Meteor shower

Leonid Meteor Shower

A meteor shower, also known as a "meteor storm," is a celestial event where a large number of meteors are seen within a very short period. These meteors are small fragments of cosmic debris entering Earth's atmosphere at extremely high speed, leaving a streak of light that very quickly disappears. Most of the small fragments of cosmic debris are smaller than a grain of sand, so almost all fragments are burnt up 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

Each time a periodic comet swings by the Sun, it produces large amounts of small particles, called meteoroids, which will eventually spread out along the entire orbit of the comet to form a meteoroid "stream". If the Earth's orbit and the comet's orbit come close enough at some point, then the Earth will pass through this stream for a few days at roughly the same time each year, producing a meteor shower. The parent bodies (comets) of most known meteor showers have now been identified.

Ronald A. McIntosh of Auckland, New Zealand has offered apparently the first idea of a meteoroid stream or trail in the 1950s where meteroids freed from the comet, traveling at low speeds relative to the comet, drift mostly in front of or behind the comet. However after that the pattern becomes complex and changing in detail even if the overall structure remains the same. Some of these effects will concentrate particles of a meteor shower into a meteor storm.

Robert McNaught and David Asher among others have attempted to characterize a meteor stream which is responsible for a storm as opposed to a shower:

Meteoroid trail between fragments of Comet 73P

A cloud of meteoroids is released from the comet in a roughly spherical distribution. The first effect is simple orbital mechanics - the material drifts only a little laterally away from the comet while drifting ahead or behind the comet forming a fragmentary trail of meteoroids with more or less dense zones from repeated revolutions of the comet through the neighborhood of the Sun. The second effect is that the orbits of most 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. So the trail has a clumping, a braiding or a tangling of crescents, or filaments 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.

Combining these effects results in narrow streams of meteoroids which when they approach the Earth's position at a particular time will result in a storm while the general time period will also see meteoroids from the general less dense trail of the comet as in a regular member of the shower which are not clustered due to these influences.

There are also always a small number of meteoroids long since dispersed from any stream or trail, which 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 radiate from a single point in the sky to an observer below. 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 they appear to originate. This "fixed point" moves across the sky during the course of the shower due to the Earth moving on its orbit. See "IMO" Meteor Shower Calendar 2007(Internaltional Meteor Organization) for maps of drifting "fixed points".

Most Famous 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. 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 Important Meteor Showers

Shower	time	parent object
Quadrantids	early January	Comet 96P/Machholz 2 aka 1994o
Lyrids	late April	Comet Thatcher
Pi Puppids	late April	Comet 26P/Grigg-Skjellerup
Eta Aquarids	early May	Comet 1P/Halley
June Bootids	late June	Comet 7P/Pons-Winnecke
South Delta Aquarids	late July	Comet 96P/Machholz 2??
Perseids	mid-August	Comet 109P/Swift-Tuttle
Draconids	early October	Comet 21P/Giacobini-Zinner
Orionids	late October	Comet 1P/Halley
Southern Taurids	early November	Comet 2P/Encke
Northern Taurids	mid-November	Comet 2P/Encke
Leonids	mid-November	Comet 55P/Tempel-Tuttle
Geminids	mid-December	Asteroid 3200 Phaethon
Ursids	late December	Comet 8P/Tuttle

• Periodic Comet 96P/Machholz 2 (1994o) suffered a catastrophic breakup in 1994.[1]

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. 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. 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, although it is unlikely that Spirit or Opportunity will still be operational at that time to observe it. 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.

See also

• Meteor
• Asteroid
• Comet
• Zenith hourly rate

• List of meteor showers This list of meteor streams and peak activity times are based on data from the International Meteor Organization while most of the parent body associations are from Gary Kronk's book, Meteor Showers': A Description Catalog, Enslow Publishers, New Jersey, ISBN 0-89490-071-4.
• See Meteor Streams
• and National Geographic News - Sky-Watcher Alert: Meteor Show Peaks This Week

External links

• The International Meteor Organisation
  • IMO 2007 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)