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.