Saturn encounter of the close-in kind

Voyager 1, the robot planetary explorer, is closing in on one of the most intriguing objects in the solar system -- Saturn. Here at the Jet Propulsion Laboratory (JPL) of the US National Aeronautics and Space Administration, headquarters of the Voyager mission, the planet's image seems to be everywhere. In the control center, in scientists' workrooms, in the press center, and even in the cafeteria, TV monitors show it growing steadily larger. It is a quietly insistent reminder that on Nov. 12 a semi-intelligent, space-faring automaton is scheduled to give humanity a breathtaking close-up view of another world.

Already, the famours rings -- which once looked to their discoverer, Galileo, like a pair of indistinct cup handles -- show a wealth of detail. Besides the three main rings and the dark division between two of them that can be seen from Earth, dozens, perhaps hundreds of other bands are beginning to appear. Dark fingerlike projections that appear are a major puzzle. It now is obvious that the entire theory of Saturn's rings will have to be rethought.

Glowing with vivid oranges, blues, and greens of computer-enhanced color, Saturn itself is beginning to show intriguing detail. Perhaps because of a high-level atmospheric haze, the planet has had a rather bland appearance. But as its image grows larger, and with contrast enhanced by the computer processing , a few light and dark spots now can be seen in middle northern latitudes, plus a host of pararell light and dark bands in the Southern hemisphere. A vast equatorial jet stream -- 400 meters per second (900 miles per hour) -- appears to be blowing.

Such things whet the anticipation of JPL scientists who are braced for a flood of new knowledge about Saturn which will drastically alter their preconceived concept.

The spacecraft is following somewhat the same trajectory taken by Pioneer 11 when it swepts by Saturn Sept. 1, 1979 -- down past the outer edge of the rings, below the rings, past Saturn's cloud tops, and up again and out into the deep space beyond the solar system.

Pioneer 11 gathered valuable information on the Saturn environment, uncovered some detail of the rings, and measured some of the characteristics of Saturn. But it was not equipped to make a proper study. Its mission was a preliminary reconnaissance of Jupiter. The trip to Saturn was a bonus made possible by a favorable planetary alignment that enabled controllers to adjust the spacecraft's trajectory so that Jupiter's gravity deflected it to Saturn.

Voyager 1, and its sister spacecraft, Voyager 2, are equipped for detailed study of both Jupiter and Saturn. Indeed, Voyager 2 may also survey Uranus and Neptune if its equipment holds up. With sensors to detect magnetic fields and particles, with systems to form images at a variety of electromagnetic frequencies, they can measure key aspects of Saturn, its moons, and their environments and determine their composition at least in the outer parts.

Voyager 1 began its intensive data taking Aug. 22 and will continue it through Dec. 15. Its close encounter next week will bring the spacecraft within 1,000 kilometers (625 miles) of Titan, the largest moon in the solar system, past several other moons, and within 124,000 kilometers (77,000 miles) of the planet itself.

Next August, Voyager 2 will make comparable explorations, coming in along a somewhat different route. Between them, the twin spacecraft are expected to compile a vast data bank that will be intensively studied for decades.

There are many tantalizing questions. Titan, as large as the planet Mercury, has a largely methane atmosphere. Of what else does it consist? How deep is it? The size of the underlying body is unknown. How dense is the atmosphere? Current estimates of surface pressure range from 2 percent to 200 percent of Earth's sea-level pressure. What is Saturn's makeup? It seems to be mainly hydrogen with some helium, perhaps a small rocky core rounded by a layer of hydrogen pressed into a metallic, electrically conducting state.

Scientists know less about the Saturn system than they did about Jupiter before the Voyagers approached that planet earlier. For example, they do not even know roughly of what Saturn's 15 moons are made. But, within less than a week, they will know that and a great many more details.

Thus it is that Voyager is expected to define for the first time what the Saturn system is really like. What they find will help to define the Saturn system and help scientists understand how it formed and what it implies for the formation of the solar system as a whole.