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What makes an asteroid an asteroid and not a comet

  1. May 13, 2009 #1
    Hi all

    Have been thinking about this for sometime, what is the fundamental difference that makes an asteroid an object orbitting around the star at a lesser speed and not a comet?

    Hope my question is clear, framing it otherwise why do asteroids not turn into raging comets or comets into sedate asteroids?

    Thanks again :smile:
  2. jcsd
  3. May 13, 2009 #2


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    Comets are almost exclusively in highly elliptical orbits (why we only see them every hundred years). This is thought to be because they were objects ejected (by some form of gravitational pertubation) from the Kuiper belt or Oort Cloud. An object in such a highly elliptical orbit spends a comparatively low amount of its life near enough to the sun to remove any gasses and melt any ices (what we observe as the comet's tail). An asteroid close enough to the sun would be stripped of these elements within a fairly short period of time.

    Though, the distinction is based almost solely on their appearance. Certainly an asteroid in the outer solar system could have the same composition as a comet which streaks through the solar system with a fairly long period. The difference is, because the asteroid is in a (much more) circular orbit, there is no perceptible change in its appearance (increase in luminosity), so they go undiscovered. If someone were to magically throw it towards the inner solar system, it would appear the same.
  4. May 14, 2009 #3
    What you are basically saying is that random gravitational perturbances cause the asteroid/stellar body to assume some extra velocity which in turn is added on by the gravitational pull of the star near it and hence it becomes a comet

    Am i correct then in assuming that the flip-flop between an asteroid and comet is a matter of pure chance?
  5. May 14, 2009 #4


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    It's an arbitrary definition, like the split between a planet and an asteroid.
    Any rock can become either depending on it's interactions.
  6. May 15, 2009 #5
    comets have more ice or frozen gas to form a tail
    and fewer hard high melting stuff in them
    a rocky or metal asteroid will not form a tail
    unless it gets so close to the sun to melt
    the rock or metal

    has there ever been a rock or metal comet without ice
  7. May 15, 2009 #6


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    what does a comet become when all it's ice has evaporated?

    Most of them, it only takes a few 1000s orbits for most to lose all their volatiles.
  8. May 15, 2009 #7
    but at that point it is no longer a comet!!!!!!!!!!!!
    but a asteroid with a odd orbit
  9. May 16, 2009 #8


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  10. May 18, 2009 #9
    Any rock IMO can become an ice ball far enough into space, which possibly explains why comets originate at the Oort cloud?
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