What is an Asteroid?
An asteroid is a celestial body, small or large, which orbits the sun with the other objects that are in our solar system, things like planets. Most asteroids are smaller then planets and very few actually get to be large. Every year, literally thousands of new asteroids are discovered.
In December of 2004, Asteroid 99942, now called Apophis was discovered and many observations indicated that there was a 2.7 percent chance that it could strike Earth in 2029. There were many more observations that said that the possibility of the asteroid hitting the earth were smaller then they thought.
However, when Asteroid Apophis swings by earth in 2029, it could go into what is known as the gravitational keyhole which is a specific part of space no more than 600 meters in diameter and if it does it could present a threat of impact on April 13th, 2036.
The distinct possibility that Asteroid Apophis could impact the earth in 2036 kept it on the Torino Impact Hazard Scale that they use to determine threat levels of possible near earth asteroids and Apophis remained on Level 4 which broke the record for any asteroid being on the list. It was then lowered to Level 1.