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Planetary Society Blog - 2010 Feb 14
Calypso, only 34 x 22 x 22 kilometers in diameter, occupies the L4 Lagrangian point in Tethys' orbit -- it sits in a point 60 degrees ahead of Tethys. Cassini passed within 22,000 kilometers of the moon on February 13, 2010, for the best look yet at its smooth, icy surface. This view is a color composite from Cassini's raw images, red, green, and blue frames overlaid on a stacked composite of red, green, blue, and clear images. (Stacking mostly serves to decrease the artifacts caused by the JPEG format of the raw images.) The full-size image has been enlarged by a factor of two. Credit: NASA / JPL / SSI / color composite by Emily Lakdawalla
Cassini performed a close flyby of Mimas yesterday, the only one planned for the whole mission. The images from that encounter are not yet on the ground, but there are some preliminary goodies: Cassini got pretty close to Calypso yesterday, on the way in to Mimas. Calypso is one of the smaller moonlets of Saturn at only 34 x 22 x 22 kilometers in diameter. Here's my take on one of the two red-green-blue image sets. At least in the raw JPEG images posted to the Cassini public website, I see basically no color variation across the little moon's surface.
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* Saturn's moon Tethys has two much smaller satellites at its L4 and L5 points named Telesto and Calypso, respectively.
* Saturn's moon Dione has smaller moons Helene and Polydeuces at its L4 and L5 points, respectively.
Click here to enlarge
* The Sun–Jupiter L4 and L5 points are occupied by the Trojan asteroids.
* Neptune has Trojan objects at its L4 and L5 points.
* The Sun–Earth L4 and L5 points lie 60° ahead of and 60° behind the Earth as it orbits the Sun. They contain interplanetary dust.
* One version of the giant impact hypothesis suggests that an object named Theia formed at the Sun–Earth L4 or L5 points and crashed into the Earth after its orbit destabilized, forming the moon.
* The Earth–Moon L4 and L5 points lie 60° ahead of and 60° behind the Moon as it orbits the Earth.
They may contain interplanetary dust in what is called Kordylewski clouds.
<<Kordylewski clouds are large concentrations of dust that may exist at the L4 and L5 Lagrangian points of the Earth-Moon system. They were first reported by Polish astronomer Kazimierz Kordylewski in the 1960s, but there is still controversy as to whether they actually exist, due to their extreme faintness. It is thought by some that they could be a transient phenomenon as the L4 and L5 points are unstable, due to the perturbations of the inner planets.
The existence of a photometrically confirmable concentration of dust at the libration points was predicted by Professor J. Witkowski in 1951. The clouds were first seen by Kordylewski in 1956. Between 6 March and 6 April, 1961 he succeeded in photographing two bright patches near the L5 libration point. During the observation time the patches hardly appeared to move relative to L5. The observations were taken from the mountain Kasprowy Wierch. In 1967, J. Wesley Simpson made observations of the clouds using the Kuiper Airborne Observatory.
The existence of the Kordylewski clouds is still under dispute. The Japanese Hiten space probe, which passed through the libration points to detect trapped dust particles, did not find an obvious increase in dust levels above the density in surrounding space. Most claimed observations have been made from deserts, at sea, or from mountains. The clouds appear somewhat redder than the Gegenschein, indicating that they may be made of a different kind of particle.>>