Meteor shower
2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)
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 of time. 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
Each time a periodic comet swings by the Sun, it produces large amounts of small particles 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 intersect 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 1950's 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.
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:
A cloud of meteoroids are released from the comet in a roughly spherical distribution. The first effect is simple orbital mechanics - the material drift little laterally away from the comet while drifting ahead or behind the comet forming something of a trail of meteoroid with more or less dense zones from repeated revolutions of the comet through the neighbourhood of the Sun. The second effect is that the orbits of most repeating comets, and materials leaving them are in resonant orbits with Jupiter - 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 over many revolutions 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 light pressure which will tend to push small objects, about the size of meteoroids, away from the Sun - exactly countering the effect of the Suns gravity to a small degree while larger objects (responsible for bolides or fireballs will tend to not be affected by light pressure.
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.
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 though when not storming it's 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. 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.