The Cassini space probe will soon complete its four-year primary mission to study Saturn, its rings, and its moons. Though close to its June completion, the Cassini mission's operators will be busy today, as the spacecraft makes a very close flyby of one of Saturn's more interesting moons, Enceladus.

What interests scientists most about Enceladus is the giant geyser at its south pole. While the moon itself is a mere 500 kilometers in diameter, the geyser, which is composed mostly of micrometer-sized ice particles, extends almost three times that distance into space. The geyser makes Enceladus one of the most geologically active bodies in our solar system.

Though the flyby seems daring, at one point coming as close as 50 kilometers to the surface near the moon's equator, it will be four times that distance when it reaches the outskirts of the plume of vapor and particles. Though the ejected matter leaves the geyser at approximately 400 meters per second, the small size of the particles shouldn't pose a problem to the space probe at the speed and altitude where it will encounter them.

The team hopes to use Cassini's particle analysis equipment to get a better understanding of what kinds of materials are spewing from the planet's interior. While some of the particles are pure water ice, other components include gases like carbon dioxide and methane. Analyzing the composition will help scientists quantify and understand any differences between the plume and the envelope of material that surrounds the entire moon and understand how the plume itself was formed.

Should today's flyby conclude successfully, other, more daring flybys may be planned for the craft's proposed extended mission cycle to begin in August of this year.

NASA's Jet Propulsion Laboratory has created a movie explaining and illustrating Cassini's mission and can be found here (flash multimedia).