Phoebe Science Highlights
The Cassini spacecraft will fly by Phoebe, one of Saturn's most mysterious moons, on its final approach to Saturn. The flyby is scheduled in Friday June 11, 2004. This will be the only flyby of an outer satellite of Saturn and is the first close flyby of an irregularly shaped satellite.
Image right: Voyager 2 took these images of Saturn's outer satellite Phoebe, on Sept. 4, 1981, from 2.2 million kilometers (1.36 million miles) away. This pair shows two different hemispheres of the satellite. Image credit: NASA/JPL
Phoebe is one of the most intriguing satellites, orbiting at a distance of 12,952,000 kilometers (about 8,050,000 miles) from the planet. While most of Saturn's other satellites orbit directly in Saturn's ring plane, this moon orbits in an inclined plane (175.3 degrees). Phoebe also orbits the planet in the opposite direction (retrograde) with respect to the other satellites. These anomalies, combined with its irregular shape have led scientists to speculate on the possible origins of Phoebe. Is Phoebe a Kuiper belt object, captured into Saturn's orbit? Science data collected during Cassini's close flyby of Phoebe will help determine where the satellite came from and what mysteries its chemical composition might reveal.
What will Cassini's Science Instruments Do at Phoebe?
The Imaging Science Subsystem (ISS) will collect images with an anticipated resolution of up to 15 meters (49 feet) per pixel. These data will help determine surface properties, geological history, surface age, body shape, local topography, and distribution of surface materials.
Multi-color mapping of almost the entire surface at 0.3 to 2.1 kilometers (0.2 to 1.3 miles) per pixel is planned.
The Visual and Infrared Mapping Spectrometer (VIMS) will obtain the first ever resolved spectra of the surface of Phoebe, up to 0.5 kilometers (0.3 miles) per pixel at closest approach, with full range 0.4 to 5-micron spectra. This data will be used to derive detailed maps of Phoebe's surface, allowing scientists to understand what materials lie on the satellite's surface.
Phoebe is a fascinating target for the Composite Infrared Spectrometer (CIRS), due to its unusual surface composition compared to most of the icy satellites, and its warm temperatures, which will provide significant signal to noise data. CIRS also plans to conduct global mapping of composition and both day and night temperature variations.
The Ultraviolet Imaging Spectrograph (UVIS) will measure the reflective properties of Phoebe's ultraviolet surface, providing the first ultraviolet map of this satellite (more similar to an asteroid than to the other icy moons of Saturn). The UVIS measurements will aid in understanding Phoebe's compositional makeup and distribution of volatile chemicals.
RADAR observations will penetrate Phoebe's surface to between 2 and 20 centimeters (0.8 and 7.9 inches), and will measure the bulk density and/or the relative ice cleanliness in the upper layer of rock and soil lying just above the satellite's crust, known as the regolith.
The Radio and Plasma Wave Science Instrument (RPWS) will look for evidence of any interaction between Phoebe and the solar wind.
At about the spacecraft's closest approach to Phoebe, Cassini will cross the Hill sphere of this icy moon. Particles detected inside the Hill sphere most likely originate from the surface of the moon. The Cosmic Dust Analyzer (CDA) measurements starting at closest approach to Phoebe will provide important information about the composition of the moon's surface as well as about what processes generate the dust.
A key measurement will be the determination of Phoebe's bulk density using mass derived from radio science tracking and volume from imaging data. Based on its possible low density, this will help determine if it is a highly porous 'rubble pile', as had been suggested for one of Jupiter's similar sized moons, Amalthea. Or perhaps it might be a more compact, less porous body as might be suggested from its rough, irregular shape. The density value will also used to understand Phoebe's composition and indicate the rough proportions of rock and ice in its composition. This will ultimately assist scientists in understanding the origin of the satellite.
The flyby may also provide insight into the possible interrelationships between the dark side of Iapetus, Phoebe and Hyperion, Titan and the small outer satellites. Geologic, morphologic and compositional evidence should also point to the origin of Phoebe: the Kuiper belt, a Centaur or even the main asteroid belt.