the analysis of Doppler shift time series is outlined with specific application to solar coronal conditions. The power of the method is demonstrated and tested using synthetic data and comparing them with actual high-resolution (e.g. SoHO/SUMER) observations. Further, related recent examples of standing longitudinal oscillations in coronal loop structures observed with the new Hinode/EIS instrument are also presented. These latter observations provide an advanced ground for MHD seismology as a tool for plasma heating diagnostics in the atmosphere of the Sun.     

Magnetic Flux Ropes in the Martian Atmosphere: Global Characteristics

We report observations of magnetic fields amplitude, which consist of a series of individual spikes in the Martian atmosphere. A minimum variance analysis shows that these spikes form twisted cylindrical filaments. These small diameter magnetic filaments are commonly called magnetic flux ropes. We examine the global characteristics of magnetic flux ropes, which are observed on 5% of the elliptical orbits of Mars Global Surveyor. Flux ropes are more often observed in Venus' atmosphere (70% of the orbits). In this paper we report some of the global characteristics of the flux ropes identified in the Martian atmosphere. No flux ropes are observed in the southern hemisphere of Mars. Most of them occur at high solar zenith angles, close to the terminator plane, and at high latitude with altitudes below 400 km. The orientation of the flux ropes appears random while in the case of Venus the orientation is more horizontal near the terminator for altitudes greater than 200 km. We have identified fewer flux ropes for SZA between 40 to 60 deg and for SZA lower than 20 deg, like in the case of Venus (Elphic and Russell, 1983b). Statistically, Mars' ionosphere with SZA range between 40^circ to 60^circ is less magnetized than near the subsolar point. As the Martian ionosphere is quite often magnetized by the magnetic components of the crustal field, this crustal magnetic field seems to inhibit the flux ropes formation in the southern hemisphere. However, some orbits without crustal magnetic field, called magnetic cavities, were observed without flux ropes. So the flux ropes formation process seems to be uppressed by another factor, like the solar wind dynamic pressure for Venus (Krymskii and Breus, 1988).     

Determination des orbites des satellites artificiels

Summary The problem of the determination of the orbit of a celestial body is an old astronomical problem, of which solutions can be found in many classical books on elementary celestial mechanics. However, the introduction of new (radioelectric) means of observations for the artificial satellites have brought up new solutions of this old problem.The author reminds the definitions of the six elliptic elements of an orbit. The problem of their determination is usually divided into two separate steps: a preliminary orbit determination and the improvement of the preliminary elements. Two principal types of preliminary orbit determination exist: the Gaussian type, purely geometrical in which the positions of the body at two different times are determined, and the Laplacian type, of a more dynamical character, in which both position and velocity vectors are found for a given time. The improvement of the preliminary elements is usually obtained by a numerical solution of equations of variations of the elements, minimizing the sum of the squares of the angular distances between the computed and the observed points. The elements on which these variations are applied can be quite various. The coefficients of the equations are usually obtained numerically, although their analytical expressions can also be derived.Most of the modification proposed to the classical methods of orbit determination are more technical improvements in connection with the computation on electronic machines. The most interesting modifications are inspired by the fact that time measurements are less precise than the position (Batrakov, Iszak), the effects of this difference in errors are discussed in this paper. In some cases, using Laplace's method, no preliminary orbit is computed, and all the observations can be used at once (Barlier, Kovalevsky).The determination of orbits from radar measurements is discussed. The proposed methods are quite different in principle from the classical ones. They are actually improvements of a circular orbit whose determination is made by a method of undetermined coefficients (Baker). Some of the elements determined by these methods are quite unstable, and it is always wiser to use other informations together with radar measurements.The determination of orbits from Doppler data alone has been worked out by many investigators. It is the inverse problem of the main problem of all navigational systems. The method proposed by Patton is summarized. The principles of others are quite analogous. The results are improved when more than one receiver are used. The different types of corrections: refraction, perturbations, etc., should be introduced in later stages of the determination, and the whole frequency curve is to be used if a good determination of all elements is desired.Finally, the author quotes a method (Baker) using both Doppler and radar data, and the solution of a new problem: the determination of the orbit of a satellite of another planet from Doppler data alone (Deutsch).     

Solid component of interplanetary matter from vehicle observations

This paper contains the data of meteoric particle investigations by means of piezoelectric detectors.The data obtained on the different Soviet vehicles at distances from about 100 km to about 100000000 km from the earth are given.Measurements have demonstrated that apart from fluxes there are individual aggregations of meteoric particles with very unequal spatial density of particles in them. Linear dimensions varied within wide limits.A concept of the mass spectrum of meteoric particles is presented.Very high concentrations of minute dust particles were observed near the earth at altitudes from 100–200 km by means of ground-based methods and rocket measurements.The increased density of interplanetary matter in the vicinity of the moon during April and May 1966 was recorded by means of satellite Luna-10. By means of Luna-12 it could not be discovered whether this aggregation is the moon halo or is the result of the moon passage through some dust particle aggregation, because the investigation on satellite Luna-12 was carried out mostly during meteor showers.     

一种有效提高超大偏心率轨道初轨确定精度的新方法

针对某型号航天器的运行轨道偏心率大、轨控结束后可用于初轨确定的测轨数据少的特点,提出了一种能有效提高超大偏心率短弧段初轨确定精度的新方法。该新方法将轨道改进的思想融入初轨计算方法中,使之既适用于各种偏心率轨道的初轨计算,也适用于各种偏心率轨道的长弧段、多圈、多站数据的轨道改进;解决了传统初轨确定方法超大偏心率轨道短弧段初轨确定误差偏大的问题。通过任务实测数据的检验,新方法适用于各种偏心率轨道,并且超大偏心率轨道的初轨确定精度明显优于传统方法。     

空间电离辐射对镧系光学玻璃透射率的影响

利用60Coγ射线对镧火石玻璃和镧冕玻璃进行辐照,研究不同辐照剂量对光学透射率的影响及其在空间光学系统中使用的适应性,辐照总剂量最大达到10 kGy。结果表明:所有玻璃在辐照后可见光透射率都下降了,而在近红外波段仍然保持较高的透射率。尽管镧火石玻璃LaF10平均透射率在辐照前最小,但是辐照后LaF10透射率衰减是所有玻璃中最小的。模拟了8个不同轨道高度地球辐射环境10年累积的总剂量以及经过10 mm铝屏蔽后的累积总剂量。发现对于10年任务期,在3 000、6 000和10 000 km轨道需要增加屏蔽层厚度,而在其他5个轨道,10 mm厚的铝屏蔽可以保证镧系玻璃满足系统对透射率的要求。     

Aiming at a 1-cm Orbit for Low Earth Orbiters: Reduced-Dynamic and Kinematic Precise Orbit Determination

The computation of high-accuracy orbits is a prerequisite for the success of Low Earth Orbiter (LEO) missions such as CHAMP, GRACE and GOCE. The mission objectives of these satellites cannot be reached without computing orbits with an accuracy at the few cm level. Such a level of accuracy might be achieved with the techniques of reduced-dynamic and kinematic precise orbit determination (POD) assuming continuous Satellite-to-Satellite Tracking (SST) by the Global Positioning System (GPS). Both techniques have reached a high level of maturity and have been successfully applied to missions in the past, for example to TOPEX/POSEIDON (T/P), leading to (sub-)decimeter orbit accuracy. New LEO gravity missions are (to be) equipped with advanced GPS receivers promising to provide very high quality SST observations thereby opening the possibility for computing cm-level accuracy orbits. The computation of orbits at this accuracy level does not only require high-quality GPS receivers, but also advanced and demanding observation preprocessing and correction algorithms. Moreover, sophisticated parameter estimation schemes need to be adapted and extended to allow the computation of such orbits. Finally, reliable methods need to be employed for assessing the orbit quality and providing feedback to the different processing steps in the orbit computation process. This revised version was published online in August 2006 with corrections to the Cover Date.     

2001 Mars Odyssey Mission Summary

The 2001 Mars Odyssey spacecraft, now in orbit at Mars, will observe the Martian surface at infrared and visible wavelengths to determine surface mineralogy and morphology, acquire global gamma ray and neutron observations for a full Martian year, and study the Mars radiation environment from orbit. The science objectives of this mission are to: (1) globally map the elemental composition of the surface, (2) determine the abundance of hydrogen in the shallow subsurface, (3) acquire high spatial and spectral resolution images of the surface mineralogy, (4) provide information on the morphology of the surface, and (5) characterize the Martian near-space radiation environment as related to radiation-induced risk to human explorers. To accomplish these objectives, the 2001 Mars Odyssey science payload includes a Gamma Ray Spectrometer (GRS), a multi-spectral Thermal Emission Imaging System (THEMIS), and a radiation detector, the Martian Radiation Environment Experiment (MARIE). THEMIS and MARIE are mounted on the spacecraft with THEMIS pointed at nadir. GRS is a suite of three instruments: a Gamma Subsystem (GSS), a Neutron Spectrometer (NS) and a High-Energy Neutron Detector (HEND). The HEND and NS instruments are mounted on the spacecraft body while the GSS is on a 6-m boom. Some science data were collected during the cruise and aerobraking phases of the mission before the prime mission started. THEMIS acquired infrared and visible images of the Earth-Moon system and of the southern hemisphere of Mars. MARIE monitored the radiation environment during cruise. The GRS collected calibration data during cruise and aerobraking. Early GRS observations in Mars orbit indicated a hydrogen-rich layer in the upper meter of the subsurface in the Southern Hemisphere. Also, atmospheric densities, scale heights, temperatures, and pressures were observed by spacecraft accelerometers during aerobraking as the spacecraft skimmed the upper portions of the Martian atmosphere. This provided the first in-situ evidence of winter polar warming in the Mars upper atmosphere. The prime mission for 2001 Mars Odyssey began in February 2002 and will continue until August 2004. During this prime mission, the 2001 Mars Odyssey spacecraft will also provide radio relays for the National Aeronautics and Space Administration (NASA) and European landers in early 2004. Science data from 2001 Mars Odyssey instruments will be provided to the science community via NASA’s Planetary Data System (PDS). The first PDS release of Odyssey data was in October 2002; subsequent releases occur every 3 months.     

I: PRECISE ORBIT DETERMINATION AND GRAVITY FIELD MODELLING: Strategies for Precise Orbit Determination of Low Earth Orbiters Using the GPS

Considerable experience accumulated during the past decade in strategies for processing GPS data from ground-based geodetic receivers. First experience on the use of GPS observations from spaceborne receivers for orbit determination of satellites on low altitude orbits was gained with the launch of TOPEX/POSEIDON ten years ago. The launch of the CHAMP satellite in July 2000 stimulated a number of activities worldwide on improving the strategies and algorithms for orbit determination for Low Earth Orbiters (LEOs) using the GPS. Similar strategies as for ground-based receivers are applied to data from spaceborne GPS receivers to determine high precision orbits. Zero- and double-differencing techniques are applied to obtain kinematic and/or reduced-dynamic orbits with an accuracy which is today at the decimeter level. Further developments in modeling and processing strategies will continuously improve the quality of GPS-derived LEO orbits in the near future. A significant improvement can be expected from fixing double-difference phase ambiguities to integer numbers. Particular studies focus on the impact of a combined processing of LEO and GPS orbits on the quality of orbits and the reference frame realization. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Towards a Global Unified Model of Europa’s Tenuous Atmosphere

Despite the numerous modeling efforts of the past, our knowledge on the radiation-induced physical and chemical processes in Europa’s tenuous atmosphere and on the exchange of material between the moon’s surface and Jupiter’s magnetosphere remains limited. In lack of an adequate number of in situ observations, the existence of a wide variety of models based on different scenarios and considerations has resulted in a fragmentary understanding of the interactions of the magnetospheric ion population with both the moon’s icy surface and neutral gas envelope. Models show large discrepancy in the source and loss rates of the different constituents as well as in the determination of the spatial distribution of the atmosphere and its variation with time. The existence of several models based on very different approaches highlights the need of a detailed comparison among them with the final goal of developing a unified model of Europa’s tenuous atmosphere. The availability to the science community of such a model could be of particular interest in view of the planning of the future mission observations (e.g., ESA’s JUpiter ICy moons Explorer (JUICE) mission, and NASA’s Europa Clipper mission). We review the existing models of Europa’s tenuous atmosphere and discuss each of their derived characteristics of the neutral environment. We also discuss discrepancies among different models and the assumptions of the plasma environment in the vicinity of Europa. A summary of the existing observations of both the neutral and the plasma environments at Europa is also presented. The characteristics of a global unified model of the tenuous atmosphere are, then, discussed. Finally, we identify needed future experimental work in laboratories and propose some suitable observation strategies for upcoming missions.     

Solar Fine-Scale Structures. I. Spicules and Other Small-Scale, Jet-Like Events at the Chromospheric Level: Observations and Physical Parameters

Over the last two decades the uninterrupted, high resolution observations of the Sun, from the excellent range of telescopes aboard many spacecraft complemented with observations from sophisticated ground-based telescopes have opened up a new world producing significantly more complete information on the physical conditions of the solar atmosphere than before. The interface between the lower solar atmosphere where energy is generated by subsurface convection and the corona comprises the chromosphere, which is dominated by jet-like, dynamic structures, called mottles when found in quiet regions, fibrils when found in active regions and spicules when observed at the solar limb. Recently, space observations with Hinode have led to the suggestion that there should exist two different types of spicules called Type?I and Type?II which have different properties. Ground-based observations in the Ca?ii H and K filtergrams reveal the existence of long, thin emission features called straws in observations close to the limb, and a class of short-lived events called rapid blue-shifted excursions characterized by large Doppler shifts that appear only in the blue wing of the Ca?ii infrared line. It has been suggested that the key to understanding how the solar plasma is accelerated and heated may well be found in the studies of these jet-like, dynamic events. However, while these structures are observed and studied for more than 130 years in the visible, but also in the UV and EUV emission lines and continua, there are still many questions to be answered. Thus, despite their importance and a multitude of observations performed and theoretical models proposed, questions regarding their origin, how they are formed, their physical parameters, their association with the underlying photospheric magnetic field, how they appear in the different spectral lines, and the interrelationship between structures observed in quiet and active regions on the disk and at the limb, as well as their role in global processes has not yet received definitive answers. In addition, how they affect the coronal heating and solar wind need to be further explored. In this review we present observations and physical properties of small-scale jet-like chromospheric events observed in active and quiet regions, on the disk and at the limb and discuss their interrelationship.     

Neutron measurements in space

The experimental measurements of the neutron flux and energy spectrum in space since 1964 are reviewed and related to the theoretical predictions. A discussion of the neutron sources is presented. The difficulties associated with neutron measurements of both the atmospheric neutron leakage flux and solar neutrons are included. Particular emphasis is placed upon the neutron leakage flux and energy measurements at energies greater than about 1 MeV. The possibilities of CRAND as a source for the energetic trapped protons are discussed in light of recent measurements of the 10–100 MeV neutron flux. The current status of the solar neutron flux observations is also presented.The primary purposes of neutron measurements in space have been to determine the neutron leakage flux from the atmosphere of the Earth and the solar neutron flux. As a consequence of the inefficient methods for neutron detection and the difficulties of conducting the measurements in the presence of the galactic and solar cosmic-ray backgrounds, the experimental results are very conflicting. It is the purpose of this review to interpret and discuss recent neutron measurements. In order to understand these results the theoretical predictions of the neutron fluxes and energy spectra from possible neutron sources will be briefly presented. Since comparisons of the different neutron measurements depend critically upon the experimental techniques, we will briefly discuss neutron detection methods applicable to space measurements. The emphasis will be upon measurements since 1964 made outside the Earth's atmosphere, but considerable reference will be made to high energy neutron experiments conducted within the Earth's atmosphere at < 10g cm^-2 altitude. A review of earlier neutron measurements of terrestrial and solar neutrons has been made by Haymes (1965).