Partners include the U.S. Air Force, the U.S. Department of Energy, and academic and industrial scientists from 19 countries. Around the world, more than 250 scientists study the data collected by Cassini.

"Enceladus has really captured our imagination," Planetary Division Director James Green said during an August 1 briefing at NASA headquarters. "In reality, it's nothing like we've ever seen here on Earth."

The Cassini scientists have many questions. They want to know, for example, how a body 505 kilometers wide has enough internal heat to drive a spewing plume of mostly water into space at distances much greater than its size.

They also want to know the exact composition of the plume, and whether one or more of the geographic features (called tiger stripes) found on the moon's surface in 2005 are contributing to the plume.

There is one way to find out.

Close Encounter
Four NASA spacecraft have traveled the 1.5 billion kilometers from Earth to Saturn. Pioneer 11 was first to fly past Saturn in 1979. Voyager 1 flew past a year later, followed by its twin, Voyager 2, in 1981.

Cassini and its probe, Huygens, are the first spacecraft to explore Saturn's system of rings and moons from orbit. After a seven-year voyage, Cassini entered Saturn's orbit in 2004 and started sending back images and data. The orbiter and the probe both had arrays of instruments and cameras for gathering images in different conditions and light spectra, from visible light to the infrared.

In December 2004, Cassini ejected the Huygens probe. After a 20-day coast, the cone-shaped probe descended into the cloudy atmosphere of Titan, another of Saturn's moons. Three sets of parachutes slowed the probe and provided a stable platform for scientific measurements of the atmosphere's chemical composition and clouds. (See related article.)

Nearly two and a half hours after entering Titan's atmosphere, the probe landed near the equator. It survived the impact and communicated with the spacecraft for a few minutes after landing on Titan's frozen surface. Huygens' traveled farther than any human-built object to land on a celestial body.

Cassini's four-year mission includes more than 70 orbits around Saturn and its moons. A flyby of Enceladus in February 2005 swept within 1,180 kilometers of the moon's wrinkled surface. Another in July 2005 brought the spacecraft within 172 kilometers of the surface.

In March 2008, Cassini will be within 30 kilometers to 100 kilometers of the surface, and the spacecraft "will clearly be immersed in the plume," Green said. "Being right there in situ, having that material thrown up from the surface allows us to take a good look at what might be underneath those tiger stripes and below the surface."

The spacecraft will not be able to take a sample of the plume, but a range of instruments will allow the scientists to determine the size of particles in the plume, along with its temperature, composition and structure.

Proposed Trajectory
The full plume towers over the 505-kilometer-wide moon and is at least as tall as the moon's diameter. Individual jets contribute to the plume and, as reported in Science magazine in 2006, imaging scientists believe the jets are geysers erupting from pressurized subsurface reservoirs of liquid water whose temperature is above 0 degrees Celsius.

Over the next two months, the project team at NASA's Jet Propulsion Laboratory at the California Institute of Technology will analyze Cassini's proposed trajectory and the safety of that path.

"This is something Cassini wasn't designed to do but we think it can accomplish safely," said Alan Stern, associate administrator for the Science Mission Directorate at NASA headquarters. "We didn't know these plumes existed, much less be able to get in and sample them, flying right down over the deck on this little icy world. It is just going to be spectacular."

USINFO is produced by the Bureau of International Information Programs, U.S. Department of State.

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