Thursday, December 3, 2009

Geminids Meteor Shower

What is a meteor shower?

Sometimes while looking at the dark night sky full of stars, suddenly a star appears to fall off the sky. We call it a “Shooting or Falling Star”. On some particular nights of the year we can observe a large number of shooting stars. These celestial events are called Meteor Showers.
Meteors are 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".

A geminids streaking past orion

Does it mean that when meteor showers occur we expect fireballs from the skies falling on earth, or does it actually look like a shower of meteors? Let’s explore the facts. A meteor is the visible path of a meteoroid (a small sand size particle) that enters the Earth's atmosphere. They can be seen on any night and they fall at random. But when the number of meteors is large, it is called a meteor shower or meteor storm. The shower always happen on a particular day or a time period as it is associated usually with comets. During meteor showers, which usually last a few days, the majority of the meteors appear to come from a particular point in the sky, called the radiant of the shower. The meteor shower is commonly named after the constellation in which this radiant is found, and occurs annually during a well-defined time period.

One of the most Spectacular Meteor Showers of the Year-Geminids

After last month's major meteor shower “Leonids”, this month i.e. on 14 December 2009, will bring us one more major meteor shower this year, called the “Geminids”. This is one of the best meteor showers of the year and never seems to disappoint observers.
How does this shower occur?

As comets move about their orbits they leave a stream of debris because dust and rocky material is liberated from the head as the ices vaporize. If the earth’s orbit crosses this debris, this leads to an increased number of meteors as the debris burns up while encountering the Earth’s atmosphere. When an object like a dust or rock floating in space enters the Earth's atmosphere at extremely high speeds, it heats up due to friction with the air. Its temperature increases very rapidly and it starts radiating light. Now this object can be of a size starting from small dust grains (1/100th of millimeter) to the size of pebbles (few 100 of millimeter). These falling, radiating objects are called meteors. Sometimes these Meteors can reach the surface of the Earth, these are called Meteorites.

Origin of the debris:

In the vast empty space between the planets there are huge regions containing mixture of dust and ice particles. These regions are caused by streams of debris from comets and sometimes asteroids. Comets are like dirty snowballs. They orbit around the sun and as they come closer to the sun some of its ice vaporizes and with this vapor some material is also thrown off the comet. It forms streams of debris along the orbit of the comet. When the Earth’s orbit passes through this stream, some of this space debris enters the atmosphere, they radiate and we observe this phenomena as a Meteor Shower. Interestingly, in the case of Geminids, the source is not a comet but a rocky asteroid named 3200 Phaethon, which is in a highly elliptical 1.4 year orbit.
Meteor showers are named after the nearest bright star or a particular constellation that is close to the radiant position at the peak (maximum) of the shower. So the Geminids got its name because its radiant position lies in the constellation Gemini.
The Geminids
The first Geminid meteors suddenly appeared in the mid-1800's. Those early showers were unimpressive, boasting a mere 10-20 shooting stars per hour. Since then, however, the Geminids have grown in intensity until today it is one of the most spectacular annual showers. In 1996, the last time the Geminids appeared in a dark moon-less sky, observers saw upto 110 per hour. The ZHR (Zenithal Hourly Rate) is about 50-100.
Most well known meteor showers, like the Perseids and Leonids, are old. They've been observed for hundreds or even thousands of years. The earliest record of a modern-day meteor shower is probably a notation in Chinese annals dated 36 AD, regarding the Perseids, where it is said that "more than 100 meteors flew thither in the morning."
The Geminids are a different story. The first Geminid meteors suddenly appeared in the mid-1800's. Those early showers were unimpressive, displaying a mere 10-20 shooting stars per hour. Since then, however, the Geminids have grown in intensity until today it is one of the most spectacular annual showers. In 1998 observers counted as many as 140 per hour (zenithal hourly rate). Meteor observers with clear skies should see close to that many this year if the Geminids continue to intensify.

After the discovery of the Geminids in 1862 astronomers began searching for the parent comet. Most meteor showers result from debris that that boils off a comet's nucleus when it passes close to the Sun. This debris orbits the Sun along with the comet, forming a thin, elongated stream of meteoroids that become shooting stars when they hit Earth's atmosphere. Years of searching proved to no avail until finally, in 1983, NASA's Infrared Astronomical Satellite discovered a curious object moving in the same orbit as the Geminid meteoroid stream. The orbital match was so good that it had to be the source of the debris, but to the surprise of many it wasn't a comet. The source of the Geminids was apparently a rocky asteroid. 3200 Phaethon, as the asteroid is now known, is in a highly elliptical 1.4 year orbit that brings it within 0.15 AU (astronomical units) of the Sun. It made its closest recent approach to Earth in December 1997 when it passed within 0.31 AU of our planet.

How does an asteroid produce a meteoroid debris stream?

Comets do it easily whenever they pass close enough to the sun to heat their frozen nucleus. Tiny bits of ice and dust naturally bubble away into interplanetary space. Rocky asteroids are made of tougher stuff, however, so it is unclear how bits of 3200 Phaethon would break or boil off to form a meteoroid stream. When Phaethon passes by the sun it doesn't develop a cometary tail, but bits and pieces do break off to form the Geminid meteoroids. Many astronomers now believe that Phaethon is an extinct or dormant comet that has accumulated a thick crust of interplanetary dust grains. Phaethon's thick mantle gives it the outward appearance of an asteroid, but underneath lies the nucleus of a comet.
Source of light: When meteoroids enter the Earth's atmosphere, they collide with air molecules. Those collisions sputter away the outer layers of the particle, creating a vapor of sodium, iron and magnesium atoms. Electrons are knocked into larger orbits 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.
Sounds: Meteors do not normally cause audible sounds, hence will pass by unnoticed if not seen. A sonic boom is sometimes heard for very bright Geminids meteors.
In 2008, viewing of the shower was restricted due to a full moon washing out the fainter meteors. 2009 will see the Geminids close to new moon phase so expect a good viewing.
How to observe Meteors?
Meteor watching is one of the easiest forms of astronomy. Anyone can go out in the early-morning hours, lie back in a chair or on ground, and wait for the occasional shooting star. Make sure that you have selected an area where light pollution is at its minimum. Plan to start your watch around midnight. By then Gemini constellation (radiant of Geminids) will be fairly high above the horizon towards the East. The hour or two before dawn should be best of all.
No trees or buildings should intrude into your view except maybe at the very edges. Get a sleeping bag for protection against cold, and dew. You'll also need a watch and a dim, red-filtered torchlight to read it by. Give your eyes at least 15 minutes to adjust to the dark. Settle in, look up, and relax. That’s it, now just sit back and enjoy the spectacle.
Now if you would like to count and estimate how many of these are from the Geminids, then you can do the following:
The simplest thing to do is just to count the number of "shower" (S) and "non-shower" (NS) meteors that you see. Shower meteors will seem to come from the Gemini constellation. Trace the path of a meteor backwards across the sky. If the line comes near the radiant, then you have observed a shower meteor. If the line goes elsewhere, then you have observed a non-shower meteor. Watch the sky at least 50° up, and pick a direction away from the radiant. Keep your field of vision filled with sky. Meteors can provide some of the more spectacular sights in the sky so stop reading about it and get out there! This year the viewing should be good as it is close to a new moon.