Rings Discovered Around a Moon for the First Time 144
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.'"
pff (Score:5, Funny)
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That's no moon. (And that's no ring. It's a debris field.)
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Re:pff (Score:4, Informative)
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Natural satellites of planets are moons.
Sheesh
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2. A secondary planet, or satellite, revolving about any member of the solar system; as, the moons of Jupiter or Saturn. [1913 Webster]
From WordNet (r) 2.0 [wn]:
6: any natural satellite of a planet; "Jupiter has sixteen moons"
So, y'know. STFU.
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March 13, 1781 [wikipedia.org]
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Here ya go...well sort of:
http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=2901 [nasa.gov]
space crud in crud's shoes (Score:1)
This no troll, just curious question.
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Pre-emptive comment (Score:3, Funny)
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Dumb question: Why are they 2 dimensional? (Score:5, Interesting)
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.
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I think it boils down to spin and gravity.
Re:Dumb question: Why are they 2 dimensional? (Score:5, Informative)
Will answer your question much better than I could.
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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.
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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 spira
Re:Dumb question: Why are they 2 dimensional? (Score:5, Informative)
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.)
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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.
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. :-)
Re:Dumb question: Why are they 2 dimensional? (Score:5, Informative)
Re:Dumb question: Why are they 2 dimensional? (Score:5, Informative)
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.
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Re:Dumb question: Why are they 2 dimensional? (Score:5, Funny)
The Flying Spaghetti Monster [wordpress.com] makes flat plate-like shapes because spaghetti likes to rest on plates. See, it all falls into place logically.
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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.
Re:Dumb question: Why are they 2 dimensional? (Score:5, Informative)
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
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Cool post.
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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?
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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
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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
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But it does matter. If the plane is 1,000 light years thick, then anything within that field, like a ring at 45 degrees to the plane, would remain at that angle as the attraction on all particles would be the same as the whole ring as it is blanketed by the same force vector.
I just agree that that was true, and then clearly stated that if the galaxy were NOT thick disk, something slightly different would happen. (That's the "Even if" part.) Why are you disagreeing and then repeating what I just said was true?
However anything above the plane and (eg) about 75,000 light years out, would travel on a ballistic curve toward the center.
You're confusing two things: stars that oscillate above and below the galactic mid-plane (which they do... they don't get pulled into a monolayer as you seem to be suggesting) and solar systems, which is what we *were* talking about. A solar system orbits its star firs
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It takes energy to change the orbital plane.
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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 orb
Photo. (Score:5, Informative)
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Funny timing (Score:4, Interesting)
Re:Funny timing (Score:5, Interesting)
Now, my version is heavily modded (and it's the alpha version), but I can see Apollo still orbiting good ol' Moon in Celestia. And witness a nice dawn together with Apollo. *sigh* it's a pity that you go through that military drill to become an astronaut. I surely would like to be one.
Essentially, that's the same as putting a satellite around Earth, as Earth orbits Sun like Moon orbits Earth.
What's even more interesting: you could put a spacecraft in the Lagrange-point between Earth and Moon, so it wouldn't move - well with respect to Earth and Moon, of course.
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*sigh* it's a pity that you go through that military drill to become an astronaut. I surely would like to be one.
I hear there's an opening on a Mars mission... Given NASA funding shortages, they might be ok to dispense with all the military drill for that one.
Or, if you're young enough, mmm, just wait about 15-20 years (boo-HEY!). Commercial space flight is inevitable in the near future. Of course, trained astronauts will probably get first dibs, but you never know...
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I mean, technically you don't have to tell him he's not coming back.
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of course Nasa could always higher a comcast help desk monkey to put him on hold for a while.
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What's even more interesting: you could put a spacecraft in the Lagrange-point between Earth and Moon, so it wouldn't move - well with respect to Earth and Moon, of course
Thank You!! Lagrangian Point - that's the term I've been trying to remember for the longest time. Incidentally, if a massive enough object were to pass through this point relative to the earth and the moon, the moon would fall into the earth. Although the likelihood of such an occurrence is, er, astronomical...
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Actually, you'd probably be better not making the force radial by trying to kill some angular momentum instead. Assuming you *want* to smash the Moon into the Earth.
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Its too bad L1-3 are unstable.. you'll still need small maneuvering thrusters/attitude gyros etc. to keep your craft from straying too far from the equilibrium point. L4 and L5 are dynamically stable, but there's also a lot of other cruft just lying around there that you'd have to shield against.
But yes, Lagrange points are awesome and we need to exploit them more!
Aikon-
From the Earth To the Moon (Score:3, Insightful)
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perhaps you meant http://www.shatters.net/celestia/ [shatters.net] ?
At least following the first google link there looks like the right page, and links to a sourceforge project named celestia.
Mycroft
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(Actually, you have to be more than European. You have to be a citizen of one of the countries that finances the astronauts, which currently doesn't include my country. Which yesterday suggested they'd remove the funding to the worlds second best telescope array [bbc.co.uk])
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Yes. In fact, there's already a mission planned: the Lunar Reconnaissance Orbiter [nasa.gov] (LRO). Set for launch later this calendar year, the LRO will be put into a low polar lunar orbit for about 1 year. Among its objectives are the creation of high-resolution lunar maps (it is equipped with a laser altimeter), seek suitable landing ellipses for future craft, and search for evidence of water ice and other resources.
Aikon-
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Forgot to mention: The LRO orbital period is roughly 2 hours.
Aikon-
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No, unfortunately the resolution is not that high.. If I recall correctly, certain regions of the moon will be mapped with a resolution of about 100m/pixel while most of the moon will be mapped at about 1-2km/pixel.
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We already have, many times, starting with Apollo 8 which orbited around the moon. Actually, I'm not 100% certain that was the first. Nevertheless, we have put satellites in orbit, that is, man-made objects in stable orbital configurations, around the moon already. We even did it with slide-rules and computers far less powerful than what's in your cell phone.
Wow. (Score:3, Interesting)
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).
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If the central body is spherical symmetrical, its rotation is entirely irrelevant. If it has any asymmetry, things get more interesting. A lone satellite (or satellites that don't interact significantly) will have their orbits precess in space, but they won't tend toward the equator. However, if you have interacting satellites, all sorts of things can happen. In the case of rings/disks, collisions betweens bodies averag
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That's no moon... (Score:1)
(i'm so sorry)
Obviously... (Score:2)
We're all DOOMED.
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Damnit, it's supposed to be in-your-face-Bender-style-yelling, Slashdot, stop complaining.
uh? (Score:1)
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Can we make some rings around our own moon? (Score:3, Funny)
It'd be a really nice decoration.
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We've heard in the laundry commercial (ages ago... (Score:2)
"You've got 'Ring around the collar'..."
Now, we find we've got "Rings around URhea..."
What's next? "Rings around Uranus?"
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obligatory (Score:2)
Maybe Rings/Dusty Halo (Score:4, Informative)
Does this mean (Score:3, Funny)
Yeah, in mythology they were husband and wife (Score:5, Interesting)
You joke, but Saturn's (Cronus's) wife in mythology was named Rhea. A bit of a coincidence that.
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* Except the shepherds Pan and Daphnis, of course. Who are named for, um, shepherds.
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The coincidence I was pointing out was that out of all of Saturn's many moons, it just so happens to be Rhea that has a ring just like Saturn does. The GGP made a joke about the fact that they both have rings is sort of like them being married. No one knew Rhea had a ring when they gave the satellite that name.
Please (Score:1)
Fractal astronomy! (Score:1)
Cool, fractal astronomy! Does the moon's ring have rings itself?
And in next year's news... (Score:3, Funny)
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Let's name it Recursia
Aw, how cute (Score:1)
It's rule #7 at work. (Score:2)
Space junk? (Score:2)
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A simple solar power ion drive for minor adjustments, and on the outside, information like we put on the Voyager missions.
Have it do a burst of light and radio noise once every 100 years. Actually, maybe 4 times once every 100 years. So it would be easier to find.
The next step in long term space capsules.
Also I would like to put a Cube large enough it can be seen from earth with a moderate telescope. H
There's a ring around the earth! (Score:2)
Wasn't there a movie about this? (Score:5, Funny)
Misleading (Score:2)
What about that old song? (Score:2)
I see a ring around Uranus...
Please....... (Score:2)
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