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OBSERVATIONS OF COORBITAL SATELLITES OF SATURN IN 2004-2005

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@bdl.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 only
during the Earth and Sun crossings of Saturn's ring plane which occur once
in 14 years. Recently other ways of observing these satellites were found.
This paper contains precalculated dates of possible observations of Janus
and Epimetheus in 2004-2005.



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, as well as Voyager encounters
with Saturn in October 1980 and August 1981 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. [1] 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 2004. The circumstances of these rare events for
the years 2004-2005 have been precalculated by the authors. Our aim is to
encourage observers to observe these satellites, planning observations in
advance.



2. Coorbital satellites ephemeris in 2004-2005

The opposition of Saturn occurs in winter and the satellites will be
observable from July 28, 2004 to July 8, 2005 (it should be noted that
these time limits are strongly dependent on the values of Saturn's
equatorial and polar radii and the outer radius of Saturn's A Ring). Being
observed at infrared wavelengths, the planet-ring system will be visible as
a single ring interrupted by a gap at its Southern part. This gap will be
minimum at the outer edge of the A Ring. 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). All necessary
data on the events are contained in a table partly presented in this paper
(see Table 1), while the complete table is available on the Internet. Each
table line contains information on one single event. Table columns contain
the following data: the first column gives Julian Date (in TT), then
follows the date in a form YYYY-MM-DD, where YYYY denotes year, MM is a
month number, DD is a day. Next column gives the moment of the event with a
precision to within 1 second. It should be repeated that the duration of
each passing near the Saturn's pole is about 30-40 minutes, so the
observations may be begun 15-20 minutes before the indicated moment. The
next column contains the designation of the satellite, which will be
observed at this time: S10 is Janus, S11 is Epimetheus. The next column
describes the nature of the event: "pole" means that the ring is occulted
by the disk of Saturn and the corresponding satellite is in superior
conjunction; "shade" means that the satellite passes near the shaded part
of Saturn's ring. The complete table is located at

http://lnfm1.sai.msu.ru/neb/nss/coorb/nsscooa.htm

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. Special software elaborated by one of
the authors may effectively assist in that choice and visually reproduce
all circumstances of the event [2]. The reasons why the satellites should
be observed simultaneously with other satellites and advantages of such
observations when the images of two satellites are located on the same CCD
frame are discussed in [3].

Observational techniques for the observations of such apparent events is
described in [1]. The authors (Nicholson et al.) 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).
Table 1. Excerpts from the table containing data on coorbital satellites
observations in 2004-2005.
|2453214.549980 2004-07-28 01h11m58s S11 pole |
|2453214.980649 2004-07-28 11h32m08s S10 pole |
|2453215.244525 2004-07-28 17h52m06s S11 pole |
|2453215.675449 2004-07-29 04h12m38s S10 pole |
|2453215.939070 2004-07-29 10h32m15s S11 pole |
|2453216.370248 2004-07-29 20h53m09s S10 pole |
|2453216.633615 2004-07-30 03h12m24s S11 pole |
|2453217.065048 2004-07-30 13h33m40s S10 pole |
|2453217.328148 2004-07-30 19h52m31s S11 pole |
|2453217.759847 2004-07-31 06h14m10s S10 pole |
|2453218.022693 2004-07-31 12h32m40s S11 pole |
|2453218.454647 2004-07-31 22h54m41s S10 pole |
|2453218.717238 2004-08-01 05h12m49s S11 pole |
|... ... ... ... ... |
|2453417.815695 2005-02-16 07h34m36s S10 pole |
|2453417.823125 2005-02-16 07h45m18s S10 shade |
|2453417.984421 2005-02-16 11h37m33s S11 pole |
|2453417.991793 2005-02-16 11h48m10s S11 shade |
|2453418.510332 2005-02-17 00h14m52s S10 pole |
|2453418.517890 2005-02-17 00h25m45s S10 shade |
|2453418.678757 2005-02-17 04h17m24s S11 pole |
|2453418.686257 2005-02-17 04h28m12s S11 shade |
|2453419.204970 2005-02-17 16h55m09s S10 pole |
|2453419.212655 2005-02-17 17h06m13s S10 shade |
|2453419.373082 2005-02-17 20h57m14s S11 pole |
|2453419.380709 2005-02-17 21h08m13s S11 shade |
|2453419.899607 2005-02-18 09h35m26s S10 pole |
|2453419.907420 2005-02-18 09h46m41s S10 shade |
|2453420.067407 2005-02-18 13h37m03s S11 pole |
|2453420.075173 2005-02-18 13h48m14s S11 shade |
|2453420.594245 2005-02-19 02h15m42s S10 pole |
|2453420.602184 2005-02-19 02h27m08s S10 shade |
|2453420.761744 2005-02-19 06h16m54s S11 pole |
|2453420.769626 2005-02-19 06h28m15s S11 shade |
|2453421.288894 2005-02-19 18h56m00s S10 pole |
|... ... ... ... ... |
|2453556.079308 2005-07-04 13h54m12s S10 pole |
|2453556.774131 2005-07-05 06h34m44s S10 pole |
|2453557.468954 2005-07-05 23h15m17s S10 pole |
|2453558.163776 2005-07-06 15h55m50s S10 pole |
|2453558.858599 2005-07-07 08h36m22s S10 pole |
|2453559.553421 2005-07-08 01h16m55s S10 pole |


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 January, 2014.

Acknowledgements. This work was supported by the Russian Foundation for
Basic Research, project no. 03-02-17441.

References
1. 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.
2. Emel'yanov N.V.: 1996. "A new tool for providing observations of
planetary satellites with ephemerides" - Astronomy Letters. V. 22, n. 1,
p. 135-137.
3. Emel'yanov N.V. : 2001. "Visible Encounters of the Outermost Satellites
of Jupiter" - Solar System Research, v. 35, n. 3, p. 209-211.


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