Short-period comets

Discussion in 'Astronomy, Exobiology, & Cosmology' started by Zeno, Jan 21, 2018.

  1. Zeno Registered Senior Member

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    I have a few questions on something I've been thinking about. First of all, have there been any new short-period comets? The second question is how is it possible for short-period comets to be replenished? It seems that if an object drifted into the inner solar system from the Kuiper Belt or the Oort Cloud, instead of becoming a short-period comet it would just be pulled in by the Sun's gravity and perish.
     
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  3. DaveC426913 Valued Senior Member

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    Just because it is a short-period comet does not in-and-of-itself mean it will be pulled in by the sun and perish, any more than a long period comet would. A s-p orbit is still an orbit.

    Excepting of course, that it will be more susceptible to planetary perturbation. But that door swings both ways. Perturbation can just as easily nudge a comet into a longer orbit.

    What would exhaust the s-p comets faster is collisions with planets. They spend more time in the area of higher impact probability.

    That being said, I imagine s-p comets are probably unstable on a cosmic timescale.
     
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  5. Zeno Registered Senior Member

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    I guess the point I'm trying to make is that the solar system is billions of years old so short-period comets need to be replenished. I fail to see how that can occur. If the replenishment is coming from the Kuiper Belt or the Oort Cloud why would they go around the Sun instead of just being pulled in by the Sun's gravity and perishing?
     
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  7. Michael 345 Bali in Nov closer Valued Senior Member

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    Would that not be due to the comet having enough speed to escape the sun's gravity and travel in orbit?

    Speed would be the key

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  8. DaveC426913 Valued Senior Member

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    What is wrong with perturbations in the orbits of long-period comets, turning them in to short-period comets?

    Why would they not go around the sun? Why would they be pulled straight into the sun?

    It is in orbit. It can't fall into the sun - because it has transverse motion - which can't go away.

    The sun's disc is only 1/100th of an AU - a very, very tiny target from 100,000 AUs distance.
    That's a ratio of 1:10,000,000.

    A comet, falling from the Oort cloud has to be that perfectly-aimed to have a chance at hitting the sun. That is the equivalent of trying to hit a marble on the ground by dropping a grain of sand from the edge of space.

    Imagine this scenario:

    You are in a weather balloon, at 62 miles altitude. Your job is to hit a marble on the ground by dropping a grain of sand.

    Your platform is not motionless. The weather balloon is moving in the wind.

    You must wait until you are right over the marble, and then you must give the marble a single random tap backwards - exactly enough to cancel its forward motion from being on the balloon. That tap must stop all motion of the grain of sand, so that it falls exactly vertically - and I mean exactly - to within one part in 10 million - if it's going to hit that marble 62 miles below.

    If you miss - even by 1 mm - your grain of sand will not perish, and will bounce back up and fall again - forever missing the marble.
     
    Last edited: Jan 23, 2018
  9. Zeno Registered Senior Member

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    215
    It seems that the only way for a new short-period comet to be created would be for a long-period comet to have its orbit perturbed by passing close to a planet. An extremely rare and unlikely event. Other than that, the new comet would have to have been forcefully thrusted towards a point near the Sun so that it can go around the Sun. I don't really understand what would cause that to occur. The new comet doesn't have to be aiming towards the Sun in order to be pulled in by the Sun's gravity. If it is moving into the inner solar system at a slow enough speed it will just gradually be pulled in by the gravity of the Sun.
    Have there been any document cases of an absolutely new short-period comet?
     
    Last edited: Jan 25, 2018
  10. exchemist Valued Senior Member

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  11. Janus58 Valued Senior Member

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    But it won't be moving at slow enough speed in the inner solar system because is has been gaining speed the whole time during its long fall in to the Sun from its point of origin. By the time a body which left the the Kuiper belt reached Earth orbit distance, it would be moving at ~41.5 km/sec, this is ~39% greater than the Earth's orbital velocity and not much less than escape velocity for the Sun at that distance. Unless its trajectory at that point is very nearly pointed directly at the Sun, it will miss, swing around the Sun and head back out to deep space. For an object originating at inner Oort cloud distance, this velocity jumps to ~42 km/sec at Earth orbit distance. It is really hard to have something falling in from the outer solar system hit the Sun, especially if it started from an existing orbit.
    Again, these are going to be rare events, so the chances of catching one "in the act" as it were are not likely. But these events are not needed to produce short-term comets.

    Of course you might be using a different definition for "short term" comet, as the standard definition is further divided in to Halley-type (>20 yr periods) and Jupiter-family(<20 yr periods). So if you consider Jupiter-family comets as short term and Halley_types as long term, then yes you could say that these shorter term comets originated as longer term comets. But since all the Jupiter-family comets have orbits that closely coincide with Jupiter's orbit, it is not a far stretch to conclude that they got their orbits from close encounters with Jupiter. We have seen evidence of comets having their trajectories altered by Jupiter (Shoemaker-Levy 9 actually was pulled into orbit around Jupiter.)
     
  12. Michael 345 Bali in Nov closer Valued Senior Member

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    I went there just for a looksee

    Did you know, to quote the site

    The Kuiper belt is a hypothetical massive flattened disc of billions of icy planetesimals supposedly left over from the formation of the solar system

    (my highlight)

    Last I heard Kuiper belt was established as real or did I miss something?

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  13. DaveC426913 Valued Senior Member

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    As a wise man once said 'God moves in mysterious ways, but as for his reps on Earth, they hardly move at all.' or somesuch.

    That's s Christian website.
     
  14. Michael 345 Bali in Nov closer Valued Senior Member

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    Interesting

    When I made my comment I thought I was being sort of glib and original (?)

    I had no idea the comment was already out there

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  15. Xelasnave.1947 Valued Senior Member

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    Lets imagine a comet setting off from way out where ever.
    Does it drop to a point where the Sun actually is or does it drop to a point where the Sun was?

    If we were able to fire a shot at the Sun we would aim at a point where the Sun appears to be now or would we fire at a position we calculate the Sun will be in eight minutes?
    Does the message of gravity travel at C? If so do things drop to a place where the Sun was rather than to where it actually is?
    Alex
     
  16. Janus58 Valued Senior Member

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    How you aim depends on your own motion motion relative to the Sun is. If you start in a circular or near circular orbit, you "aim" in the opposite direction of your orbital motion so your shot will drop into the Sun ( you have to cancel out almost all of your orbital velocity in order for your bullet to hit the Sun). If you have no tangential velocity, you just aim at the Sun.

    But what I think your question is really is about is "does gravity undergo aberration?". If you are in orbit around the Sun, the apparent direction that its light comes from is a little bit "ahead" of the actual Sun's position. The greater your orbital velocity the larger the angle. For Mercury, this works out to be about 33 sec of an arc between where we would see the Sun vs. the true direction of the Sun. This is called aberration. For something out in the Kuiper belt, it is ~ 3.2 sec of an arc.

    So what about gravity? The apparent direction of gravity will come from the actual direction of the Sun. Gravity does not show aberration. This makes sense, because if gravity did show aberration, then the direction that the Sun's gravity pulls on the Earth would come from slightly in the direction of the Earth's orbital direction and the Earth would be slowly speeding up as it orbited.
     
  17. Michael 345 Bali in Nov closer Valued Senior Member

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    That's what really puzzles me. As I understand it does travel at light speed

    Ignore for the moment any movement and think Earth / Sun - two stationary bodies
    Light takes 8 minutes to get here
    Gravity takes the same
    Light which hits the Earth has a arc window
    Gravity has same arc window
    We don't get all of the sun's light because of the arc
    We don't get all of sun's gravity because of distance

    How does that square up?

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  18. Xelasnave.1947 Valued Senior Member

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    Thank you for all your post but it is this sentence that explains things for me.
    Alex
     
  19. Xelasnave.1947 Valued Senior Member

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    Perhaps not.
    Alex
     
  20. Michael 345 Bali in Nov closer Valued Senior Member

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    How not so?

    The light fans out from the Sun and we only get the percentage from a small portion

    Gravity strength decreases with distance. The small portion of the Sun from which we get the light surely would be the same portion we are being affected by, from its gravity?

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  21. Xelasnave.1947 Valued Senior Member

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    I said perhaps because I dont know.
    I would think it reasonable to assume we get light from all the parts of the Sun we can see....but dont look at it to check as that could damage your eyes...and dont look to see if in fact looking will damage your eyes.
    If you jump off the tallest building you will find you are going faster just before you hit the ground than the speed you were going 10 feet into your downward trip.
    Caution do not jump off any buildings to test the validity of what I suggest.
    Does this show gravity is less because you choose a very tall building.
    Does the fact you stop when you hit the ground establish gravity as a weak force?
    I cant answer that. I have read the bible cover to cover and I am afraid there is nothing in there to answer your question.
    But thats good because it means you can invent any answer you like.
    Perhaps Janus58 covers it reasonably well.
    Alex
     
  22. iceaura Valued Senior Member

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    New discoveries of short-period comets are fairly common:
    http://www.rochester.edu/newscenter/rochester-scientist-discovers-new-comet-143482/
    https://twitter.com/RonBaalke/status/953760725398208512 (nice animation)
    etc.
    Obviously some of these are very likely to have recently entered new orbits, or they would almost certainly have been noticed before.
    That long period comets will sometimes be captured in short period orbits is a mathematical "certainty" ( very high theoretical probability).
    And that the supply of long period comets is replenished frequently is indicated by their more rapid fading and dissipation on average - they only survive a few dozen orbits, on average.

    So we have a theoretical likelihood and a matching likelihood based on circumstances of observation - you do the math.
     
    Last edited: Jan 26, 2018
  23. exchemist Valued Senior Member

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    Let's be accurate please. It is a creationist website. The two are very far indeed from synonymous.
     
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