A meteor shower, some of which are known as a "meteor storm", or "meteor outburst", is a celestial event where a group of meteors are observed to radiate from one point in the sky. These meteors are small fragments of cosmic debris entering Earth's atmosphere at extremely high speed. They vaporize due to friction with the air, leaving a streak of light that usually very quickly disappears. For bodies with a size scale larger than the atmospheric mean free path (10 cm to several metres) this visible light is due to the heat produced by the ram pressure (not friction, as is commonly assumed) of atmospheric entry. Most of the small fragments of cosmic debris are smaller than a grain of sand, so almost all fragments disintegrate and never hit the earth's surface. Fragments which do contact Earth's surface are called meteorites.

The causes of meteor showers

Comet Encke's meteoroid trail is the diagonal red glow

Meteoroid trail between fragments of Comet 73P

Irish astronomer George Johnstone Stoney (1826-1911), collaborating with British astronomer Arthur Matthew Weld Downing (1850-1917), and independently Adolf Berberich of the Königliches Astronomisches Rechen Institut (Royal Astronomical Computation Institute) in Berlin, Germany, had apparently first offered in the 1890s the idea of a meteoroid stream or trail, when they calculated how meteroids, once freed from the comet and traveling at low speeds relative to 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. ^[1] 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.

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^[2] and David Asher^[3], and Esko Lyytinen of Finland, were first to apply this method in the West. ^[4][5] Peter Jenniskens has published predictions for future dust trail encounters, resulting in a "meteor storm" or "meteor outburst", for the next 50 years. ^[6]

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.

Meteor showers originate from fixed points in the sky

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 among the constellations. 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".

Notable meteor showers

Perseid and Leonid meteor showers

The most visible meteor shower in most years are the Perseids, which peak on August 12th of each year at over 1 meteor a minute.

The most spectacular meteor shower is probably the Leonids, the King of Meteor Showers.^[7] Approximately every 33 years the Leonid shower produces a "meteor storm" with hundreds of thousands of meteors per hour. These Leonid storms gave birth to the term "meteor shower" since most meteor showers produce only a few meteors per hour, rarely producing as many as one meteor every 30 seconds. The last two massive Leonid storms were in 1933 and 1966. The anticipated storm of 1999 was much less spectacular. When the Leonid shower is not storming it is less active than the Perseids.

Other notable meteor showers

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^[10], 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. Because of the similar air pressure at altitudes for meteors only the relatively slower motion because of increased distance from the sun should marginally decrease meteor brightness.^[11] 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 is expected on December 20 2007. Other showers speculated about are a "Lambda Geminid" shower associated with the Eta Aquarids of Earth (ie both associated with Comet 1P/Halley), a "Beta Canis Major" shower associated with Comet 13P/Olbers, and "Draconids" from 5335 Damocles.^[12]

See also

• American Meteor Society (AMS)
• Asteroid
• Comet
• List of meteor showers
• Meteor
• North American Meteor Network
• Radiant, a point in the sky, from which (to a planetary observer) meteors appear to originate.
• Zenith Hourly Rate

References

External links

• North American Meteor Network
• The International Meteor Organisation
  • IMO 2008 Meteor Shower Calendar
• The American Meteor Society
• Meteor Showers Online, by Gary W. Kronk
• Meteor Showers, by Sky and Telescope
• Upcoming Meteor Showers, by Sky and Telescope
• Basics of Meteor Observing, by Sky and Telescope
• The Space Book by Eonitus
• Six Not-So-Famous Summer Meteor Showers Joe Rao (SPACE.com)
• Meteor Shower Photos and Info (AOL Research & Learn)
• Meteor Streams
• National Geographic News - Sky-Watcher Alert: Meteor Show Peaks This Week
• Infography about Meteor Showers
• Meteor showers Astronomy Cast episode #8, includes full transcript in PDF-format.

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