File:Iapetus mosaic color.jpg
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.

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Cassini space probe to flyby Saturn's moon Iapetus