The spacecraft Galileo, underway since October, 1989, has now completed eight of the scheduled eleven Jovian revolutions in its primary mission to explore the Galilean satellites of our solar system's largest planet. This spacecraft, crippled by the failure of its umbrella-shaped antenna to deploy, has faithfully made its observations and then slowly and painfully transmitted its discoveries back to scientists anxiously waiting on Earth. Planetary scientists have had some of their suspicions confirmed and have made many new discoveries. The four largest satellites of Jupiter, discovered by the Italian scientist Galileo in 1610, are slowly yielding their secrets.

        Galileo was scheduled to study Io, the Galilean satellite closest to Jupiter, as the spacecraft was captured by the planet in December, 1995. Problems with the onboard tape recorder caused that part of the mission to be scrapped lest some of the important data relayed back to the orbiter by the probe descending into Jupiter's atmosphere be lost. Io orbits in Jupiter's radiation belts. Because of damage done by this radiation to Galileo's electronics, no other close flybys of this moon are planned for the primary mission. Still, the orbiting explorer has made distant observations of Io on each orbit of Jupiter. It has documented changes in Io's surface and found more active volcanoes, first discovered by the Voyager spacecraft more than 17 years ago. Closer approaches are tentatively planned as part of Galileo's extended mission if the equipment is still functioning well after completing its primary mission this November.

        The next closest to Jupiter of the Galilean satellites is Europa, the smallest of the four. It has been visited by Galileo twice and is scheduled to be photographed from less than 700 miles on Galileo's eleventh orbit. It is observations of this moon that have revealed some of the most stunning possibilities. This ice-covered moon shows evidence of recent and on-going motion of parts of its icy surface. Scientist believe this could be the result of a liquid water ocean beneath the ice. Primitive life forms may have evolved in this ocean, just as scientists believe living organisms may have first formed on young Earth. The interior of the moon may be kept warm by tidal stresses imposed on it by Jupiter's strong gravitational field as the moon orbits its parent planet.

        The third most distant of the Galilean satellites is Ganymede, the largest planetary satellite in the solar system. This moon is larger than Pluto or Mercury. Galileo has completed all four of is close passages of this satellite. High resolution photographs of its craters have been returned, but the most exciting discovery is this moon's magnetic field and magnetosphere. The magnetosphere is a region surrounding the moon from which high energy solar- wind particles are excluded by the magnetic field. Radiation belts similar to Earth's Van Allen belts are formed here. Radio noise and static have been detected that indicate an ionosphere, and its gravitational field suggests that the core of Ganymede is probably iron. Ganymede "closely resembles a planet like Earth rather than other moons in the solar system," according to a NASA press release.

         Callisto, the most distant of the Galilean moons, has been visited only once by Galileo, but is scheduled for a second close flyby later this month and a third in September. Last December's flyby showed a heavily cratered surface with a surprising lack of very small craters and some smoothing of larger craters. Scientists suggest that some process, perhaps the sliding of material down craters' sides, is responsible for the obliteration of the smallest craters and softening of the edges of the larger craters.

         Tomchin Planetarium's final presentations of the skyshow "The Voyager Encounters" will take place at 7, 8, and 9 p.m. on Friday, June 13, 1997. The shows are free, but reservations are required. Please call 293-3422, Ext. 1443, to make free reservations if you would like to see one of these shows.