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Size limit of Comets

  1. May 25, 2009 #1
    Hi all

    Just a quick question

    What would be the maximum size to which a comet could actually go to to be able to maintain an elliptical orbit so as to follow kepler's laws

    To reword it what would be the least amount of eccentricity that a comet could have and what would this be dependent on?
  2. jcsd
  3. May 25, 2009 #2


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    I don't think there is a limit to the body size. Sure, when the bodies get large, dense and close there will be some relativistic effects at work, but as long as we are talking about 'normal' bodies they obey Kepler's laws regardless of their size.

    Or do you mean that the orbit gets modified because of the matter ejected from the comet? Even than, it is not a matter of comet size, but of the amount of the matter ejected vs comet mass. These are not directly related. Somehow related, but it is much more complicated - amount of mass ejected will depend on the comet composition, albedo, distance from the Sun and so on.
  4. May 26, 2009 #3
    No not really am looking at the elliptical orbit being maintained with a very low e as size increases. I hope am correct in saying that larger the body larger would be the elliptical orbit.

    So if we are going to have a comet say the size of jupiter, would it be possible to have an elliptical orbit
  5. May 26, 2009 #4


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    Jupiter already is in an elliptical orbit. There would be zero orbital difference if instead of a ball of hydrogen, it was mainly ice, dust, and rock.

    In a very good approximation, the mass of the orbiting body does not determine the orbital shape. This is completely determined by its angular momentum and the mass of the star.
  6. May 26, 2009 #5
    If I am allowed to extrapolate does this mean that jupiter if in an orbit of a comet could become a comet, obviously this has to happen way before it has become a gas giant
  7. May 26, 2009 #6


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    Define comet.
  8. May 28, 2009 #7
    First we have to consider what is being asked.

    Elliptical Orbit - As stated above, most planets are in a slightly elliptical orbit, other items like what was named Pluto are on greater elliptical orbits. Comets as we see them in the inner system are on an elongated elliptical orbit. This is not a pattern for growth, but decay. Once a comet gets into a pattern like this the eventual destruction is guaranteed, and it's life span is limited to it's ability to dodge the planets, moons, other comets, and meteorites in the inner system, as well as erosion from the strong solar wind the inner solar system is subjected to(gives it a tail).

    Comets are formed far out at the edge and beyond the edge of the planetary system. How large can they grow to? well it is likely they could grow to earth size or larger, and may have in the early solar system. That could vary well have been what supplied the lighter elements that makeup the atmosphere of the inner planets. The initial formation of the planets were either heavy elements in a cloud of light elements or a clump of asteroids that formed a planet, then from comets, or other stellar bodies came the lighter gashes elements depositing them on the solid bodies.

    In short, they can grow as large as their is mass to grow with, but once they start heading inward toward us, they are dieing.

    Despite what NASA says about the one probe that did a surface analysis of one Comet, we know very little about the composition, and many are likely different compositions from one another. We can tell by the tail, and the light coming off them a general idea of their base element, but no idea of percentages, or if they have other element layers or how they formed. What made the frozen gas come together just that way. A lot of theory but little real knowledge.
  9. May 29, 2009 #8
    (astronomy) a relatively small extraterrestrial body consisting of a frozen mass that travels around the sun in a highly elliptical orbit

    My question is relative to what?? :smile:
  10. May 29, 2009 #9
    Fair enough but is it a possible that a large enough comet can displace the position of planets? Say the way shoemacher levy jostled jupiter

    Thats alright , so is it again possible to have a comet at any size, would it be able to maintain an elliptical orbit
  11. May 29, 2009 #10


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    Does Jupiter share characteristic with this comet definition? Just putting it in the elliptical orbit will not make it relatively small, nor will make it consist of a frozen mass.
  12. May 29, 2009 #11
    Yes and sort of...

    Size yes.

    Maintain, is a word not used often in astrophysics, since everything is measured in it's time to die. Any orbit can be maintained for a time, but how long?? Possible yes, but for a relatively short period. The one test we had on a comet showed it is very loosly held together, this explains why shoemacher levy 9 broke up so completely, and easily. It is likely if a comet was as large as earth or even mars, it would break up into dozens or hundreds of pieces if it came too close to the inner planets/sun.

    I use words like 'probable', or 'likely' because there is no definate information to go on, since we have never seen anything like that.
  13. Jun 1, 2009 #12
    Cool answers my question

  14. Jun 2, 2009 #13
    I would think the smaller the bit of ice
    the more likely it is to be flung inward
    by coming too close to a bigger bit of stuff
    and few if any bigger bits of ice get knocked
    out of the ort cloud to come near the sun and
    give us a show
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