Cassini Captures Calypso Close-Up : Big Pic

March 27, 2010 -- This is Calypso, one of Saturn's 60 known moons, as photographed by NASA's Cassini Equinox mission. Measuring only 21 kilometers (or 13 miles) across at its widest point, this potato-shaped Saturnian satellite has a surprisingly smooth surface.

VIDEO: As Cassini's primary mission came to an end (just before it was re-named the "Cassini Equinox mission"), it had already notched up an amazing array of achievements. James Williams gives a recap of the spacecraft's greatest hits.

It is thought that Calypso is as cratered as the other moons in the Saturn system, but it is covered in fine dust, filling in the impact craters. As the spacecraft passed within 21,000 kilometers (13,000 miles) on Feb. 13, Cassini's narrow-field camera managed to capture this view with a resolution of only 128 meters (or 420 feet) per pixel, showing just how dust-covered and featureless the moon is.

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Calypso is a special moon as it is engaged in a gravitational dance with a larger moon called Tethys, trailing its orbit by 60 degrees. There's another moon called Telesto (slightly larger than Calypso, but equally as dust covered) leading Tethys in its orbit by 60 degrees. Both Calypso and Telesto are known as "Tethys Trojans," two of only four known "Trojan Moons" in the solar system. (The other two trojan moons are called Helene and Polydeuces, leading and trailing the larger moon Dione, also orbiting Saturn.)

BIG PIC: Tethys may have the gravitational upper-hand over Calysto, but from Cassini's point of view earlier this year, the large moon Titan swallowed Tethys (in a manner of speaking).

So why is Calypso (and Telesto) trapped in this 60 degree trojan orbit? Calypso is sitting in a region of gravitational calm known as a Lagrangian Point (in this case, it's the "L₄ point") where the combined gravitational pull of both Saturn and Tethys balance out creating an island of gravitational stability. As the Saturnian system evolved, its many moons gradually growing from dust and smaller rocks sticking together, Calypso found itself being dragged around its orbit by the motion of Tethys' L₄ point (like a dog on a gravitational leash).

Before now, only very fuzzy photographs of Calypso have been available (originally shot by Voyager 2 in Aug. 1981 and later by Cassini -- from over 100,000 km away -- in Sept. 2005), so this stunning Calypso portrait is the most detailed ever taken, definitely one for the Cassini hall of fame.

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--Ian O'Neill, Discovery News

Image credit: NASA/JPL/Space Science Institute

Source: NASA Cassini Equinox mission website