Orbit computation for transneptunian objects

Using statistical orbital ranging, we systematically study the orbit computation problem for transneptunian objects (TNOs). We have automated orbit computation for large numbers of objects, and, more importantly, we are able to obtain orbits even for the most sparsely observed objects (observational arcs of a few days). For such objects, the resulting orbit distributions include a large number of high-eccentricity orbits, in which TNOs can be perturbed by close encounters with Neptune. The stability of bodies on the computed orbits has therefore been ascertained by performing a study of close encounters with the major planets. We classify TNO orbit distributions statistically, and we study the evolution of their ephemeris uncertainties. We find that the orbital element distributions for the most numerous single-apparition TNOs do not support the existence of a postulated sharp edge to the belt beyond 50 AU. The technique of statistical ranging provides ephemeris predictions more generally than previously possible also for poorly observed TNOs.     

Accurate ionospheric delay model for real-time GPS-based positioning of LEO satellites using horizontal VTEC gradient estimation

Ionospheric delays compensation is a mandatory step for precise absolute and relative positioning of Low Earth Orbit Satellites (LEO) by GPS measurements. The most frequently used ionosphere model for real-time GPS-based navigation in LEO is an isotropic model proposed by Lear, which uses the Vertical Total Electron Content (VTEC) above the receiver and a mapping function for TEC evaluation along a given ray path. Based on significant assessed results available for ground-based GPS receivers, we propose the use of a different model relying on the thin shell assumption and a bilinear horizontal variation of the VTEC as a function of latitude and longitude in the shell. It is expected that this model is capable of better describing horizontal gradients in the ionosphere, thus improving ionospheric delay estimation, especially in intense ionospheric conditions. This model is referred to as Linear Thin Shell (LTS). LTS performance in estimating undifferenced and double-differenced ionospheric delays is checked by comparing measured and predicted delays computed using flight data from the GRACE mission. Results show that the LTS always outperforms the isotropic model, especially in case of high solar activity. Moreover, the LTS model provides a higher performance uniformity over a wide range of ionospheric delays, thus ensuring good performance in different conditions. The results obtained demonstrate that the LTS model improves the ionosphere delays estimation accuracy by 20 and 40% for undifferenced and double-differenced delays, respectively. This suggests the LTS model can effectively contribute to improving precision in LEO positioning applications.     

Biogeochemistry of the Madeira river basin

A biogeochemical characterization of the Madeira river basin has been made to evaluate the local and global effects of possible alterations in the ecosystem caused by recent intensive occupation in Rondonia state. During the period April 1983—January 1986, sampling was made both by land and river along the tributaries and main channel of the Madeira river. The parameters analysed lead to a detailed study of the physicochemical quality of the waters of the basin and their relationship with the local geology, associated with the transport of solid material and the hydrological behavior of the ecosystem.Penman's method adapted to tropical rainforest conditions was used to evaluate the potential evapotranspiration for the basin. Estimated potential evapotranspiration was 1420 mm/y, 77% due to the energy balance. Real evapotranspiration was 94% of the estimated potential and the main residence time of the rain water in the basin was 2 months. The isotopic behavior of Hydrogen and Oxygen in the river waters of the region was typical of great rivers, the values being more positive during the dry season and more negative during the rainy season. An isotopic gradient of ¹⁸O 0.038 ()/100 km, was established from Porto Velho station to the estuary, which was considered low when compared with the value of 0.063 ()/100 km, obtained for the Amazon river.In general, the waters of the tributaries were poor in dissolved ion species when compared with the main channel of the Madeira river. Seasonal variation in the transport of suspended sediment kept the same pattern, greater transport being observed on rising water than during high water. A transport of 2.85 million tons per day was observed in the Madeira river near the mouth.     

Chaotic dynamics of planet‐encountering bodies

The dynamics of two families of minor inner solar system bodies that suffer frequent close encounters with the planets is analyzed. These families are: Jupiter family comets (JF comets) and Near Earth Asteroids (NEAs). The motion of these objects has been considered to be chaotic in a short time scale,and the close encounters are supposed to be the cause of the fast chaos. For a better understanding of the chaotic behavior we have computed Lyapunov Characteristic Exponents (LCEs) for all the observed members of both populations. LCEs are a quantitative measure of the exponential divergence of initially close orbits. We have observed that most members of the two families show a concentration of Lyapunov times (inverse of LCE) around 50–100yr. The concentration is more pronounced for JF comets than for NEAs, among which a lesser spread is observed for those that actually cross the Earth's orbit (mean perihelion distance q < 1.05 AU). It is also observed that a general correspondence exists between Lyapunov times and the time between consecutive encounters. A simple model is introduced to describe the basic characteristics of the dynamical evolution. This model considers an impulsive approach, where the particles evolve unperturbedly between encounters and suffer ‘kicks’ in semimajor axis at the encounters. It also reproduces successfully the short Lyapunov times observed in the numerical integrations and is able to estimate the dynamical lifetimes of comets during a stay in the Jupiter family in correspondence with previous estimates. It has been demonstrated with the model that the encounters with the largest effect on the exponential growth of the distance between initially nearby orbits are neither the infrequent deep encounters, nor the frequent and far ones; instead, the intermediate approaches have the most relevant contribution to the error growth. Such encounters are at a distance a few times the radius of the Hill's sphere of the planet (e.g. 3). An even simpler model allows us to get analytical estimates of the Lyapunov times in good agreement with the values coming from the model above and the numerical integrations. The predictability of the medium‐term evolution and the hazard posed to the Earth by those objects are analysed in the Discussion section. This revised version was published online in July 2006 with corrections to the Cover Date.     

UESAC — The Uppsala-ESO Survey of Asteroids and Comets

The Uppsala-ESO Survey of Asteroids and Comets was undertaken to find previously undetected comets in the vicinity of Jupiter. Over 15000 positions of moving objects have been detected on 74 plates obtained from the European Southern Observatory in Chile and from the Anglo-Australian Observatory in Australia in 1992 and 1993. Two or more positions were secured for about 3300 asteroids and orbits have so far been calculated for 1944 asteroids. The main bulk of these asteroids are previously undetected. We present absolute magnitudes and diameters for asteroids which have an accurate orbit. The magnitude and diameter distributions are compared to the results of the Palomar-Leiden Survey of Faint Minor Planets.     

A meteorite crater on Earth formed on September 15, 2007: The Carancas hypervelocity impact

Abstract— On September 15, 2007, a bright fireball was observed and a big explosion was heard by many inhabitants near the southern shore of Lake Titicaca. In the community of Carancas (Peru), a 13.5 m crater and several fragments of a stony meteorite were found close to the site of the impact. The Carancas event is the first impact crater whose formation was directly observed by several witnesses as well as the first unambiguous seismic recording of a crater‐forming meteorite impact on Earth. We present several lines of evidence that suggest that the Carancas crater was a hypervelocity impact. An event like this should have not occurred according to the accepted picture of stony meteoroids ablating in the Earth's atmosphere, therefore it challenges our present models of entry dynamics. We discuss alternatives to explain this particular event. This emphasizes the weakness in the pervasive use of “average” parameters (such as tensile strength, fragmentation behavior and ablation behavior) in current modeling efforts. This underscores the need to examine a full range of possible values for these parameters when drawing general conclusions from models about impact processes.     

Mineralogy of suspended sediment in three karst springs

Springs in karstic carbonate rocks frequently carry a sediment load as well as a dissolved load. Analysis of morphology and mineralogy of suspended sediment from three contrasting karst springs reveals a suite of clastic particles that reflect both source areas and processes that take place within the aquifer. Nolte Spring in Lancaster County, Pennsylvania, USA, discharges sediment of apparently precipitated calcite, indicating that at some point in the aquifer or vadose zone, water exceeds saturation with respect to calcite. Sediment morphologies and chemical conditions in the aquifer point to two different scenarios for this precipitation. The other two springs, Arch Spring in Blair County, Pennsylvania and Bushkill Spring in Northampton County, Pennsylvania, show no evidence of calcite precipitation. Arch Spring discharges mainly layer silicates, while Bushkill Spring discharges mainly silica.

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Resumen Los manantiales en rocas carbonatadas karstficadas tienen frecuentemente una carga de sedimentos y de sólidos disueltos. Los análisis de la morfología y mineralogía de los sedimentos suspendidos de tres manantiales kársticos contrastados revela un conjunto de partículas clásticas que reflejan ambas áreas fuente y los procesos que tienen lugar dentro del acuífero. Nolte Spring en el Condado de Lancaster, Pennsylvania, USA, descarga sedimento de calcita aparentemente precipitada indicando que en cierto punto del acuífero o en la zona vadosa, el agua supera la saturación con respecto a la calcita. La morfología de los sedimentos y las condiciones químicas en el acuífero apuntan a dos escenarios diferentes para esta precipitación. Los otras dos manantiales, Arch Spring en el Condado de Blair, Pennsylvania y Bushkill Spring en el Condado de Norhtampton, Pennsylvania, no muestran evidencia de precitación de calcita. Arch Spring descarga principalmente filosilicatos mientras que Bushkill Spring descarga fundamentalmente Sílice.

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Résumé Les sources situées en contexte carbonaté karstique charrient fréquemment, outre les matières dissoutes, une charge solide. La morphologie et la minéralogie des sédiments en suspension issus de trois sources karstiques de caractéristiques distinctes ont été analysées : une série de particules clastiques, reflétant à la fois les secteurs des sources et les processus internes de l’aquifère, en ressort. Les sédiments issus de Nolte Spring (Comté de Lancaster, Pennsylvanie, Etats-Unis) sont constitués de calcite apparemment précipitée: ceci indique que l’eau atteint la sursaturation vis-à-vis de la calcite à un certain point de l’aquifère ou de la zone non-saturée. La morphologie des sédiments et les conditions chimiques au sein de l’aquifère mènent à deux scénarii différents pour expliquer cette précipitation. Les deux autres sources, Arch Spring (Comté de Blair, Pennsylvanie) et Bushkill Spring (Comté de Northampton, Pennsylvanie), ne présentent aucun indice de précipitation de calcite. Arch Spring exporte essentiellement des phyllosilicates, alors que la silice est majoritaire à Bushkill Spring.

     

Constraints to Uranus' Great Collision III: The Origin of the Outer Satellites

The theory of satellite loss resulting from a giant impact on Uranus (Parisi and Brunini 1997, Planet. Space Sci.45, 181-187) is revisited, in the light of the discovery of its five outer moons (Gladman et al. 1998, Nature392, 897-899; Gladman et al. 2000, Icarus147, 320-324; erratum 148, 320). Physical conditions and dynamical constraints in the great collision scenario and restrictions in the possible mechanisms for the origin of the outer uranian satellites are obtained from the knowledge of their actual orbital properties. We conclude that the existence of these moons implies that their origin must be connected to a breakup process. Other scenarios for their origin cast serious doubts on the occurrence of a giant collision at the end of Uranus' formation process to account for its large spin axis obliquity.     

An Asteroid in a Earth-like Orbit

The dynamical evolution of an asteroid with orbital elements strikingly similar to the Earth is analysed. The object, 1991 VG, was discovered by the Spacewatch telescope during a particular encounter with the planet. 1991 VG experienced a temporary satellite capture by the Earth, a phenomena that is recurrent in its dynamical history. The possible origin of this puzzling object is discussed, including the suggestion that 1991 VG could be a piece of lunar ejecta after a great impact. This revised version was published online in July 2006 with corrections to the Cover Date.