Photos from the Voyagers, although distant, show that the border between the two is fairly sharp - although, near the edge, there are some craters on the light side with light rims but dark floors, and some small light spots on the dark side.

This is yet another of the Saturn system's major puzzles - was the dark material erupted up out of Iapetus' interior during its early days, or has it been dumped onto the moon's leading face from an outside source? The "exogenous" (outside-source) theory is favored, because nobody can come up with a good explanation for why Iapetus' interior would vomit up such dark material when none of Saturn's other moons shows a trace of it.

One popular theory is that, over the eons, a steady trickle of material has been blasted off Saturn's dark little outer moon Phoebe by meteor impacts, spiraled in toward Saturn because of various orbital forces, and - since Phoebe is in a "retrograde" orbit going in the opposite direction to the inner moons - eventually smacked head-on into Iapetus, coating its leading side while leaving its shielded trailing side uncoated (with those light spots near its edge being small recent meteor craters that punched through the thin blanket of Phoebe dust).

But there are problems with that theory, too. Spectra indicate that the black material on Iapetus has a distinct reddish tinge unlike Phoebe.

It may be that the incoming rain of micrometeors from Phoebe, as they smash into Iapetus' ice at high speed, are boiling traces of methane in the ice to create new, dark organic material.

But there's also the problem of those craters on Iapetus' light side near the boundary, with dark floors but light rims and central peaks - they may just have happened to scoop up some of the Phoebe dust as it came in over their rims at a low angle, but they do look very much as though they are places where big meteoroids blasted through an outer layer of light-colored ice to expose an underlying layer of dark material, the exact opposite of the "deposited dark blanket" theory.

We just do not know - and we need far better closeup photos of Iapetus' surface features, and VIMS spectra of its light and dark surface materials, to solve the puzzle.

So on September 10, 2007, Cassini will fly by Iapetus at 1000 km distance, at a point in the moon's orbit where parts of both the dark and light regions - and the boundary between them - are clearly lit by the Sun.

Iapetus is one of Saturn's most scientifically important moons, like Enceladus.

But, unfortunately, it orbits the planet at a considerable distance (3.6 million km), and its orbit is tilted 15 degrees relative to Saturn's equator and the orbits of the inner moons, all of which makes it particularly hard to arrange flybys of it by Cassini. As a result, there is only one such flyby scheduled during the primary four-year orbital tour.

Later during Phase IV - on March 12, 2008 - Cassini will make a third flyby of Enceladus at 1000 km range, the last close flyby of any moon other than Titan during the four-year tour.

Cassini's orbit around Saturn, as I said, will steadily shrink during Phase IV.

After its 44th and final Titan flyby on May 28, 2008, it will be in a near-polar orbit (tilted 75 degrees to Saturn's equator), with an apoapsis just beyond Titan's orbit, a periapsis so close to Saturn that it will fly past the ring's edge at only 23,000 km range, and a period of only a week, less than half Titan's orbital period.

34 days later - exactly four years after it first entered orbit around Saturn - Cassini's Primary Tour of the Saturn system ends.

But on that same day, it will be in position to make still another Titan flyby.

And - if it's anything like most spacecraft - it will still be working well enough to commence an optional extended mission.

Indeed, Cassini's working lifetime in Saturn orbit, with a little luck, may easily run a decade or more.

Cassini uses a set of "reaction wheels" - motorized flywheels - to make the most of its attitude maneuvers, only occasionally using its thrusters to hold itself steady while the wheels "unload" built-up spin.

And so, if its thrusters were used only for such part-time attitude control, their fuel supply could keep the craft stable and working for seveal decades.

However, it's a safe bet that Cassini will also use its thrusters for occasional orbital maneuvers to set up more Titan flybys for an extended orbital tour.

Exactly what that tour will be, we just do not know at this point - for it will certainly depend (as Galileo's extended missions did) on the discoveries it makes during that first busy and exciting four years, which are bound to be numerous and surprising.

But we can predict a few likely goals.