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IAA Transactions, No. 8, ``Celestial Mechanics'', 2002
Dynamics of the Saturnian system with regard to
high precision observations
A. Vienne 1;2 , W. Thuillot 1 , J. E. Arlot 1
1 IMCCE/Observatoire de Paris
2 Universit'e de Lille, France
The acquisition of high precision astrometric observations of Solar System ob­
jects can be applied to the improvement of the dynamical models and to explore
some specific problems of celestial mechanics. In particular, in the Saturnian sys­
tem, the next investigation of the CASSINI space probe and the interpretation of
these data will require the use of an improved dynamical model. This is why cam­
paigns of observations have been recently organized. We present here new precise
observations of the major satellites in relation with the celestial mechanics prob­
lems we study in this system. The study of the interplay of tides and resonances
in the Mimas­Tethys system, for example, requires to know the magnitude of
the secondary resonance that is the value of the eccentricity of Tethys, and to
know the magnitudes of tides that is the secular acceleration in the longitude of
Mimas. Others studies require also to know constraints: theory of motion of the
eight major satellites, tidal effects in the other satellites, motion of the co--orbital
satellites. New recent observations of the major satellites have then been obtained
and will allow us to measure the magnitudes of the parameters of the theoretical
studies. Some of these observations are already reduced. We present them and
the method applied to obtain the high precision required.
Several years ago, a theory of motion of the eight major satellites of Saturn has
been built in a dynamically consistent way, in which the satellites are considered
all together; its only parameters are explicitly the initials conditions, the masses
of the satellites and the oblateness coefficients of Saturn. Its internal precision is
a few tens of kilometers (TASS, Vienne & Duriez, 1995).
This study has shown some new terms in the mean longitude of Mimas which
depend of the Tethys eccentricity. Because of this eccentricity and tidal effects
due to dissipation in Saturn, the system can enter in one or several secondary
resonances. Champenois and Vienne (1999) have shown that the system may have
been trapped in a secondary resonance or may have behaved in a chaotic way
on capture in the present main resonance. The probability of capture into the
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present resonance may be much higher (up to 1) than 0:04 found previously. The
evolution, under tidal effects, of the dynamics of this system extremely depends
upon the value of the eccentricity of Tethys which is not well known at present.
Since 1990, the observations of Saturn satellites are mainly CCD ones and of­
ten published in pixels. So no astrometry is really done upon them. The problem
is that generally there is no reference stars which would allow to do an astromet­
ric reduction. But we succeed in extracting the astrometric data from these kind
of observations. Our recent reductions (Vienne et al., 2001 and Peng et al., 2002)
lead to 6919 observations for which the precision reachs 0: 00 064 (0: 00 075 for the
270 positions of Mimas). All others available CCD observations of others authors
(3350 positions) are given in pixels only. Their precision is estimated to 0: 00 086
(0: 00 141 for the 15 positions of Mimas). These observations had led to a new esti­
mation of the eccentricity of Tethys: e 0 = 0:00021 \Sigma 0:00008 . More investigations
are necessary to reduce the uncertainty.
In 1995, mutual occultations and eclipses of the satellites occurred. Many
observers then planned to observe these events. Sixty--six such observations of
events were successfully performed in several countries (Thuillot et al., 2001).
The analysis of these sixty­six observations are in progress. We expect that the
precision on the relative position will reach about 0: 00 01.
Some others studies are also in progress such as the tidal effects in the other
satellites or the motion of the co--orbital satellites. In all cases we show that
high precision observations are necessary to well studying the dynamics of the
saturnian system, and in particular to understand the evolution of this planetary
system.
References
1. Champenois S., Vienne A. The role of secondary resonances in the evolution
of the Mimas­Tethys system. Icarus, 1999, 140, 106--121.
2. Thuillot W., Arlot J. E. et al. The PHESAT95 catalogue of observations of
the mutual events of the Saturnian satellites. Astron. Astrophys., 2001, 371,
343.
3. Vienne A., Duriez L. TASS1.6: Ephemerides of the major Saturnian satel­
lites. Astron. Astrophys., 1995, 297, 588--605.
4. Vienne A., Thuillot W., et al. Saturnian satellites observations made in Brazil
during the 1995 opposition with an astrometric analysis. Astron. Astrophys.,
2001, 380, 727--733.
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