The observations carried out by CASSINI during its passage close to Phoebe are the only ones that the probe will collect during its 4 years mission around Saturn. Indeed, Phoebe is too distant from Saturn - almost 13 million kilometers - to allow a new meeting. Whereas the Voyager probes had been able to lead only one coarse study of Phoebe at the beginning of the 1980’s, the data collected at short distance by CASSINI will allow a much more detailed study of this object, eccentric in many regards. It has an inclined, retrograde and chaotic orbit, indicating that it most probably acts of an asteroid captured after the formation of the Kronien system. © NASA/GSFC The image above represents a thermal cartography of the hemisphere of Phoebe visible during the phase of approach. It was obtained by the instrument CIRS (Composite Infra-Red Spectrometer) a hundred minutes before the flyby at short distance. The left image is a cartography of the intensity of the thermal radiation emitted by the surface of the satellite around 16 microns in wavelength, while the middle image is its transcription in terms of surface temperature. The zones exposed to the Sun reach a temperature of 106 K (or -167 degrees C) while the temperature of the shadow areas is between 85 K (or -188 C) and below 75 K (or -198 C, the CIRS detection limit). The thermal emission of the "night" area of the satellite is clearly detected whereas the image taken by the ISS camera in the visible (on the right) does not show any detail in this zone. In the terminating area (limit between night and day), the topography of the ground prints its mark on the distribution of the temperatures : the bottom of the craters and depressions is appreciably colder because of the lack of illumination. In addition to the detailed study of the topography and nature of the ground by the ISS camera (Imaging Sub-System) and the thermal cartography carried out by CIRS, one also expect from this exceptional flyby the multispectral cartography in the visible and the infra-red by VIMS (Visible Infrared Mapping Spectrometer), the cartography of reflectance in ultraviolet ray by UVIS (Ultraviolet Imaging Spectrometer), the survey of the surface layers to a depth of a few centimetres by the RADAR, the search for an interaction between Phoebe and solar wind by RPWS (Radio and Plasma Wave Science), as well as a better determination of the period of rotation and mass of the satellite. But the most spectacular is still to come, with the placing in orbit and the first meeting with the satellite Titan, all this in about thirty hours. The first images taken during orbital insertion are expected on July 1 at 12h39 UT.
CASSINI enters the Saturn system
On June 11 at 20h56 UT, the CASSINI probe (launched in October 1997) made its official entry in the Saturn system by flying closely over Phoebe, the most remote satellite of the planet with rings. This flyby, carried out 2000km away and at the speed of 6.4 km/s, is the prelude to placing CASSINI in orbit around Saturn which will occur on July 1st at 2h36 UT, at the end of a tour of 3,5 billion km. Some researchers from Paris Observatory are associated to the majority of the experiments of the Cassini-Huygens mission.