Cassini flew past Iapetus on New Year's Eve 2004, capturing the four visible light images that were put together to form this global view. The scene is dominated by a dark, heavily-cratered region, called Cassini Regio, that covers nearly an entire hemisphere of Iapetus.
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Cassini's Closest Visit of Iapetus
On Sept. 10, the Cassini spacecraft performs its closest flyby during the entire mission of the odd moon Iapetus, passing by about 1,640 kilometers (1,000 miles). Iapetus is a world of sharp contrasts. The leading hemisphere is as dark as a freshly-tarred street, and the white, trailing hemisphere resembles freshly-fallen snow. Scientists want to know more about the composition of the dark material that coats Iapetus. They also want to learn more about Iapetus' distinctive walnut shape and the chain of mountains along its equator.
The Cassini space probe will make a close flyby today of Saturn's third largest moon Iapetus. For reasons that are not well-understood, one side of Iapetus is much darker than the other side. Explanations for the dark side include meteorite deposition as well as possible debris from other moons of Saturn. 

Also, the Cassini probe had previously discovered an equatorial ridge on Iapetus during a flyby in 2004 which compounds Iapetus' already non-spherical shape. The origin of the ridge is also not well understood and scientists have proposed a variety of explanations for its presence. This second flyby will be approximately  1,600 km from the surface and will be about 100 times closer than the previous flyby.  Scientists hope that the detailed observation using radar and photography will provide insight into Iapetus's odd shape and coloring.

Iapetus was first discovered by Giovanni Cassini, for whom the probe is named, in 1671. He was also responsible for discovering that one side of Iapetus was substantially darker than the other.  

This flyby will likely be the last close flyby of one of Saturn's moons by the Cassini probe.