APOD: Stereo Helene (2013 Jan 05)

[APOD Robot]

Stereo Helene

Explanation: Get out your red/blue glasses and float next to Helene, small, icy moon of Saturn. Appropriately named, Helene is one of four known Trojan moons, so called because it orbits at a Lagrange point. A Lagrange point is a gravitationally stable position near two massive bodies, in this case Saturn and larger moon Dione. In fact, irregularly shaped ( about 36 by 32 by 30 kilometers) Helene orbits at Dione's leading Lagrange point while brotherly ice moon Polydeuces follows at Dione's trailing Lagrange point. The sharp stereo anaglyph was constructed from two Cassini images (N00172886, N00172892) captured during a close flyby in 2011. It shows part of the Saturn-facing hemisphere of Helene mottled with craters and gully-like features.

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[Case]

... about 36 by 32 by 30 kilometers ...
[The image] shows part of the Saturn-facing hemisphere of Helene ...

Wikipedia lists 43.4×38.2×26 km from a P.C. Thomas study (2010, Cornell University) on the "Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission".

Table 3
Properties of irregularly-shaped saturnian satellites.
Moon: Helene
Radius "a" (Saturn-facing): 21.7 ± 0.5 km
Radius "b" (orbit-facing): 19.1 ± 0.3 km
Radius "c" (polar radius): 13.0 ± 0.3 km
Mean radius (radius of a sphere of same volume): 17.6 ± 0.4 km

The 3D effect is huge. To me, in this image it looks like the depth is much more that the width, like the small profile angle on a football. But the numbers seem to suggest that we’re looking at the biggest possible profile, namely the Saturn-facing side.

[orin stepanek]

I thought this picture looked familar! http://apod.nasa.gov/apod/ap110623.html

http://asterisk.apod.com/discuss_apod.php?date=110623

[neufer]

I thought this picture looked familar! http://apod.nasa.gov/apod/ap110623.html

http://asterisk.apod.com/viewtopic.php?f=9&t=28410

http://apod.nasa.gov/apod/ap120502.html
http://asterisk.apod.com/discuss_apod.php?date=120502

[cdavenport]

So, if either of these ice moons were orbiting the Sun instead of Saturn, they might be comets?

[quigley]

Is Helene made up of water ice? Is it chunks of ice that appear to flow into the "crater"-like shapes to form the gullies?

[jsmunroe]

Is there anyway you could do a "cross-eyed" stereography for those of us who don't have the special glasses? That is, put the images next to eachother in such a way that they one can either cross-their eyes or stare parallel at the images in order see the three dimensional image? Most of the time, I do this myself, but the images are scaled differently for this one.

[Chris Peterson]

Is there anyway you could do a "cross-eyed" stereography for those of us who don't have the special glasses? That is, put the images next to eachother in such a way that they one can either cross-their eyes or stare parallel at the images in order see the three dimensional image? Most of the time, I do this myself, but the images are scaled differently for this one. :(

I find anaglyphs to be an entirely unsatisfactory way of viewing 3D images, so for anything interesting, the first thing I do myself is construct a proper stereo pair. Here's mine for this set of images:

[neufer]

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Is there anyway you could do a "cross-eyed" stereography for those of us who don't have the special glasses? Most of the time, I do this myself, but the images are scaled differently for this one.

I find anaglyphs to be an entirely unsatisfactory way of viewing 3D images, so for anything interesting, the first thing I do myself is construct a proper stereo pair. Here's mine for this set of images:

[dnwdfw@verizon.net]

Do objects captured in Trojan orbits rotate relative to the objects they lead/follow? Is the dynamic such that they're gravitationally locked to their parent bodies? Is there a practical limit to the size of a Trojan object?