NASA's Cassini spacecraft will pass the geyser moon Enceladus and Titan within less than 48 hours. The alignment of the moons means that Cassini can pass them both with no maneuver in between.

Lights on the Enceladus plumes
Cassini will make its closest approach to Enceladus late at night on May 17 Pacific time, which is in the early hours of May 18 UTC. The spacecraft will pass within about 435km of the moon's surface.

The main scientific goal at Enceladus will be to watch the sun play peekaboo behind the water-rich plume emanating from the moon's south polar region. Scientists using the ultraviolet imaging spectrograph will be able to use the flickering light to measure whether there is molecular nitrogen in the plume. Ammonia has already been detected in the plume and scientists know heat can decompose ammonia into nitrogen molecules. Determining the amount of molecular nitrogen in the plume will give scientists clues about thermal processing in the moon's interior.

Measure Titans underground ocean
Cassini will primarily be doing radio science during this pass to detect the subtle variations in the gravitational tug on the spacecraft by Titan, which is 25 percent larger in volume than the planet Mercury. Analyzing the data will help scientists learn whether Titan has a liquid ocean under its surface and get a better picture of its internal structure. The composite infrared spectrometer will also get its southernmost pass for thermal data to fill out its temperature map of the smoggy moon.

Cassini has made four previous double flybys and one more is planned in the years ahead. The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency.

More information on the Enceladus flyby, dubbed "E10," is available at: http://saturn.jpl.nasa.gov/mission/flybys/enceladus20100518/

More information on the Titan flyby, dubbed "T68," is available at: http://saturn.jpl.nasa.gov/mission/flybys/titan20100520/