Astronomers use two impact hazard scales to describe the risks associated with Near Earth Objects (NEOs).
The simpler Torino Scale runs from 0 to 10. Objects rated 0 are either highly unlikely to strike Earth, or too small to survive the passage into the atmosphere, while an object meriting a 10 would be a certain impact large enough to cause global catastrophe.
Low numbers are assigned to newly discovered objects requiring further observation, while numbers above 5 are assigned to objects that may threaten Earth with various degrees of devastation.
The Palermo Technical Scale, on the other hand, combines the likelihood of collision with scale of potential devastation in a more mathematical way, incorporating a background risk level for objects of the same size, set as 0. Objects with positive values are potential threats, while those with values above 2 merit more in-depth observation.