With comets, we understand their orbital paths and know exactly where the comet will be and at what time. For the most part, asteroids remain the same. Most of the asteroids in our solar system exist in the asteroid belt, located between Mars and Jupiter. Asteroids, however, are much less predictable. Because asteroids are not as visible as comets, they can seem to appear out of nowhere. We know the orbits of many of the asteroids that come close to the Earth's orbit, also called Apollo asteroids. The problem is that there are still thousands of them that we do not know about yet. This is why asteroids are one of the most feared objects in our solar system. That fear is well founded, for if a large enough asteroid were to impact the Earth it could destroy life, as we know it.

The chances of such an impact are extremely small (about 1 every 250,000 years or so), but due to the catastrophic nature of such an impact we study the skies closely, searching for any such objects. Many programs, such as NASA's Near Earth Object (NEO) Program actively search for asteroids that could impact Earth. After finding such asteroids they calculate the asteroids orbit to see if it will cross the path of our planet. It is estimated that we will need about 10 years warning to send spacecraft out to an asteroid to prevent its impact.

Any asteroid smaller than 50 meters will simply burn up when it enters our atmosphere. Asteroids between one and two kilometers in size will cause extensive damage to an area. Asteroids over two kilometers in size can affect the entire globe.

This dust blocks sunlight from reaching the surface. As a result, crops throughout the planet would die from the lack of sunlight and starvation and disease would overcome much of the population. Even worse are the extremely large asteroids whose impact would block the sun long enough to wipe out the entire population. It was an extremely large asteroid (15 kilometers in size) that caused the extinction of the dinosaurs 65 million years ago.