288 research outputs found
The Evolution of the Networks of Observers of Phenomena
8 p.International audienceAstrometric observations are commonly measurements of angular positions of bodies on the celestial sphere in a given reference frame. However, the precision of such measurements is limited by the instruments and by the receptors used for these observations. Astronomers saw very early that celestial bodies were often involved in phenomena such as eclipses of the Sun or the Moon. Such phenomena correspond to specific positions of the involved bodies in space. At least, one may say that the topocentric positions of the Moon and the Sun are very close at the time of an eclipse. This is an astrometric observation. Since the possible phenomena are numerous in the solar system thanks to the velocity of the moving objects, astronomers made predictions of such events and made observations as precise as possible in order to deduce astrometric positions from these events. Of course, a model was necessary for that purpose but this was the main way to improve our knowledge of the solar system at the beginning of astronomical research
Estimating the accuracy of satellite ephemerides using the bootstrap method
International audienceContext: The accuracy of predicted orbital positions depends on the quality of the theorical model and of the observations used to fit the model. During the period of observations, this accuracy can be estimated through comparison with observations. Outside this period, the estimation remains difficult. Many methods have been developed for asteroid ephemerides in order to evaluate this accuracy. Aims: This paper introduces a new method to estimate the accuracy of predicted positions at any time, in particular outside the observation period. Methods: This new method is based upon a bootstrap resampling and allows this estimation with minimal assumptions. Results: The method was applied to two of the main Saturnian satellites, Mimas and Titan, and compared with other methods used previously for asteroids. The bootstrap resampling is a robust and practical method for estimating the accuracy of predicted positions
A new catalogue of observations of the eight major satellites of Saturn (1874-2007)
The original publication in Astronomy & Astrophysics is available at www.aanda.org.International audienceContext : The lastest catalogue of observations includes about 51 000 observations (over 3500 nights) of Saturn's satellites from 1874 to 1989. Since 1989, many observations have been published, often in different formats, based on the publication. Aims : Our new catalogue of observations of the eight major satellites of Saturn includes the observations of the previous catalogues, newly published data and also old observations left out of the previous catalogue. The observations are tabulated in a consistent format. Methods : We give, for each observation, the corrections applied for reduction such as refraction, aberration or phase effects. Furthermore, when it was possible, the instrument and catalogue are also indicated. Results : The new catalogue presents more than 130 000 observations (over 6000 nights) of the eight major satellites of Saturn from 1874 to 2007
Eclipses of the inner satellites of Jupiter observed in 2015
During the 2014-2015 campaign of mutual events, we recorded ground-based
photometric observations of eclipses of Amalthea (JV) and, for the first time,
Thebe (JXIV) by the Galilean moons. We focused on estimating whether the
positioning accuracy of the inner satellites determined with photometry is
sufficient for dynamical studies. We observed two eclipses of Amalthea and one
of Thebe with the 1 m telescope at Pic du Midi Observatory using an IR filter
and a mask placed over the planetary image to avoid blooming features. A third
observation of Amalthea was taken at Saint-Sulpice Observatory with a 60 cm
telescope using a methane filter (890 nm) and a deep absorption band to
decrease the contrast between the planet and the satellites. After background
removal, we computed a differential aperture photometry to obtain the light
flux, and followed with an astrometric reduction. We provide astrometric
results with an external precision of 53 mas for the eclipse of Thebe, and 20
mas for that of Amalthea. These observation accuracies largely override
standard astrometric measurements. The (O-C)s for the eclipse of Thebe are 75
mas on the X-axis and 120 mas on the Y-axis. The (O-C)s for the total eclipses
of Amalthea are 95 mas and 22 mas, along the orbit, for two of the three
events. Taking into account the ratio of (O-C) to precision of the astrometric
results, we show a significant discrepancy with the theory established by
Avdyushev and Ban'shikova in 2008, and the JPL JUP 310 ephemeris.Comment: 7 pages, 10 figures, 4 table
Astrometric observations of Phobos and Deimos during the 1971 opposition of Mars
Accurate positional measurements of planets and satellites are used to
improve our knowledge of their dynamics and to infer the accuracy of planet and
satellite ephemerides. In the framework of the FP7 ESPaCE project, we provide
the positions of Mars, Phobos, and Deimos taken with the U.S. Naval Observatory
26-inch refractor during the 1971 opposition of the planet. These plates were
measured with the digitizer of the Royal Observatory of Belgium and reduced
through an optimal process that includes image, instrumental, and spherical
corrections to provide the most accurate data. We compared the observed
positions of the planet Mars and its satellites with the theoretical positions
from INPOP10 and DE430 planetary ephemerides, and from NOE and MAR097 satellite
ephemerides. The rms residuals in RA and Dec. of one position is less than 60
mas, or about 20 km at Mars. This accuracy is comparable to the most recent CCD
observations. Moreover, it shows that astrometric data derived from
photographic plates can compete with those of old spacecraft (Mariner 9, Viking
1 and 2).Comment: 5 pages, 3 figure
Taking the opportunity of the Gaia reference star catalogue for observing the Solar system in the past
International audienceThe Gaia astrometric catalogue of reference stars will provide proper motions of stars until mag 18 with an accuracy better than 6 mas over one century. So, we may reduce all astrometric observations of Solar System objects made since the end of the XIXth century with an accuracy better than the present accuracy of the best reference star catalogues such as the UCAC2 or UCAC4. This should solve or considerably reduce the problems of biases in ephemerides because of zonal errors in the catalogues. We performed tests on photographic plates and, thanks to the use of sub-micrometric scanners, we succeeded to improve the reduction of plates made in the 1960's for planetary satellites. Even with an accuracy less than the expected one of the future Gaia catalogue, we show a systematic shift of the ephemerides during the last decades
An observation of a mutual event between two satellites of Uranus
We present observations of the occultation of Umbriel by Oberon on 4 May,
2007. We believe this is the first observed mutual event between satellites of
Uranus. Fitting a simple geometric model to the lightcurve, we measure the
mid-event time with a precision of 4 seconds. We assume previously measured
values for the albedos of the two satellites (Karkoschka 2001), and measure the
impact parameter to be 500 +/- 80 km. These measurements are more precise than
estimates based on current ephemerides for these satellites. Therefore
observations of additional mutual events during the 2007-2008 Uranian equinox
will provide improved estimates of their orbital and physical parameters.Comment: 4 pages, 3 figures, accepted for publication in MNRAS Letter
Constraining multiple systems with GAIA
GAIA will provide observations of some multiple asteroid and dwarf systems.
These observations are a way to determine and improve the quantification of
dynamical parameters, such as the masses and the gravity fields, in these
multiple systems. Here we investigate this problem in the cases of Pluto's and
Eugenia's system. We simulate observations reproducing an approximate planning
of the GAIA observations for both systems, as well as the New Horizons
observations of Pluto. We have developed a numerical model reproducing the
specific behavior of multiple asteroid system around the Sun and fit it to the
simulated observations using least-square method, giving the uncertainties on
the fitted parameters. We found that GAIA will improve significantly the
precision of Pluto's and Charon's mass, as well as Petit Prince's orbital
elements and Eugenia's polar oblateness.Comment: 5 pages, accepted by Planetary and Space Science, Gaia GREAT-SSO-Pis
Astrometric positions for 18 irregular satellites of giant planets from 23 years of observations
The irregular satellites of the giant planets are believed to have been
captured during the evolution of the solar system. Knowing their physical
parameters, such as size, density, and albedo is important for constraining
where they came from and how they were captured. The best way to obtain these
parameters are observations in situ by spacecrafts or from stellar occultations
by the objects. Both techniques demand that the orbits are well known. We aimed
to obtain good astrometric positions of irregular satellites to improve their
orbits and ephemeris. We identified and reduced observations of several
irregular satellites from three databases containing more than 8000 images
obtained between 1992 and 2014 at three sites (Observat\'orio do Pico dos Dias,
Observatoire de Haute-Provence, and European Southern Observatory - La Silla).
We used the software PRAIA (Platform for Reduction of Astronomical Images
Automatically) to make the astrometric reduction of the CCD frames. The UCAC4
catalog represented the International Celestial Reference System in the
reductions. Identification of the satellites in the frames was done through
their ephemerides as determined from the SPICE/NAIF kernels. Some procedures
were followed to overcome missing or incomplete information (coordinates,
date), mostly for the older images. We managed to obtain more than 6000
positions for 18 irregular satellites: 12 of Jupiter, 4 of Saturn, 1 of Uranus
(Sycorax), and 1 of Neptune (Nereid). For some satellites the number of
obtained positions is more than 50\% of what was used in earlier orbital
numerical integrations. Comparison of our positions with recent JPL ephemeris
suggests there are systematic errors in the orbits for some of the irregular
satellites. The most evident case was an error in the inclination of Carme.Comment: 9 pages, with 3 being online materia
Strong tidal dissipation in Saturn and constraints on Enceladus' thermal state from astrometry
Tidal interactions between Saturn and its satellites play a crucial role in
both the orbital migration of the satellites and the heating of their
interiors. Therefore constraining the tidal dissipation of Saturn (here the
ratio k2/Q) opens the door to the past evolution of the whole system. If
Saturn's tidal ratio can be determined at different frequencies, it may also be
possible to constrain the giant planet's interior structure, which is still
uncertain. Here, we try to determine Saturn's tidal ratio through its current
effect on the orbits of the main moons, using astrometric data spanning more
than a century. We find an intense tidal dissipation (k2/Q= (2.3 \pm 0.7)
\times 10-4), which is about ten times higher than the usual value estimated
from theoretical arguments. As a consequence, eccentricity equilibrium for
Enceladus can now account for the huge heat emitted from Enceladus' south pole.
Moreover, the measured k2/Q is found to be poorly sensitive to the tidal
frequency, on the short frequency interval considered. This suggests that
Saturn's dissipation may not be controlled by turbulent friction in the fluid
envelope as commonly believed. If correct, the large tidal expansion of the
moon orbits due to this strong Saturnian dissipation would be inconsistent with
the moon formations 4.5 Byr ago above the synchronous orbit in the Saturnian
subnebulae. But it would be compatible with a new model of satellite formation
in which the Saturnian satellites formed possibly over longer time scale at the
outer edge of the main rings. In an attempt to take into account for possible
significant torques exerted by the rings on Mimas, we fitted a constant rate
da/dt on Mimas semi-major axis, also. We obtained an unexpected large
acceleration related to a negative value of da/dt= -(15.7 \pm 4.4) \times 10-15
au/day
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