A -  Comet: A comet is made up of cloud of dirty ice, non-volatile grains, and poisonous gases, called the nucleus. Comets travel in elliptical paths around the sun. Away from the sun, they show like pale points of light and the nucleus gases are frozen. In contrast, they show like a glowing object when they approach the sun. After that, they turn around and start a long journey in deep space.
  B -  Asteroids: These are very small planets that largely vary in their size. They move in elliptical trajectories around the sun, located normally between Mars and Jupiter (in a zone normally called: Asteroid Belt). Their total mass does not reach the mass of the earth’s moon. However, they are still a threat to earth. Some scientists believe that asteroids are the remaining of a destroyed planet.
  C -  Meteoroids: This category includes two sub-categories. In general, the term meteoroids refer to the objects orbiting the sun, similar to the asteroids, but with a relatively smaller size. The first type is called Meteors. These are the small meteoroids, whose sizes do not exceed small spheres. From their friction with the earth’s atmosphere, they burn causing light in the sky. They are termed also as the shooting stars. The other type is called Meteorites These are larger objects. They form large craters in the earth’s crust.

Scientists estimate the mass of the meteoroids that fall on the earth every day at around 3000 tons. However, most of these meteoroids are small meteors. The relatively larger ones are accompanied by a large explosion and glowing, but this happens at an altitude of 50 Km. Actually meteoroids offer for the scientists an opportunity to study the conditions of their formation; by analyzing these objects, it was discovered that they are composed of iron, magnesium, silicon, aluminum and other elements.
On the other side, comets as a matter of fact were the objects that drew attention to the issue of the impact idea. In 1994, the comet Shoemaker-Levy 9 collided with Jupiter in a spectacular astronomical event. Scientists studied that event and they started searching in the neighboring solar systems for any NEOs that may pass by the earth. However, still the chance for a comet to hit the earth is very low. Still, another threat comes from the comet, which is its long tail. This tail is composed of poisonous gases, extended along a distance of tens of millions of kilometers. In fact, the earth has gone through that experience three times (1819/1861/1910). However, our atmosphere’s high density protected us from the poisonous gases.
The real threat comes from the asteroid 1997 XF11. This asteroid was discovered by NASA. This mile-wide asteroid is expected to pass by the earth in the October 2028. In fact, a mile-wide asteroid colliding with Earth would have the equivalent destructive force of 200 atomic bombs. However, this one was estimated to pass at a distance of 960,000 kilometers. NASA described such a distance, according to their calculations, as a "comfortable" one and there is no need to worry. However, fate does not believe in calculations!!