Explanation: Why does this moon look like a sponge? To better investigate, NASA and ESA sent the Saturn-orbiting robotic spacecraft Cassini zooming past Saturn's moon Hyperion, once again, earlier this week. One of the images beamed back to Earth is featured above, raw and unprocessed. Visible, as expected, are many unusually shaped craters with an unusual dark material at the bottom. Although Hyperion spans about 250 kilometers, its small gravitational tug on Cassini indicates that it is mostly empty space and so has very low surface gravity. Therefore, the odd shapes of many of Hyperion's craters are thought to result from impacts that primarily compress and eject surface material -- instead of the more typical round craters that appear after a circular shock wave that explosively redistributes surface material. Cassini is on track for another flyby of Saturn's Dione in about two weeks.
Re: APOD: Flyby Image of Saturn's Sponge Moon... (2015 Jun 0
Posted: Wed Jun 03, 2015 4:33 am
by Nitpicker
Wow, its density is only about half that of water's. The thing might just float (for a while at least) in a deep enough ocean. Edit: It looks so much like pumice ... Ann beat me to it.
Re: APOD: Flyby Image of Saturn's Sponge Moon... (2015 Jun 0
Re: APOD: Flyby Image of Saturn's Sponge Moon... (2015 Jun 0
Posted: Wed Jun 03, 2015 4:56 am
by the old blind man
Where's the Shrike?
Re: APOD: Flyby Image of Saturn's Sponge Moon... (2015 Jun 0
Posted: Wed Jun 03, 2015 5:15 am
by Qweenie
Hyperion always reminds me of the surface structure we see in snowbanks in late winter, when we have cold days (about -20C) and colder nights, but the sun is strong enough to sublimate snow. You get exactly the same pitted structure. Each cavity focuses the sun's energy within the cavity, so they get larger.
Re: APOD: Flyby Image of Saturn's Sponge Moon... (2015 Jun 0
Posted: Wed Jun 03, 2015 5:15 am
by geckzilla
The joke is that Hyperion either has something to hide or only has one side. It tumbles erratically and is difficult to follow so the Cassini team never knows what they'll get from it. So far all the passes show more or less the same side. Want to see the other side? Send another probe to Saturn. Maybe Hyperion is just helping future planetary scientists have job security.
Re: APOD: Flyby Image of Saturn's Sponge Moon... (2015 Jun 0
Posted: Wed Jun 03, 2015 5:28 am
by gorade
Could it be that Hyperion has once been a comet that has become captured by Saturn? It 's spongy structure is the remains after ice has been vaporised away, perhaps after many sun passages.
Re: APOD: Flyby Image of Saturn's Sponge Moon... (2015 Jun 0
Posted: Wed Jun 03, 2015 6:59 am
by Ann
geckzilla wrote:The joke is that Hyperion either has something to hide or only has one side. It tumbles erratically and is difficult to follow so the Cassini team never knows what they'll get from it. So far all the passes show more or less the same side.
The joke's on us.
Ann
Re: APOD: Flyby Image of Saturn's Sponge Moon... (2015 Jun 0
Posted: Wed Jun 03, 2015 7:35 am
by Elbabb
The captured comet is a good one but I like some sort of artificial ablative layer to serve as a heat shield for atmospheric entry
Re: APOD: Flyby Image of Saturn's Sponge Moon... (2015 Jun 0
Posted: Wed Jun 03, 2015 9:14 am
by hoohaw
Hey, even Saturn would float in your bathtub: "Saturn has the lowest density of all the planets in the Solar System. The actual number is 0.687 grams per cubic centimeter. "
Re: APOD: Flyby Image of Saturn's Sponge Moon... (2015 Jun 0
Posted: Wed Jun 03, 2015 10:41 am
by Boomer12k
"Splish-Splash, I was takin' a bath...long about Saturday Night...."
I wonder if the darker material at the bottom is from melting, like a Moon Crater...
:---[===] *
Re: APOD: Flyby Image of Saturn's Sponge Moon... (2015 Jun 0
Posted: Wed Jun 03, 2015 11:21 am
by Dad is watching
geckzilla wrote: It tumbles erratically and is difficult to follow so ...
How is this possible from a solid body in orbit? Shouldn't it rotate/spin on its center of mass? Does this mean that its center of mass changes indicating that it is a less than solid object? Or is it just that there have been insufficient observations to allow predictions of future positions/orientations?
Re: APOD: Flyby Image of Saturn's Sponge Moon... (2015 Jun 0
Posted: Wed Jun 03, 2015 11:54 am
by henrystar
Dad is watching wrote:
geckzilla wrote: It tumbles erratically and is difficult to follow so ...
How is this possible from a solid body in orbit? Shouldn't it rotate/spin on its center of mass? Does this mean that its center of mass changes indicating that it is a less than solid object? Or is it just that there have been insufficient observations to allow predictions of future positions/orientations?
Great question! NO, no, no, under gravitation it is quite possible for moons or even planets to tumble endlessly in an essentially unpredictable way. Several moons in the solar system act this way. (If our own Moon were in such a condition, would we have EVER figured it out?) Anyway, here's the story from wikipedia: http://en.wikipedia.org/wiki/Chaos_theory
Re: APOD: Flyby Image of Saturn's Sponge Moon... (2015 Jun 0
1) it is very irregularly shaped,
2) has a fairly eccentric orbit (eccentricity = 0.123),
3) and is near a much larger moon, Titan.
These factors combine to restrict the set of conditions under which a stable rotation is possible. The 3:4 orbital resonance between Titan and Hyperion may also make a chaotic rotation more likely. Hyperion is the only moon in the Solar System known to rotate chaotically.>>
Re: APOD: Flyby Image of Saturn's Sponge Moon... (2015 Jun 0
Posted: Wed Jun 03, 2015 1:45 pm
by Chris Peterson
Dad is watching wrote:
geckzilla wrote: It tumbles erratically and is difficult to follow so ...
How is this possible from a solid body in orbit? Shouldn't it rotate/spin on its center of mass? Does this mean that its center of mass changes indicating that it is a less than solid object? Or is it just that there have been insufficient observations to allow predictions of future positions/orientations?
A single body will always rotate about its center of mass. If it is tumbling, that means it rotates on more than one axis. External forces can cause the actual axis of rotation (or axes of tumbling) to change, possibly predictably, more likely chaotically.
I'm not sure if Hyperion is actually tumbling. I think it rotates around one axis, the orientation of which changes chaotically. But that's close enough to "tumble" in the colloquial sense. Although Hyperion is unique among planetary moons in this respect, similar dynamics are common in asteroids.
Re: APOD: Flyby Image of Saturn's Sponge Moon... (2015 Jun 0
Posted: Wed Jun 03, 2015 2:22 pm
by ta152h0
two dark spots, that suck in light
Re: APOD: Flyby Image of Saturn's Sponge Moon... (2015 Jun 0
Re: APOD: Flyby Image of Saturn's Sponge Moon... (2015 Jun 0
Posted: Wed Jun 03, 2015 3:04 pm
by neufer
Click to play embedded YouTube video.
Re: APOD: Flyby Image of Saturn's Sponge Moon... (2015 Jun 0
Posted: Wed Jun 03, 2015 5:10 pm
by FloridaMike
I don't understand the path the camera is following in the video.
Re: APOD: Flyby Image of Saturn's Sponge Moon... (2015 Jun 0
Posted: Wed Jun 03, 2015 5:18 pm
by Craine
the old blind man wrote:Where's the Shrike?
^5 for Dan Simmons
Re: APOD: Flyby Image of Saturn's Sponge Moon... (2015 Jun 0
Posted: Wed Jun 03, 2015 5:35 pm
by geckzilla
There's a new video of Pluto's moon, Nix, showing its chaotic rotation. It seems something like Hyperion.
Click to play embedded YouTube video.
(I had no idea there is a difference between chaotic rotation and tumbling. Today I learned something new...)
Re: APOD: Flyby Image of Saturn's Sponge Moon... (2015 Jun 0
Posted: Wed Jun 03, 2015 7:06 pm
by Craine
Anybody have an idea how long it would take for such a chaotic rotation or tumbling to stabilize?
What I mean is...I always thought objects such as Hyperion and Nix are subject to tidal forces which should gradually stabilize them until they show the same face to their parent (like our moon). And those tidal forces are relatively larger for more irregular objects (which Hyperion and Nix are).
Seems to me it may take a few million years, but then they should be fairly stable....unless they get bombarded regularly with sufficient incoming mass to throw that rotation off again?
Re: APOD: Flyby Image of Saturn's Sponge Moon... (2015 Jun 0
Posted: Wed Jun 03, 2015 7:30 pm
by BMAONE23
geckzilla wrote:There's a new video of Pluto's moon, Nix, showing its chaotic rotation. It seems something like Hyperion.
Click to play embedded YouTube video.
(I had no idea there is a difference between chaotic rotation and tumbling. Today I learned something new...)
I thought that it's orbit would have been less chaotic...Guess I'll have to Nix that idea
Re: APOD: Flyby Image of Saturn's Sponge Moon... (2015 Jun 0
Posted: Wed Jun 03, 2015 7:40 pm
by neufer
Chris Peterson wrote:
A single body will always rotate about its center of mass. If it is tumbling, that means it rotates on more than one axis.
External forces can cause the actual axis of rotation (or axes of tumbling) to change, possibly predictably, more likely chaotically.
Chris Peterson wrote:
I'm not sure if Hyperion is actually tumbling. I think it rotates around one axis, the orientation of which changes chaotically. But that's close enough to "tumble" in the colloquial sense. Although Hyperion is unique among planetary moons in this respect, similar dynamics are common in asteroids.
Hyperion is no doubt tumbling quite significantly in an unstable mode (i.e., an angular momentum vector aligned far from either its major or minor axis) thereby making it highly susceptible to external tidal forces from Titan & Saturn.
Chris Peterson wrote:
Although Hyperion is unique among planetary moons in this respect, similar dynamics are common in asteroids.
Most asteroids are subjected to very weak tidal forces.
However, many asteroids have chaotic orbits in period resonance with planets:
http://en.wikipedia.org/wiki/4179_Toutatis wrote:
<<4179 Toutatis (too-TAH-tis) is an Apollo, Alinda, and Mars-crosser asteroid with a chaotic orbit produced by a 3:1 resonance with the planet Jupiter, a 1:4 resonance with the planet Earth, and frequent close approaches to the terrestrial planets, including Earth. Its rotation combines two separate periodic motions into a non-periodic [but NON-chaotic] result; to someone on the surface of Toutatis, the Sun would seem to rise and set in apparently random locations and at random times at the asteroid's horizon. It has a rotation period around its long axis (Pψ) of 5.38 days. This long axis is precessing with a period (Pφ) of 7.38 days.>>
Flyby Image of Saturn's Sponge Moon Hyperion
Hyperion is no doubt 
