Observations of the Galilean moons of Jupiter in 2013–2015 at Pulkovo

Observational results are presented for Jupiter and its Galilean moons from the Normal Astrograph at Pulkovo Observatory in 2013–2015. The following data are obtained: 154 positions of the Galilean satellites and 47 calculated positions of Jupiter in the system of the UCAC4 (ICRS, J2000.0) catalogue; the differential coordinates of the satellites relative to one another are determined. The mean errors of the satellites’ normal places in right ascension and declination over the entire observational period are, respectively: ε_α = 0.0065″ and ε_δ = 0.0068″, and their standard deviations are σ_α = 0.0804″ and σ_δ = 0.0845″. The equatorial coordinates are compared with planetary and satellite motion theories. The average (O–C) residuals in the two coordinates relative to the motion theories are 0.05″ or less. The best agreement with the observations is achieved by a combination of the EPM2011m and V. Lainey-V.2.0|V1.1 motion theories; the average (O–C) residuals are 0.03″ or less. The (O–C) residuals for the features of the positions of Io and Ganymede are comparable with measurement errors. Jupiter’s positions calculated from the observations of the satellites and their theoretical jovicentric coordinates are in good agreement with the motion theories. The (О–С) residuals for Jupiter’s coordinates are, on average, 0.027″ and–0.025″ in the two coordinates.     

Astrometric Observations of the Galilean Moons of Jupiter at the Pulkovo Normal Astrograph in 2016−2017

We present the results of observations of the Galilean moons of Jupiter carried out at the Normal Astrograph of the Pulkovo Observatory in 2016?2017. We obtained 761 positions of the Galilean moons of Jupiter in the system of the Gaia DR1 catalog (ICRF, J2000.0) and 854 differential coordinates of the satellites relative to each other. The mean errors in the satellites’ normal places and the corresponding root-mean-square deviations are ε_α = 0.0020′′, ε_δ = 0.0027′′, σ_α = 0.0546′′, and σ_δ = 0.0757′′. The equatorial coordinates of the moons are compared to the motion theories of planets and satellites. On average, the (O–C) residuals in the both coordinates relative to the motion theories are less than 0.031′′. The best agreement with observations is achieved by a combination of the EPM2015 and V. Lainey-V.2.0|V1.1 motion theories, which yields the average (O–C) residuals of approximately 0.02″. Peculiarities in the behavior of the (O–C) residuals and error values in Ganymede have been noticed.     

Photographic observations of Saturn’s moons at the MAO NAS of Ukraine in 1961–1990

A catalog of 1385 astrometric positions of Saturn’s moons S2–S9 has been compiled with Tycho-2 as a reference frame from photographic observations obtained at the Main Astronomical Observatory, National Academy of Sciences of Ukraine, in 1961–1990. Astronegatives have been digitized with an Epson Expression 10000XL commercial scanner in 16-bit grayscale with a resolution of 1200 dpi. Reduction has been performed in the LINUX-MIDAS-ROMAFOT software supplemented with additional modules. The internal positional accuracy of the reduction is 0.09…0.23′′ for both coordinates and 0.27…0.37^m for the photographic magnitudes of the Tycho-2 catalog. The calculated topocentric positions of the moons are compared online with the IMCCE ephemeris data (DE405 + TASS1,7). Moon-minus-moon differential coordinates are found for most of the moons and compared with theoretical data (http://lnfm1.sai.msu.ru/neb/nss/nssephmr.htm).     

Accurate Mars Express orbits to improve the determination of the mass and ephemeris of the Martian moons

The determination of the ephemeris of the Martian moons has benefited from observations of their plane-of-sky positions derived from images taken by cameras onboard spacecraft orbiting Mars. Images obtained by the Super Resolution Camera (SRC) onboard Mars Express (MEX) have been used to derive moon positions relative to Mars on the basis of a fit of a complete dynamical model of their motion around Mars. Since, these positions are computed from the relative position of the spacecraft when the images are taken, those positions need to be known as accurately as possible. An accurate MEX orbit is obtained by fitting two years of tracking data of the Mars Express Radio Science (MaRS) experiment onboard MEX. The average accuracy of the orbits has been estimated to be around 20–25 m. From these orbits, we have re-derived the positions of Phobos and Deimos at the epoch of the SRC observations and compared them with the positions derived by using the MEX orbits provided by the ESOC navigation team. After fit of the orbital model of Phobos and Deimos, the gain in precision in the Phobos position is roughly 30 m, corresponding to the estimated gain of accuracy of the MEX orbits. A new solution of the GM of the Martian moons has also been obtained from the accurate MEX orbits, which is consistent with previous solutions and, for Phobos, is more precise than the solution from the Mars Global Surveyor (MGS) and Mars Odyssey (ODY) tracking data. It will be further improved with data from MEX-Phobos closer encounters (at a distance less than 300 km). This study also demonstrates the advantage of combining observations of the moon positions from a spacecraft and from the Earth to assess the real accuracy of the spacecraft orbit. In turn, the natural satellite ephemerides can be improved and participate to a better knowledge of the origin and evolution of the Martian moons.     

Astrometric studies of the series of photographic and CCD observations of the Saturnian system with the 26-inch Pulkovo refractor in 1995-2007

The results of the reduction, investigation, and comparison of the photographic observations of the major Saturnian satellites and CCD observations with an ST6 CCD camera obtained at the 264nch Pulkovo refractor in 1995–2007 are presented. A comparison of the observational results with the TASS 1.7 theory of motion of the Saturnian satellites has served as the basis for investigating and comparing the series of observations. The period-averaged (O-C) residuals and observational errors have been calculated. A comparison of the series of CCD and photographic observations has shown the same external accuracy of the observations at a higher internal accuracy of the CCD observations than that of the photographic ones. A comparison of the Pulkovo results with those of other authors has shown them to be close in accuracy. The accuracy of the theory has been estimated by comparing simultaneous (on the same night) CCD and photographic observations. The errors of the observations and the theory have been found from this comparison to be the following: 0.081“ and 0.067” for the observations and 0.077“ and 0.115” for the theory (inxandy, respectively). An analysis of the dependence of (O-C)x,y for three satellites (the sixth, seventh, and eighth) on the satellite positions in Saturn-centered orbits has revealed systemat ic deviations for the seventh satellite in both coordinates. The positions of Saturn have been determined from satellite observations without measuring its images on photographic plates with accuracies of 0.121“ and 0.105” in right ascension and declination, respectively.     

Atronomical CCD observations of the main Saturn’s satellites at Pulkovo Observatory in 2004–2007

The results of astrometric observations of Saturn’s satellites (S1–S8) obtained using a 26-inch refractor and a normal astrograph at Pulkovo Observatory in 2004–2007 are given. High-accuracy equatorial coordinates of Saturn’s satellites in the system of the UCAC2 reference catalog and the relative “satellite-satellite” positions suitable for specifying their motion theories are obtained. The observations are compared with the DE405 + TASS1.7 and INPOP06 + TASS1.7 theories of motion. The root-mean-square errors of the obtained satellite positions lie within the range of 10–50 mas, as far as the intrinsic convergence is concerned, and 20–70 mas, as far as the extrinsic one is concerned. The observation results are included into the astrometrical database of the Pulkovo Observatory (www.puldb.ru).     

Method for determination of the celestial coordinates of the center of the Earth

A method for determining the celestial coordinates of the center of the disk of the Earth from satellite observations with an accuracy of 1–5 arcseconds is suggested. The choice of the most suitable wavelength range is substantiated.     

Astrometric observations of major satellites of Saturn with a 26-inch refractor

The results of observations of Saturn and its satellites with the 26-inch refractor at Pulkovo are presented. Over the observing period from January 2008 until May 2009, results were found from more than 5000 CCD frames suitable for measurement. On the basis of these frames, 183 positions of major satellites of Saturn (with the exception of Mimas) were obtained. The astrometric reduction was based on the Turner method, with the use of the UCAC2 catalog as a reference. The obtained equatorial coordinates of satellites were compared with the TASS 1.7 theory, and results of comparison are presented. The accuracy of observed positions is 0.05″ on average. Positions of Saturn, calculated on the basis of positions of satellites and their theoretical saturnocentric coordinates according to the TASS 1.7, and the differential coordinates of satellites relative to each other, are also given.     

The accuracy of observations of the Galilean satellites of Jupiter taken at the Abastumani Astrophysical Observatory of the Academy of Sciences of Georgia

The sets of photographic observations of the Galilean satellites of Jupiter taken at the Abastumani Astrophysical Observatory of the Academy of Sciences of Georgia are analyzed here. Positional observations of the system of Jupiter were made in the period from 1985 to 1994 with the use of the double Zeiss astrograph in order to determine the exact coordinates of Jupiter and its satellites. The accurate positions of the satellites and Jupiter itself, as well as their stellar (equatorial) coordinates relative to the stars of the currently available catalogs and the relative ??satellite ?? satellite?? coordinates were obtained from the observations. From the comparison of the observation results with the modern theories of motion of satellites, the accuracy in determining the positions of the satellites and Jupiter was analyzed. The results of observations are presented in the Pulkovo database of observations of Solar System bodies that is accessible to users at http://www.puldb.ru.     

The disruption of planetary satellites and the creation of planetary rings

The Voyager spacecraft discovered that small moons orbit within all four observed ring systems coincident with the discovery of narrow and dusty rings around Jupiter, Saturn, Uranus and Neptune. These moons can provide the source for new rings if they are catastrophically disrupted by a comet or large meteoroid impact. This hypothesis for ring origins provides a natural mechanism for the ongoing creation of planetary rings. While it relieves somewhat the problem of explaining the continued existence of rings with apparently short evolutionary lifetimes, it raises the problem of explaining the continued existence of small moons, and the coexistence of moons and rings at comparable locations within the Roche zones of the giant planets. This problem has been studied in some detail recently, and the present work is a review of our current understanding of the processes in satellite disruption that pertain to the creation of planetary rings and the collisional cascade of circumplanetary bodies. Significant progress has been made. Narrow rings are produced by disruption of small moons in numerical simulations, and a self-consistent model of the collisional cascade can explain present-day moon populations. Absolute timescales and initial moon populations remain uncertain due to our poor knowledge of the impactor population and uncertainties in the strength of planetary satellites. More pressing are the qualitative issues that remain to be resolved including the nature of reaccretion of the debris and the origin of Saturn's rings.     

Analytical theory of motion of phoebe,the ninth satellite of saturn

The literal solution of the restricted three body problem obtained by the authors up to the eleventh order with respect to the minor parameter is applied to the investigation of the motion of Phoebe, the ninth satellite of Saturn. As distinct from the existing analytical theories of the motion of the satellite, in the present paper the planetary perturbations are taken into account. A comparison with the modern numerical theory of the motion of Phoebe has shown that the new analytical theory of the satellite motion represents observations with the same degree of accuracy.     

用太阳系主要天体的位置建立特大地震的时间预测模型

根据某类事件发生时太阳系主要运动天体的位置，通过对其历史资料的归算，尝试并设计出一种具有普适性的事件时间预测模型．以1900-1980年间全球所有8级及以上的99次地震为例，定义与天体特征位置(视黄经)相关的发震率，初步建立预测发震时刻的概率曲线．指出这可能成为研究地震预报问题的新途径．     

Astrometry of Iapetus, Ariel, Umbriel, and Titania from eclipses and occultations

Highly accurate astrometric positions obtained from eclipses and occultations of planetary satellites are reported. These measurements may be used to test existing ephemerides, to improve upon them, and to fit system constants such as satellite masses and planetary zonal harmonics. Eclipse and occultation photometry of 5 uranian satellite mutual events has resulted in precise astrometry for 3 of these moons. Relative satellite positions were determined with an uncertainty of less than 10 milli-arcseconds for 4 of the events. These observations plus two additional data from C. Miller and N.J. Chanover (private communication) indicate that predictions based on the SPICE [Acton, C.H., 1996. Planet. Space Sci. 44, 65-70] ephemeris URA083 and those from the LA06 ephemeris in a paper by Arlot et al. [Arlot, J.-E., Lainey, V., Thuillot, W., 2006. Astron. Astrophys. 456, 1173-1179] are significantly more accurate than predictions generated by Christou [Christou, A.A., 2005. Icarus 178, 171-178] using the GUST86 ephemeris in the along-track component of motion. The observations indicate that Ariel, Umbriel and Titania are lagging behind their predicted positions for all of the ephemerides, but by varying distances and significance levels. Analysis of data recorded by Hidas et al. [Hidas, M.G., Christou, A.A., Brown, T.M., 2008. Mon. Not. R. Astron. Soc. 384, L38-L40] suggests a similar lag for Oberon. Photometry recorded during the ingress portion of a saturnian eclipse of Iapetus on 2007 May 5 indicates that the middle of the event occurred at geocentric UTC 02:14:58. At that moment the center of the satellite disk facing the Sun was intersected by a solar-centered ray refracted at a minimum altitude of 240 km above the 1-bar pressure level in the planet's atmosphere. The uncertainty in the timings due to observational scatter was only 5 s which equates to 16 km of Iapetus motion, but other factors increased the overall uncertainty to 111 km or 16 milli-arcseconds at the distance of Saturn from the Sun. The astrometric result is fit very well by the SPICE ephemeris SAT288.     

Current problems of dynamics of moons of planets and binary asteroids based on observations

The general approach to studying the dynamics of moons of planets and asteroids consists in developing more and more accurate models of motion based on observational data. Not only the necessary ephemerides, but also some physical parameters of planets and moons are obtained this way. It is demonstrated in the present study that progress in this field is driven not only by the increase in accuracy of observations. The accuracy of ephemerides may be increased by expanding the observation time interval. Several problems arise on the way toward this goal. Some of them become apparent only when the procedure of observational data processing and use is examined in detail. The method used to derive astrometric data by processing the results of photometric observations of mutual occultations and eclipses of planetary moons is explained below. The primary contribution to the error of astrometric results is produced by the unaccounted noise level in photometric readings and the inaccuracy of received values of the albedo of moons. It is demonstrated that the current methods do not allow one to eliminate the noise completely. Extensive additional photometric measurements should be performed at different angles of rotation of moons and in different spectral bands of the visible wavelength range in order to obtain correct values of the albedo of moons. Many new distant moons of the major planets have been discovered in the early 21st century. However, the observations of these moons are scarce and were performed over short time intervals; as a result, some of the moons were lost. The necessity of further observations of these Solar System bodies is pointed out in the present study. Insufficient knowledge of asteroid masses is an obstacle to improving the accuracy of the ephemerides of Mars. The basic method for determining the masses of large asteroids consists in analyzing their influence on the motion of Mars, the Earth, and spacecraft. The masses of more than 100 large asteroids were determined this way. One of the principal techniques for Earth-based measurement of the masses of asteroids involves astrometric observations of binary asteroids. The determination of relative coordinates is made rather difficult by the apparent proximity of components. The success of these efforts depends on the availability of instrumentation and the expertise of observers skilled in adaptive optics and speckle interferometry. Collaboration between different research teams and observers is absolutely necessary.     

A compiled catalogue of reference stars around 227 ICRF extragalactic radio sources

A compiled catalogue of 21 440 stars with magnitudes between 10 and 17 is prepared from original observations made at the end of the 20th century to the beginning of the 21st century. The catalogue contains 227 fields of the celestial sphere centered at ICRF extragalactic radio sources with declinations of ?17 to +89°. The field size is 40′ for both right ascension and declination. The internal accuracy of positions for both coordinates is no worse than 0.1″. A comparison of the stellar positions with the UCAC2 and CMC13 catalogues shows that the average external accuracy is approximately 0.05–0.15″. The positions of 10 795 stars with declinations to +50° are given for the epoch and equinox of J2000.0, whereas the positions of other stars are given for the epoch of observation.     

Astrometric observations of planetary satellites at the Abastumani Astrophysical Observatory

We present and discuss the results of the astrometry project during which we observed the satellites of Mars, Jupiter, Saturn, Uranus, and Neptune at the Abastumani Astrophysical Observatory (Georgia) between 1983 and 1994. Observations at the Abastumani Observatory were performed with the double Zeiss astrograph (DZA: D/F = 400/3024 mm) and AZT-11 telescope (F = 16 m). We processed a large array of observations and determined exact coordinates of the planets and their satellites in a system of reference stars of modern catalogues as well as relative coordinates of the satellites. The results were compared with modern ephemerides using the MULTI-SAT software. The comparison enabled us to estimate the accuracy of observations (their random and systematic uncertainties) and the accuracy of modern theories of the motion of planets and their satellites. Random uncertainties of observations are estimated to be 0.10??C0.40?? for various objects and observational conditions. Observational results obtained for Uranus, Neptune and the satellites Titania and Oberon were shown to deviate appreciably and systematically from theories of their motion. The results of observations are presented in the Pulkovo database for Solar System bodies that is available at the website http://www.puldb.ru.     

The 1966 observations of the coorbiting satellites of Saturn S10 and S11

Most of the positions of faint satellite images obtained during the 1966 Saturn ring plane crossing fit the period of the coorbital satellites 1980 S1 and 1980 S3. In 1966 the satellites were separated by 137° in orbital longitude. Until the mutual interaction of the satellites is understood and applied to derive the precise orbital motion, the 1966 and 1980 observations cannot be linked.     

X-ray probes of magnetospheric interactions with Jupiter's auroral zones, the Galilean satellites, and the Io plasma torus

Remote observations with the Chandra X-ray Observatory and the XMM-Newton Observatory have shown that the jovian system is a source of X-rays with a rich and complicated structure. The planet's polar auroral zones and its disk are both powerful sources of X-ray emission. Chandra observations revealed X-ray emission from the Io plasma torus and from the Galilean moons Io, Europa, and possibly Ganymede. The emission from the moons is due to bombardment of their surfaces by highly energetic magnetospheric protons, and oxygen and sulfur ions. These ions excite atoms in their surfaces leading to fluorescent X-ray emission lines. These lines are produced against an intense background continuum, including bremsstrahlung radiation from surface interactions of primary magnetospheric and secondary electrons. Although the X-ray emission from the Galilean moons is faint when observed from Earth orbit, an imaging X-ray spectrometer in orbit around one or more of these moons, operating from 200 eV to 8 keV with 150 eV energy resolution, would provide a detailed mapping of the elemental composition in their surfaces. Surface resolution of 40 m for small features could be achieved in a 100-km orbit around one moon while also remotely imaging surfaces of other moons and Jupiter's upper atmosphere at maximum regional resolutions of hundreds of kilometers. Due to its relatively more benign magnetospheric radiation environment, its intrinsic interest as the largest moon in the Solar System, and its mini-magnetosphere, Ganymede would be the ideal orbital location for long-term observational studies of the jovian system. Here we describe the physical processes leading to X-ray emission from the surfaces of Jupiter's moons and the properties required for the technique of imaging X-ray spectroscopy to map the elemental composition of their surfaces, as well as studies of the X-ray emission from the planet's aurora and disk and from the Io plasma torus.     

Influence of a second satellite on the rotational dynamics of an oblate moon

The gravitational influence of a second satellite on the rotation of an oblate moon is numerically examined. A simplified model, assuming the axis of rotation perpendicular to the (Keplerian) orbit plane, is derived. The differences between the two models, i.e. in the absence and presence of the second satellite, are investigated via bifurcation diagrams and by evolving compact sets of initial conditions in the phase space. It turns out that the presence of another satellite causes some trajectories, that were regular in its absence, to become chaotic. Moreover, the highly structured picture revealed by the bifurcation diagrams in dependence on the eccentricity of the oblate body’s orbit is destroyed when the gravitational influence is included, and the periodicities and critical curves are destroyed as well. For demonstrative purposes, focus is laid on parameters of the Saturn–Titan–Hyperion system, and on oblate satellites on low-eccentric orbits, i.e. \(e\approx 0.005\).     

Satellite formation,II

The satellite formation model of Harris and Kaula (Icarus24, 516–524, 1975) is extended to include evolution of planetary ring material and elliptic orbital motion. This model is more satisfactory than the previous one in that the formation of the moon begins at a later time in the growth of the earth, and that a significant fraction of the lunar material is processed through a circumterrestrial debris cloud where volatiles might have been lost. Thus the chemical differences between the earth and moon are more plausibly accounted for. Satellites of the outer planets probably formed in large numbers throughout the growth of those planets. Because of rapid inward evolution of the orbits of small satellites, the present satellite systems represent only satellites formed in the last few percent of the growths of their primaries. The rings of Saturn and Uranus are most plausibly explained as the debris of satellites disrupted within the Roche limit. Because such a ring would collapse onto the planet in the course of any significant further accretion by the planet, the rings must have formed very near or even after the conclusion of accretion.