Editing Meteor shower (section)

==Dynamical evolution of meteoroid streams ==

Shortly after Whipple predicted that dust particles traveled at low speeds relative to the comet, Milos Plavec was the first to offer the idea of a ''dust trail'', when he calculated how meteoroids, once freed from the comet, would drift mostly in front of or behind the comet after completing one orbit. The effect is simple [[celestial mechanics]]&nbsp;– the material drifts only a little laterally away from the comet while drifting ahead or behind the comet because some particles make a wider orbit than others.<ref name="Jenniskens P. 2006" /> These dust trails are sometimes observed in comet images taken at mid infrared wavelengths (heat radiation), where dust particles from the previous return to the Sun are spread along the orbit of the comet (see figures).

The gravitational pull of the planets determines where the dust trail would pass by Earth orbit, much like a gardener directing a hose to water a distant plant. Most years, those trails would miss the Earth altogether, but in some years, the Earth is showered by meteors. This effect was first demonstrated from observations of the 1995 [[alpha Monocerotids]],<ref>Jenniskens P., 1997. Meteor steram activity IV. Meteor outbursts and the reflex motion of the Sun. Astron. Astrophys. 317, 953–961.</ref><ref>Jenniskens P., Betlem, H., De Lignie, M., Langbroek, M. (1997). The detection of a dust trail in the orbit of an Earth-threatening long-period comet. Astrohys. J. 479, 441–447.</ref> and from earlier not widely known identifications of past Earth storms.

Over more extended periods, the dust trails can evolve in complicated ways. For example, the orbits of some repeating comets, and meteoroids leaving them, are in [[resonant orbits]] with [[Jupiter]] or one of the other large planets&nbsp;– so many revolutions of one will equal another number of the other. This creates a shower component called a filament.

A second effect is a close encounter with a planet. When the meteoroids pass by Earth, some are accelerated (making wider orbits around the Sun), others are decelerated (making shorter orbits), resulting in gaps in the dust trail in the next return (like opening a curtain, with grains piling up at the beginning and end of the gap).  Also, Jupiter's perturbation can dramatically change sections of the dust trail, especially for a short period comets, when the grains approach the giant planet at their furthest point along the orbit around the Sun, moving most slowly. As a result, the trail has a ''clumping'', a ''braiding'' or a ''tangling'' of ''crescents'', of each release of material.

The third effect is that of [[radiation pressure]] which will push less massive particles into orbits further from the Sun&nbsp;– while more massive objects (responsible for [[bolide]]s or [[Glowworm (astronomy)|fireball]]s) will tend to be affected less by radiation pressure. This makes some dust trail encounters rich in bright meteors, others rich in faint meteors.
Over time, these effects disperse the meteoroids and create a broader stream. The meteors we see from these streams are part of ''annual showers'', because Earth encounters those streams every year at much the same rate.

When the meteoroids collide with other meteoroids in the [[Zodiacal dust|zodiacal cloud]], they lose their stream association and become part of the "sporadic meteors" background. Long since dispersed from any stream or trail, they form isolated meteors, not a part of any shower. These random meteors will not appear to come from the radiant of the leading shower.