The Torino scale is a system that predicts the chances of objects in space hitting the Earth. Established in 1999, the Torino scale rates the potential of an asteroid to cause harm by looking at its size, and the probability that it will hit the Earth. The scale ranges from zero to ten—zero means no danger and ten means that an asteroid is guaranteed to hit the Earth, and large enough that an impact would cause worldwide destruction.
This scale is meant to be an easier way for people to understand the risk associated with asteroid impacts. However, there has been a public debate about whether the scale is helpful. Some say there are other systems that are more effective. Others say the Torino scale isn't beneficial because it's unlikely for an asteroid to impact Earth.
If an asteroid appears to be approaching Earth, it is observed continually until its orbit can be determined. If astronomers determine there is no chance for an object to hit our planet, the object is assigned a zero on the Torino scale. But if there is a chance the asteroid could harm the Earth in the next 100 years, it will be given a higher value.
The highest rank any asteroid has received is a level four. This indicates a one percent chance of a collision causing a regional disaster. This happened in December of 2004. Fortunately, the object in question did not impact our planet.
Astronomers predict that, on average, the Torino scale should receive our attention once every 1,000 years, though total destruction events only occur approximately once every 100,000 years (or longer). The asteroid impact that wiped out the dinosaurs would be rated a 10 on the scale.
The Torino scale does not command much public interest, because asteroids that have a chance to cause damage to the Earth are very rare. But it is a helpful tool to keep us safe and aware of our Solar System.
[Source: The Planetary Society; Space.com; NASA]