APOD: Prometheus Rising Through Saturns F Ring (2010 Oct 19)

[APOD Robot]

Prometheus Rising Through Saturns F Ring

Explanation: What is that dark streak below Prometheus? Although it may look like a shadow or a trail blazed by sweeping up material, computer simulations indicate that the dark streak is better understood as an empty path pulled away by the gravity of Saturn's small moon. The particles don't follow Prometheus so much as glide sideways past where Prometheus used to be. One dark streamer is created during each pass of Prometheus through the F-ring that it shepherds. The streamers were unpredicted and first discovered in 2004 on high resolution images taken by the robotic Cassini spacecraft orbiting Saturn. Close inspection of the surface of Prometheus itself in the above image shows interesting structure and craters. The Cassini spacecraft arrived at Saturn in 2004 and, as it continues to function well, is now expected to continue to send back data and images from the distant ringed world until 2017.

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[/b]

[owlice]

Ah... ah... ah... ah...

(speechless!!!)

[neufer]

Click to play embedded YouTube video.

http://asterisk.apod.com/vie ... 47#p128682

[biddie67]

Long live Cassini !!!

OK folks - referring back to the photo for APOD on Aug 10, both that photo and this photo of Prometheus has it moving in the same direction through the F-ring. How does it get back to the outside of the ring in order to come through it in such a regular pattern? Is it moving in a spiral kind of pattern and touching the F-ring at only one point in its spiral path??

[Chris Peterson]

OK folks - referring back to the photo for APOD on Aug 10, both that photo and this photo of Prometheus has it moving in the same direction through the F-ring. How does it get back to the outside of the ring in order to come through it in such a regular pattern? Is it moving in a spiral kind of pattern and touching the F-ring at only one point in its spiral path??

Well, it's in an unstable orbit because of its proximity to other moons that perturb it. But the main motion with respect to the F-ring isn't in and out, but up and down. Prometheus is slightly inclined with respect to the rings, so every orbit it passes through the ring plane twice, once up and once down. That's when it collides with the F-ring.

[León]

Prometheus with his shadow

[moodle]

Can anyone identify the little dot in the lower right corner of the photo?

[orin stepanek]

arrived

pretty sneaky; each letter was a link I liked the movies of Prometheus making the streamers.
[youtube] http://www.youtube.com/watch?v=2mX2Xpnst9w [/youtube]

[biddie67]

.... arrived ....

Sure missed the embedded string of links last night - too sleepy. Great way to have a quick review of Cassini's approach to Saturn!!!

[thongar]

Reading the caption on this beautiful picture, I am reminded of the following:

Chief Technical Writer - Sir, What is this Ground/Water Interface?

New Technical Writer - Why it's the Beach, Chief.

Chief Technical Writer - Then why the heck don't you use Beach?

When a large object (moon?) travels through a fluid medium (F Ring?) pushing said fluid (tiny Ice particles or the F Ring) aside, the mysterious action is usually identified as a Wake.... at least in English.

but then it could also be "Plowing the field" but still ships through water, airplanes through thin clouds, Rocks through orbiting Ice fields. Its a Wake, my friends.

Don't look technical when the obvious is staring you in the face

[emc]

Its a Wake, my friends.

Click to play embedded YouTube video.

Yeah! I beat neufer!

[neufer]

I liked the movies of Prometheus making the streamers.

Click to play embedded YouTube video.

Which answers biddie67's question:

OK folks - referring back to the photo for APOD on Aug 10, both that photo and this photo of Prometheus has it moving in the same direction through the F-ring. How does it get back to the outside of the ring in order to come through it in such a regular pattern? Is it moving in a spiral kind of pattern and touching the F-ring at only one point in its spiral path??

Prometheus makes just a small nick on the inside of the F-ring and then pulls out.

The spiral wake is all in the ring dynamics resulting from that nick.

[robo]

So Prometheus with its elongated shape, does it tumble or is it like a bullet? I'm assuming the later, but was interested to know for certain.

[Chris Peterson]

So Prometheus with its elongated shape, does it tumble or is it like a bullet? I'm assuming the later, but was interested to know for certain.

Neither. It is tidally locked to Saturn- that is, the same side always faces Saturn. So it is rotating about a single axis, with its rotation period equal to its orbital period, like the Moon.

[neufer]

Can anyone identify the little dot in the lower right corner of the photo?

S/2004 S 6?

Pandora Orbital period: 0.628504213 d
S/2004 S 6 Orbital period: 0.6180116 d
Prometheus Orbital period: 0.612990038 d

S/2004 S 6

---

<<S/2004 S 6 is the provisional designation of a dusty object seen orbiting Saturn very close to the F ring. It is not clear whether it is only a transient clump of dust, or if there is a solid moonlet at its core.

It was first seen by scientists in images taken by the Cassini-Huygens probe on October 28, 2004 and announced on November 8 that year. It appears to be the best tracked object in this region with at least five probable sightings in the period till late 2005. In comparison, two objects in the F ring's vicinity (S/2004 S 3 and S/2004 S 4) that were first seen several months earlier have not been recovered with any confidence. Nevertheless, it continues to be unclear whether there is a solid core to S/2004 S 6 or whether it is just a transient dust clump that will dissipate on a timescale of years or months. Notably, an imaging sequence covering an entire orbital period at 4 km resolution taken on November 15, 2004 (soon after S/2004 S 6's discovery) failed to recover the object, while it has been seen again later. The lighting conditions in S/2004 S 6's part of the orbit were different during these two observations, however, with the discovery being made when the region was strongly backlit by the sun. A suggested resolution of the absence in November is that S/2004 S 6's visibility is primarily due to a diffuse cloud of fine dust that is much brighter in forward scattered light (the conditions of the discovery image), and that the solid core (if any) is small.

S/2004 S 6 has been seen both inside and outside the main F ring, and its orbit must cross the ring. Careful calculations show that the object periodically plows through the ring material, coming within 1.5 km of the densest core e.g. on 9 April, 2005. It has been suggested that a spiral structure in the tenuous material surrounding the F ring may have been a consequence of this. The dusty halo seen in images is sizeable, being around 2000 km in lengthwise extent. The solid object, if any, would be no greater than 3−5 km in diameter based on brightness. Additional evidence came in 2008, as it appears that S/2004 S 6 or a body like it is required to explain the dynamics of the F Ring.>>

[BMAONE23]

So Prometheus with its elongated shape, does it tumble or is it like a bullet? I'm assuming the later, but was interested to know for certain.

Neither. It is tidally locked to Saturn- that is, the same side always faces Saturn. So it is rotating about a single axis, with its rotation period equal to its orbital period, like the Moon.

In the You Tube link provided by Orin

I liked the movies of Prometheus making the streamers.

Click to play embedded YouTube video.

it does appear to tumble, or is it just a trick of light?

[neufer]

[quote="B"MAONE23"]

In the You Tube link provided by Orin
it does appear to tumble, or is it just a trick of light?[/quote]
Prometheus probably wobbles but it doesn't tumble (or fall down).

Even the Earth's moon wobbles back and forth some;
and Prometheus physically interacts with the F-ring
and chaotically interacts with Pandora (and Atlas):

The orbit of Prometheus appears to be chaotic, as a consequence of a mean motion resonance with Pandora. The most appreciable changes in their orbits occur approximately every 6.2 years, when the periapsis of Pandora lines up with the apoapsis of Prometheus and the moons approach to within about 1400 km.

[FrogSplash]

Can anyone identify the little dot in the lower right corner of the photo?

I asked that myself. I'm guessing it's a little speck of the F ring.

FS

[León]

OK folks - referring back to the photo for APOD on Aug 10, both that photo and this photo of Prometheus has it moving in the same direction through the F-ring. How does it get back to the outside of the ring in order to come through it in such a regular pattern? Is it moving in a spiral kind of pattern and touching the F-ring at only one point in its spiral path??

Well, it's in an unstable orbit because of its proximity to other moons that perturb it. But the main motion with respect to the F-ring isn't in and out, but up and down. Prometheus is slightly inclined with respect to the rings, so every orbit it passes through the ring plane twice, once up and once down. That's when it collides with the F-ring.

The other side, apparently looks darker

[neufer]

The other side, apparently looks darker

And the "dark side" of the Earth's Moon is the Far Side?

Art Neuendorffer

[Chris Peterson]

Prometheus probably wobbles but it doesn't tumble (or fall down).

I agree. It's hard to imagine how something this out-of-round, in a highly perturbed environment, wouldn't show something akin to libration.

[emc]

Prometheus probably wobbles but it doesn't tumble (or fall down).

I agree. It's hard to imagine how something this out-of-round, in a highly perturbed environment, wouldn't show something akin to libration.

Wow! No libration… that is amazing. What does that imply? Prometheus in a harmonic orbit with Saturn… isn't that incredible?.. I mean how many things have to coalesce in order for that to happen? Our Moon has libration... I expected this to be the norm.

[Chris Peterson]

Prometheus probably wobbles but it doesn't tumble (or fall down).

I agree. It's hard to imagine how something this out-of-round, in a highly perturbed environment, wouldn't show something akin to libration.

Wow! No libration… that is amazing. What does that imply? Prometheus in a harmonic orbit with Saturn… isn't that incredible?.. I mean how many things have to coalesce in order for that to happen? Our Moon has libration... I expected this to be the norm.

Well, if I haven't gotten the wrong number of negatives in there, I think I'm saying that there is libration.

Having synchronous rotation (that is, being tidally locked to Saturn) is completely to be expected- it would be hard to explain it otherwise.

[neufer]

Our Moon has libration... I expected this to be the norm.

It should be noted that the Moon's libration has to do with the Moon's constant spin rate while in an elliptical orbit (eccentricity ~ 0.055).

Since Prometheus has an eccentricity of only 0.0022 its corresponding 'steady state' libration should be quite small.

The harmonic tidal forces on Prometheus are quite large, however, while the damping forces (due to feedback from Prometheus's tides on Saturn) are relatively small so some dynamical wobbling should be expected.

[Chris Peterson]

It should be noted that the Moon's libration has to do with the Moon's constant spin rate while in an elliptical orbit (eccentricity ~ 0.055).

Since Prometheus has an eccentricity of only 0.0022 its corresponding 'steady state' libration should be quite small.

The harmonic tidal forces on Prometheus are quite large, however, while the damping forces (due to feedback from Prometheus's tides on Saturn) are relatively small so some dynamical wobbling should be expected.

When posting the earlier I debated whether this wobble could properly be called "libration". A little research into the meaning of the term convinced me it could- that libration refers to wobble in a body with a synchronous rotation, regardless of the forces responsible for that wobble.