The Bordeaux Automatic Transit Circle: First Catalogues, Current and Future Programs

The Bordeaux automatic meridian circle has been used for routine differential observations for three years. Current programs concern NPZT stars, faint nearby stars from the Gliese catalogue, Mars, Uranus, Neptune, Jupiter and Saturn satellites, and minor planets. From 1984 on, 5000 faint stars from the HIPPARCOS Input Catalogue will be observed.

The Satellites of Uranus

The Uranian satellite system contains five known members, all of which are difficult to study owing to their faintness and proximity to Uranus. The photometry of these objects is not in a satisfactory state, nor is the photovisual spectrophotometry. Infrared observations are in some sense easier and more precise because Uranus itself is faint in the near infrared and interferes but little with the satellite studies. The near infrared work reveals water ice or frost on the satellite surfaces, perhaps in a very pure state. The satellites are most similar to Ganymede in terms of the strength of the ice bands, but subtle differences, now under study, may be present. The diameters and masses of the satellites, while not readily measurable directly, can be estimated from generalizations about the surface geometric albedos, assumed mean densities, and certain dynamical arguments. Most of the data and theories are consistent with bodies with radii in the range 160-520 km (similar to the larger asteroids) and albedos on the order of 0.5, consistent with ice and snow. The mean densities are probably similar to those of the icy Saturn satellites, about 1.3 g/cm3. Uranus’ satellites probably formed after the event that caused the planet to tilt to the presently observed obliquity.

Saturn Satellites and Minor Planets Positions in February 1933 at Esq, La Silla

In february 1983. we have observed the Satellites of Saturn and Minor Planets at the ESO, La Silla, Chile. The instrument GRO (D=40cm; f=4m) was used. Having clear skies, magnitude 18 could be attained. Measures and reductions were performed at the Observatoire royal de Belgique with the Ascorecord measuring machine (0,1µ) and by means of five reference stars on the UNIVAC computer of the Royal Institute of Meteorology (SAO Star Catalogue, Least Squaras and Dependences Methods).

Relation Between Light Variations of Solar System Satellites and Their Interaction with Interplanetary Medium

The behaviour of the available light curves of Jupiter and Saturn satellites is shown to change steadily from inner to outer satellites.The change in reflectivity of satellites with orbital motion could be explained assuming that dark meteoroidal material accumulates on a given portion of the satellite's surface, which was once mainly covered with ice or snow. The preferential impact location on the satellite's surface could be a consequence of the perturbing action of the planet on the motion of interplanetary material.