Revised albedos of Trojan asteroids (911) Agamemnon and (4709) Ennomos

CCD‐photometry was performed for two Jupiter Trojan asteroids (911) Agamemnon and (4709) Ennomos for which the diameters were obtained from occultation events. New data on rotation periods, lightcurve amplitudes, color indices, magnitude–phase slopes, and absolute magnitudes were obtained for these asteroids. We have used the diameters from occultations (166 and 99 km) and new data on absolute magnitudes at the instant occultation (7.95 and 8.85 mag) to revise their albedos to 0.042 (911 Agamemnon) and 0.052 (4709 Ennomos).     

Lunar occultation of Saturn. I. The diameters of Tethys,Dione, Rhea,Titan, and Iapetus

The diameters of Tethys, Dione, Rhea, Titan and Iapetus were determined from observations of their March 30, 1974, lunar occultations, made with the Mauna Kea 224 and 61 cm telescopes. Light curves were obtained simultaneously in four colors, and the difference between the time of occultation at the two telescopes provided a direct measurement of the slope of the lunar limb, found to be small in all cases. The satellite diameters were determined by least-squares fits of model occultation light curves to the data. In these fits the diameter and degree of limb darkening of the satellite are correlated variables, requiring the limb darkening to be specified before the diameter can be determined, or vice versa. However, for Titan the signal-to-noise ratio is sufficiently high to allow some assessment of the amount of limb darkening, which was found to be substantial. Titan's diameter must be at least 5800 km, much larger than the currently accepted value of 5000 km, making it the largest satellite in the solar system. This larger diameter implies a low mean density. For the other four satellites arguments are presented in favor of accepting the occultation diameters corresponding to limb darkened disks. Except for Titan, the lunar occultation diameters generally agree with previous diskmeter and radiometric determinations.     

上海天文台GPS掩星技术研究现状

简要地介绍了GPS/LEO卫星无线电掩星技术的基本原理、意义以及国际上的最近动态 ,列举了几个主要的LEO卫星计划 ;重点分析了GPS掩星探测地球大气技术中需要注意的若干问题 ,并推出可能采取的改进方案和相应的技术路线。上海天文台在GPS/LEO空基气象学中已经成功地提出 :通过通约LEO/GPS轨道方法 ,实现掩星地面观测点控制的新思路。作为一个应用实例 ,针对东中国海洋上空大气状况对中国东部经济发达的地区特别是上海地区的天气有着重要影响的背景 ,提出利用通约LEO/GPS卫星可对该地区大气实施监控。为了充分利用GPS/LEO掩星观测资源 ,上海天文台开展了振幅反演的计算方法研究 ;提出了在振幅反演中存在着无线电信号几何衰减和物理衰减两种不同的衰减机制 ,并考虑了它们的数学模型以及对反演结果的影响。简要地叙述了掩星质量因子的定义和计算方法。作为国内天文地球动力学研究中心 ,上海天文台将与国内外一些合作单位考虑建立GPS/LEO掩星技术观测处理的软件系统。作为空基GPS气象学的推广 ,还考虑进行山基和飞机载掩星观测实验 ,论证用这些方法监测局部大气剖面的可行性以及它们在国民经济和科学研究上的作用     

New observations of surface markings on Jupiter's satellites

Visual and photographic observations of the Galilean satellites of Jupiter made in September 1973 with the 108 cm reflector at Pic-du-Midi Observatory are presented. A method of estimating the contrasts and albedos of surface markings on the satellites during transit by comparing them with the adjacent surface of Jupiter is described. Results for Io, Europa, and Ganymede give albedo ranges of 0.28 to 0.67, 0.45 to 0.67, and 0.22 to 0.47, respectively. These are geometric albedos for a phase angle of 5°. The percentages of the disks covered by high albedos are consistent with the conclusions of previous workers regarding the fraction of exposed water ice on the surface.     

Occultation of ε Geminorum by Mars: IV. Oblateness of the martian upper atmosphere

The oblateness of the Martian upper atmosphere was determined from analysis of photoelectric observations of the 8 April 1976 occultation of ε Geminorum by Mars at seven stations. The oblatness is 0.0096 ± 0.0023, consistent with a mean equator-to-pole temperature difference in excess of ～ 50°K, vertically averaged from the surface to the occulation altitude of ～70 km. The astrometric solution provides precise determination of the occultation path relative to the Martian shadow, and absolute vertical alignment of upper atmospheric temperature profiles obtained by inversion of occultation light curves. The observations can be compared directly with models of atmospheric tides computed for the conditions at the suboccultation regions on Mars.     

Astrometric results of observations of mutual occultations and eclipses of the Galilean satellites of Jupiter in 2002-2003

We suggest a new approach and develop an original method for deriving astrometric data from the photometry of mutual occultations and eclipses of planetary satellites. We decide to model not the relative apparent motion of one satellite with respect to another satellite but the deflection of the observed relative motion with respect to the theoretical motion implied by appropriate ephemerides.We have attempted to reduce the results of photometric observations of the Gallilean satellites during their mutual occultations and eclipses in 2002-2003. The data of observation for 319 light curves of 106 mutual events were received from the observers. The reliable 245 light curves were processed with our method. Eighty six apparent relative positions have been obtained.Systematic errors arise inevitably while deriving astrometric data. Most of them are due to factors that are unrelated to the methods for deriving astrometric data. The systematic errors are more likely due to incorrect excluding the effect of background on photometric counts. In the case of mutual occultations, the flux drop is determined to a considerable degree by the ratio of the mean albedos of the two satellites. Some mutual event observations revealed wrong adopted values of the mean albedos.     

Asteroid Observations at Low Phase Angles: II. 5 Astraea, 75 Eurydike, 77 Frigga, 105 Artemis, 119 Althaea, 124 Alkeste, and 201 Penelope

The results of photometric observations of seven main-belt asteroids with moderate surface albedos are presented. New magnitude-phase dependences were obtained for these asteroids: 5 Astraea (down to a phase angle of 0.3°, S type), 75 Eurydike (0.1°, M), 77 Frigga (0.9°, M), 105 Artemis (0.3°, C), 119 Althaea (0.3°, S), 124 Alkeste (0.1°, S), and 201 Penelope (0.5°, M). The parameters of approximating functions and the amplitudes of the opposition effect of these asteroids were determined. The obtained data allowed us to improve values of the rotation periods for some of them: 5 Astraea (16.815±0.002 h), 105 Artemis (18.56±0.01 h), 119 Althaea (11.466±0.001 h), and 124 Alkeste (9.907±0.001 h).     

Particle transfer in decelerating regions

Equations connecting the transmission and reflection functions of a finite medium to the reflection function of a semi-infinite one are used to get the albedos in the slowing-down region. The transport equation is solved by the modified Eddington method using Fermi's backward-forward scattering model, modulated to allow for different orders of backward-forward and isotropic scattering. Numerical results for the energy albedos are obtained and compared.On leave of absence from Atomic Energy Center, Inchass, Egypt.     

The occultation of Beta Scorpii by Jupiter. III. Simultaneous high time-resolution records at three wavelengths

Simultaneous high time-resolution records at three wavelengths of the May 13, 1971 Jupiter occultation of Beta Scorpii are presented. These observations are unique and contain important information about the structure and composition of the Jovian atmosphere. An equally favorable Jupiter occultation should not occur again for several centuries.     

The Hoher List observations of the occultation of 28 Sagittarii by Titan

Observations of the occultation of 28 Sgr by Titan are presented. Apparently being located close to the central line of the occultation track, a double peaked central flash was observed. Some characteristic times of the phenomenon are given.     

Independent radio-occultation studies of venus' atmosphere

Two independent analyses of the dual-frequency radio-occultation experiment performed by Mariner 10 at Venus are presented. Using closed-loop frequency data obtained at NASA's Goldstone facility, we have computed S- and X-band pressure-temperature profiles for Venus' neutral atmosphere, and an S-band profile of the nightside ionosphere. Neutral atmosphere dispersion between the two frequencies is negligible (less than 0.1% in refractivity), as expected for a CO₂ atmosphere. The results confirm those obtained by Howard et al. (1974) from the same S-band data with an accuracy of ±5°K at a given pressure level, though there is a discrepancy of 1 km in the radial scale between the two analyses. These two Mariner 10 profiles are compared with the Mariner 5 occultation profile and in situ measurements by Veneras 8, 9, and 10. The occultation was also monitored at the Owens Valley Radio Observatory, though only at X-band. Despite the much lower quality of these data, a reasonable neutral atmosphere refractivity profile above 65 km was obtained from the occultation entry. Uncertainties in the calculated temperatures, however, are too large to permit useful comparison with previous results. The existence of real anomalies in both the amplitude and frequency of the signal during exit from occultation is confirmed.     

A measurement of the ozone concentration from 65 to 75 km at night

The concentration of ozone between 65 and 75 km has been determined from measurements of the attenuation of moonlight at 2570 Å made from a Moon-pointing rocket payload. The results support earlier rocket measurements, but are in marked disagreement with some recent data obtained by Hays and Roble using the stellar occultation technique.     

Coordinate systems and lunar observing station positions

Satellite geodesy has yielded the locations of more than fifty stations in a single coordinate system referred to the Earth's center of mass with accuracies in the five to ten meter range. The following different methods have been used at Goddard to accomplish this.Dynamical solutions have been obtained for the locations of some fifty key stations using data from the GEOS satellite program. The distribution of observations about the stations is illustrated in terms of the data obtained for a typical station such as the one at Edinburg, Texas. Geopotential coefficients were held fixed in these solutions. The results of these dynamical determinations implied geodetic datum shifts which were then used to arrive at positions for some two hundred additional stations.Another approach involved the adjustment of the coordinates of seventeen stations on the basis of observations of short arcs of GEOS satellite orbits. These results were found to be consistent with those obtained through ground surveys to about five meters rms in each coordinate.Simultaneous solutions for station locations and geopotential coefficients have also yielded values for positions of some sixty stations, again in a coordinate system defined in terms of the Earth's center of mass.Lunar laser ranging and lunar occultation observing programs involve knowledge of the positions of the observing sites. In some cases the lunar observing program itself yields station coordinate information. In other cases greater reliance is placed upon independent determinations of site locations. The location of an occultation observation site at Olifantsfontein, for example, has been obtained in a center-of-mass system in both the dynamical and simultaneous satellite solutions. It is anticipated that a dynamical satellite solution will be extended in 1973 to obtain center-of-mass coordinates for a station in New Zealand. This will make it possible to tie an occultation site in that region to a dynamically determined coordinate system referred to the mass center. Coordinates for stations at Organ Pass, New Mexico, determined in both the dynamical and simultaneous solutions, and Edinburg, Texas, found in both the dynamical and short-arc adjustments, provide the basis for referring the location of a facility such as the McDonald Observatory to a center-of-mass system either through accurate ground surveying techniques or by means of a satellite geodesty tie. The latter approach has already been used, for example, to fix the position of an isolated site on Madagascar relative to a reference point in Africa and, in turn, to a center-of-mass coordinate system.Estimates of the accuracies of the satellite determinations are discussed.Theoretical aspects of coordinate systems associated with the Earth and the Moon are also considered.Communication presented at the conference on Lunar Dynamics and Observational Coordinate Systems held January 15–17, 1973 at the Lunar Science Institute, Houston, Tex., U.S.A.     

Combining asteroid models derived by lightcurve inversion with asteroidal occultation silhouettes

Asteroid sizes can be directly measured by observing occultations of stars by asteroids. When there are enough observations across the path of the shadow, the asteroid’s projected silhouette can be reconstructed. Asteroid shape models derived from photometry by the lightcurve inversion method enable us to predict the orientation of an asteroid for the time of occultation. By scaling the shape model to fit the occultation chords, we can determine the asteroid size with a relative accuracy of typically ∼10%. We combine shape and spin state models of 44 asteroids (14 of them are new or updated models) with the available occultation data to derive asteroid effective diameters. In many cases, occultations allow us to reject one of two possible pole solutions that were derived from photometry. We show that by combining results obtained from lightcurve inversion with occultation timings, we can obtain unique physical models of asteroids.     

A closer look at main belt asteroids 1: WF/PC images

We present new reconstructions of images of main belt Asteroids 9 Metis, 18 Melpomene, 19 Fortuna, 216 Kleopatra, and 624 Hektor, made with the uncorrected Wide-Field/Planetary Camera (WF/PC) on the Hubble Space Telescope (HST). Deconvolution with the MISTRAL algorithm demonstrates that these asteroids are clearly resolved. We determine diameters, albedos, and lower limits to axial ratios for these bodies. We also review the process used to restore the aberrated images. No surface features or companions are found, but the rotation of 216 Kleopatra is clearly seen. The asteroidal albedos are similar to those determined by other procedures.     

Statistical analysis of the results of champ occultation data processing at Shanghai astronomical observatory

With the development of the CPS (Global Positioning System) it has become possible to retrieve accurate profiles of atmospheric temperature, pressure and moisture from CPS occultation data. Using the inversion module developed in Shanghai Astronomical Observatory, (SHAG), we obtained atmospheric profiles from more than 2700 CHAMP occultation events observed in the period 2002 Aug. 1–17. The retrieved profiles are compared with the data of the European Center for Medium-range Weather Forecasts (ECMWF), and their errors are analyzed. An optimal method of statistical analysis is proposed and applied. The statistical results show that GPS occultation data may contribute valuably to numerical weather forecast and long-term monitoring of the earth's climate.     

Keck observations of near-Earth asteroids in the thermal infrared

We present the results of thermal-infrared observations of 20 near-Earth asteroids (NEAs) obtained in the period March 2000-February 2002 with the 10-m Keck-I telescope on Mauna Kea, Hawaii. The measured fluxes have been fitted with thermal-model emission continua to determine sizes and albedos. This work increases the number of NEAs having measured albedos by 35%. The spread of albedos derived is very large (p_v=0.02−0.55); the mean value is 0.25, which is much higher than that of observed main-belt asteroids. In most cases the albedos are in the ranges expected for the spectral types, although some exceptions are evident. Our results are consistent with a trend of increasing albedo with decreasing size for S-type asteroids with diameters below 20 km. A number of objects are found to have unexpectedly low apparent color temperatures, which may reflect unusual thermal properties. However, the results from our limited sample suggest that high thermal-inertia, regolith-free objects may be uncommon, even amongst NEAs with diameters of less than 1 km. We discuss the significance of our results in the light of information on these NEAs taken from the literature and the uncertainties inherent in applying thermal models to near-Earth asteroids.     

非圆轨道GPS/LEO掩星反演地球大气参数的算法及讨论

在非圆轨道GPS和LEO卫星条件下，给出一种较为直接的GPS／LEO掩星反演地球大气参数技术中弯曲角序列的迭代算法，并在理论上对该迭代法的收敛性进行了严格的数学证明．利用GPS掩星反演模拟程序，定量估算了卫星圆轨道假设对GPS／LEO掩星反演地球大气参数的影响，并验证了在非圆轨道条件下各种迭代法的一致性．指出了文献中给出的级数展开迭代算法的不足之处．     

One-dimensional variational retrieval of refractivity profiles by GPS occultation

In recent years, the technique of Global Positioning System/Low Earth Orbit (GPS/LEO) radio occultation has opened up a new route for exploring the earth atmosphere. With this technique, the vertical profiles of pressure, temperature, water vapor of the earth atmosphere can be retrieved from GPS/LEO occultation data, thus making the latter a potentially valuable data resource for meteorological and atmospheric sciences. The technique may effectively improve the quality of the meteorological profiles, and so improve the current numerical weather forecasting model. In the one-dimensional variational (1DVAR) assimilation retrieval technique in current use, the profiles of atmospheric water vapor and temperature, as well as the sea level pressure are retrieved by assimilating the atmospheric refractivity or bending angle profile from GPS/LEO occultation data. In the system independently developed at Shanghai Observatory, the 1DVAR assimilation of CPS occultation data is accomplished by the Levenberg-Marquardt method with the analytical data from the European Center for Medium Range Weather Forecasting (ECMWF) as the background field, the refractivity profile obtained from CHAMP occultation data as the observational quantity. The result was checked with sounding balloon data near the occultation points.     

An Emulation Research on the Radio Occultation Exploration of Martian Ionosphere

The technical system of the Sino-Russian joint satellite-to-satellite Mars ionosphere occultation is analyzed and introduced. The analogue computation of the observed values of the radio waves of the ionosphere occultation event is carried out by adopting the three-dimensional ray tracking method and the electron density profile inversion is conducted by means of the simulated occultation observational data, with the result showing that the emulation algorithm is reliable. By taking advantage of the emulation method the case computation and analysis of the inversion errors caused by the observational error of the occultation radio wave phase and the satellite orbital error are respectively carried out, and it is obtained from the result that the effect of the phase measuring error of the 5% circle on the result of the daytime ionosphere occultation exploration may be neglected, while the absolute error of the night electron density measurement is less than 4 × 108 m^?3, and the main effect of the satellite orbital error on the occultation leads to the lifting or falling of the ionospheric height. The result shows that the technical system of the Sino-Russian joint Mars ionosphere occultation exploration is advanced. It can be expected that the high accuracy electron density profile is obtained and the technical system can be applied to the exploration of the lunar ionospheric environment.