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Consistency of Meteor Shower Dates (i.e. the peak of Perseids always on Aug 13th)

  1. Nov 10, 2009 #1
    First off, love the site and I use it all the time to get my mind working and I really appreciate everyone who puts the time in to answer questions and contribute their two cents to questions that are, at the moment, unanswerable.

    My question is regarding meteor showers and what mechanisms allow them to consistently occur at the same moment in the earth's orbit year after year. I guess the crux of my question concerns how ejected materials from comets is able to hold a stationary position in the solar system so that the earth passes through the cloud at a predictable time. I'm hoping that a better understanding of this question will help me establish a more intuitive understanding of orbital motion and the effects of gravity on small bodies. The are a few sub questions that I've come up with while trying to figure this out. To simplify (I hope) I've used the Perseid meteor shower as a discussion case:

    1. From what I've read the Perseids are ejected material from the passage of Comet Swift-Tuttle which has an orbital period of about 133 years. Additionally, wikipedia says that much of the material entering Earth's atmosphere during the Perseids comes from a passage of Swift-Tuttle approximate 1000 years ago. My question here is how has Swift-Tuttle's orbit been consistent enough over a thousand years that it passes through roughly the same spot in the solar system 8 orbits in a row. When I picture this I imagine large bodies in the solar system coming close enough to perturb it's orbit.
    2. Why is it that material "ejected" from the comet remains stationary after a thousand years? It seems that material "ejected" from the comet would still have a velocity which would cause it to scatter. I could possibly buy that it scatters together in a cloud but this would seemingly throw off the timing of Earth encountering the cloud.
    3. How is it that after 1000 passages of Earth through the filament cloud there is still material left for Earth to capture? It seems like, due to the fact that the Earth passes through the filament cloud every 1 year and the cloud only gets renewed every 133 years, Earth should clear out an empty swath of space in the filament cloud and thus turn the Perseids into an event that happens every 133 years.
    4. How does the gravity of the sun affect the material left behind by Swift-Tuttle? If we were to launch a space mission inside of the Earth-Sun L1 point and leave a car-sized object there with 0 velocity wouldn't we see it begin to accelerate toward the sun (plainly wouldn't an object not interacting with any other bodies interact with the largest gravity well in the galaxy)?

    thanks for all of your help in advance!
  2. jcsd
  3. Nov 10, 2009 #2


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    It doesn't hold a stationary position. Instead, comets eject debris along their entire orbit. Since the orbit itself changes very little, Earth intersects the orbit at the same time every year.

    The solar system is overwhelmingly dominated by the Sun; the second most massive object, Jupiter, is only a thousandth of its mass. Unless a comet passes very close to a planet--which is extremely unlikely because most of the solar system is empty space--it's going to follow more or less the same orbit around the Sun every single time. (If the comet and the Sun were the only objects that existed in the universe, the comet would follow precisely the same path every orbit. This is a consequence of the inverse-square law of gravitation.)

    Meteoroid streams are formed in two ways: either the volatile substances on a comet sublimate due to the Sun's heat and cause small chunks to break off, or the comet disintegrates. Neither process ejects material at a high speed.

    The filament cloud is orbiting the Sun, so Earth passes through different parts of it every year. Also, the meteoroid stream is billions of kilometers long (since it's scattered along the entire orbit) and millions of kilometers wide. A 0.012 million km rock passing through it for 2-3 days isn't going to do much.
  4. Nov 11, 2009 #3
    Thanks for the help!

    If I'm understanding what you say here correctly I think it answers most of the questions I had. The filament cloud is orbiting the sun along the same path as the comet. Therefore, as you said, the segment of the stream the earth passed through the year before is either closer to the sun or further away (I'm not sure if the Earth passes through the comet's path on its way in or out of the solar system).

    I guess one lingering question I have is in regards to the sublimation that you described. As pieces of the comet break off why is it that they lose velocity relative to the comet and are therefore left behind? It seems natural to me because I'm used to seeing air resistance do similar things in our atmosphere but I can't understand how it occurs in a vacuum. Is it the energy release that results from a state change the material of the comet goes through? Chunks that break off the comet loses velocity with respect to the comet but still have enough to continue along its path?
  5. Nov 12, 2009 #4
    I'll take this one. Pictures of comets near the sun (and near earth counts as near the sun) show two tails. One is caused by radiation pressure the other is caused by the solar wind. I'm sure you have heard of the solar wind, a very tenuous stream of high-temperature gas that flows from the sun all the way out of the solar system.

    The net effect of these two is to spread small particles from a comet in a wide band. Note that the tails always point away from the sun. As the comet approaches the sun, this is back along the orbital trajectory. As the comet passes near the sun you would think that the solar wind and radiation pressure would push the particles away from the comet's orbit. They do, but orbital mechanics comes into play. The net effect is that the particles move faster, but in a similar orbital track. When this orbit gets far away from the sun, it diverges from the comet's orbit. Since this orbit is longer, even though the particles are moving "faster", when the particles get back near the sun, they are close to the comet's original orbit, but behind the comet. So the net effect, whichever way the solar wind and sun's radiation push small particles, is that they end up following the comet around the sun.

    Oh, small particles includes the dust grain sized particles which are the largest number of meteorites. Bigger particles get moved less, which is why the 33 year period for spectacular shows. The earth has to pass very close to the comet's orbit and the comet. The comet's orbital period is not the same as the earth's. In the case of Swift-Tuttle, every 33 years the earth and comet are in almost the same places in their orbits.
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