The classification of space debris makes it very clear what a meteor is. When a solid particle hits the earth’s atmosphere, friction and ram pressure from its passage through the air heats the particle and the surrounding gasses. When it is hot enough to glow it is a meteor. Before the path of the particle has become visible, it is a meteoroid, and if it becomes one of the rare objects which reaches the ground before disintegrating, it becomes a meteorite - prized by scientists and collectors alike.
For an object to produce a meteor it generally needs to be at least pebble sized. Anything smaller is unlikely to generate enough heat to be seen before it is entirely disintegrated. The process begins about 75 miles above the ground, with the visible streak seen between 40 and 75 miles. By 30 miles above the ground the show is usually over.
An estimated one million meteors pass through the atmosphere every day. The majority of these are simply freely moving particles in space, and consequently they hit the atmosphere randomly, leaving a glowing trail that could point in any direction. Analysis of the meteor trajectories suggests that these particles drift into the earth’s orbit from the asteroid belt.
A less common phenomenon is the meteor shower. In a shower, multiple meteors can be observed over a short period of time. By tracing back the trails, it becomes clear that all of the objects are radiating from a single point in the sky. This is a perspective effect resulting from the parallel paths followed by the particles. The effect can be compared with the way parallel lines seem to converge at a point far in the distance.
The reason numerous particles are found to be moving in parallel and at the same velocity is that, unlike random meteors, these objects have a very particular and relatively small source. Meteor showers are usually generated by debris stripped from comets. Periodic comets leave a trail behind them as they orbit the sun. Sometimes, a comet’s trajectory takes it through the earth’s orbit (hopefully without colliding with us!) When the earth subsequently reaches the same point in the orbit, it intercepts the debris left behind and a meteor shower is the result. Once the comet has left a trail, the meteor shower becomes a yearly event.
There are dozens of meteor showers every year, but only a few are notable enough for the casual astronomer to pay them any attention. The two most well known and spectacular showers are the Perseids and the Leonids. The Perseids, arriving in mid-August, were formed by the comet Swift-Tuttle. The Leonids, mid-November, were formed by comet Tempel-Tuttle. As with all meteor showers, they are named after the constellation from which they appear to radiate.
A good meteor shower will contain around 100 meteors per hour, but the rate can go up and occasional meteor ‘storms’ have produced light shows of 1,000 meteors per hour. If you want to observe the showers yourself, you need nothing more than your own eyes. Binoculars will allow you to see dozens more fascinating things in the night sky, but watching a meteor shower is a simple case of looking up. Check out a meteor shower calendar such as the one at the International Meteor Organization website.