Comets and asteroids are two common astronomical objects in space that are identified by astronomers. They each have different attributes and behave in certain ways.
Comets: Dirty Snowballs Flying Through Space
Comets originate in the outer solar system and orbit around the sun. Their orbits are divided into long or short periods. A short period comet has an orbit from 20 to 200 years and a long period comet can have an orbit of thousands of years.
Comets are composed of ice, rock, space dust and frozen gasses. While they are in the outer solar system comets remain frozen but when comets come into the inner solar system and get closer to the sun the volatile elements of the comet heat up and the comet begins off-gassing causing a tail of gas to stream out of the center. The tail of a comet always points away from the sun. Comets leave a trail of debris that can become meteors as they streak across the sky and enter the atmosphere of earth.
The Leonidsis a well known meteor shower that is associated with the comet Tempel-Tuttle. They are called the Leonids because they appear to originate and radiate from the constellation of Leo. As the earth moves through the particles left by the comet’s tail a spectacular meteor shower is displayed on earth. The Leonids shower is somewhat predictable and in 2008 they were active between November 14th and 22nd, in 2009 November 10th to the 21st and as of this writing are predicted for 2010 for November 10th to the 21st again.
In 1994 a comet collided with Jupiter. Two well known comets are the Hale-Bopp Comet that appeared in 1997 and Halley’s Comet which last appeared in 1986 and is predicted to appear again in 2061. Comets have in times past and in contemporary time caused great fear and superstition leading to radical behaviors and predictions of the end of life. Some of you may remember or have read about the mass suicide of the Heaven's Gate religious cult in connection to the Hale-Bopp Comet appearance.
Asteroids: Rocks in Orbit
Asteroids are big rocks that come in all shapes and sizes and are made of various materials. They orbit the sun and there are some are large enough to be called minor planets. There is one discovery recently of an asteroid with its own moon. There are two primary asteroid belts; the main belt is between the orbits of Mars and Jupiter and the second belt called Near-Earth Asteroids (NEA’s) comes in closer proximity to the earth.
There are groups of asteroids whose orbital paths intersect with Earth’s orbital path. These near earth asteroids are monitored and diversion tactics are studied. You may have seen the 1998 movie Armageddon with Bruce Willis, Billy Bob Thornton and Ben Affleck in which an asteroid the size of Texas is heading for the Earth.
A Croissant Orbited by an Egg
Ida, a croissant shaped asteroid in the main belt was the first asteroid to be discovered to have it’s own moon in orbit. Ida’s moon is called Dactyl which is shaped like an egg.
Many movies, books, stories and superstitions have arisen from the appearances of and interactions with astronomical objects. Appearances of heavenly objects have inspired countless poets and dreamers as well as scientists and scholars.