A new reduction of old observations: a challenge for the next decade

Program available at: http://www.imcce.fr/hosted_sites/naroo/program.htmlInternational audienceThe solar system objects are fast moving objects, needing to build accurate dynamical models in order to make ephemerides and to predict the positions of these objects in the future. More, the building of such dynamical models helps to understand the formation and the evolution of the objects of the solar system. For that purpose, astrometric observations of their positions are made since years. Because of the progress of the observational techniques, the astrometric accuracy of the observations is increasing, years after years

Improvements of astrometry from ground based observatories

International audienceThe analysis of planetary satellites or asteroids ground based observations is quite similar. Our first goal was to improve the astrometric reduction for digitized photographic plates in case of too few reference stars and to propose solutions. We here present astrometric techniques used for old observations of the Galilean satellites made with the 26-inch refractor of USNO in Washington DC from 1967 to 1998 (Pascu, 1977, 1979, 1994). We present some solutions that may be applied to asteroids or any small solar system bodies observations. In fact, we were able to determine an observational error and we discuss the expected improvements necessary in the frame of the GAIA FUN SSO program. A real application in the frame of the Gaia program deals with the new analysis of old photographic plate observations

Improvement of the old reductions of irregular satellites using the first publications of the data

Program available at: http://www.imcce.fr/hosted_sites/naroo/program.htmlInternational audienceThe use of long-exposure photographic plates made the discovery of additional natural satellites possible. The first satellite to be discovered in this manner, Phoebe, was found in 1899 by W.H. Pickering. A reliable model of satellite motion must be constructed with high accurate observations and with data spreading over a period as long as possible. Some old literatures have given the positions of the natural satellites and the reference stars for determining its positions. At that time, the catalogue they used did not contain enough stars, so there were not many catalogue stars on the plates. Because of the unsatisfactory precision of the old catalogue at the epoch and the imprecise measurements on the plate, the positions of the reference stars were not very precise. At present, although we have no plates, we can reduce the positions of the natural satellites from the row data provided in these literatures with modern precise astrometric catalogues, such as PPM and UCAC2. The purpose of this work is to reduce the old observations of Phoebe (Saturn IX), in order to improve its orbit

ODIN: a new model and ephemeris for the Pluto system

Reproduced with permission. Copyright ESO. The original publication is available at www.aanda.org.International audienceBecause of Pluto's distance from the Sun, the Pluto system has not yet completed a revolution since its discovery, hence an uncertain heliocentric distance. In this paper, we present the fitting of our dynamical model ODIN (Orbite, Dynamique et Intégration Numérique) to observations. The small satellites P4 and P5 are not taken into account. We fitted our model to the measured absolute coordinates (RA, DEC) of Pluto, and to the measured positions of the satellites relative to Pluto. The masses we found for the bodies of the system are consistent with those of previous studies. Yet the masses of the small satellites Nix and Hydra are artificially constrained by the number of observations of Charon. The best way to improve the determination of their masses would be to use observations of P4 and P5, but there are still not enough published observations. Concerning the heliocentric distance of the system, we compared the value we obtained using ODIN and those of other models. The difference between the models far exceeds the uncertainty needed (about 1000 km) for the mission New Horizons. A new astrometric reduction of old photographic plates may be an efficient way to constrain this distance. The ephemeris for Pluto's satellites is available on the web page of the IMCCE at http://www.imcce.fr/hosted_sites/saimirror/nssreq9hf.htm. The complete version of the ephemeris is available as a SPICE kernel at http://www.imcce.fr/~beauvalet/. Appendices A and B are available in electronic form at http://www.aanda.orgTable A.2 is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/553/A1

New constraints on Saturn's interior from Cassini astrometric data

This work has been supported by the European Community’s Seventh Framework Program (FP7/2007-2013) under grant agreement 263466 for the FP7-ESPaCE project, the International Space Science Institute (ISSI), PNP (INSU/CNES) and AS GRAM (INSU/CNES/INP). The work of R. A. J. was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. N.C. and C.M. were supported by the UK Science and Technology Facilities Council (Grant No. ST/M001202/1) and are grateful to them for financial assistance. C.M. is also grateful to the Leverhulme Trust for the award of a Research Fellowship. N.C. thanks the Scientific Council of the Paris Observatory for funding. S. Mathis acknowledge funding by the European Research Council through ERC grant SPIRE 647383

Astrometry: optical ground-based observations for ephemerides development

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Equinoxes on Jupiter and Saturn in 2009: call for observations

International audienceThe equinox time on the giant planets The equinox occurs on the giant planets as on Earth, when the Sun crosses the equatorial plane i.e. every 6 years for Jupiter, 14 years for Saturn, 42 years for Uranus and 82 years for Neptune. The main large satellites of these planets have their orbits in the equatorial plane and, except Neptune, each planet have several major satellites. Note that at the time of the equinox, the Earth will also cross the equatorial plane of the planet since, as seen from the giant planets, the Earth is very close to the Sun. Then, the satellites will eclipse and occult each other mutually for a groundbased observer during a one-year period around the equinox. These events are observed with photometric receivers since the light from the satellites will decrease during the events. The light curve will provide information on the geometric configuration of the satellites at the time of the event with an accuracy of a few kilometers, not depending on the distance to Earth of the satellite system. Then, we are able to get an astrometric observation with an accuracy several times better than using direct imaging for astrometric positions. The equinox on Jupiter in 2009 The equinox on Jupiter will occur on June 22, 2009 allowing mutual events during about one year around the equinox time (mutual events will occur from January 2009 to April 2010 not regularly due to the small inclinations of the orbits of the satellites on the Jovian equator) as shown on figure n°1. observable events should be numerous. The negative declination of Jupiter will make the observations easier from southern sites but they are not sufficient as seen on figure n°5 and a special effort should be made in the northern hemisphere to make observations even when Jupiter is low above the horizon. The Galilean satellites of Jupiter are very bright (magnitude 5) and their observation is possible even with a very small telescope. Amateur astronomers are able to make such observations of a high astrometric accuracy (similar to space observations): for that purpose they need to follow very precise recommendations. The photometric accuracy should be good enough for further analysis. Such a photometric accuracy may be reached even with cheap camera and we encourage the potential observers to read the Technical notes dedicated to the observation of the mutual events and the recommendations to be followed by the observers on the web site http://www.imcce.fr/phemu09. Note that all the light curves must be dated in Universal Time with an accuracy of 0.1 second of time to be useful for scientific purpose. GPS may be used for that purpose. Such events occur every 6 years and we have experience of these observations. In the past, many campaigns of observations were made [1] providing a large amount of accurate data allowing to improve the dynamics of the system and we know how useful are the observations made by amateur astronomers. The predictions of the mutual events have been made [2] and are also available at the web address given above. We provide below examples of light curves to be obtained on figures n°2 and 3. On figure n°2, the light curve is quite symmetrical (the surface of Europa is quite uniform) but on figure n°3, the light curve is not symmetrical (the surface of Callisto is less uniform) providing information of the surfaces of the satellite and on the laws of reflexion of light. This information allows to improve the astrometric accuracy of the observations by helping to find the center of mass or center of figure of the satellite from the observation of its center of light or photocenter. The equinox on Saturn in 2009 The equinox on Saturn will occur on August 12, 2009 allowing events as for the Jovian system as shown on figure n°4. The mutual events will occur from September 2008 to June 2010. observable events will be less numerous and a special effort has to be made by the observers. The declination of Saturn is favourable for both hemispheres. Such event occurs every 14 years and the satellites are fainter than the Jovian ones. We have experience of these observations. In the past, two campaigns of observations were made [3] providing accurate data allowing to improve the dynamics of the system. The predictions of the mutual events have been made [4] and are also available at the web address: http://www.imcce.fr/phemu09. Call for observations These observations have shown their high interest allowing quantifying the dissipation of energy due to the tidal effects inside the satellites. A constraint on the internal structure of the satellites may be deduced from the analysis of these observations thanks to the theoretical models developed either at JPL or at IMCCE. As we did in the past, we need the participation of as many observers as possible since the events occur at any time and that the satellites must be above the horizon to be observed. Observing sites at all possible longitudes are required. For the last campaigns, we had about 40 sites of observations as shown on figure n°5. More observing sites are required in the southern hemisphere for the Jovian events. The Saturnian events are more difficult to catch (fainter satellites closer to the bright planet) but are observable easily with a CCD camera and a telescope with a not too short focal length. All explanations are provided on Technical notes available on the web site of IMCCE at: http://www.imcce.fr/phemu09. 2009 will be the International Year of Astronomy supported by IAU. We proposed that the opportunity of the equinoxes on Jupiter and Saturn were used to ask professional and amateur astronomers to make together observations of the mutual events presented in this communication. It will be a commemoration of the observations by Galileo four centuries ago. He extensively observed the Jovian system discovering the eclipses by Jupiter. Such an international campaign of observation will gather professional and amateurs for a common scientific work as we did in the past as shown by common publications (see [1] and [3] ). Do not hesitate to join the network of observers to participate to observations! Contact: [email protected] Web site: http://www.imcce.fr/phemu09 References [1] Arlot J.E. et al. (2006) Astron. Astrophys., 451, 733-737. [2] Arlot J.E. (2008) Astron. Astrophys., 478, 285-298. [3] Thuillot W.. et al. (2001) Astron. Astrophys., 371, 343-349. [4] Arlot J.E. et al. (2008) Astron. Astrophys., 485, 293- 298

The projects of IMCCE concerning the natural planetary satellites

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Predictions of the mutual events of the Galilean satellites of Jupiter occurring in 2009-2010

International audienceAims: Mutual occultations and eclipses of the Galilean satellites of Jupiter will occur in 2009-2010. This paper provides predictions of these events as well as useful information for their observations. Such events are uncommon, since they only occur every 6 years when the Earth and the Sun pass through the common orbital plane of the Galilean satellites. The present dynamical models of motion of the Galilean satellites include the effects of the internal structure of the satellites on their motion. Only accurate observations such as mutual events may fit the parameters depending on the internal structure. Methods: For these predictions, we used the best theoretical models of the motion of the Galilean satellites since the prediction of events is very sensitive to the accuracy of the relative calculated positions of the satellites. Results: Therefore, we encourage professional and amateur astronomers to join the networks of observers in order to get as many observations as possible. Data on the predictions of the events and interactive software for their visibility are available on the web server of the IMCCE (www.imcce.fr/phemu09)