What is the Torino scale?
The Torino scale rates the hazard of an asteroid or comet impact from 0, no hazard, to 10, certain global catastrophe, to help communicate risk to the public.
The Torino scale is a simple way for scientists to communicate the risk of an asteroid or comet hitting Earth. It was adopted in 1999 at a conference in Turin, Italy, from which it takes its name.
The scale. It runs from 0 to 10 and combines two things: the probability of impact and the energy it would release, which depends on the object's size and speed.
- 0, white: no hazard. Almost all objects are rated 0. - 1, green: normal. A routine discovery with an extremely unlikely chance of collision; usually downgraded to 0 after more observations. - 2 to 4, yellow: meriting attention by astronomers. A close encounter with a small chance of impact. - 5 to 7, orange: threatening. A serious chance of impact by an object that could cause regional or global damage. - 8 to 10, red: certain collisions, ranging from localised destruction to a global catastrophe.
Examples. In December 2004, the asteroid Apophis briefly reached level 4, the highest ever, before more observations ruled out an impact in 2029. In early 2025, asteroid 2024 YR4 reached level 3 before being downgraded to 0.
Why it exists. Impact probabilities often rise at first and then drop to zero as observations improve. The scale helps avoid unnecessary alarm while making clear when attention is needed.
Other scales. Scientists also use the more technical Palermo scale, which compares an object's risk with the background risk from all objects of similar size.
Most of the time, every known object is rated 0, and ratings above 0 are usually lowered as soon as more observations arrive.