April 2, 2010

Cassini Visits Titan and Dione Next Week

In a special double flyby early next week, NASA's Cassini spacecraft will visit Saturn's moons Titan and Dione within a period of about a day and a half, with no maneuvers in between. A fortuitous cosmic alignment allows Cassini to attempt this doubleheader, and the interest in swinging by Dione influenced the design of its extended mission.

The Titan flyby, planned for Monday, April 5, will take Cassini to within about 7,500 kilometers (4,700 miles) of the moon's surface. The distance is relatively long as far as encounters go, but it works to the advantage of Cassini's imaging science subsystem. Cassini's cameras will be able to stare at Titan's haze-shrouded surface for a longer time and capture high-resolution pictures of the Belet and Senkyo areas, dark regions around the equator that ripple with sand dunes.

In the early morning of Wednesday, April 7 in UTC time zones, which is around 9 p.m. on Tuesday, April 6 in California, Cassini will make its closest approach to the medium-sized icy moon Dione. Cassini will plunge to within about 500 kilometers (300 miles) of Dione's surface.

This is only Cassini's second close encounter with Dione. The first flyby in October 2005, and findings from the Voyager spacecraft in the 1990s, hinted that the moon could be sending out a wisp 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. Like Enceladus, Saturn's more famous moon with a plume, Dione features bright, fresh fractures. But if there were a plume on Dione, it would certainly be subtler and produce less material.

Cassini plans to use its magnetometer and fields and particles instruments to see if it can find evidence of activity at Dione. Thermal mapping by the composite infrared spectrometer will also help in that search. In addition, the visual and infrared mapping spectrometer will examine dark material found on Dione. Scientists would like to understand the source of this dark material.

Cassini has made three previous double flybys and another two are planned in the years ahead. The mission is nearing the end of its first extension, known as the Equinox mission. It will begin its second mission extension, known as the Solstice Mission, in October 2010.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate in Washington. The Cassini orbiter was designed, developed and assembled at JPL.

Titan T-67: A More Complete Picture

This is one of the two most important Titan encounters for the Imaging Science Subsystem (ISS) in the extended mission. These high (about 7,000 kilometer) flybys were designed to provide very long, low phase, high-resolution views of Titan's surface.

On this high-altitude encounter, ISS will perform high-resolution observations during and after closest-approach along the equator from eastern Belet across the trailing hemisphere to western Senkyo, imaging Senkyo at very low phase angles (less than five degrees). The Composite Infrared Spectrometer (CIRS) captures the farthest north vertical profiles of the extended mission, observing the composition and temperature profile at 70 degrees north, and possibly observing the break up of winter/spring vortex.

Dione 'D2' Flyby: Search for Activity

Cassini swoops down to within about 500 kilometers (311 miles) of Dione to "sniff" the moon. Particle and fields instruments will try to determine if Dione is actively spewing particles.

The Composite Infrared Spectrometer (CIRS) begins observations on the approach to this Dione flyby by making a map of the satellite while Dione is in Saturn's shadow, so the heat from the sun is not measured, but any heat from Dione is gauged. The Ultraviolet Imaging Spectrograph (UVIS) and Visible and Infrared Mapping Spectrometer (VIMS) will also image Dione. The Cassini Plasma Spectrometer then takes control for closest-approach to observe the interaction between Dione and Saturn's magnetosphere, with most of the other instruments also taking data. UVIS then will map Dione's surface albedo in ultraviolet to measure composition, while other Optical Remote Sensing instruments  are observing the terrain. The Imaging Science Subsystem (ISS) follows UVIS to do a 20-panel mosaic with the other remote sensing instruments also taking data, to get compositional, thermal and geological information about Dione's sub-Saturnian hemisphere.


On the Net

Cassini-Huygens Mission