APOD: Saturn, Tethys, Rings, and Shadows (2015 Apr 05)

[APOD Robot]

Saturn, Tethys, Rings, and Shadows

Explanation: Seen from ice moon Tethys, rings and shadows would display fantastic views of the Saturnian system. Haven't dropped in on Tethys lately? Then this gorgeous ringscape from the Cassini spacecraft will have to do for now. Caught in sunlight just below and left of picture center in 2005, Tethys itself is about 1,000 kilometers in diameter and orbits not quite five saturn-radii from the center of the gas giant planet. At that distance (around 300,000 kilometers) it is well outside Saturn's main bright rings, but Tethys is still one of five major moons that find themselves within the boundaries of the faint and tenuous outer E ring. Discovered in the 1980s, two very small moons Telesto and Calypso are locked in stable locations along Tethys' orbit. Telesto precedes and Calypso follows Tethys as the trio circles Saturn.

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[Boomer12k]

And SILENCE..... Silence....

:---(===) *

[hoohaw]

It is so glorious to have PHOTOGRAPHS --- that look like artist's impressions!

[Dad is watching]

Great photo. I was wondering about the pixel resolution and relative velocities. Would the velocities of the spacecraft and the objects being photographed be sufficient to cause motion blur. I am assuming that the light levels are low enough to require a longer exposure. (A 'flash' on the camera would be useless.) Or is any motion blur restricted to the resolution of a single pixel, making it of no consequence?

[Chris Peterson]

Great photo. I was wondering about the pixel resolution and relative velocities. Would the velocities of the spacecraft and the objects being photographed be sufficient to cause motion blur. I am assuming that the light levels are low enough to require a longer exposure. (A 'flash' on the camera would be useless.) Or is any motion blur restricted to the resolution of a single pixel, making it of no consequence?

The Sun is still very bright at the distance of Saturn. These are basically daylight images, so the exposures are short (well under a second, I imagine). So at the distance of these objects, there is no significant motion blur.

In some cases, with very close flybys, spacecraft change their orientation dynamically during exposures to eliminate motion blur. But not here.

[geckzilla]

It's not moving fast enough or taking long enough exposures for motion blur, but it's sure moving fast enough to be a pain in the butt aligning images when several exposures are taken for color imagery. All the moons have to be scooted around to match up with one image and often the edge of Saturn has a bit of a coloration to it, confounding the possibility that Saturn's atmosphere actually does turn a little blue on the edge. Apparently there are some very strict procedures for scientific processing of color imagery from the Cassini mission. Well, that's not surprising at all, actually. Just not a well-known fact.

[ta152h0]

How nuch longer can Cassini operate ? Crash in to one of the moons with the camera rolling ?

[geckzilla]

How nuch longer can Cassini operate ? Crash in to one of the moons with the camera rolling ?

Its mission was extended so it still has a couple more years. A quick Googling says sometime in 2017 it will be directed into Saturn's atmosphere. I know one thing they are very concerned about is contaminating the moons. Apparently since Saturn is so big and presumably inhospitable to life as we know it, the probe's addition to it will be negligible compared to possible contamination of impact (plus potentially flying, orbiting debris!) against one of the small moons.

[WilliamAWelch]

Why would they crash Casini? Why not just let it fly until it can't take pictures/do science any more?

[Chris Peterson]

Why would they crash Casini? Why not just let it fly until it can't take pictures/do science any more?

Because if they don't destroy it while they still have control, it will eventually crash into a moon, potentially contaminating it with Earth microbes. That's why the Galileo probe was crashed into Jupiter, too.

[Roberto Capacci]

Awesome