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Why do we only have one moon?

  • 22-10-2012 09:21PM
    #1
    krd
    Banned (with Prison Access) Posts: 3,455 ✭✭✭


    I was thinking, why is it that the earth is not surrounded by more orbiting bodies?

    We have a big moon, but there doesn't seem to be anything else there. Saturn and Jupiter both have lots of rocks orbiting them. But we don't.

    And the same could be said for the sun. Why does it not have rings like Jupiter?


Comments

  • #2
    Neewbie_noob
    Banned (with Prison Access) Posts: 543 ✭✭✭Neewbie_noob


    krd wrote: »
    I was thinking, why is it that the earth is not surrounded by more orbiting bodies?

    We have a big moon, but there doesn't seem to be anything else there. Saturn and Jupiter both have lots of rocks orbiting them. But we don't.

    And the same could be said for the sun. Why does it not have rings like Jupiter?

    Because we don't. That's why.


  • #3
    tuxy
    Closed Accounts Posts: 14,983 ✭✭✭✭tuxy


    The Earth has 100% more moons than Venus!


  • #4
    eyescreamcone
    Banned (with Prison Access) Posts: 2,562 ✭✭✭eyescreamcone


    Cutbacks ??


  • #5
    Neewbie_noob
    Banned (with Prison Access) Posts: 543 ✭✭✭Neewbie_noob


    eyescreamcone wrote: »
    Cutbacks ??

    Dey took errr jeerbs moons


  • #6
    endacl
    Registered Users, Registered Users 2 Posts: 22,447 ✭✭✭✭endacl


    We kind of have two. Kind of...

    Google cruithne.


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  • #7
    tuxy
    Closed Accounts Posts: 14,983 ✭✭✭✭tuxy


    I'm not sure why you are trying to compare a terrestrial planet to a gas giant.
    You should compare like with like.
    Here's a list of the terrestrial planets and the number of moons each one has.
    Mercury 0, Venus 0, Earth 1 and Mars 2

    So nothing unusual there.


  • #8
    krd
    Banned (with Prison Access) Posts: 3,455 ✭✭✭krd


    tuxy wrote: »
    I'm not sure why you are trying to compare a terrestrial planet to a gas giant.

    Yes...obviously there's a difference........but I wanted know why there's a difference.

    We can orbit man made satellites, what I'm wonder is, why we don't even have a few rocks up there.
    You should compare like with like.
    Here's a list of the terrestrial planets and the number of moons each one has.
    Mercury 0, Venus 0, Earth 1 and Mars 2

    Okay, Mars has two moons, but the smaller moon is more of a rock.

    I think there's a problem with three moons with a planet like ours. That, the orbits would be wildly chaotic. And very quickly they'd smash into each other. So four and whatever billion years ago, when the earth was forming, if there were other large molten bodies orbiting, they'd have smashed into us, or each other very quickly.

    Though. I still can't think why we don' at least have a few rocks orbiting us.

    So nothing unusual there.

    Unusualality, is relative.


  • #9
    krd
    Banned (with Prison Access) Posts: 3,455 ✭✭✭krd


    endacl wrote: »
    We kind of have two. Kind of...

    Google cruithne.

    That's scary......Have you seen the film Melancholia?


  • #10
    enda1
    Registered Users, Registered Users 2 Posts: 5,166 ✭✭✭enda1


    tuxy wrote: »
    The Earth has 100% more moons than Venus!

    So what, Venus has half a moon?

    Probably to help OP, our moon is so large that its gravity or our own would gather most space debris.


  • #11
    Victor
    Registered Users, Registered Users 2 Posts: 78,693 ✭✭✭✭Victor


    The argument can be made that the moon is not a moon, but a twin planet of earth.

    The Sun to Jupiter have cleared a disproportionate zone between them of anything but the largest objects (the asteroid best is composed of objects stuck between the two), so it is difficult for the inner planets to get moons.


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  • #12
    endacl
    Registered Users, Registered Users 2 Posts: 22,447 ✭✭✭✭endacl


    krd wrote: »

    That's scary......Have you seen the film Melancholia?
    Ah its not really. It's just physics acting on a big lump o' rock. Nothing mystical.


  • #13
    Capt'n Midnight
    Moderators, Recreation & Hobbies Moderators, Science, Health & Environment Moderators, Technology & Internet Moderators Posts: 94,795 Mod ✭✭✭✭Capt'n Midnight


    Moon was formed when PLANET SMASHED INTO EARTH !

    http://www.theregister.co.uk/2012/10/18/moon_made_from_earth_collision/
    That old theory that the Moon was formed out of fragments of Earth blasted into space after a massive planetary collision 4.5 billion years ago has gained support from two new studies.

    ....
    If the Earth was spinning through a day of just two hours, it would be near the point when it would start to fly apart itself from rotation forces, making it much easier to throw away loose bits of itself. After that, the so-called "evection" resonance between the orbiting bodies of the Earth round the Sun and the Moon round the Earth eventually changed the planet's rotation rate to the 24-hour day we have today.

    The other part of the mystery is the lack of iron on the Moon, which would be taken care of by the planet that smashes into the Earth, named Theia. In computer simulations, Theia has to be half the mass of Mars and strike at 20km/s in order to penetrate into the core of the Earth and throw material out some of which escapes Earth's gravity. In this scenario, Theia's iron core merges with the Earth's core, leaving the mantle to make the Moon, with a similar isotopic composition but a low iron content.
    Note: the spinning earth cancels out gravity (sorta)


    One of the definitions of a planet is sweeping debris from it's orbit or something, something the size of our moon would clear out most stuff apart from the trojan points. Cf. the resonances of rings of Saturn and the resonances of moons of Jupiter


    Pluto and Charon are another example of a large "moon"
    Our moon is bigger than Pluto



    Oh and for all the Star Trek fans - http://www.theregister.co.uk/2012/10/22/tiny_tractor_beam/


  • #14
    amen
    Registered Users, Registered Users 2 Posts: 2,781 ✭✭✭amen


    I still can't think why we don' at least have a few rocks orbiting u

    ahh but we do. http://science.nasa.gov/science-news/science-at-nasa/2006/09jun_moonlets

    There could be others that we just can't see.
    And the same could be said for the sun. Why does it not have rings like Jupiter

    The sun is extremely massive and over the last 4 billions years or so has simply gobbled up everything close to it.


  • #15
    krd
    Banned (with Prison Access) Posts: 3,455 ✭✭✭krd


    amen wrote: »
    ahh but we do. http://science.nasa.gov/science-news/science-at-nasa/2006/09jun_moonlets

    That's interesting. I'm still curious as to why we don't have more bodies in near earth orbit. We get lots of meteorites. I woud expect by the law of large numbers, that a few at least would fall into a stable earth orbit.

    There could be others that we just can't see.

    I don't know what any of the space agencies have. But I would expect the objects would be highly visible through radar. Like our space junk.
    The sun is extremely massive and over the last 4 billions years or so has simply gobbled up everything close to it.

    The sun is not the cookie monster. Bodies may gravitate towards each other, and eventually crash, but if they fall into a regular orbit, they won't crash into each other.

    Orbiting is essentially falling. Angular momentum is largely conserved, so the bodies never quite get around to crashing into each other.

    A real interesting thing about the rings of Saturn and Jupiter, is there are huge number of bodies, but they fallen into a very regular orbits - I don't really know much about the maths of chaos theory, but it seems, two bodies and you have realtively smooth regularity, three bodies it gets crazy very quickly, but once you expand it to millions of bodies (like the rocks around Saturn) they're is a wonderful order to their orbits.


  • #16
    Capt'n Midnight
    Moderators, Recreation & Hobbies Moderators, Science, Health & Environment Moderators, Technology & Internet Moderators Posts: 94,795 Mod ✭✭✭✭Capt'n Midnight


    krd wrote: »
    That's interesting. I'm still curious as to why we don't have more bodies in near earth orbit. We get lots of meteorites. I woud expect by the law of large numbers, that a few at least would fall into a stable earth orbit.
    The law of large numbers means that a lot of them would fall into the region where earths gravational attraction is greater than the suns.

    Because our moon is so large it becomes a three body problem rather than lots of asteroids happily co-orbiting a planet.

    Three body problems aren't stable , cf., chaos theory and strange attractors. You basically have 5 stable LeGrange points. and three of them aren't all that stable so really just two places where debris can accumulate , 60 degrees along or behind the moon on it's orbit around the earth. (stuff has been found at these points relative to planets orbiting the sun too)


    if you have ubuntu try this gravity simulator , use A, J to populate O to change colour

    apt-get install planets


  • #17
    amen
    Registered Users, Registered Users 2 Posts: 2,781 ✭✭✭amen


    Quote:
    The sun is extremely massive and over the last 4 billions years or so has simply gobbled up everything close to it.
    The sun is not the cookie monster. Bodies may gravitate towards each other, and eventually crash, but if they fall into a regular orbit, they won't crash into each other.

    yes you are correct. I just didn't explain myself well.


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