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Comment by olddustytrail | Hacker News Reader

See, I knew this but now I think of it, it doesn't make sense.

The dust can't just hang there like it does in an atmosphere so why isn't it moving either in orbit or falling towards the sun? And if it is, how can it occur at the same time of year every year?

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The dust is ejected from the comet at relatively low speeds and the particles therefore remain in orbits that are very similar to the comet’s. Lots of objects in similar-but-different orbits quickly diverge such that they spread out along the entire orbital ellipse. So think about it more as a stream of objects in the shape of the comet’s ellipse. The whole area of the comet’s orbit is a “high traffic zone” for small particles, and where those streams cross the earth’s orbit you get meteor showers.

Imagine two small pebbles ejected from a comet at the same time, one in the prograde direction (ie. in the comet’s direction, ahead of it) and one retrograde. For the first pebble, at the point 180 degrees opposite on its orbital ellipse, it’s orbit has been raised by (say) a few thousand km, while the second pebble’s orbit has been lowered by a few thousand km. If you zoom out you can barely tell the difference - on a solar system scale the ellipses look almost identical. But the difference in velocities means that they will quickly start to diverge in time. Both pebbles will do a full orbit and return roughly to the same point in space where they were ejected. But the retrograde particle (ie. the one “dumped out the back” of the comet instead of the one “fired ahead” of the it) will arrive there much earlier - it went faster and had less distance to cover. Repeat with millions or particles and you get a cloudy stream along the ellipse.

The dust gets blown off the comet by the Sun evaporating ice particles where it was trapped on the comet. I imagine much of these dust particles would have negative velocities relative to the Sun. So it would take some time for them to start "falling" towards the Sun. In the case of the Lyrids, the Thatcher comet creating this dust layer returns every 417 years so the "dust cloud" gets renewed. Also, the trajectory of the particles that stay in the region where Earth orbits is alerted by the passage of the planets and their gravity pull. As a consequence there are "outbursts" of meteor showers every 60 years - as the dust clumps up to become denser. That's a layman's 2 cents opinion :)

https://en.m.wikipedia.org/wiki/Comet_dust

https://earthsky.org/astronomy-essentials/everything-you-nee...

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