Report of the Working Group on Satellites for the period 1972-1975
                            (B. Morando)

A - New Satellites of Jupiter


One of the most interesting developments of the triennium is 
the discovery, by C.T. Kowal at Palomar, of certainly one and 
probably two new satellites of Jupiter. Of photovisual magnitude 20, 
J XIII was found on plates obtained with the 122-cm Schmidt 
telescope on three consecutive nights in September 1974. Very 
fortunately, the object could then also be recorded on a pair of 
exposures (using an image tube) with the Steward Observatory's 229-
cm f/9 reflector at Kitt Peak some ten nights later, and as a result 
K. Aksnes and B. G. Marsden were reasonably confident that the 
object was indeed a satellite and that it belonged to the group of 
direct satellites J VI, J VII and J X. Five additional observations 
made during the following three months by Kowal and by E. Roemer 
with the Steward Observatory's reflector enabled the orbit 
computation to be refined (a = 0.0743 ± 0.0007 AU, e = 0.147 ± 
0.003, i = 26¡.72 ± 0¡.05; see Astron. J. 80, 460, 1975), and 
further observations could be made during the 1975 opposition by Kowal, 
Roemer, and also with the 208-cm reflector at the McDonald Observatory.

The probable fourteenth satellite, of photovisual magnitude 21, 
was picked up with the 122-cm Schmidt telescope on three consecutive 
nights in September-October 1975. Roemer was again able to observe 
this object a few nights later, but the object's proximity to 
Jupiter during the following dark of the Moon precluded any further 
observations, for by early December the object had become too faint, 
its position too uncertain and its motion too close to the sidereal 
rate for identification. The orbit determination leaves little doubt 
that the object is indeed a satellite, but it is not possible to 
decide whether its orbit is direct or retrograde. Presumably  this 
object will eventually be rediscovered at some opposition in the future.


B - Observations and Related Analyses

A world-wide campaign was organized to make observations of the very favorable 
mutual occultations and eclipses of Jupiter's Galilean satellites in 1973-74. 
Predictions had been prepared by S.W. Milbourn and J.V. Carey, R.T. 
Brinkmann, K. Aksnes and J.E. Arlot (Handb. Br. Astron. Assoc. for 1973; 
09.099.069; 10.099.015; 10.099.026; 11.099.201).

The photoelectric campaign, organized on the initiative of 
Brinkmann and R.L. Millis (09.099.014), has provided light curves of 
more than 100 events; these observations have been filed in the data 
bank established at the Lowell Observatory. Early analyses by R.E. 
Murphy and K. Aksnes (Nature 244, 559, 1973) and by J.R. Vermilion 
et al. (12.099.229) of a few grazing occultations of Europa by Io 
had suggested the presence of a bright north polar cap on Europa. 
The light curves of these occultations were considerably deeper than 
predicted. Later, K. Aksnes and F.A. Franklin (Center for Astrophys. 
Prepr. N¡ 339, 1975) showed that most of  this discrepancy was 
probably due to latitude errors of up to 400 km in the ephemerides 
of Io and Europa given by Sampson's theory. Although the light-curve 
analyses have not yet been completed, the results by T. Nakamura 
(11.099.217), Arlot et al. (12.099.226), Aksnes and Franklin 
(Astron. J. 80, 56, 1975) and T.C. Duxbury  et al. (Icarus 25, 1, 1975) 
show that accurate information about the positions and radii of the 
Galilean satellites can be derived from observations of mutual events.

Astrometric observations of the natural satellites of the giant planets have 
been resumed at the McDonald Observatory after a lapse of almost 20 years. 
This work is being conducted by J.D. Mulholland, with the collaboration of 
P.J. Shelus and R.I. Abbot. The list of objects observed includes 
Jupiter VI-XIII, Saturn I-IX, Uranus I-V and Neptune I and II. A special 
effort is made to secure observations of difficult objects. Some aspects 
of this work are reported in Astron. J. 80, 723, 1975.

At the University  of Arizona, Roemer has continued to make 
observations of satellites with the Steward 229-cm reflector and 
also with the 154 cm f/13.5 reflector at the Catalina Station of the 
Lunar and Planetary Laboratory. In addition to the observations of J XIII 
mentioned above, she obtained observations of J X and J XI in September 1974, 
of Phoebe in January 1974 and of Nereid on ten nights during 1973-75.

With the 155 cm reflector at Harvard Observatory's Agassiz 
station, Aksnes and co-workers obtained more than 100 plates of 
satellites, mostly of Saturn I-IX, between December 1972 and April 1974.

In 1971 and 1972, observations of the Galilean and Saturnian 
satellites were made by C.F. Peters with the 60 cm reflector at the 
Table Mountain Observatory (10.099.067).

The number of observations obtained in the U.S.S.R. has 
significantly  increased. At the Pulkovo Observatory, by means of a 
short-focus astrograph, a normal astrograph and a 66 cm refractor, 
some 800 positions of eight of Saturn's satellites and 200 positions 
of the Galilean satellites have been obtained since 1972. In 
addition, 14 positions of Phobos and 22 positions of Deimos were 
obtained at the opposition in 1973. The observers were T.P. 
Kiseleva, N.V. Fatchikhin, N.M. Bronnikova and A.A. Kiselev.

At the Nikolaev Observatory, G.K. Gorel' obtained 160 positions 
of the Galilean satellites and took more than 20 plates of Saturn's 
satellites. At Abastumani, 60 positions of Phobos and 114 positions 
of Deimos were obtained by A. Sh. Khatisov and G.N. Salukvadze in 
1973. At the same observatory differential positions were measured 
of Ganymede with respect to the star SAO 186658 during the 
occultation in 1972 (11.099.213). The satellites of Jupiter and 
Saturn were also photographed in Tashkent.

In 1974 C. Critescu remeasured the plates of Jupiter's satellites obtained 
by G. Petrescu at the Bucharest Observatory in 1934 and at the Paris 
Observatory in  1936. All the plates were measured at least twice.

Arlot has made a detailed analysis of the observations of the 
Galilean satellites in relation to Sampson's theory (IAU Colloq. 28; 
Thesis, University of Paris). S. Ferraz-Mello, using all the 
photographic observations made since 1930, has found a deviation of 
the timescale defined by the motions of the satellites from 
Ephemeris Time; the deviation can be expressed by 0.006 (t-1900.0) 
minutes (where t is in years). Using the best available observations 
(those of Petrescu in 1934 and of D. Pascu in 1968) Ferraz-Mello 
also showed that the errors due to the periodic terms in the tables 
are of the order of  0".1.

E.A. Whitaker and R.J. Greenberg (10.101.011) have remeasured 
all the available plates showing Uranus V and established that this 
satellite has both a pronounced orbital eccentricity and inclination 
(to the plane of the other satellites).

Last but not least, Duxbury has reported that the observations 
of  the Galilean satellites made by Pioneer 10 will lead to data 
that can be used for the purposes of improving ephemerides.


C. Theoretical Work

The motions of the satellites of Jupiter and Saturn are being 
extensively investigated. D.T. Vu and J.L. Sagnier have studied 
Sampson's theory of the motions of  the Galilean satellites of 
Jupiter (11.099.221). Vu is now programming the algorithms of 
Sagnier's theory in view of  its application to the planetary 
problem in general, and to the Galilean system in particular. At the 
Jet Propulsion Laboratory (JPL), J. Lieske has also been working on 
Sampson's theory (10.099.005; 11.099.206; IAU Colloq. 28).

Ferraz-Mello has developed a theory of the motions of the 
Galilean satellites to the second order of the inclinations and 
eccentricities. Integro-differential equations that appear in the 
perturbation problem and nonlinear equations in the angular 
variables have been solved by  Krasinsky's method (IAU Symp. 62). 
Ferraz-Mello has also compared classical observational data with 
Sampson's theory when the masses used are those determined at JPL 
from Pioneer 10 and 11. He shows that it is impossible to reconcile 
the value of one of the free-oscillation coefficients in the 
longitude of Ganymede with the JPL mass of  Callisto (Revista Soc. 
Astron. Brasil, in press).

A numerical study of the Galilean system is being made by P.E. Nacozy 
(University of Texas at Austin) and M. Sato (University of Sao Paulo).

E.N. Lemekhova, at the Institute for Theoretical Astronomy 
(ITA) in Leningrad, continued to improve the parameters and to make 
a comparison with the parameters in Marsden's theory of the Galilean 
satellites. Peters has made an analysis of the ephemerides of the 
Galilean satellites  (10.099.066).

The motions of the outer satellites of Jupiter have been 
studied at Tomsk University by T.V. Bordovitsina and L.E. Bykova 
(10.099.088; 10.099.089). Corrections to the mass of Jupiter have 
been obtained from the motions of satellites VI, IX and X 
(10.099.090; Trudy Tomsk. Univ. 251, 84, 1973). The results are in 
good agreement with other modern determinations.

Ross' theory of Jupiter VI has been compared with numerical 
integration by R. Dvorak and B. Morando, and similar work is 
underway for Jupiter VII. A. Bec and Morando are contemplating the 
construction of an analytical theory of the motion of Jupiter VI 
using programs prepared for lunar theory.

A.T. Sinclair has attempted to understand the origins of the 
commensurabilities among the satellites of Saturn and Jupiter. It 
appears that those in the Saturn system can be satisfactorily  
explained by the action of tidal forces, but this explanation does 
not work for the Jupiter system (08.100.018; 11.100.201; Monthly 
Notices Roy. Astron. Soc. 171, 59, 1975). He also gives a new theory 
for the motion of Iapetus, fitted to recent observations and using 
analytical expressions for the short-period perturbations and a 
numerical integration for the motion of the orbital plane 
(12.100.214). Another paper by Sinclair is on the orbits of Tethys, 
Dione, Rhea and Titan (H.M. Nautical Almanac Office Techn. Note n.36, 1974).

A new analytical theory for the motion of pairs of satellites 
in mean-motion resonance is being constructed and applied to 
Enceladus and Dione by W.H. Jefferys and L.M. Ries (Astron. J. 80, 876, 1975).

Greenberg has completed a study  of the Laplace relation among 
the satellites of Uranus and has been analyzing commensurabilities 
among the apsidal precession periods of the satellites of Saturn and 
Uranus (Monthly Notices Roy. Astron. Soc. 170, 295, 1975, 173, 121, 1975).

V.A. Shor (ITA) has continued his research on the motions of 
the satellites of Mars. From observations during 1877-1971 he 
obtained a new system of  parameters for Struve's theory of these 
satellites. The theory has been compared with ground-based 
observations made in 1973 and with the data acquired during the 
Mariner 9 experiment (IAU Colloq. 28).