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OBSERVATIONS OF COORBITAL SATELLITES OF SATURN

Emel'yanov N., Varfolomeev M.
Sternberg State Astronomical Institute, Moscow, e-mail : emelia@sai.msu.ru
Institut de MИcanique CИleste et de Calcul des иphИmИrides, Paris,
e-mail : emelia@imcce.fr

Abstract. To improve the orbital parameters and masses of coorbital
satellites of Saturn, Janus and Epimetheus, new observations of these
satellites are necessary. Usually these satellites are observable from the
Earth only during the Earth and Sun crossings of Saturn's ring plane which
occur once in 14 years. Other ways of observing these satellites were
found.



1. Introduction

Saturn coorbital satellites, Janus and Epimetheus, have very interesting
dynamics: when viewed in a reference frame rotating at the average mean
motion of the satellites, they move on horseshoe-shaped orbits. The
complete libration period around the horseshoe is 8.01 years, while the
average minimum planetocentric angular separation is 5.6 degrees or about
14900 km. The values of the libration period and the distance of mutual
approach are very sensitive to the masses of the satellites, which may be
improved from their observations. In other words, new observations of the
satellites lead to a more precise determination of their masses and orbital
parameters. Unfortunately, Janus and Epimetheus are difficult to be
observed being very faint objects and moving very close to the bright A
Ring of Saturn. Their magnitudes are 14 and 15 respectively. The only
favorable opportunities to obtain the positions of the satellites occur
during the ring-plane crossings which take place each 14 years. The events
in December 1966, March 1980 and August 1995, Voyager encounters with
Saturn in October 1980 and August 1981, as well as Cassini encounters with
Saturn in 2004-2006 make the only points on the three periods of libration.
That is why observers seek other ways to observe these satellites. The
solution found by Nicholson et al. (1992) was to exploit the strong
planetary methane and hydrogen absorption at (=2.0-2.4 (m. Using the
infrared camera, the light of the disk may be eliminated. Saturn's rings,
however, remain bright in this spectral region. It is a good idea to use
the disk of Saturn to occult or to eclipse the bright ring and to observe
the satellites as they pass through superior conjunction. Such a situation
occurs only four times during Saturn's 29-year orbital period. The relative
positions of the Earth, Saturn and the Sun restrict the useful windows for
observations. The next epoch when these observations are possible arrives
in 2013. Our aim is to encourage observers to observe these satellites,
planning observations in advance.



2. Time intervals for the proposed observations of coorbital satellites of
Saturn.



The time intervals when the edge of the planet occult the external edge of
A ring but does not occult yet the orbit of the satellites Janus and
Epimetheus are following



2013/11/20 - 2014/11/11,

2020/01/05 - 2020/03/29,

2020/06/19 - 2020/12/30,

2029/05/17 - 2030/04/24,

2033/10/10 - 2033/11/22,

2034/06/27 - 2034/09/03,

2035/01/26 - 2035/06/04.



These dates were found using the following values of the Saturn's system:

polar radius of Saturn 54364 km (Davies et al., 1996),

external radius of A ring 136770 km (Esposito et al., 1984),

demi-grand axis of satellite orbit 152026.268 km (Nicholson et al.,1992).



Being observed at infrared wavelengths, the planet-ring system will be
visible as a single ring interrupted by a gap at its Southern part.
Nevertheless, the coorbital satellites Janus and Epimetheus will be visible
from the Earth when passing near the gap or near the shaded part of the
ring. The duration of each passing is about 30-40 min. The images in Figs.
1-3 show the typical Saturn-ring-satellites configurations during these
events (these are examples of how negative images will look like, with the
exception of the fact that on the real negative images Saturn will not be
visible at all).

For the observations to be scientifically valuable, it is necessary to
choose appropriate periods of observations and point the camera on the
objects in such a way that some other Saturnian satellite (e.g., Mimas or
Enceladus) appears in the view field. It is preferably that the satellites
should be observed simultaneously with other satellites. These
circumstances may be found using the Natural Satellite Service via internet
by the addresses:

http://lnfm1.sai.msu.ru/neb/nss/index.htm

or

http://www.imcce.fr/page.php?nav=fr/ephemerides/generateur/saimirror/nssephe
.php

Observational techniques for the observations of such apparent events
is described in (Nicholson et al., 1992). The authors of this article made
the observations of Janus and Epimetheus in July and August, 1990, with the
infrared camera at the Palomar Hale telescope. From the images obtained
with this camera they have deduced two astrometric positions of Janus and
three of Epimetheus.



Fig. 1. Saturnian system on August 23, 2004, at 4.51222 hours (TT) (S10 (
Janus, S2 ( Enceladus).


Fig. 2. Saturnian system on October 15, 2004, at 5.16722 hours (TT) (S11 (
Epimetheus, S1 ( Mimas).

Fig. 3. Saturnian system on March 14, 2005, at 0.72472 hours (TT) (S10 (
Janus, S1 ( Mimas, S2 ( Enceladus).




3. Conclusion

The authors hope that this paper will be helpfull to those who wish to
observe these events in the Saturnian system. As was said, the occasions
when these satellites may be observed are very rare, so every possibility
must be used. The next ring plane crossing will take place in September,
2009, while the next possibility to observe Janus and Epimetheus in the
superior conjunction will arrive only in November 2013.


References

Davies, M. E.; Abalakin, V. K.; Bursa, M.; Lieske, J. H.;
Morando, B.; Morrison, D.; Seidelmann, P. K.; Sinclair, A. T.;
Yallop, B.; Tjuflin, Y. S. (1996) Report of the IAU/IAG/COSPAR Working
Group on Cartographic Coordinates and Rotational Elements of the Planets
and Satellites: 1994.
Celestial Mechanics and Dynamical Astronomy. V. 63. P. 127-148.

Esposito, L. W.; Cuzzi, J. N.; Holberg, J. B.; Marouf, E. A.; Tyler, G. L.;
Porco, C. C. (1984) Saturn's rings - Structure, dynamics, and particle
properties. IN: Saturn. Tucson, AZ, University of Arizona Press, 1984, p.
463-545.

Nicholson P.D., Hamilton D.P., Matthews K., Yoder C.F.: 1992. "New
observations of Saturn's coorbital satellites" - Icarus. V. 100, n. 2,
p. 464-484.



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