Asteroid occultations today and tomorrow: toward the GAIA era

Context: Observation of star occultations is a powerful tool to determine shapes and sizes of asteroids. This is key information necessary for studying the evolution of the asteroid belt and to calibrate indirect methods of size determination, such as the models used to analyze thermal infrared observations. Up to now, the observation of asteroid occultations is an activity essentially secured by amateur astronomers equipped with small, portable equipments. However, the accuracy of the available ephemeris prevents accurate predictions of the occultation events for objects smaller than ~100 km. Aims: We investigate current limits in predictability and observability of asteroid occultations, and we study their possible evolution in the future, when high accuracy asteroid orbits and star positions (such as those expected from the mission Gaia of the European Space Agency) will be available. Methods: We use a simple model for asteroid ephemeris uncertainties and numerical algorithms for estimating the limits imposed by the instruments, assuming realistic CCD performances and asteroid size distribution, to estimate the expected occultation rate under different conditions. Results: We show that high accuracy ephemerides which will be available in the future will extend toward much smaller asteroids the possibility of observing asteroid occultations, greatly increasing the number of events and objects involved. A complete set of size measurements down to ~10 km main belt asteroids could be obtained in a few years, provided that a small network of ground-based 1m telescopes are devoted to occultation studies

Daily Processing of Solar System Object Observations by Gaia

6 p.International audienceBy definition, an alert system has to be activated as fast as possible in response to a selected event. For this reason, a pipeline performing fast processing of Gaia observations of Solar System objects (SSO) is being implemented. Its output will concern moving objects that are not matched against an up-to-date catalogue of known asteroids. These "new" Gaia asteroids will have an approximate short-arc orbit determined which can be used to disseminate alerts toward ground-based observers. Without a preliminary orbit (or a bundle of possible orbits) ground-based recovery would be extremely hard, especially for NEOs passing close to Earth, due to the fact that Gaia will orbit around the Lagrangian point L2, thus resulting in a large parallax

L’impact de l’aménagement hydro-agricole sur la santé des populations au Burkina : le cas de Bagré

De nombreux travaux de recherche ont montré l'apparition ou la recrudescence de certaines maladies liées à l'eau, notamment le paludisme et les schistosomiases, à la suite de la modification du milieu par les hydro-aménagements. Une comparaison de la fréquentation de deux formations sanitaires dans la zone du barrage de Bagré, dont l'une dans les périmètres rizicoles et l'autre en dehors, a mis en évidence une opposition nette entre, d'une part, la structure du registre des maladies et, d'autre part, l'engagement des populations dans l'effort de prévention et de soin. Par ailleurs, l'évolution des cas de paludisme présentés au dispensaire appelle des études approfondies sur les causes d'une diminution du recours au soin : la diversification des itinéraires thérapeutiques, la fréquentation d'autres centres de soins, l'amélioration de l'immunité des riverains immédiats seront des hypothèses à privilégier. En attendant, il est indispensable d'intégrer les aspects sanitaires dans les aménagements hydro-agricoles.Extensive research showed the appearance or the recrudescence of certain diseases related to water, in particular malaria and schistosomiases, following the modification of the sector by hydro-installations. A comparison of patient visits of two medical formations in the zone of the dam Bagré, one in the rice perimeters and the other outside, highlighted a clear opposition between the structure of the documentation of the diseases on the one hand, and the commitment of the population in the effort of prevention and care on the other hand. In addition, the evolution of the cases of malaria presented to the community clinic demands in-depth studies on the causes of a reduction in the recourse to care: the diversification of therapeutic routes, patient visits of other care facilities, the improvement of the immunity of the immediate residents will be assumptions to be given greater importance. While waiting, it is essential to consider health issues in hydro-agricultural installations

Testing the inversion of asteroids' Gaia photometry combined with ground-based observations

We investigated the reliability of the genetic algorithm which will be used to invert the photometric measurements of asteroids collected by the European Space Agency Gaia mission. To do that, we performed several sets of simulations for 10 000 asteroids having different spin axis orientations, rotational periods and shapes. The observational epochs used for each simulation were extracted from the Gaia mission simulator developed at the Observatoire de la C\^{o}te d'Azur, while the brightness was generated using a Z-buffer standard graphic method. We also explored the influence on the inversion results of contaminating the data set with Gaussian noise with different $\sigma$ values. The research enabled us to determine a correlation between the reliability of the inversion method and the asteroid's pole latitude. In particular, the results are biased for asteroids having quasi-spherical shapes and low pole latitudes. This effect is caused by the low lightcurve amplitude observed under such circumstances, as the periodic signal can be lost in the photometric random noise when both values are comparable, causing the inversion to fail. Such bias might be taken into account when analysing the inversion results, not to mislead it with physical effects such as non-gravitational forces. Finally, we studied what impact on the inversion results has combining a full lightcurve and Gaia photometry collected simultaneously. Using this procedure we have shown that it is possible to reduce the number of wrong solutions for asteroids having less than 50 data points. The latter will be of special importance for planning ground-based observations of asteroids aiming to enhance the scientific impact of Gaia on Solar system science.Comment: Accepted in MNRA

An optimal Mars Trojan asteroid search strategy

Trojan asteroids are minor planets that share the orbit of a planet about the Sun and librate around the L4 or L5 Lagrangian points of stability. Although only three Mars Trojans have been discovered, models suggest that at least ten times this number should exist with diameters >= 1 km. We derive a model that constrains optimal sky search areas and present a strategy for the most efficient use of telescope survey time that maximizes the probability of detecting Mars Trojans. We show that the Gaia space mission could detect any Mars Trojans larger than 1 km in diameter, provided the relative motion perpendicular to Gaia's CCD array is less than 0.40 arcsec per second.Comment: 6 pages, 6 figures, 3 tables, accepted for publication in MNRAS. arXiv admin note: substantial text overlap with arXiv:1111.112

Detection of inner Solar System Trojan Asteroids by Gaia

The Gaia satellite, planned for launch by the European Space Agency (ESA) in 2013, is the next generation astrometry mission following Hipparcos. While mapping the whole sky, the Gaia space mission is expected to discover thousands of Solar System Objects. These will include Near-Earth Asteroids and objects at Solar elongations as low as 45 degrees, which are difficult to observe with ground-based telescopes. We present the results of simulations for the detection of Trojan asteroids in the orbits of Earth and Mars by Gaia.Comment: 4 pages, 3 figures, based on a talk presented at the Gaia-FUN-SSO-2 International Workshop, Paris Observatory, 19-21 September 2012. Part of the proceedings of that worksho

Australian participation in the Gaia follow-up network for solar system objects

The Gaia satellite, planned for launch by the European Space Agency (ESA) in 2013, is the next-generation astrometry mission following Hipparcos. Gaia’s primary science goal is to determine the kinematics, chemical structure, and evolution of the Milky Way Galaxy. In addition to this core science goal, the Gaia space mission is expected to discover thousands of Solar System objects. Because of orbital constraints, Gaia will only have a limited opportunity for astrometric follow-up of these discoveries. In 2010, the Gaia Data Processing and Analysis Consortium (DPAC) initiated a program to identify ground-based optical telescopes for a Gaia follow-up network for Solar System Objects to perform the following critical tasks: confirmation of discovery, identification of body, object tracking to constrain orbits. To date, this network comprises 37 observing sites (representing 53 instruments). The Zadko Telescope, located in Western Australia, was highlighted as an important network node because of its southern location, longitude, and automated scheduling system. We describe the first follow-up tests using the fast moving Potentially Hazardous Asteroid 2005 YU55 as the target