A lucky cosmic alignment will allow NASA's Cassini spacecraft to swing up close to two of Saturn's moons back-to-back this week.
The probe is scheduled to make a flyby of Titan, Saturn's largest moon, on Monday. Then in the early hours of Wednesday Cassini will make a close approach to the medium-sized icy moon Dione.
The special alignment of the moons at this time means that the spacecraft should not have to make any maneuvers in between to adjust course.
The Titan flyby will take Cassini to within about 4,700 miles (7,500 kilometers) of the moon's surface. Though the craft has been closer on past swings near this intriguing body, this time the slightly wider view will allow Cassini's cameras to stare at Titan's haze-shrouded surface for a longer time. Scientists hope the probe will be able to capture high-resolution pictures of two dark regions around the equator, called Belet and Senkyo, that appear to ripple with sand dunes.
On Wednesday Cassini will make its closest approach ever to Dione, plunging to within about 300 miles (500 km) of the moon's surface. It will be the spacecraft's second flyby of this moon, which is less famous than its siblings Titan and ice-covered Enceladus.
Based on the first Cassini flyby of Dione in October 2005, and data taken earlier by the Voyager spacecraft, scientists think the moon could be sending out wisps of charged particles into the magnetic field around Saturn and potentially exhaling a diffuse plume that contributes material to one of the planet's rings.
Dione wouldn't be the only Saturnian moon with a plume: Enceladus is famous for its geysers full of water vapor that shoot off the surface. But Dione's plume would likely be subtler and produce less material than those on Enceladus, which have been dubbed "Cold Faithful."
Cassini will use onboard instruments that measure magnetic fields and mass, electrical charges and densities of atomic particles on Dione to search for signs of activity.
The Cassini orbiter was launched in 1997 as part of the Cassini-Huygens mission, a joint project of NASA, the European Space Agency and the Italian Space Agency. Cassini arrived at Saturn in 2004, and was slated for decommissioning in September of this year, but was recently granted a life extension through 2017.
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