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Rings Discovered Around a Moon for the First Time
Riding with Robots writes "It turns out that one of the Ringed Planet's moons has rings of its own. The robotic spacecraft Cassini at Saturn has discovered that the icy moon Rhea is orbited by an extensive debris field and at least one ring, the first such system found. 'Many years ago we thought Saturn was the only planet with rings,' said one mission scientist. 'Now we may have a moon of Saturn that is a miniature version of its even more elaborately decorated parent.'"
Wake me up when they find a moon orbiting a ring.
Pre-emptive semi-funny comment involving the Goatse guy, a ring, and mooning.
Beware: In C++, your friends can see your privates!
That's no ring!
You get used to seeing them and maybe don't question it, but why do so many structures in 'outer space' -- low gravity, three-dimensional space -- take on essentially two-dimensional forms? Consider rings around planets, planetary systems around stars, and galaxies, at least. They are all flat discs.
I asked an astrophysicist I know and she said, 'that's the way the math works out'. Ah, thanks. Maybe someone here can be more enlightening.
Disclaimer: For all you nitpickers, I know there are more than three dimensions, and that the structures are not truly two-dimensional. Unless string theory applies here, I think we can leave those facts out of the discussion.
Here's a photo of Rhea from nasa.gov. Gives some nice background information on the moon as well.
512 MB RAM, 20 GB disk, 200 GB transfer, five datacenters. $19.95/month.
Just last week my son said something that made me wonder, "could we put a satellite in orbit around our moon"?
If you could reason with religious people, there would be no religious people
That's really cool. I was so into the planets when I was young. Loved the Voyager missions (even made a model of the probe out of Contrux... and it was accurate too), and watched as many Nova specials about the Voyager missions as possible. That kid is not dead, he's just taken a place inside of me. I keep an occasional glance at the Cassini mission, just like the Galileo mission to Jupiter.
This is, indeed, a surprise discovery and hopefully there might be more material to study concerning this ring-type.
On a somewhat related-note: It is ironic that this moon has a ring whereas two moons hang out in Saturn's outer rings (they are called the Shepherd Moons).
-Aegis Runestone-
I think it boils down to spin and gravity.
http://www.faqs.org/faqs/astronomy/faq/part4/section-15.html
Will answer your question much better than I could.
How we know is more important than what we know.
(i'm so sorry)
This proves that the global warming skeptics are horribly right. Global warming is being caused by disturbances in the solar system. However, it turns out that this is actually an invading Cylon fleet of six basestars, and the wreckage we see, is sadly, the Battlestar Galactica.
We're all DOOMED.
This is my sig.
I'm pretty sure most planets have a ring of debris around them. Uranus has a very dim ring. Earth has a ring but it isn't visible with out special gear.
Perhaps there's something relatively simple we can do, to add rings around our moon. Like shooting a missle at an asteroid in the asteroid belt, *just so,* or perhaps the next time a comet comes by.
It'd be a really nice decoration.
http://saturn.jpl.nasa.gov/news/press-release-details.cfm?newsID=820
"You've got 'Ring around the collar'..."
Now, we find we've got "Rings around URhea..."
What's next? "Rings around Uranus?"
Previously: "Linux... Toward the Sunrise..." Now: "Linux... Toward the-- No, now, part of Every Sunrise"
that's no ring...
intellectual property law is philosophically incoherent. it is your moral duty to ignore it or sabotage it
Despite JPL's press-release filled with certainty, this is not a definite detection. The imaging instrument has not seen any ring or halo around Rhea in spite having looked. This does not prove that the putative ring is not there (more observations are planned), but it is contrary evidence and suggests we start asking ourselves what else might cause these data.
dont. apt-get install openarena
How we know is more important than what we know.
they got engaged ?
Does does it's ring have another ring too? Oh well, the more i think about it the more i'm convinced Saturn must have piercings too. Love that fetish
:wq!
despite what many have said...
it really boils down to how does a galaxy form. compare a true 3-d object and nebula very round, nothing attracting anything to the middle. so nothing coalesces into planets, stars, and asteroids.
the trick here is the spiral galaxies all have a VERY large gravity source in the center. everything without sufficient angular momentum gets sucked in. so things in odd orbits, that aren't on a narrow plane... get sucked in to the middle. EVEN way out here on the edge of the spiral galaxy, things not on the plane of central gravity mass are imbalanced, and get sucked in, so stuff stays flat... round objects, stars, planets, gas giants form. even asteroid belts, even debris belts far outside the solar system. it all happened way before our star was even finished forming, when the super galaxies that formed all the heavy metals (uranium etc) dissolved into nebula and spiral and normal galaxies, and 'dark matter'
we're in spiral galaxy, so things tend to form in rings and other flat things. if you slowed something down, or put it in an odd orbit it would eventually reach the middle of the galaxy just from gravity. no angular momentum, and it's gone. the closer it gets the faster it goes. I'm not so sure about how non spiral galaxies are, but then the science on non spiral galaxies are far less, we basically only get the stars to look at, and the black holes if any...
https://www.gnu.org/philosophy/free-sw.html
> Now we may have a moon of Saturn that is a miniature version of its even more elaborately decorated parent.
Cool, fractal astronomy! Does the moon's ring have rings itself?
The next discovery will be that one of the rocks orbiting Rhea itself has a ring around it.
http://alternatives.rzero.com/
No, it is not. However, circumference / pi = dia-Rhea.
It's not how the math works out, it's collisions. When inelastic bodies collide, their post-collision velocities tend to be nearer the (mass-weighted) average of the original velocities. For bodies orbiting a planet, the average motion is generally in the equatorial plane. Thus, for rings (or gas disks around a variety of astronomical bodies), you get flattened features. Saturn's main rings (C, B, and A) are so optically think (think "dense" if you will) that they're very, very flat. Measurements suggest that the B and A rings may be as little as a few meters thick because of all the collisions.
the baby has a ring like the father.
Table-ized A.I.
IANAA (I am not an astrophysicist) but from the physics and astrophysics classes I've taken I can venture a guess. Of course I may be wrong so feel free to correct me if I am.
;). Anyway, the point is you're going to get rotation in a plane. So when the solar system begins to take shape this would be the plane in which it rotates. Planets form in a similar fashion to a solar system, so the spin of the planet would be in a plane and hence the debris which is caught in the planet's gravity would similarly rotate in this plane.
When interstellar gas contracts to form a solar system it has a certain angular momentum. Now let's assume it has a counter-clockwise rotation about the z-axis as well as a counter-clockwise rotation about the x-axis. Then really it has a counter-clockwise rotation in a plane which intersects the origin at 45 degrees between the x-axis and z-axis. Okay I think I totally screwed that example up... It's too late at night to think in 3-dimensions I think
Of course this is all theory on how solar systems/planets form, but to my understanding this is why. I'm sure the explanation for a galaxy would be very similar. At least this is how I understand it to be.
Maybe it's space junk from an ancient civilisation.
Drill baby drill - on Mars
We're sending so much crap out into orbit that we've built our own ring.
Whoever told you this was wrong.
Inclination of the orbit has nothing to do with the total angular moment. h = sqrt(G M a (1-e^2)), where h is the specific angular moment, G is Newton's constant, a is the semi-major axis of the orbit, M is the central body's mass (I'm assuming a point source), and e is the eccentricity. Note the lack of the inclination in there. If you think about it, it *has* to be ascent: unlike e and a, the reference plane (and therefore I) is really arbitrary. There are often better choices than others, but they're in no way absolute.
The existence (especially the high frequency of) elliptical and irregular galaxies supports this idea that disks aren't inherently required, even if they are very common.
Our solar system's flatness and the rings or Saturn is also entirely unrelated to the galaxy's shape. If it where related, you'd expect the solar system's plane to be the same as the galaxy (it isn't: prove it to yourself and look at the line of the planets in the night sky and compare it to the line that the galaxy makes). Likewise, Saturn's rings are tilted relative to the ecliptic plane by 26 degrees so that they line in Saturn's equatorial plane.
Why are things flat? Collisions. Collisions average out velocities so they tend to a single plane. (How flat you get depends on collision frequency and any pressure support.)
You nearly nailed it. :-D All you need to throw in there is how collisions average velocities/orbits and you'll be competing for my job. ;-)
it's dia-Rhea?
...yeah, I said it. yeah it's missing an 'r'.
rings + Rhea = diameter + Rhea ~= diarhea
or death to rhea or die-rhea?
ok, sorry...back to the basement....
br/
There is a reason that we have a word, satellite. A satellite orbits a planet.
There is a reason that we have a word, moon. The moon is a satellite that orbits the earth.
However, the converse is NOT true: A satellite is a moon that orbits the earth.
QED
To put it as simply as possible: Because things can't spin in three dimensions.
and maybe don't question it, but why do so many structures in 'outer space' -- low gravity, three-dimensional space -- take on essentially two-dimensional forms? Consider rings around planets, planetary systems around stars, and galaxies, at least. They are all flat discs.
The Flying Spaghetti Monster makes flat plate-like shapes because spaghetti likes to rest on plates. See, it all falls into place logically.
Table-ized A.I.
All this talk about orbiting the moon and the Lagrange point reminded me of Jules Verne's "From the Earth To The Moon", a surprisingly accurate description of lunar travel written 140 years ago. I only wish space travel were as simple as he described.
Take a mass and put it onto a string, now attempt to get it to spin in three-dimensions. You should be finding that quite impossible, rotational forces cause it to all to line up into a single plane.
You should also notice that rings appear around the middle of the planets, directly in between the two poles. This is because this is where the spin is, so your rotation is going to go to the outermost point it can get to but being unable to escape the gravity us stuck at this widest point.
Try this some day. Take a bit of rope with a ball at the end of it. A tennis ball will do nicely. Bowling balls are just asking for trouble. Now hold the end of the rope and spin around as fast as you can. You now represent a planet, the tennis ball represents a part of a ring and the rope represents gravity. Try not to get dizzy and fall down. Falling down and throwing up doesn't represent anything in astronomy. That's engineering.
Notice that the ball spins in a more or less flat circle. Inertia carries it forwards and the rope pulls it towards you. There really isn't any force pushing it up or down, so it will naturally orbit in a flat plane.
Okay, whoopdie doo. I just told you that a circle is flat. What you're really asking is why millions of little rocks in a ring will all orbit in the same plane instead of going off and doing their own thing, each orbiting in slightly different directions forming a huge cloud.
Are you still spinning that ball around? Good. Now, pick up another one in your other hand and start spinning it as well. Chances are that both balls are spinning at the same speed at opposite ends of the same circle, so everything is fine. Here's where the demonstration gets a bit tricky. You need to unhinge your arms so that you can spin both balls at different angles and slightly different speeds. Since I don't want you to need to undergo major surgery in the name of physics I'll just skip to the ending and tell you what would happen if you could do that.
The balls are going to hit each other. It may not happen right away, but if you have objects moving in intersecting orbits it _will_ happen. If you had a few million balls all spinning around at different angles you would have a better representation of the rings we're talking about with a lot more collisions, but that requires a whole lot of rope and we don't have that much.
Now we can get back to the original question. Why do all these rocks form flat rings? I could tell you that that's the only way that they won't hit each other, but that doesn't answer the question of how they got there. Suppose that you took about a million little rocks and put them all in random orbits around a planet. At the start they would form a spherical cloud around it -- A ha! A three dimensional structure, just like you were asking for. But the question is "How long can it last?"
All of those rocks are going to start hitting each other, and every time they do they're going to transfer momentum. With enough objects traveling in enough different orbits that's going to happen a _lot_. Do you want to know how much? Look up at the moon some time and count the craters. Back when the solar system was young and not quite so flat, things were smashing into one another all the time. Every time they collided they scrupulously obeyed the law of conservation of momentum and shifted into different directions. Eventually the total momentum of that spherical cloud started to average out and more and more rocks found themselves orbiting in the same flat plane. Why did that happen? Simply because those were the ones that got hit less. Like your friend the astrophysicist said, "That's the way the math works out". It's all about averages, and when you're dealing with millions of rocks smacking into one another over billions of years, that's what matters.
But if we're dealing with _averages_ and _statistics_, why is everything so perfectly flat? Why are all of the planets, moons and rings all in the same plane, and why do all of the billions of stars in the Galaxy move in the same flat orbits?
The simple answers to those questions are "It's not", "They don't" and "That doesn't happen". While the planets all move in
At first I read that as "Rings Discovered Around the Moon for the First Time."
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You joke, but Saturn's (Cronus's) wife in mythology was named Rhea. A bit of a coincidence that.
Qu'on me donne six lignes écrites de la main du plus honnête homme, j'y trouverai de quoi le faire pendre.
This is one of those moment where reading at Slashdot actually gets you to learn something.
Cool post.
I rather be free in hell than a slave in heaven.
Doh! I should have read the FAQ! It's been a long time since I checked out Usenet FAQs; what a great resource (and explanation).
Good explanation. Most illuminating!
For extra credit: why does the universe have all this angular momentum to begin with? Where did that come from? Why doesn't a proto-solar system just collapse into a sphere?
It's a great description, but I think the heart of the matter still needs clarity:
/. post.
I could see that, over billions of years, any collision that could happen would happen, and it would eliminate intersecting orbits (as well as average out the objects' momentum). That wasn't why I posted.
But those two balls only hit each other (assuming the ropes magically don't cross) if the ropes are the same length. If we could tip Jupiter's orbit, no matter what angle of inclination we used, it would not collide with other planets. It does not need to be in a plane for that purpose.
The hand-waving -- and the reason for my question -- is why a sphere (or some 3D system) resolves so often into a plane, I get the sense that it's the result of the momentum averaging out, but it's not quite clear. Heck, why not two non-intersecting discs on different planes, or ten? Maybe it's one of those issues for which there is no substitute for the math -- not even a
Thanks.
how come that is redundant? its a reference to the death star for crying out loud!
_ In Egypt Networks: Network Solutions with a Twist
What is the smallest size of space rock that can aspire to have some space debris orbiting around it, either as ring or as rock?
This no troll, just curious question.
Chat with other atheists http://secularchat.org
In all seriousness, I finally grocked the answer to my question over a bowl of spaghetti. I will leave irrefutable proofs to others ...
Do I really need to mention more?
His noodly appendage touches us all in ways we cannot even imagine!The existence (especially the high frequency of) elliptical and irregular galaxies supports this idea that disks aren't inherently required, even if they are very common.
But its mostly older galaxies without much dust, plasma, and debris that take such form. In other words, elliptical (roundish) galaxies have little or no friction or collisions among stars. The stars don't interact very often (with each other or with diminishing plasma and dust). The fact that most elliptical galaxies are "cleaner" than spirals is evidence for this.
Also, the average width of the particles in Saturn's rings in proportion to the space between them is much much larger than the ratio of the star sizes to their separation in older galaxies. Models suggest that ring particles can and do collide often.
Our solar system's flatness and the rings or Saturn is also entirely unrelated to the galaxy's shape.
That's because the collapsing solar disk's average momentum may have been shaped by forces stronger than the galaxy's movement, such as a nearby supernova explosion soon before collapse of the solar disk. Such explosions are often oblique or bipolar in shape, meaning they may press on one side of a plasma cloud more than another. Plus, there's lots of nearby stars in formation clusters giving unpredictable gravity kicks.
Table-ized A.I.
Nevermind, I understand it now ... for others still unsure after Minwee's clever explanation, I recommend the FAQ linked further up the thread. Thanks again Minwee.
Tolkien was right. One ring to rule them all.
The force of gravity is 'directional', as all matter of the galaxy is already on the same plane. So as the solar system matures further, rings and moons will eventually be forced into the same plane.
A simple system like 2 masses rotating around each other has a greater gravitational attraction in one plane. A third mass initially rotating around this simple system in a different plane, will eventually force this triad into a plane common to all 3 masses.
Don't be apathetic. Procrastinate!
But its mostly older galaxies without much dust, plasma, and debris that take such [round] form.
I should clarify this a bit. It begs the question of *how* they got semi-spherical to begin with. Most medium-to-large galaxies that we see are thought to have been involved in a couple of collisions and mergers over the years. If two dusty/gassy galaxies collide, the dust and gas will be subject to the disk-forming forces described earlier and pull or form the stars with them into a disk.
The older a galaxy gets, the less unbound gas and dust it will have because it eventually gets turned into or sucked into low-activity stars and black holes. Plus, new gas/dust is not generated as much from star deaths because there are less new stars over time. Over time what is left is small orange stars that have a long life and don't barf out much when their life ends, or neutron stars and black-holes which are pretty much done exploding. Space cleans itself of gas/dust over time via this entropy. Elliptical galaxies appear orange-yellow compared to spirals (blue-white) because of the population of these older-style stars: no clouds means no more big bright short-lived blue/white stars.
If two older galaxies collide, then there is less gas and dust to force it into a disk shape. It will thus more likely to be an elliptical (spheroid) galaxy.
The collision does seem to generate a short period of new star formation as plasma collides, but not enough to disk-ify the galaxy. This may be why such galaxies are elliptical (slightly flat) instead of fully spheroid. They did have a short period of disk-ifying influence upon merge.
(Of course, this theory could be wrong, but its a model that makes sense: less dust, less flat.)
Table-ized A.I.
Every time you moon me,
I see a ring around Uranus...
Eventually the total momentum of that spherical cloud started to average out and more and more rocks found themselves orbiting in the same flat plane. Why did that happen? Simply because those were the ones that got hit less. Like your friend the astrophysicist said, "That's the way the math works out". It's all about averages, and when you're dealing with millions of rocks smacking into one another over billions of years, that's what matters.
Bah, that's just an excuse for the motor companies to keep us from getting flying cars!
Table-ized A.I.
Beacuse an orbit remains in the same plane if left undisturbed.
It takes energy to change the orbital plane.
sudo ergo sum
Uranus already has rings: http://apod.nasa.gov/apod/ap960430.html
Does this also mean that, given time, all the space debris around the earth will clutter in a ring?
Mind you: I do realize that we generate more debris than nature will (if at all) be able to move into a ring.
why do so many structures in 'outer space' -- low gravity, three-dimensional space -- take on essentially two-dimensional forms?
Imagine two objects (A, and B) are orbitting a planet and their orbits are at an angle to each other (in orbits that do not intersect). Now, not only is there a gravitational pull between each object and the planet, but there is a minute pull between the two objects themselves. That means that A and B are trying to pull toward each other. No matter where they are in their orbits, they will always try to pull toward each other. Now, it takes a lot of energy to change an orbit, but the tiny pull between the two objects will, over time, pull the two orbits into the same plane.
The same thing happens when you have two objects, or two hundred million objects.
When our name is on the back of your car, we're behind you all the way!
No Ur-anus postings. Or has the Goatse guy already made an appearance?
== First cross river, then insult alligator.
Also, the average width of the particles in Saturn's rings in proportion to the space between them is much much larger than the ratio of the star sizes to their separation in older galaxies. Models suggest that ring particles can and do collide often. This was exactly my point. That's because the collapsing solar disk's average momentum may have been shaped by forces stronger than the galaxy's movement, such as a nearby supernova explosion soon before collapse of the solar disk. Such explosions are often oblique or bipolar in shape, meaning they may press on one side of a plasma cloud more than another. Plus, there's lots of nearby stars in formation clusters giving unpredictable gravity kicks. Also true, and also my point.
So, I learned something today! I always thought that the rings were flat due to a gravity "bulge" like around the equator. Now I know differently, so MY question is: Do the rings not lie in the same plane as the equator? thanks!
Great fleas have little fleas upon their backs to bite 'em,
And little fleas have lesser fleas, and so ad infinitum.
And the great fleas themselves, in turn, have greater fleas to go on;
While these again have greater still, and greater still, and so on.
- Augustus De Morgan
Seriously, you should fucking teach. That was a fantastic explaination and made perfect sense. Thank you for contributing.
Er, not really. For the most part, the galactic disk's effects would only be to make the planets move "up and down" (relative to the galactic plane) somewhat faster than you'd expect for an isolated system. It does not pull them into the same plane. Why? Because the z-directed forces on both sides of galactic-plane crossing are symmetrical.
If your statement were true, Saturn's rings would probably be in the ecliptic plane by now. They're a much older system, dynamically-speaking, than the solar system (let alone the galaxy).
The problem is the methodologies used to study other galaxies and solar systems is different than that used to study locations nearby. We also know more granular pieces of information about our own system than the distant ones. Only recently have we been able to tell if there is a gas giant orbiting a star, but we can say how many planets are in our system.
As for the number of planets in our solar system, the problem isn't that we discovered new information about Pluto that declassified it, the problem is that when it was discovered the scientific community didn't have any guidelines setup as to what size a celestial body had to be to be considered a planet. Once a standard was set and guidelines enacted, they realized that Pluto no longer had the characteristics that we now use to define a planet. Most of this confusion is just because since we discovered Pluto, we then discovered many other objects that we didn't classify as planets but that shared more characteristics with Pluto than Pluto did with the rest of the planets.
I assume we missed a tiny ring of debris around Saturn's moon for so long because its so small. I think a majority of what we know about the moons of the outer planets is from the Voyager space probes which were kind of limited to a linear path.
I also want to say your post doesn't deserve a -1 rating since you have a good point just need some information.
That's the theory. Most of the debris is low enough its orbits will decay, but the leftover stuff would form a ring as time approached infinity.
Rule #7: A thing with a smaller version of itself is cute.
You probably meant to link to this: http://www.shatters.net/celestia/ Celestria website
Huh?
I doubt it. Wouldn't our large moon is too much of an influence for a stable ring to form? Also the sparser the debris field, the longer the process will take since objects will not make contact that often. I think Most satellites would lose enough energy in a collision of this type to fall back to Earth.
True, and I've just convinced myself that you're right. I forgot that it depends on the 'depth' of the galactic plane, which is significant.
A 2D plane, arising from a point, had no depth. A plane in a 3D universe, does.
Anything enveloped along the depth of the plane (in this case 1000 light years), will have an effect on masses independent of spin, which masks the effect out anyway.
So rings, moons, planets and even solar systems can and do have a variance of up to 20 degrees within the galactic plane.
Don't be apathetic. Procrastinate!
That's also very important, but it wasn't what I was talking about. Even if the galactic plane were thinner than the solar system's scale, it wouldn't matter. The forces would be symmetrical on either side of the plane crossing. (Not that galactic forces are even significant in the parts of the solar system that are coplanar anyway.)
The symmetrical forces above and below the plane wouldn't have an effect as the ring is well within the thickness of the plane.
However anything above the plane and (eg) about 75,000 light years out, would travel on a ballistic curve toward the center.
Hard to prove.
It would be interesting statistical physics to find out how many systems' plane is aligned to the galactic plane or at azimuth (90 degrees). That would be the cruncher. It would also be interesting to find out if there are gravity variations caused by the varying degrees of mass rotating around the center. If you had an observer at the edge of the galaxy, measuring the gravitational forces over a galactic period, would it vary? Are the spirals balanced? Is the plane uniform?
If the plane is microns thick, then the symmetrical forces that you are talking about will indeed have an effect.
Don't be apathetic. Procrastinate!
If the star is OUT of the galactic midplane, you still don't get a change in the inclination, although you do shift the location of the orbit (if memory serves). It would be interesting statistical physics to find out how many systems' plane is aligned to the galactic plane or at azimuth (90 degrees). Been done, they're randomly oriented. Random motions in the gas clouds that collapse to form stars and planets overwelm the effects of the galaxy and the galaxy simply doesn't torque the planes into its own.