Anything that comes close to the earth from outer space is known as a near-earth object (NEO). These include asteroids and comets that may have been pushed out of their normal direction and then begin to head to earth. While both comets and asteroids are included in this group, there are things that make them quite alike as well as different.
They are believed to be left over material from the beginning of our solar system, over 4 billion years ago. In the early years of earth’s creation, many hit the earth and if you look at the moon you can still see the craters that are left by the impacts. Both asteroids and comets played a major role in building our solar system, as we believe that in hitting the planets, they actually became part of the planets.
The difference between them is mainly what they are made of. Comets are made of rock, ice and organic compounds. They are sometimes called ‘dirty snowballs’. Although they are thought to have originally been made in the farthest sections of the solar system, they travel specific paths due to both planetary and the sun’s gravitational pull. As a comet nears the heat source of the sun, the ice melts and creates a gas. When traveling, the gas is reflected by the sun and we can sometimes see it from the earth. The gaseous ‘tail’ can be as long as thousands of miles. Scientists believe that when the earth was first forming, the water that is contained in the comets hit the earth and contributed to developing our oceans. It’s also believed that this affected the climate and possibly deposited carbon-based molecules that may have helped to start life on the planet. In ancient times, people thought that seeing a comet could be considered a ‘sign’. Some considered it bad, while other cultures thought it was good.
Asteroids are either made up of rock or rock and some metals, like nickel and iron. Some asteroids look like one big piece while others are actually clusters of smaller pieces that are being held together with the gravity from the whole asteroid. There are a small amount of asteroids that are actually burned out comets who lost all of their ice long ago and drift in space. Almost all of the asteroids originate in the asteroid belt that is between Jupiter and Mars. Jupiter’s massive gravity acts like a kind of guardian, keeping most of the asteroids away from earth.
Asteroids bang and knock into each other in the asteroid belt, and occasionally the force is strong enough to send one spinning into the solar system. This puts all of the planets at risk from being hit. Some of the smaller asteroids have come close to the earth and break up in the atmosphere. We call these ‘shooting stars’. There are, however, a lot of larger asteroids in the solar system that could hit earth and do a lot of damage. Thankfully, we have Jupiter to help keep them away.
Comets were never really considered as dangerous, because are usually on a consistent path far from the planets. This changed in 1994, when Comet Shoemaker-Levy 9 hit Jupiter. Jupiter’s gravity made the comet break up into smaller pieces and both scientists and the world watched as the impact created large dark spots on the giant planet. Since then, many countries have developed their own programs and are working together to watch for any asteroids and comets that could get near the earth.
NASA - Near Earth Object Program