# Moons vs rocks **URL:** \ **Category:** Factual Questions\ **Created:** [March 20, 2000, 10:57am UTC](https://boards.straightdope.com/t/moons-vs-rocks/8951 "2000-03-20T10:57:00Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![Danielinthewolvesden](https://avatars.discourse-cdn.com/v4/letter/d/ce7236/32.png) [@Danielinthewolvesden](https://boards.straightdope.com/u/Danielinthewolvesden)\ **Post date:** [March 20, 2000, 10:57am UTC](https://boards.straightdope.com/t/moons-vs-rocks/8951/1 "2000-03-20T10:57:00Z")
It seems like everytime I turn around, a new, smaller “moon” of jupiter or saturn (or…) is “discovered”. However, most of these are very small and just jagged chunks of rock. Is there a dividing line between debis & “moons”? Clearly a baseball sized rock can’t be called a “moon” ( or else saturn would have thousands of them). Where do prof astronomers draw the line, or is there no line? Now, of course everytime we find a new space rock, we COULD just ask Cece, but he is so busy running the world empire… ---
**Author:** ![AWB](https://avatars.discourse-cdn.com/v4/letter/a/bb73d2/32.png) [@AWB](https://boards.straightdope.com/u/AWB)\ **Post date:** [March 20, 2000, 1:59pm UTC](https://boards.straightdope.com/t/moons-vs-rocks/8951/2 "2000-03-20T13:59:00Z")
Everything webwise I looked at didn’t give me a definitive answer. In general, any _natural_ object in orbit of a planet is a moon. Mars’ moons are 6 & 12 miles in diameter, but are far from spherical. Saturn’s moon’s Pan and Atlas are 12 & 18 miles in diameter, and interact with the rings. Atlas is a shepherd moon of Saturn’s A ring. Though they look continuous from the Earth, the rings are actually composed of innumerable small particles each in an independent orbit. They range in size from a centimeter or so to several meters. A few kilometer-sized objects are also likely. Saturn’s outermost ring, the F-ring, is a complex structure made up of several smaller rings along which “knots” are visible. Scientists speculate that the knots may be clumps of ring material, or mini moons. Saturn’s Titan, however, is larger than the planets Mercury and Pluto. So, sometimes their called moons, sometimes mini-moons, moonlettes, and just particles. * * * Wrong thinking is punished, right thinking is just as swiftly rewarded. You’ll find it an effective combination. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [March 20, 2000, 2:01pm UTC](https://boards.straightdope.com/t/moons-vs-rocks/8951/3 "2000-03-20T14:01:00Z")
I still think the Earth/Moon system is a binary planet system. ---
**Author:** ![RM\_Mentock](https://avatars.discourse-cdn.com/v4/letter/r/e274bd/32.png) [@RM\_Mentock](https://boards.straightdope.com/u/RM_Mentock)\ **Post date:** [March 20, 2000, 3:14pm UTC](https://boards.straightdope.com/t/moons-vs-rocks/8951/4 "2000-03-20T15:14:00Z")
> [@](#): > > Originally posted by Johnny L.A.: > **I still think the Earth/Moon system is a binary planet system.** What are your definitions, then? Do you think Saturn is a binary? * * * rocks ---
**Author:** ![AWB](https://avatars.discourse-cdn.com/v4/letter/a/bb73d2/32.png) [@AWB](https://boards.straightdope.com/u/AWB)\ **Post date:** [March 20, 2000, 3:18pm UTC](https://boards.straightdope.com/t/moons-vs-rocks/8951/5 "2000-03-20T15:18:00Z")
An arguement that the Earth/Moon (Terra/Luna) is _almost_ a binary planet system is that the center of their mutual orbit is only 1000 miles (or KM, can’t recall) below the Earth’s surface. If it were above the Earth to any degree, I’d say it was definately a binary planet system. * * * Wrong thinking is punished, right thinking is just as swiftly rewarded. You’ll find it an effective combination. ---
**Author:** ![aschrott](https://avatars.discourse-cdn.com/v4/letter/a/aca169/32.png) [@aschrott](https://boards.straightdope.com/u/aschrott)\ **Post date:** [March 21, 2000, 6:46am UTC](https://boards.straightdope.com/t/moons-vs-rocks/8951/6 "2000-03-21T06:46:00Z")
my favorite site for this info is: [http://csep10.phys.utk.edu/astr161/lect/index.html](http://csep10.phys.utk.edu/astr161/lect/index.html) it posits that some of the Jovian moons may actually be captured asteroids rather than part of the original planetary system. ---
**Author:** ![manhattan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/manhattan/32/7_2.png) [@manhattan](https://boards.straightdope.com/u/manhattan)\ **Post date:** [March 20, 2000, 7:53pm UTC](https://boards.straightdope.com/t/moons-vs-rocks/8951/7 "2000-03-20T19:53:00Z")
::Test Post: Please ignore this post. If you see multiposts above, please ignore them too. I’ll get a mop. ---
**Author:** ![Ursa\_Major](https://avatars.discourse-cdn.com/v4/letter/u/a88e57/32.png) [@Ursa\_Major](https://boards.straightdope.com/u/Ursa_Major)\ **Post date:** [March 20, 2000, 8:02pm UTC](https://boards.straightdope.com/t/moons-vs-rocks/8951/8 "2000-03-20T20:02:00Z")
I’ve also heard an arguement for calling our solar system a binary system (Sun/Jupiter). That’s wrong, of course. Why is it wrong? Because it’s obviously NOT a binary system! I think the criteria for making such a distinction is the same for moons/rocks, ships/boats, horses/ponies: “You know one when you see one!” ---
**Author:** ![RM\_Mentock](https://avatars.discourse-cdn.com/v4/letter/r/e274bd/32.png) [@RM\_Mentock](https://boards.straightdope.com/u/RM_Mentock)\ **Post date:** [March 20, 2000, 8:27pm UTC](https://boards.straightdope.com/t/moons-vs-rocks/8951/9 "2000-03-20T20:27:00Z")
Ursa Major That might be AWB’s argument. Let’s see, did I get anything wrong here: SunMass = 1.989 10^30 kg SunRadius = 1390000000 / 2 m JupiterMass = 1.900 10^27 kg JupiterDist = 778330000000 m The ratio of SunMass to JupiterMass is 1046.84, but the distance to Jupiter is 1119.9 times the SunRadius, so the center of their mutual orbit is outside the sun. * * * rocks ---
**Author:** ![aschrott](https://avatars.discourse-cdn.com/v4/letter/a/aca169/32.png) [@aschrott](https://boards.straightdope.com/u/aschrott)\ **Post date:** [March 20, 2000, 9:07pm UTC](https://boards.straightdope.com/t/moons-vs-rocks/8951/10 "2000-03-20T21:07:00Z")
the reason people discuss our solar system as being nearly binary is that, had Jupiter been about 100 times more massive (apparently not so significant in astronomical terms) it would have been a star rather than a planet. It’s more of a cool “what if” thing than a concept that actually applies. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [March 20, 2000, 10:27pm UTC](https://boards.straightdope.com/t/moons-vs-rocks/8951/11 "2000-03-20T22:27:00Z")
[quote] SunMass = 1.989 10^30 kg JupiterMass = 1.900 10^27 kg [/quote > [@](#): > > had Jupiter been about 100 times more massive (apparently not so significant in astronomical terms) it would have been a star rather than a planet. If Jupiter needs to be 100 times more massive to be a star, then if the Sun were 100 times _less_ massive it would NOT have been a star. Not sure where this is all leading… * * * _It is too clear, and so it is hard to see._ ---
**Author:** ![Jophiel](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jophiel/32/66_2.png) [@Jophiel](https://boards.straightdope.com/u/Jophiel)\ **Post date:** [March 20, 2000, 10:27pm UTC](https://boards.straightdope.com/t/moons-vs-rocks/8951/12 "2000-03-20T22:27:00Z")
Obviously, the moon would win. The tooth size to jaw ratio on Luna is about 120% that on your average rock and… Oh wait… this isn’t one of _those_ threads? * * * “I guess one person can make a difference, although most of the time they probably shouldn’t.” ---
**Author:** ![Danielinthewolvesden](https://avatars.discourse-cdn.com/v4/letter/d/ce7236/32.png) [@Danielinthewolvesden](https://boards.straightdope.com/u/Danielinthewolvesden)\ **Post date:** [March 21, 2000, 9:45am UTC](https://boards.straightdope.com/t/moons-vs-rocks/8951/13 "2000-03-21T09:45:00Z")
OK, but lets get back to the question I originally posed. As AWB pointed out there are chunks in Saturns rings in the several meters to maybe a kilometer. Why are these not moons? Why isn’t Saturn listed as having 100 moons, or whatever? They are natural, and in orbit. Can some yayhoo at some observatory “find” another dozen moons (and name them after the 7 Dwarves+?). Terra could very well have a meter-sized hunk-o-rock orbiting it, would that mean we have 2 moons? Do we have any contributors who are pro’s in this field? ---
**Author:** ![EnigmaOne](https://avatars.discourse-cdn.com/v4/letter/e/d2c977/32.png) [@EnigmaOne](https://boards.straightdope.com/u/EnigmaOne)\ **Post date:** [March 21, 2000, 10:48am UTC](https://boards.straightdope.com/t/moons-vs-rocks/8951/14 "2000-03-21T10:48:00Z")
On concrete answer for the OP, despite scouring all of my references. As an aside: {{{I still think the Earth/Moon system is a binary planet system.}}}—Johnny L.A. Asimov, Isaac - Asimov On Astronomy, Chapter 9: Just Mooning Around. pp 129-142. Paraphrased: The Moon is too far outside of the Roche limit to be a true satellite of the Earth, and too massive to have been captured by the Earth. Neither does it orbit the Earth in the plane of the Earth’s equator. Rather it revolves about the Earth in a plane approximating that of the ecliptic…something that would be expected of a planet. The math works out perfectly. Funny you should mention that concept as I’m currently working out the math on revised satellite discoveries, masses, orbits, etc. for my students–something different for the last day of this semester. * * * – **Kalél** [TheHungerSite.com](http://www.thehungersite.com) “If ignorance is bliss, you must be orgasmic.” “Well, there was that thing with the Cheese-Wiz…but I’m feeling **much** better now!” – John Astin, Night Court ---
**Author:** ![aschrott](https://avatars.discourse-cdn.com/v4/letter/a/aca169/32.png) [@aschrott](https://boards.straightdope.com/u/aschrott)\ **Post date:** [March 21, 2000, 1:29pm UTC](https://boards.straightdope.com/t/moons-vs-rocks/8951/15 "2000-03-21T13:29:00Z")
Saturn’s ring system is inside the Roche limit. This means that the tidal gravitational forces from the planet would rip apart any orbiting mass that attempted to accrete into a moon. I think that’s why none of the large chunks are considered actual satellites. In fact, one theory for the formation of Saturn’s ring system is that there WAS a moon, but the planet’s gravity ripped it into all those little pieces. But then, there are a couple of really small moons that orbit between ring sections and act as “shepherds” (AWB’s post), so I guess that shoots down my theory… (I know, you wanted experts…but it looks like you’re still hangin’ with us interested onlookers :)) ---
**Author:** ![Jophiel](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jophiel/32/66_2.png) [@Jophiel](https://boards.straightdope.com/u/Jophiel)\ **Post date:** [March 21, 2000, 10:13pm UTC](https://boards.straightdope.com/t/moons-vs-rocks/8951/16 "2000-03-21T22:13:00Z")
Just as an aside… > [@](#): > > Can some yayhoo at some observatory “find” another dozen moons (and name them after the 7 Dwarves+?) No. According to the International Astronomical Union (the guys in charge of deciding what in space is named what), the planetary moons are named after mythological figures (except the moons of Uranus who are named after classic literary fictional figures) that generally have something to do with the god in the planet itself is named after. If you were to discover a moon, it simply gets a code listing the planet, where it is and when it was found (a new moon for Neptune would simply be called “S/2000 N3” for example meaning “satellite found in 2000AD, 3rd one around Neptune”) and doesn’t get a name until it’s been confirmed. Even then, if you want a chance of “your” name being applied to it, you’d better play by the IAU’s rules or else you’ll find the chunk of rock you dubbed “Marijuana Land” will come to be known as “Amphitrite” (look it up) in the astronomical journals. * * * “I guess one person can make a difference, although most of the time they probably shouldn’t.” ---
**Author:** ![Jophiel](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jophiel/32/66_2.png) [@Jophiel](https://boards.straightdope.com/u/Jophiel)\ **Post date:** [March 21, 2000, 10:16pm UTC](https://boards.straightdope.com/t/moons-vs-rocks/8951/17 "2000-03-21T22:16:00Z")
Before anyone corrects me, I wasn’t thinking. Neptune already has 8 moons. Let’s make Amphtrite “S/2000 N9”. * * * “I guess one person can make a difference, although most of the time they probably shouldn’t.”