Emily LakdawallaJul 26, 2007

The best view of Helene yet

Cassini's current orbit has been a great one for the imaging of Saturn's icy satellites; there have been lots of images of Tethys, Dione, Enceladus, Hyperion, and a bunch of mutual events of various moons with each other and/or Saturn and its rings. However, the most unusual item is Cassini's closest approach yet to Helene. Helene is a very small moon, a lump approximately 36 by 32 by 30 kilometers in size. Unlike most of Saturn's smaller moons, this one isn't close to the rings; instead, it orbits among the much larger round satellites. This one sits in Dione's leading Lagrange point ("L4"), a position 60 degrees ahead of Dione in the same orbit where the gravity of Saturn and Dione balance each other. (There is also a trailing co-orbital named Polydeuces.)

Just to set up the context, here's the best image we had of Helene before Cassini arrived. I went in to the newly calibrated and geometrically corrected Voyager Saturn data set on Helene to get this one, not that it mattered very much; Helene is only a few pixels across, and not much is visible except that it's lumpy and has at least one really big crater.

Voyager 2's view of Helene

NASA / JPL / PDS Rings Node / Seti Institute

Voyager 2's view of Helene
Voyager 2 captured this image of Helene on August 25, 1981, from a distance of about 341,000 kilometers. This image has been enlarged by a factor of 3.
Helene in day and night

NASA / JPL / SSI / Color by Gordan Ugarkovic

Helene in day and night
These images of Dione's orbital companion Helene were taken by Cassini on February 25, 2006 from a distance of about 68,000 kilometers. The moonlet was lit at a high phase angle so only a crescent is lit by the Sun. Both views are made from the same set of images. The one on the left shows the sunlit crescent, while the one on the right shows details on the night side visible in yellowish Saturnshine.
Helene

NASA / JPL / SSI

Helene
Cassini passed within 36,000 kilometers of Helene to take this image on July 20, 2007. Helene is the leading co-orbital companion to Dione and is a small, irregular body measuring approximately 36 by 32 by 30 kilometers in size. This image has been enlarged by a factor of two.

For a better sense of the topography, here's a crossed-eye stereo pair. To see the 3-D view, stare at one and cross your eyes until the two images overlap in the middle of your vision, then try to focus your vision to get the crossed image to resolve into three dimensions.

Crossed-eye stereo image of Helene

NASA / JPL / SSI / Stereo by Phil Stooke

Crossed-eye stereo image of Helene
Cassini captured these two images of Helene at slightly different times on July 20, 2007. Since Cassini's point of view shifted in the time between the two images, they can be used to create a 3-D view.

Hyperion at a scale of 1 km/pixel
Hyperion

Phoebe at a scale of 1 km/pixel
Phoebe

Pan at a scale of 1 km/pixel
Pan
Atlas at a scale of 1 km/pixel
Atlas
Telesto at a scale of 1 km/pixel
Telesto
Helene at a scale of 1 kilometer per pixel
Helene

Prometheus at a scale of 1 km/pixel
Prometheus

Pandora at a scale of 1 km/pixel
Pandora

Epimetheus at a scale of 1 km/pixel
Epimetheus

Janus at a scale of 1 km/pixel
Janus

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