In addition to the planets in our solar system, and the moons which orbit them, a vast number of smaller objects are known to exist. Thousands of comets have been discovered, and the number of asteroids is in the millions. What are these astronomical bodies, and how do they differ?
What Are Asteroids?
Asteroids resemble large boulders in space. Like the inner planets of the solar system, they are made of rock and metal, and were probably formed in the solar system's inner region. Asteroids tend to have circular orbits, and most of them are found orbiting the sun in the region between the planets Mars and Jupiter, which is known as the asteroid belt.
Asteroids can be as small as a few feet across, or as large as several miles in width. They cannot be seen from earth with the naked eye, and have therefor been generally unnoticed throughout history. Only relatively modern astronomical equipment has made detection of asteroids possible.
What Are Comets?
Comets, however, have been observed by mankind throughout history. This is because comets emit a tail which may be visible from earth. This tail consists of dust and ionized gas, and is formed by the pressure of solar radiation hitting the comet as it nears the sun.
Unlike asteroids, comets have very elongated orbits, which bring them close to the sun at certain times, then back out to extreme distances from the sun. A comet's orbit will make it visible from earth on a predictable schedule. Comet Halley, for example, can be seen from earth every 76 years. Other comets may take hundreds or even thousands of years to return.
Most of the time comets exist in the cold, outer regions of the solar system, where they are believed to have been formed. Because of this, they resemble dirty snowballs, being composed mostly of ice, with some rock and other organic (carbon-based) compounds. The solid part of the comet, called the nucleus, may be as small as one mile across, but some comets may approach 100 miles in diameter.
A comet has no tail during most of its orbit. Only as it approaches the sun is the tail created. Because of the pressure of the solar wind, the tail always faces away from the sun. As the comet goes around the sun, the tail does not follow directly behind the comet, as one might imagine. Instead the tail moves so that it is always facing away from the sun, relative to the position of the comet.
Asteroids That Once Were Comets?
Each time a comet approaches the sun, more of its ice is vaporized. After enough orbits, most of a comet's ice may be gone. It is possible that some objects now considered to be asteroids are comets which have had all of their ice vaporized and blown away. If enough rock remained, it would be identified as an asteroid when discovered.
Asteroids and Comets: Our Past and Our Future
Asteroids and comets are probably remnants from the formation of the solar system. Studying them may provide insights into the origin of our own planet, but the study of asteroids and comets is equally important to our future.
We know the dinosaurs were made extinct by an asteroid collision. An equivalent strike today would probably wipe out the human race. The probability of such an event occurring is very low, but cannot be dismissed. The Shoemaker-Levy 9 comet impact with Jupiter in 1994 shows that such catastrophic impacts do happen. We can only hope that continued study of asteroids and comets will provide humanity with enough warning to save itself.