Wednesday's flyby will take Cassini within 30 miles of the surface of Enceladus at closest approach, the unmanned probe will be about 120 miles above the moon as it sweeps through the edge of the geysers and measures their chemical makeup.

The carefully orchestrated event will take Cassini "deeper than we've been before," mission scientist Carolyn Porco of the Space Science Institute said in an e-mail.

Scientists long believed Enceladus, the shiniest object in the solar system, was cold and still because it resides hundreds of millions of miles from the sun. But recent evidence shows the Arizona-size satellite is geologically active, with a significant atmosphere and a relatively warm south pole.

In 2005, Cassini surprised scientists when it snapped images of geyser-like eruptions of ice particles and water vapor spewing from the south pole. The dramatic images effectively put Enceladus (en-SELL'-uh-duhs) on the short list of places within the solar system most likely to have conditions suitable for extraterrestrial life.