The Leonids ([ˈli.əˌnɪdz] lee-uh-nids) are a prolific meteor shower associated with the comet Tempel-Tuttle. The Leonids get their name from the location of their radiant in the constellation Leo: the meteors appear to stream from that point in the sky. The 2009 display peaking on November 17 may produce more than 500 meteors an hour.
A meteor shower is a celestial event 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 speeds on parallel trajectories. Most meteors are smaller than a grain of sand, so almost all of them disintegrate and never hit the Earth's surface. Fragments that do survive impact with Earth's surface are called meteorites. Intense or unusual meteor showers are known as meteor outbursts and meteor storms, which may produce greater than 1,000 meteors an hour.
History
Earth moves through the meteoroid stream of particles left from the passages of the comet. The stream comprises solid particles, known as meteoroids, ejected by the comet as its frozen gases evaporate under the heat of the Sun which begins to warm the comet as it comes within the orbit of Jupiter. A typical particle is no bigger than fine dust. The main source of light of a meteor is caused by the solar wind, which fragments and atomizes the dust, and the resulting spray of microscopic debris collides with individual atoms of the atmosphere ionizing the air. The air molecules recombine and cool by giving off photons. Larger particles leave a stream of smaller particles and form a tail, which can leave a glowing trail in the atmosphere. Leonids in particular are well known for having such bright meteors. The meteoroids left by the comet are organized in trails in orbits similar to though different from that of the comet. They are differentially disturbed by the planets, in particular Jupiter (see also a full explanation by McNaught & Asher (1999)). The ensemble of the trails compose the meteoroid stream. Old trails are spatially not dense and compose the meteor shower background (a few meteors per minute), happening around November 17, but changing every year. Conversely, young trails are spatially very dense and the cause of meteor storms when the Earth enters one of these structures. Usual counts during a storm exceed 1000 meteors per hour, to be compared to the annual background (1 to 2 meteors per hour) and the shower background (a few per hour).
The Leonids are famous because their meteor showers, or storms, can be, and have been in a few cases, among the most spectacular. Because of the superlative storm of 1833 and the recent developments in scientific thought the Leonids have had a major effect on the development of the scientific study of meteors which had previously been thought to be atmospheric phenomena. The meteor storm of 1833 was of truly superlative strength. One estimate is over one hundred thousand meteors an hour, but another, done as the storm abated, estimated in excess of two hundred thousand meteors an hour over the entire region of North America east of the Rocky Mountains. It was marked by the Native Americans, slaves and owners, and many others. That same 1833 shower, near Independence, Missouri, was taken as a sign to push the growing Mormon community out of the area.
Other great Leonid storms were seen in 1866 and 1867. When the storms failed to return in 1899, it was generally thought that the dust had moved on and storms were a thing of the past. Then, in 1966 a spectacular storm was seen over the Americas. Leading up to the 1998 return, an airborne observing campaign was organized to mobilize modern observing techniques by Peter Jenniskens at NASA Ames Research Center. This resulted in spectacular footage from the 1999, 2001 and 2002 storms producing up to 3,000 Leonid meteors per hour. Initially, the exact location of the dust was unknown. A graph published in Sky and Telescope adapted from Comet 55P/Tempel-Tuttle and the Leonid Meteors(1996, see p.6) shows relative positions of the Earth and Tempel-Tuttle and marks where Earth encountered dense dust. This showed that the particles are behind and outside the path of the comet, but paths resulting in powerful storms were very near paths of nearly no activity. The work of David Asher, Armagh Observatory and Robert H. McNaught, Siding Spring Observatory, and independently of Esko Lyytinen in Finland, following on from the pioneering research by Kondrat'eva, Reznikov and colleagues at Kazan, is considered by most meteor experts as the breakthrough in modern analysis of meteor storms. Whereas previously it was hazardous to guess if there would be a storm or little activity, the predictions of Asher and McNaught timed bursts in activity down to five minutes. However, the relative brightness of the meteors is still not understood. The double spikes in Leonid activity in 2001 and in 2002 were due to the passage of the comet's dust ejected in 1767 and 1866. The 1833 storm was not due to the recent passage of the comet, but from a direct hit with the 1800 dust and the 1966 storm was from the 1899 passage of the comet. Examples of other streams accounting for spikes in activity include the 2004 June Bootids. Peter Jenniskens has published predictions for the next 50 years.
However, a close encounter with Jupiter is expected to perturb the comet's path, and many streams, making storms of historic magnitude unlikely for many decades.
In 2009 Leonids active between November 10-21, ranging from 300 to 400 predicted per hour
"There are no guarantees in meteor work ... observers should be alert as often as conditions allow throughout the shower, in case something unexpected happens, this year 2009 the best viewing spots will be Southern Asia, Northern and Eastern Australia".
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
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| Comet Encke's meteoroid trail is the diagonal red glow |
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, 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 vaporizes 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 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.
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| Meteoroid trail between fragments of Comet 73P |
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. 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, 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 David Asher, and Finland's Esko Lyytinen were the first to apply this method in the West. Peter Jenniskens has published predictions for future dust trail encounters, resulting in "meteor storms" or "meteor outbursts" for the next 50 years.
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.
Notable meteor showers
Perseid and Leonid meteor showers
The most visible meteor shower in most years are the Perseids, which peak on August 12 of each year at over 1 meteor a minute. A useful tool to calculate how many meteors per hour are visible from your observing location is found here: http://leonid.arc.nasa.gov/estimator.html.
The most spectacular meteor shower is probably the Leonids, the King of Meteor Showers[10] which peaks on a day near 17 November. Approximately every 33 years the Leonid shower produces a "meteor storm", peaking at rates of thousands of meteors per hour. These Leonid storms gave birth to the term "meteor shower", when it was first realised during the November 1833 storm that the meteors radiated from near the star Gamma Leonis. The last Leonid storms were in 1999, 2001 (two), and 2002 (two). Before that, there were storms in 1767, 1799, 1833, 1866, 1867, and 1966. When the Leonid shower is not storming it is less active than the Perseids.
Extraterrestrial meteor showers
Any other solar system body with a reasonably transparent atmosphere can also have meteor showers. For instance, Mars is known to have meteor showers, although these are different from the ones seen on Earth because the different orbits of Mars and Earth intersect orbits of comets in different ways.
Although the Martian atmosphere has less than one percent of the density of Earth's at ground level, at their upper edges, where meteoroids strike, the two are more similar. Because of the similar air pressure at altitudes for meteors, the effects are much the same. Only the relatively slower motion of the meteoroids due to increased distance from the sun should marginally decrease meteor brightness. This is somewhat balanced in that the slower descent means that Martian meteors have more time in which to ablate.
On March 7, 2004, the panoramic camera on Mars Exploration Rover Spirit recorded a streak which is now believed to have been caused by a meteor from a Martian meteor shower associated with comet 114P/Wiseman-Skiff. A strong display from this shower was expected on December 20, 2007. Other showers speculated about are a "Lambda Geminid" shower associated with the Eta Aquariids of Earth (ie both associated with Comet 1P/Halley), a "Beta Canis Major" shower associated with Comet 13P/Olbers, and "Draconids" from 5335 Damocles.
source : wikipedia
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