Accurate theories of motion of natural satellites are required to broaden our knowledge about the Solar System and to plan and realize space missions to other planets. In order to improve such theories it is necessary to make very accurate observations.
An efficient method for the study of planetary satellites are photometric observations of their mutual eclipses and occultations often referred to as mutual events. Observations of these rare phenomena provide a wealth of data. The surface characteristics and accurate astrometric data can be deduced from them [16, (Mallama 1992]; [8, Descamps et al. 1992]). The mutual positions of satellites as inferred from photometry of mutual events are several dozen and even several hundred times more accurate than those based on photographic observations. In the dynamics of natural satellites the effects are present that cannot be reliably interpreted on the basis of currently available observations. That is why we should take the maximum advantage of the "gift" offered by nature, the mutual events in satellite systems. Unfortunately, the mutual events are rare: they occur only when the Earth and the Sun cross the orbital planes of the satellites. A special effort must be made by the observers during these occurrences.
The first observations of a mutual event in Saturnian satellites were published in 1921 [7, (Comrie 1921]). These visual observations of the eclipse of Rhea by the shadow of Titan were made at five british observatories. The astrometric data derived from photometric observations of mutual occultations and eclipses of the Galilean satellites in 1973 and 1979 and four Saturnian satellites in 1980 were published in [1, (Aksnes et al. 1984]). The Bureau des Longitudes has been systematically launching international campaigns of observations of mutual phenomena in the system of the Galilean satellites [2, (Arlot 1978]; [3, Arlot 1984]; [6, Arlot et al. 1990]; [4, Arlot 1996]) and of the first eight Saturnian satellites [5, (Arlot & Thuillot 1993]).
Mutual events of major Saturn's satellites are confined to six-months epochs that occur once every 15 years. About 300 mutual events occur during each epoch. Each phenomenon typically lasts from 1 to 10 min and therefore it is accessible to observations at a specific group of observatories only. The last epoch of mutual events of Saturnian satellites took place in the second half of 1995. The worldwide distribution of events over observatories are discussed in [17, Nasonova (1996)]. It was shown that part of all events is accessible to observation only at observatories of the FSU and nowhere else. It turns out that a failure to perform observations at these observatories may result in a loss of 20% of valuable observational data [17, (Nasonova 1996]).
In 1995 the observations has been organized on a number of observatories of the CIS (FSU) with the aim to observe mutual events in the system of Saturnian satellites. The goal of these observations was to make a contribution in the whole data base of observations of the major Saturnian satellites in order to improve the theory of their motion using the advantages of the rare events, mutual eclipses and occultations of the satellites.
Of interest are not only the photometry of mutual phenomena but also measurements of satellite positions at times close to such events. CCD observations of close apparent approaches are an opportunity to get more accurate measurements of the mutual positions of satellites. Photographic observations during these periods provide independent check on photometric results.
The staff of FAI AS RK had previous experience in this field gained during their photometric observations of mutual events of Galilean satellites in 1985. The results of their observations are published in [13, (Grigorjeva et al. 1986a,b]).
In the framework of the campaign of observatories of the CIS the only successful photometric observations turned out to be those made at CL SAI and at two observatories - in Assy and Almaty - of FAI AS RK. In addition, CCD position observations were performed at MAO RAS in Pulkovo and at two observatories of FAI AS RK. Within the framework of this campaign MAO RAS also performed photographic observations of Saturnian satellites.
During a mutual occultation of two satellites the combined flux decreases due to partial or complete screening of one satellite by the disk of the other satellite. Photometric observations with high time resolution make it possible to obtain the light curve of the combined flux of both satellites during their mutual occultation. This light curve depends on a number of factors: the mutual position of satellites (i.e., their coordinates), reflective properties of satellite surfaces, sizes and shapes of their bodies. During a mutual eclipse one of the satellites enters the umbra or penumbra of another satellite resulting in a decrease of the flux of the former. This decrease, in turn, is reflected in the measured light curve. During a mutual eclipse the combined light of the two satellites is measured. In some cases, when the angular separation between the two satellites is sufficiently large, it is possible to separately measure the magnitude of the eclipsed satellite. In the case of mutual eclipse, like in that of mutual occultation, the flux from satellites depends on their coordinates, sizes, shapes, and reflective properties.
The light curves of mutual events can be used to solve the inverse problem - to determine the differences of apparent coordinates of satellites at certain instants of time. Thus, photometry of mutual events of natural satellites provides an opportunity to obtain high-precision astrometric information. Given sufficient number of photometric observations, it is also possible to determine the sizes and shapes of satellites and the distribution of albedo on their surfaces.
Mutual events in the system of major Saturnian satellites occurred from April, 1995 to February, 1996. The period of possible observations of these events at the observatories mentioned above was much shorter - from July to December, 1995. Each event lasted several minutes. Up to 50 mutual events could be observed at each observatory. However, for various reasons (weather conditions and limited technical potential of observatories) photometric observations were performed only for 12 events of which only three proved to be successful. The ephemerides were computed at the Celestial Mechanics Department of the SAI by an original technique using a special software [10, (Emel'yanov 1996]) and have been published before [11, (Emel'yanov et al. 1994]).
In this paper we describe the results of observations of
Saturnian satellites made within the framework of the
campaign. The files with results
available in electronic form are given in:
ftp cdsarc.u-strasbg.fr
username: anonymous
password: (type your e-mail adress here)
cd /pub/A+AS/[this volume number]/[this page number]
or
ftp ftp.sai.msu.ru
username: anonymous
password: (type your e-mail adress here)
cd /pub/PEOPLE/emelia/phemurus.
The file names are
given below. The data can also be obtained on a request
submitted to the first author of this paper at the following
address: This email address is being protected from spambots. You need JavaScript enabled to view it.
.
The authors would like to acknowledge the use of the data by making a reference to this paper. Below we describe the results subdividing the observations into the following groups:
Copyright The European Southern Observatory (ESO)