Science Blog -- NASA's Cassini spacecraft is set to cap off 2004 with an encounter of Saturn's ying-yang moon Iapetus (eye-APP-eh-tuss) on New Year's Eve. This is Cassini's closest pass yet by one of Saturn's smaller icy satellites since its arrival around the ringed giant on June 30 of this year. The next close flyby of Iapetus is not until 2007. 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.