Meteor showers (some of which are known as "meteor storms" or "meteor outbursts") are celestial events in which a number of meteors are observed to radiate from one point in the night sky. These meteors are caused by streams of cosmic debris called meteoroids entering Earth's atmosphere at extremely high speed on parallel trajectories. Most are smaller than a grain of sand, so almost all meteoroids disintegrate and never hit the Earth's surface. Fragments which do contact Earth's surface are called meteorites.
The radiant point :
Because meteor shower particles are all traveling in parallel paths, and at the same velocity, they will all appear to an observer below to radiate away from a single point in the sky. This radiant point is caused by the effect of perspective, similar to railroad tracks converging at a single vanishing point on the horizon when viewed from the middle of the tracks. Meteor showers are almost always named after the constellation from which the meteors appear to originate. This "fixed point" slowly moves across the sky during the night due to the Earth turning on its axis, the same reason the stars appear to slowly march across the sky. The radiant also moves slightly from night to night against the background stars (radiant drift) due to the Earth moving in its orbit around the sun. See "IMO" Meteor Shower Calendar 2007 (International Meteor Organization) for maps of drifting "fixed points."
Meteor showers are named after the nearest bright star with a Greek or Roman letter assigned that is close to the radiant position at the peak of the shower, whereby the declension of the Latin possessive form is replaced by "id" or "ids". Hence, meteors radiating from near the star delta Aquarii (declension "-i") are called delta Aquariids. The International Astronomical Union's Task Group on Meteor Shower Nomenclature and the IAU's Meteor Data Center keep track of meteor shower nomenclature and which showers are established.
The origin of meteoroid streams :
A meteor shower is the result of an interaction between a planet, such as Earth, and streams of debris from a comet.
Comets can produce debris by water vapor drag, as demonstrated by Fred Whipple in 1951 [1] , and by breakup. Whipple envisioned comets as "dirty snowballs," made up of rock embedded in ice, orbiting the Sun. The "ice" may be water, methane, ammonia, or other volatiles, alone or in combination. The "rock" may vary in size from that of a dust mote to that of a small boulder. Dust mote sized solids are orders of magnitude more common than those the size of sand grains, which, in turn, are similarly more common than those the size of pebbles, and so on. When the ice warms and sublimates, the vapor can drag along dust, sand, and pebbles. Each time a comet swings by the Sun in its orbit, some of its ice vapourises and a certain amount of meteoroids will be shed. The meteoroids spread out along the entire orbit of the comet to form a meteoroid stream, also known as a "dust trail" (as opposed to a comet's "dust tail" caused by the very small particles that are quickly blown away by solar radiation pressure).
Recently, Peter Jenniskens [2] has argued that most of our short-period meteor showers are not from the normal water vapor drag of active comets, but the product of infrequent disintegrations, when large chunks break off a mostly dormant comet. Examples are the Quadrantid and Geminid showers, which originated from a breakup of asteroid-looking objects 2003 EH1 and 3200 Phaethon, respectively, about 500 and 1000 years ago. The fragments tend to fall apart quickly into dust, sand, and pebbles, and spread out along the orbit of the comet to form a dense meteoroid stream, which subsequently evolves into Earth's path.
The dynamical evolution of meteoroid streams :
Shortly after Whipple predicted that dust particles travelled at low speeds relative to the comet, Milos Plavec was the first to offer the idea of a dust trail, when he calculated how meteroids, once freed from the comet, would drift mostly in front of or behind the comet after completing one orbit. The effect is simple orbital mechanics - 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.[2] 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 [3][4], and from earlier not widely known identifications of past Leonid storms.
In the 1890s, Irish astronomer George Johnstone Stoney (1826-1911) and British astronomer Arthur Matthew Weld Downing (1850-1917), were the first to attempt to calculate the position of the dust at Earth's orbit. They studied the dust ejected in 1866 by comet 55P/Tempel-Tuttle in advance of the anticipated Leonid shower return of 1898 and 1899. Meteor storms were anticipated, but the final calculations showed that most of the dust would be far inside of Earth's orbit. The same results were independently arrived at by Adolf Berberich of the Königliches Astronomisches Rechen Institut (Royal Astronomical Computation Institute) in Berlin, Germany. Although the absence of meteor storms that season confirmed the calculations, the advance of much better computing tools was needed to arrive at reliable predictions.
In 1985, E. D. Kondrat'eva and E. A. Reznikov of Kazan State University first correctly identified the years when dust was released responsible for several past Leonid meteor storms. In anticipation of the 1999 Leonid storm, Robert H. McNaught[5] David Asher,[6] and Finland's Esko Lyytinen were the first to apply this method in the West.[7][8] Peter Jenniskens has published predictions for future dust trail encounters, resulting in "meteor storms" or "meteor outbursts" for the next 50 years.[2]
Over longer periods of time, the dust trails can evolve in complicated ways. One effect is that the orbits of some repeating comets, and meteoroids leaving them, are in resonant orbits with Jupiter or one of the other large planets - so many revolutions of one will equal another number of revolutions of the other. So over time since Jupiter will have the same relative position intermittently and it will tend to pull meteoroids into keeping that relative position. 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), 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 change sections of the dust trail dramatically, especially for short period comets, when the grains approach the big 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 individual release of material.
The third effect is that of radiation pressure which will push less massive particles into orbits further from the sun - while more massive objects (responsible for bolides or fireballs) 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 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 main shower.
