On Dec. 31, 2004, NASA’s Cassini spacecraft flew by Iapetus, Saturn’s third largest moon. [‘On This Day in Space’ Video Series on Space.com](https://www.space.com/39251-on-this-day-in-space.html)

Iapetus was discovered by the Italian astronomer Giovanni Cassini in 1671. NASA’s Cassini craft flew by the moon multiple times while it was in orbit around Saturn. This was the first close flyby of Iapetus, and Cassini came within 77,000 miles of its surface. NASA's Deep Space Network received the first data from Cassini's flyby on New Year's Eve at 11:47 p.m. Pacific Time. The Cassini Orbiter captured incredible, up-close images of Iapetus. The images revealed that Iapetus kind of looks like a yin-yang symbol. One hemisphere is as dark as coal, but the other is much brighter. This earned it the nickname "the yin-yang moon." Cassini also discovered a huge ridge that runs along the equator, which earned it a second nickname: the "walnut moon."