Rhea and Tethys as the third target of the Cassini–Huygens's mission

NASA, ESA and ASI started the Cassini-Huygens mission in 1997 to study the Saturn system, including its moons and its rings. The probe consists of the NASA Cassini orbiter, named from the Italian scientist Giovanni Domenico Cassini who discovered many things about the Saturn system such as the moons Rhea and Tethys, and the ESA Huygens lander, named from the Dutch scientist Christiaan Huygens.

This short movie of Tethys and Rhea will be stitched together from 27 images taken 60 seconds apart. 

Rhea and Tethys are the ones worthy to be investigated because of its mysteries. Moreover the occultation is also one of the best moment to accurately estimate their orbits that change over time, determinating the shape of Rhea and the observations of its tenuous atmosphere.

Tethys is one of the brightest objects in the solar system because of its extremely water-ice-rich composition that remains unexplained. Its surface is characterized by many cracks caused by fractures in the ice and it hides intriguing secrets: there are red arcs on the surface discovered by NASA’s Cassini spacecraft combining clear, green, infrared and ultraviolet spectral filters to create the enhanced-colour views. About how they are generated and why they are red there are not definite answers yet. Maybe it is the result of gas jets, but some scientists have also suggested that they could be fractures in the ground or exposed ice with chemical impurities.
Furthermore, this mission will greatly improve our knowledge about the unknown Rhea's internal structure. Rhea is an ice body and before the Cassini's mission scientists assumed that the moon had a rocky core. Anyway after the first flyby the measurements claimed that the axial dimensionless moment of inertia coefficient implying a homogeneous structure with a small compression of the ice inside and some models suggest that maybe there is a internal liquid-water ocean.

Rhea also allows scientists to study how often meteoroids impact with the surface. For this purpose a spacecraft camera provides some images of its surface at different wavelengths. What is more, in this case the meteorite bombardments become particularly useful to the cosmic dust analyzer instrument which can detect the dusty debris that leave the surface after these tiny impacts necessary to estimate the bombardment and the contamination rate of Saturn system.

The third target would also allow scientists to observe if there is the ring system that on March 6, 2008, they announced Rhea might have. This hypothesis was suggested because when Cassini passed by Rhea, scientists observed changes about the flow of electrons trapped by the magnetic field of Saturn and it was later found a set of ultraviolet-bright spots distributed along the equator perhaps generated by ring materials impact. Nevertheless, no evidence of ring material was found and scientists are looking for another explanation.

Choosing this target allows us to find answers to some of these greatest Saturn system mysteries and face new questions and other amazing science adventures.


These are some useful links for the Cassini Scientist for a Day's details (click on acronyms): European Space Agency (ESA), National Aeronautics and Space Administration (NASA) e National Institute for Astrophysics (INAF).