Starship Asterisk*

biddie67 wrote:just out of curiosity ----- in the formula above, P = Sqrt[(A*A+B*B)/3(A*A-B*B)] ,
I would have thought that the 3rd measurement "C" would have to be also used in the equation .......

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

Chris Peterson wrote:
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.

• I) gravity gradient stabilization and
  
  II) the corresponding pitch oscillation wobble/libration

• 1) near spherical satellites like the moon have rather long pitch oscillation wobble/libration periods
  [not to be confused with the simple standard kinematic lunar libration]
  
  2) but oblong satellites like Prometheus have pitch oscillation wobble/libration periods
  comparable to the orbital period (as appears to be the case in the youtube video)

• The pitch oscillation wobble/libration of
  a gravity gradient stabilized ellipsoid of dimensions A, B, C
  is P times the orbital period where:
  
  • P = Sqrt[(A*A+B*B)/3(A*A-B*B)]
  for Prometheus: A=119km, B=87km & C=61km
  
  so the equation above gives: P ~ 1.05 !!!
  
  Hence, the pitch oscillation wobble/libration of Prometheus
  is almost in resonance with the orbital period of F-ring forcing

neufer wrote: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.

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

emc wrote:

Chris Peterson wrote:

neufer wrote: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 wrote:

neufer wrote: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.

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

León wrote:
The other side, apparently looks darker

Chris Peterson wrote:

biddie67 wrote: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.

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

http://en.wikipedia.org/wiki/Prometheus_(moon) wrote:
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.

Chris Peterson wrote:

robo wrote: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.

orin stepanek wrote:
I liked the movies of Prometheus making the streamers.

Click to play embedded YouTube video.

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

http://en.wikipedia.org/wiki/S/2004_S_6 wrote: 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.>>

robo wrote: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.

orin stepanek wrote:
I liked the movies of Prometheus making the streamers.

Click to play embedded YouTube video.

biddie67 wrote: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??

thongar wrote:Its a Wake, my friends.

.... arrived ....

arrived