A meteor, sometimes called a "shooting star," can be the brightest object in the night sky, yet meteoroids are the smallest bodies in the solar system that can be observed by eye.
Meteor shower sometimes occur when the Earth passes thru the orbit of a comet. Some occur with great regularity: Every year in August 9 and 13,we have the Perseid meteor shower, when the Earth passes thru the orbit of Comet Swift-Tuttle. In October we have the Orionid meteor shower, from Comet Halley. Meteors observed to radiate from a single point in the sky, seen over a limited period, usually of several hours or days. Meteor showers occur when the Earth's orbital motion causes it to cut through a stream of meteoric material. Each day as many as 4 billion meteors, most miniscule in size, fall to earth.

Their masses total several tons, seemingly a large amount, but negligible compared to the earth's total mass of 6,600,000,000,000,000,000,000 tons. A more typical, average rate for a good regular shower would be around 50 meteors per hour. Shower members are identified by the fact that their trails, if traced back on a sky map, all appear to intersect at a single point, called the radiant. The effect is one of perspective. The meteors are in fact caused by material entering the upper atmosphere along parallel tracks.

A very bright meteor, called "fireball", may weigh many kilograms, but even a meteor weighing less than a gram can produce a beautiful trail. Some of these are large enough to survive their trip through the atmosphere and impact the ground, then we call them Meteorites. Fireballs are sometimes followed by trails of light and they are visible for about 30 minutes. Some, called bolides, explode with a loud sound and sometimes they caused shadows on the ground.

What are meteors? - Meteors are better known as "shooting stars": startling streaks of light that suddenly appear in the sky when a dust particle from outer space evaporates high in the Earth's atmosphere. We call the light phenomenon in the atmosphere a "meteor", while the dust particle is called a "meteoroid".

Size - A Leonid meteor of magnitude +5, which is barely visible with the naked eye in a dark sky, is caused by a meteoroid that weights only 0.00006 gram and is only 0.5mm in diameter. Most visible Leonids are between 1 mm and 1 cm in diameter.

Speed - That tiny particle can cause a light so bright that it can be seen over distances of hundreds of kilometers. The reason is the astronomical speed of the meteoroids. Just before they enter the Earth's atmosphere, Leonid meteoroids travel at 71 kilometers per second, or some 2,663 times as fast as a fast pitch in baseball, or, if you want, around the Earth in 3.8 minutes!

Source of light - When meteoroids enter the Earth's atmosphere, they collide with numerous air molecules. Those collisions sputter away the outer layers of the particle, creating a vapor of sodium, iron and magnesium atoms. In subsequent collisions, electrons are knocked into orbits at larger mean distances from the nucleus of the atoms. When the electrons fall back to their rest positions, light is emitted. This is the same process as in gas discharge lamps.

Colors of meteors - The color of many Leonids is like the color of our sodium discharge lamps. For the same reason: meteoroids contain traces of sodium. The color of a meteor is an indication of its composition and the excitation temperature: sodium atoms give an orange-yellow light, iron atoms a yellow light, magnesium a blue-green light, calcium atoms may add a violet hue, while silicon atoms and molecules of atmospheric nitrogen give a red light.

Sounds - Meteors do not normally cause audible sounds. Hence, they will pass by unnoticed if not seen. But watch out for hissing sounds that have been reported for very bright meteors. These sounds are thought to be due to very low frequency (VLF) radio waves interacting with the local environment. A sonic boom is sometimes heard for very bright Leonid meteors, called fireballs, that appear near your own observing site high in the sky. If the particle is larger than the mean free path of the air molecules, a high Mach number shock wave forms in front of the meteoroid. Very rarely, this shock wave penetrates deep enough in the atmosphere that it can be heard. It sounds like the sonic boom of an airplane, but as a distant rumble.

Wake and Afterglow - The brief glow behind the meteor head is called the wake. The wake is mainly the green light of neutral oxygen atoms. Wakes last 1-10 seconds. Sometimes wake is also used to describe the area directly behind the meteor head. Very bright meteors leave a wake rich in emissions from neutral metal atoms (Na, Fe, Mg) which is called the afterglow.

The persistent train -After a rapid decay in intensity, bright Leonids often leave long enduring trains that last for 1-30 minutes (typically 4-6 minutes) at an apparent brightness of +4 to +5 magnitude. The light of these long enduring trains is from airglow-type chemistry (Na, FeO). Persistent trains last long enough to enable telescopic studies of the path of a meteor. This image was obtained at the Observatori Astronomic de Mallorca in November of 1998.

Meteor stream - Some days of the year show a larger number of meteors than usual that all have a common origin. This is a "shower of meteors", also called a "meteor stream". The Leonids are such a meteor stream November.

Radiant - All meteors of a stream appear to radiate from a single point on the sky, which is called the "radiant". The radiant of the Leonids is in the constellation Leo. The radiant is a perspective effect. All particles move in about the same orbit. An observer in the middle of the stream sees the meteors fall left and right, above and behind him. However, they all seem to come from a certain direction. That direction is the radiant.

Meteor stream activity - A cloud of dust particles in the path of a comet is called a "meteoroid stream". When the dust is still freshly ejected from the comet, it is usually in the form of a "dust trail". The dust trail evolves into a meteoroid stream as a result of perturbations by the major planets. It is not yet known exactly how this process happens. When the Earth crosses these dust components, various forms of meteor stream activity result.

Where do meteors come from? - Meteor streams are caused by the debris of comets. Leonid meteoroids move in the orbit of comet P/Tempel-Tuttle. The stuff of comets comes from interstellar space where the materials are assembled in the atmospheres of stars and in the dense molecular clouds of gas and dust between the stars. The comets are build of that material and were formed in the outer parts of the solar system, in regions beyond Saturn's orbit, at the time of the birth of our solar system.

Annual stream - Every year the Earth travels through the debris of many comets. That debris has moved far enough away from the comet orbit to collide with the Earth. The resulting meteor showers are refered to as "the annual meteor streams". In most years, the Leonids are a rather insignificant annual meteor stream. Rates peak at 13 per hour on November 17. The main activity is between November 13 and 20, but Leonids occur annually at a rate larger than 1 per hour in the period between October 31 and November 30 (in good dark skies early in the morning).

Meteor outburst - Only when the Earth travels through relatively fresh cometary matter will rates go up significantly above the normal level of annual activity. Then, and only then, can we see a meteor outburst. The Leonid storms are such meteor outbursts. There were no Leonid outbursts reported between 1970 and 1993. The first Leonid outburst of a new series associated with the upcoming return of P/Tempel-Tuttle was seen in 1994.

Meteor outburst and meteor storm. - A meteor storm is just an intense meteor outburst. We talk about meteor storms when rates increase to 1 per second (or Zenit Hourly Rate increases to 1000 per hour). The best annual streams peak at around ZHR =100 meteors per hour.

How the meteoroids leave the comet - Comets are mountains of ice and dust. When comets approach the Sun, the ices evaporate and the dust particles are ejected into orbit in geyser like fountains.

Tails and trails - The large particles remain close to the comet and form a dust coma. The smallest particles (less than 0.1 mm in size) are ejected from the dust coma by solar radiation forces and form the dust tail of a comet. The remaining material stays close to the orbit of the comet. However, small ejection velocities cause large differences in orbital period of these particles. Hence, in the next return the slow particles will lag and the fast particles will proceed the comet. The result is a trail like structure in the orbit of the comet. We see a meteor storm when the Earth crosses that trail of dust.

Credit: P. Jenniskens/NASA-ARC
Visit Leonid Mission Homepage: http://leonid.arc.nasa.gov/
Images and text are from http://leonid.arc.nasa.gov/meteor.html

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Observing Meteors

Meteors, Meteorites and Impacts

About Meteors and Meteor Showers

Frequently Asked Questions (FAQ) About Fireballs and Meteorite Dropping Fireballs

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