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My understanding of the meteor showers that repeat yearly is that the Earth passes through the orbit of a comet that is nowhere near us, and might not even exist anymore, yet it's orbit is filled with a stream of dust in the same heliocentric orbit by way ahead of or behind their cometary source by tens of millions of kilometers or more.

How do these particles spread out along the comet's orbit so far and yet remain tightly correlated transverse to the orbit? Some meteor showers like the "Unicorn shower" linked below can be quite short, lasting only a fraction of a day!

Left: From What (actually) is Jupiter doing to this year's Perseids meteor shower? Right: From Why would the Perseids meteor rate fall off after maximum faster than the increase before maximum? - click each for full size

enter image description here enter image description here

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The cometary particles need not retain the exact same orbit as the comet in order to stay close together near perihelion, thus where their orbit passes Earth's orbit.

Slight changes in the orbital parameters of the released dust particles can include slighly higher or lower aphelion, and thus change the revolution time and the time of perihelion (Earth) passage enough to spread out the particles. Over many revolutions it will eventually spread out over the whole orbit.

Additionally the orbits by the dust particles are subject to changes due to interaction with the solar wind and solar radiation which will over time change their orbits (yorp effect, Yarkovsky effect, radiation pressure etc).

This behaviour also explains that the meteor showers will be stronger in some years when Earth passes through a part where particles have not yet had time to more equally disperse.

Addendum: The other question on tails How can comets have tails if there's no air resistance in space? is related. Yet the (visible) tails are caused by either (ionized) gas or (small) dust particles (nm ... µm) which scatter the light. While these dust particles certainly also can make meteors, the larger the particles are, the less efficiently radiation pressure works on them, so their orbital evolution is much slower. And comets do release also a lot of bigger particles from µm up to cm (excact size distriubtion subject to disussion and analysis especially as follow-up to Rosetta mission, also depending on comet size and age) which are not easily visible in the visible light when observing it from Earth.

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  • $\begingroup$ I think we screwed up way back then by calling them "tails", since they don't necessarily lag behind the comet in its orbit, but are pulled away from it by gravitational forces. $\endgroup$ – Darrel Hoffman Apr 24 at 20:01
  • $\begingroup$ Gravity has nothing to do with the tails; see the linked other thread in my answer $\endgroup$ – planetmaker Apr 24 at 20:51
  • $\begingroup$ Oops, sorry, not gravity, magnetic field. Anyhow, the direction of the tail definitely does not indicate the direction of movement. $\endgroup$ – Darrel Hoffman Apr 24 at 21:16
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It's orbital mechanics. If a comet ejects a rock with a very small velocity, the rock winds up in an orbit very similar to the comet. However, that orbit may have a slightly different period. So, over many orbits, the rock drifts along the orbit with little sideways displacement. Changes in the other orbital elements don't cause a displacement that accumulates with time like the displacement caused by a change in orbital period.

Oddly, if the comet ejects the rock forward, the rock's orbital period increases, so it soon falls behind the comet, and continues to do so on the average as time goes on.

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    $\begingroup$ yep yep, concise and nice. Thanks! $\endgroup$ – uhoh Apr 24 at 23:46

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