Astronomy Picture of the Day
June 4, 2015

Sisters in the Saturnian Night
Sisters in the Saturnian Night

Credits: NASA/JPL/Space Science Institute - Credits for the additional process. and color.: Dr Paolo C. Fienga/LXTT/IPF

Is the Saturnian moon Dione (about 698 miles - or approx. 1123,319 Km - across) suddenly larger than Rhea (which is about 949 miles - or approx. 1527,26 Km - across)?

No, of course not! The NASA - Cassini Spacecraft simply captured an image when Dione was much closer to its camera, making the smaller moon appear much bigger than her larger "sister moon". Besides their beauty, images like these can help Cassini's Navigators to determine exactly where the Spacecraft is, and confirm that it is on the right course (---> path).

This view looks toward the Trailing Hemisphere of Dione; North is up. The image was taken in Visible Light with the NASA - Cassini Spacecraft Narrow-Angle Camera on April 11, 2015. The view was obtained at a distance of approximately 68.000 miles (such as about 109.435,12 Km) from Dione and at a Sun-Dione-Cassini Spacecraft, or Phase, Angle of 29°. The image scale for Dione is roughly 2165 feet (such as about 659,892 meters) per pixel. Rhea, on the other hand, was about 300.000 miles (such as approx. 482.802 Km) away from the Spacecraft, at a Phase (i.e.: Sun-Rhea-Cassini Spacecraft) angle of 30°. The image scale for Rhea is roughly 2 miles (such as about 3,21868 Km) per pixel.

This frame (which is an Original NASA - CASSINI Spacecraft's b/w and NON Map-Projected image published on the NASA - Planetary Photojournal with the ID n. PIA 18319) has been additionally processed, contrast enhanced, magnified to help the visibility of the details, Gamma corrected and then colorized - according to an educated guess (or, if you wish, an informed speculation) carried out by Dr Paolo C. Fienga - in Absolute Natural Colors (such as the colors that a human eye would actually perceive if someone were onboard the NASA - Cassini Spacecraft and then looked outside, towards the Saturnian moons "Rhea" and "Dione"), by using an original technique created - and, in time, dramatically improved - by the Lunar Explorer Italia Team. Different colors, as well as different shades of the same color, mean, among other things, the existence of different Elements (Minerals) present on the Surface of Rhea and Dione, each having a different Albedo (---> Reflectivity) and Chemical Composition.

Note: it is possible (but we, as IPF, have no way to be one-hundred-percent sure of such a circumstance), that the actual luminosity of Rhea and Dione - as it is in this frame - would appear, to an average human eye, a little bit lower than it has been shown (or, better yet: interpreted) here.

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