WASHINGTON, Sept. 7 (Xinhua) -- Spacecraft Cassini will make its only close flyby of Saturn's odd, two-toned, walnut-shaped moon Iapetus on Sept. 10, at about 1,640 km from the surface, according to mission updates from NASA.
This flyby will be 100 times closer than Cassini's 2004 encounter, and will be the last time the spacecraft will aim its instruments at this moon.
"Iapetus spun fast, froze young, and left behind a body with lasting curves," said Julie Castillo, Cassini scientist at NASA's Jet Propulsion Laboratory.
Iapetus has a ridge of surprisingly large mountains -- the so-called "belly-band" -- that lies directly on top of the equator. The moon also has a distinct difference in the brightness of its leading and trailing hemispheres, one as bright as snow and the other dark as tar.
The irregular shape, the mountain ridge and Iapetus' brightness contrast are among the key mysteries scientists are trying to solve.
An array of Cassini instruments will home in on Iapetus during the flyby. The full menu of objectives includes plans to: characterize the chemical composition of the surface; look for evidence of a faint atmosphere or erupting gas plumes; and map the nighttime temperature of the surface.
This will be the first Cassini flyby of an icy moon, other than Titan, that's close enough and slow enough to perform radar imaging with Cassini's Synthetic Aperture Radar. A large swath of terrain will be covered, including the equatorial ridge and regions of craters and basins. These measurements may provide the height of some of the features.
In coming weeks, scientists will be analyzing data from multiple instruments. Some results of that analysis will be presented at a planetary science conference in Orlando, Fla., in mid-October.
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.