Saturn watchers on the Cassini mission are preparing for another fly-by, this time of the oddly shaped moon Iapetus.
The Cassini probe is set to pass within 1,600 kilometres of the moon, roughly 100 times closer than the last encounter in 2004. It is also the last time the probe is scheduled to fly past the moon, and mission scientists are keen to capture as much data as they can.
Iapetus has long been a puzzle to scientists. It has a two-tone appearance, where the leading edge is far brighter than the trailing hemisphere. The difference is similar to the contrast between fresh snow and black tar.
Several possible explanations have been put forward, but no consensus has yet been found: could the dark matter, which has a slightly red tinge, be residue from another moon? Could it come from inside the moon? Is it comet debris, or the result of meteoroid impacts?
JPL scientist Amanda Hendrix, of Cassini's ultraviolet imaging spectrograph team, speculates that an event that disrupted the moon Hyperion, which has a similarly dark red tint, could be responsible for depositing the material on Iapetus.
Meanwhile, Dale Cruikshank, at NASA's Ames Research Center, suggests that the dark matter could be pre-biotic organic compounds: the raw stuff of life.
The moon also has a ridge of high mountains running exactly around its equator. The origin of these peaks remains mysterious, too.
The scientists involved with the mission hope this fly by will help clear up some of these mysteries. They are planning to turn Cassini's Synthetic Aperture Radar towards the little world to collect data on the size of various surface features. They also plan to study the chemical composition of the surface, search for any evidence of an atmosphere or erupting gas plumes; and map the night-time temperature of the surface.
The probe will make its closest approach on September 10th. ®
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.