NASA tracks many near-Earth objects that orbit our Sun in their Near Earth Object Program. This includes asteroids and comets that are large enough to cause catastrophic damage if they collide with Earth. As a minimum, NASA tracks everything in our solar system that is 1 km in diameter or larger. Although they have detected smaller asteroids, 1 kilometer is considered to be the threshold that could cause extinction of the human race. Anything smaller that impacts Earth would cause immense local damage, but is considered survivable to humans.
NASA tracks these objects because their orbits are typically elliptical around the Sun taking them to areas around the outer gas giants of our solar system (ie. Saturn, Uranus, Neptune and their moons). If any of these objects come too close to these giants, the resulting gravitational effects could change their trajectory.
This implies that the orbits of these near-Earth objects could CHANGE. This means even a small "tug" on these objects by the gravity of a gas giant or its moon could be enough to change the trajectory of these objects.
Also, the affect of the gravitational "tug" may not be evident right away. It may be evident only after the object makes numerous orbits in the future. So NASA tracks the orbits of most of these and models them so they know the future potential for a collision with Earth.
When you look at the diagram below (compliments of neofuel.com), the shear number of near-Earth objects being tracked by NASA is staggering. However, the scale of this drawing is very large and in reality, all the objects shown on this diagram have very little chance of colliding with Earth, at the present time. However, NASA continues to track all these objects in case their orbits change.
NASA's Near Earth Object Program is essential in tracking all objects that have the potential to strike the Earth in the future. With this program, NASA would be able to predict the probability of an Earth collision and would have enough time to plan and launch a counter-measure.
Assuming the size of the object is small enough, the most likely counter-measure would be some sort of "impactor" sent to collide with the object in hopes of altering its trajectory enough to clear the Earth. Yes, this implies that NASA would not be able to do anything against objects that are immense (i.e. like objects the size of the moon). There is nothing that can be done against objects this large.
In 2004, it was predicted that an asteroid 325 meters in diameter had a 3% chance of impacting the Earth in 2029. (This asteroid was named Apophis, for the Greek god of the underworld and enemy of the Egyptian god of the Sun, Ra). More detailed calculations of the trajectory of Apophis have now eliminated the possibility of an Earth collision, but it is still being tracked.
As stated earlier, it is impossible to predict gravitional effects on these objects in the future.
Maddalena Environmental Inc.