Shining brilliantly high in our eastern sky during the pre-dawn hours of these early November mornings is the planet Jupiter, our solar system's largest planet. Even a pair of binoculars will reveal the presence of four large moons orbiting around the giant planet. These four objects, christened Io, Europa, Ganymede, and Callisto, were discovered by the Italian astronomer Galileo Galilei with his primitive telescope back in 1610, and collectively are known as the Galilean satellites of Jupiter.

Many more satellites of Jupiter are now known, although almost three centuries would elapse after Galileo's discovery before another satellite would be discovered around this planet. As a result of spacecraft visits as well as modern astronomical instrumentation, today almost forty moons around Jupiter are known, half of these having been discovered just within the past two years. Most of these objects are very small and distant, being no more than a few miles in diameter and taking up to two years or more to orbit around Jupiter.

The four Galilean moons are all large objects, each being almost as large as Earth's moon if not larger. (Ganymede, in fact, is the largest moon in the entire solar system, and is even larger than the planet Mercury.) As a result of encounters by the twin Voyager spacecraft in 1979 and of repeated flybys by the Galileo spacecraft that has been in orbit around Jupiter since late 1995, each of the Galilean moons has been revealed to be a unique and dramatic world worthy of much continued study in its own right.

Galileo is reaching the end of its expected mission lifetime, and next September it will be directed to enter Jupiter's atmosphere in a final death plunge. But before it does so, it has one final encounter with a Jovian moon, which in fact has been taking place this week. The object being visited, however, is not one of the Galilean moons, but rather is the much smaller, fifth-discovered satellite of Jupiter, known as Amalthea.

Amalthea was discovered in 1892 by American astronomer Edward Barnard at Lick Observatory in California; it has the interesting distinction of being the last planetary moon in our solar system to be discovered visually, i.e., by someone's looking through the eyepiece of a telescope. It is closer in toward Jupiter than are the Galilean moons, and orbits around the giant planet once every twelve hours. Photographs taken at a distance by the Voyagers and by Galileo show that Amalthea is an oblong object, about 170 miles long in its longest dimension. This longest dimension remains oriented toward Jupiter as Amalthea whizzes around its parent planet.

The spacecraft images show that, not unexpectedly, Amalthea's surface contains several moderately-sized impact craters. These images also show that Amalthea's surface has a distinctly reddish coloration. This is very probably due to sulfur that has originated from volcanic eruptions on Io (the innermost Galilean moon); Jupiter's intense magnetic field spreads these sulfur particles into a wide tubular ring that surrounds the planet, and as Amalthea travels through this ring it sweeps up these particles and they are deposited onto its surface.

Shortly after 11:00 P.M. MST this past Monday, November 4, Galileo was due to pass just 100 miles above the surface of Amalthea. Unfortunately, Galileo's camera and other imaging equipment have been turned off, so we won't be able to get any close-up photographs of Amalthea like we have the Galilean moons. However, by observing how Amalthea's gravity affects Galileo's trajectory we will be able to determine Amalthea's mass, and consequently its overall density and composition. This information will in turn help in testing theories about how Jupiter's system of moons formed -- and can also be applied to ideas about the formation of our solar system as a whole.

While this week's meeting with Amalthea is Galileo's last encounter with a Jovian moon, this is not the end of science-gathering for Galileo. During the coming days and weeks Galileo will be traveling through Jupiter's dust ring, and will be making measurements of the size and speed of the dust particles within it. Galileo will also be passing closer to Jupiter than it ever has before, and will be making measurements of the extremely intense radiation environment at that distance. (It is well within the possibility that the electronics aboard Galileo will be completely fried by this environment.) The data gathered during this approach will be useful in designing future space probes that can visit the region around Jupiter and the Galilean moons.

Once it completes this swing close by Jupiter and heads back away from the planet, Galileo's mission is all but done, at least until it makes its final plunge ten months from now. Despite several severe technical problems, this intrepid little explorer has given us a tremendous amount of information about Jupiter and the Galilean moons, and will leave quite a legacy behind it.