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Space Moon Science

Rings Discovered Around a Moon for the First Time 144

Posted by Soulskill
from the chip-off-the-old-block dept.
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.'"
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Rings Discovered Around a Moon for the First Time

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  • pff (Score:5, Funny)

    by McGiraf (196030) on Thursday March 06, 2008 @08:06PM (#22671136) Homepage
    Wake me up when they find a moon orbiting a ring.
    • by User 956 (568564)
      Wake me up when they find a moon orbiting a ring.

      That's no moon. (And that's no ring. It's a debris field.)

      • Re: (Score:3, Interesting)

        by Brian Gordon (987471)
        It might not even be a ring at all..

        Due to a decrease in the number of electrons detected by NASA's Cassini spacecraft on either side of the moon, scientists suggest that rings are the likeliest cause of these electrons being blocked before they reach Cassini.
        Not very convincing.
    • Re: (Score:2, Flamebait)

      by Brian Gordon (987471)
      It's amazing just how much fascinating data these deep solar system probes have gathered. Some are even out of the solar system testing interstellar space. And how many trillions have been spent uselessly collecting dust on Mars? More solar probes please!
      • by eln (21727)
        Trillions spent collecting dust on Mars? Seriously? I doubt NASA has even managed to spend multiple trillions of dollars in its entire history, much less on any Mars missions. The Mars rovers were built, launched, and operated (for the first 90 days anyway) for $820 million. I seriously doubt the additional operating costs up to this point are anywhere near a trillion dollars, unless they've been paying their engineers several billion dollars each per year, in which case I want that job.
    • by siddesu (698447)
      all the moons of Saturn obrit a ring in their own twisted way ;)
    • by mcpkaaos (449561)
      Wake me up when they find a moon orbiting a ring.

      March 13, 1781 [wikipedia.org]
    • Re: (Score:2, Interesting)

      by Tablizer (95088)
      Wake me up when they find a moon orbiting a ring.

      Here ya go...well sort of:

      http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=2901 [nasa.gov]
         
    • 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.
  • by Jugalator (259273) on Thursday March 06, 2008 @08:07PM (#22671148) Journal
    Pre-emptive semi-funny comment involving the Goatse guy, a ring, and mooning.
    • Pre-emptive semi-funny comment involving the Goatse guy, a ring, and mooning.
      Don't get upset because you couldn't think of a witty remark.
  • by guanxi (216397) on Thursday March 06, 2008 @08:13PM (#22671180)
    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.
    • 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 think it boils down to spin and gravity.

    • by QuantumG (50515) * <qg@biodome.org> on Thursday March 06, 2008 @08:31PM (#22671308) Homepage Journal
      http://www.faqs.org/faqs/astronomy/faq/part4/section-15.html [faqs.org]

      Will answer your question much better than I could.

      • by guanxi (216397)
        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).
    • by kesuki (321456)
      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 spira
      • by CheshireCatCO (185193) on Thursday March 06, 2008 @09:40PM (#22671800) Homepage
        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.)
    • by CheshireCatCO (185193) on Thursday March 06, 2008 @09:33PM (#22671762) Homepage
      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.
    • by AstrumPreliator (708436) on Thursday March 06, 2008 @09:34PM (#22671768)
      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.

      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 ;). 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.

      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.
      • 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. ;-)
    • by Urza9814 (883915)
      To put it as simply as possible: Because things can't spin in three dimensions.
    • by Tablizer (95088) on Thursday March 06, 2008 @09:53PM (#22671870) Journal
      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 [wordpress.com] makes flat plate-like shapes because spaghetti likes to rest on plates. See, it all falls into place logically.
                   
      • by guanxi (216397)
        In all seriousness, I finally grocked the answer to my question over a bowl of spaghetti. I will leave irrefutable proofs to others ...
    • 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.
    • by Minwee (522556) <dcr@neverwhen.org> on Thursday March 06, 2008 @10:35PM (#22672100) Homepage

      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.

      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

      • This is one of those moment where reading at Slashdot actually gets you to learn something.

        Cool post.
      • 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?
      • by guanxi (216397)
        It's a great description, but I think the heart of the matter still needs clarity:

        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
    • by famebait (450028)
      Beacuse an orbit remains in the same plane if left undisturbed.
      It takes energy to change the orbital plane.
    • 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 orb
  • Photo. (Score:5, Informative)

    by palegray.net (1195047) <philip.paradis@pa3.14legray.net minus pi> on Thursday March 06, 2008 @08:16PM (#22671208) Homepage Journal
    Here's a photo of Rhea [nasa.gov] from nasa.gov. Gives some nice background information on the moon as well.
  • Funny timing (Score:4, Interesting)

    by Brad1138 (590148) <brad1138@yahoo.com> on Thursday March 06, 2008 @08:18PM (#22671226)
    Just last week my son said something that made me wonder, "could we put a satellite in orbit around our moon"?
    • Re:Funny timing (Score:5, Interesting)

      by Mantaar (1139339) on Thursday March 06, 2008 @08:33PM (#22671326) Homepage
      Yes of course we could. If you download Celestia [celestia.org] you can see all sorts of interesting things in space.
      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.
      • by JonathanR (852748)

        *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.
        • by glavenoid (636808)

          *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...

        • Well, given the recent talk about making the trip a one-way [slashdot.org] proposition, I'd say that they would probably dispense with a lot of requirements. Why waste money on a highly-paid, highly-trained astronaut he's at best a disposable commodity? Just pick some joe like the GP who really wants to get into space and ship him out.

          I mean, technically you don't have to tell him he's not coming back.
      • by glavenoid (636808)

        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...

        • Why is the L1 point special in this regard?

          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.
      • Re: (Score:3, Insightful)

        by AikonMGB (1013995)

        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-

      • 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.
      • Celestia.org is a dead domain. Just list of 'searches' based on the word celestia and an offer to sell the domain name.
        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
      • by xaxa (988988)
        If you're European then you can either be an excellent pilot from the military, or an excellent scientist: http://www.esa.int/esaHS/ESA1RMGBCLC_astronauts_0.html [esa.int]

        (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])
    • by creimer (824291)
      No, no, no. The moon is good for slamming stuff into and leaving garbage behind. Besides, why have a satellite around the moon when the weather never changes?
    • Re: (Score:3, Informative)

      by AikonMGB (1013995)

      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-

      • by AikonMGB (1013995)

        Forgot to mention: The LRO orbital period is roughly 2 hours.

        Aikon-

      • by Brad1138 (590148)
        Thanks, my thought was that the pull from Earth might have to much of an effect on it to maintain a stable orbit, but I guess that doesn't really make sense. About the orbiter, will it be able to take high enough resolution pictures to see the lunar landers and put the "Moon landing was faked" argument to rest?
        • by AikonMGB (1013995)

          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.

    • by pz (113803)
      could we put a satellite in orbit around our moon?

      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)

    by Aegis Runestone (1248876) on Thursday March 06, 2008 @08:19PM (#22671230) Homepage Journal
    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).
    • How does the rotation of a planet affect it's orbiting bodies? over time do they drift toward the equator? I thought that i'd heard years ago that there was a moon (or moons) with rings. Is this really new news, or am i mistaken?
      • I know of no moon with rings, but I may be missing an old press-release somewhere. :-)

        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
    • Depending on your definition of "in the rings", there are around 5 moons (shepherds all) already known: Pan (Encke gap), Daphnis (Keeler gap), Atlas (Roche gap), and Prometheus and Pandora (shepherding the F ring). However, I'd be careful calling this the outer rings: the E and G rings are exterior to the F ring. :-)
  • ...it's a ringed moon(TM)!



    (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.
    • by rebelcan (918087)
      DOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOMED! </Bender>

      Damnit, it's supposed to be in-your-face-Bender-style-yelling, Slashdot, stop complaining.
  • by nobey (911370)
    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.
  • by LionKimbro (200000) on Thursday March 06, 2008 @08:45PM (#22671428) Homepage
    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.
    • by glavenoid (636808)
      Please, you need to think your plan through a little more. Funding problems and whatnot. "Enjoy Pepsi - Choice of the Lunar Generation"
    • Well, i guess it will happen sooner or later with all the space junk orbiting around us...
  • http://saturn.jpl.nasa.gov/news/press-release-details.cfm?newsID=820 [nasa.gov]

    "You've got 'Ring around the collar'..."

    Now, we find we've got "Rings around URhea..."

    What's next? "Rings around Uranus?"
  • that's no ring...
  • by CheshireCatCO (185193) on Thursday March 06, 2008 @08:54PM (#22671496) Homepage
    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.
  • by zakeria (1031430) on Thursday March 06, 2008 @08:56PM (#22671514) Homepage
    they got engaged ?
    • by dido (9125) <dido&imperium,ph> on Thursday March 06, 2008 @11:50PM (#22672438)

      You joke, but Saturn's (Cronus's) wife in mythology was named Rhea. A bit of a coincidence that.

  • " A broad debris disk and at least one ring appear to have been detected by a suite of six instruments on Cassini specifically designed to study the atmospheres and particles around Saturn and its moons."
    Please let the rings be named Dia.
  • > 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?
  • by tverbeek (457094) on Thursday March 06, 2008 @09:19PM (#22671674) Homepage
    The next discovery will be that one of the rocks orbiting Rhea itself has a ring around it.
    • Re: (Score:2, Funny)

      by Tablizer (95088)
      The next discovery will be that one of the rocks orbiting Rhea itself has a ring around it.

      Let's name it Recursia
           
  • the baby has a ring like the father.
  • Maybe it's space junk from an ancient civilisation.
    • by geekoid (135745)
      YOu know, I have often wondered if we could but a satellite in an orbit that can stay there for 10K+ yaers.
      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
  • We're sending so much crap out into orbit that we've built our own ring.
  • by rumli (1066212) on Thursday March 06, 2008 @09:50PM (#22671862)

    'Many years ago we thought Saturn was the only planet with rings,' said one mission scientist...
    But they were all of them deceived, for another ring was made...
  • At first I read that as "Rings Discovered Around the Moon for the First Time."
  • Every time you moon me,
    I see a ring around Uranus...
  • No Ur-anus postings. Or has the Goatse guy already made an appearance?

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